Sickle Cell Disease

views updated May 14 2018

Sickle Cell Disease

Definition

Sickle cell disease describes a group of inherited blood disorders characterized by chronic anemia, painful events, and various complications due to associated tissue and organ damage.

Because sickle cell diseases are characterized by the rapid loss of red blood cells as they enter the circulation, they are classified as hemolytic disorders, "hemolytic" referring to the destruction of the cell membrane of red blood cells resulting in the release of hemoglobin.

Description

The most common and best-known type of sickle cell disease is sickle cell anemia, which is also called meniscocytosis, sicklemia, or SS disease. All types of sickle cell disease are caused by a genetic change in hemoglobin, the oxygen-carrying protein inside the red blood cells. The red blood cells of affected individuals contain a predominance of a structural variant of the usual adult hemoglobin. This variant hemoglobin, called sickle hemoglobin, has a tendency to polymerize into rod-like structures that alter the shape of the usually flexible red blood cells. The cells take on a shape that resembles the curved blade of the sickle, an agricultural tool. Sickle cells have a shorter life span than normally shaped red blood cells. This results in chronic anemia characterized by low levels of hemoglobin and decreased numbers of red blood cells. Sickle cells are also less flexible and stickier than normal red blood cells, and can become trapped in small blood vessels preventing blood flow. This compromises the delivery of oxygen, which can result in pain and damage to associated tissues and organs. Sickle cell disease presents with marked variability, even within families.

Carriers of the sickle cell gene are said to have sickle cell trait. Unlike sickle cell disease, sickle cell trait does not cause health problems. In fact, sickle cell trait is protective against malaria, a disease caused by blood-borne parasites transmitted through mosquito bites. According to a widely accepted theory, the genetic mutation associated with the sickle cell trait occurred thousands of years ago. Coincidentally, this mutation increased the likelihood that carriers would survive malaria infection. Survivors then passed the mutation on to their offspring, and the trait became established throughout areas where malaria was common. As populations migrated, so did the sickle cell trait. Today, approximately one in 12 African Americans has sickle cell trait.

Worldwide, it has been estimated that one in every 250,000 babies is born annually with sickle cell disease. Sickle cell disease primarily affects people of African, Mediterranean, Middle Eastern, and Asian Indian ancestry. In the United States, sickle cell disease is most often seen in African Americans, in whom the disease occurs in one out of every 400 births. The disease has been described in individuals from several different ethnic backgrounds and is also seen with increased frequency in Latino Americans—particularly those of Caribbean, Central American, and South American ancestry. Approximately one in every 1000-1400 Latino births are affected.

Causes and symptoms

Humans normally make several types of the oxygen-carrying protein hemoglobin. An individual's stage in development determines whether he or she makes primarily embryonic, fetal, or adult hemoglobins. All types of hemoglobin are made of three components: heme, alpha (or alpha-like) globin, and beta (or beta-like) globin. Sickle hemoglobin is the result of a genetic change in the beta globin component of normal adult hemoglobin. The beta globin gene is located on chromosome 11. The sickle cell form of the beta globin gene results from the substitution of a single DNA nucleotide, or genetic building-block. The change from adenine to thymine at codon (position) 6 of the beta globin gene leads to insertion of the amino acid valine-instead of glutamic acid—at this same position in the beta globin protein. As a result of this change, sickle hemoglobin has unique properties in comparison to the usual type of adult hemoglobin.

Most individuals have two normal copies of the beta globin gene, which make normal beta globin that is incorporated into adult hemoglobin. Individuals who have sickle cell trait (called sickle cell carriers) have one normal beta globin gene and one sickle cell gene. These individuals make both the usual adult hemoglobin and sickle hemoglobin in roughly equal proportions, so they do not experience any health problems as a result of having the trait. Although traces of blood in the urine and difficulty in concentrating the urine can occur, neither represents a significant health problem as a result of sickle cell trait. Of the millions of people with sickle cell trait worldwide, a small handful of individuals have experienced acute symptoms. In these very rare cases, individuals were subject to very severe physical strain.

When both members of a couple are carriers of sickle cell trait, there is a 25% chance in each pregnancy for the baby to inherit two sickle cell genes and have sickle cell anemia, or SS disease. Correspondingly, there is a 50% chance the baby will have sickle cell trait and a 25% chance that the baby will have the usual type of hemoglobin. Other types of sickle cell disease include SC disease, SD disease, and S/beta thalassemia. These conditions are caused by the co-inheritance of the sickle cell gene and another altered beta globin gene. For example, one parent may have sickle cell trait and the other parent may have hemoglobin C trait (another hemoglobin trait that does not cause health problems). For this couple, there would be a 25% chance of SC disease in each pregnancy.

Normal adult hemoglobin transports oxygen from the lungs to tissues throughout the body. Sickle hemoglobin can also transport oxygen. However, once the oxygen is released, sickle hemoglobin tends to polymerize (line-up) into rigid rods that alter the shape of the red blood cell. Sickling of the red blood cell can be triggered by low oxygen, such as occurs in organs with slow blood flow. It can also be triggered by cold temperatures and dehydration.

Sickle cells have a decreased life span in comparison to normal red blood cells. Normal red blood cells survive for approximately 120 days in the bloodstream; sickle cells last only 10-12 days. As a result, the bloodstream is chronically short of red blood cells and hemoglobin, and the affected individual develops anemia.

Sickle cells can create other complications. Due to their shape, they do not fit well through small blood vessels. As an aggravating factor, the outside surfaces of sickle cells may have altered chemical properties that increase the cells' 'stickiness'. These sticky sickle cells are more likely to adhere to the inside surfaces of small blood vessels, as well as to other blood cells. As a result of the sickle cells' shape and stickiness, blockages form in small blood vessels. Such blockages prevent oxygenated blood from reaching areas where it is needed, causing pain as well as organ and tissue damage.

The severity of symptoms cannot be predicted based solely on the genetic inheritance. Some individuals with sickle cell disease develop health- or life-threatening problems in infancy, but others may have only mild symptoms throughout their lives. Individuals may experience varying degrees of health at different stages in the life cycle. For the most part, this clinical variability is unpredictable, and the reasons for the observed variability can not usually be determined. However, certain types of sickle cell disease (i.e. SC disease) tend to result in fewer and less severe symptoms on average than other types of sickle cell disease (i.e. SS disease). Some additional modifying factors are known. For example, elevated levels of fetal hemoglobin in a child or adult can decrease the quantity and severity of some symptoms and complications. Fetal hemoglobin is a normally occurring hemoglobin that usually decreases from over 90% of the total hemoglobin to under 1% during the first year of life. This change is genetically determined, although some individuals may experience elevated levels of fetal hemoglobin due to variation in the genes that control fetal hemoglobin production. Such individuals often experience a reduction in their symptoms and complications due to the ability of fetal hemoglobin to prevent the polymerization of sickle hemoglobin, which leads to sickling of the red blood cell.

There are several symptoms that warrant immediate medical attention, including the following:

signs of infection (fever greater than >101°F or 38.3°C, coughs frequently or breathing trouble, unusual crankiness, feeding difficulties)
signs of severe anemia (pale skin or lips, yellowing of the skin or eyes, very tired, very weak)
signs indicating possible dehydration (vomiting, diarrhea, fewer wet diapers)

other signs (pain or swelling in the abdomen, swollen hands or feet, screams when touched)

These can be signs of various complications that occur in sickle cell disease.

Infections and effects on the spleen

Children with sickle cell disease who are under age three are particularly prone to life-threatening bacterial infections. Streptococcus pneumoniae is the most common offending bacteria, and invasive infection from this organism leads to death in 15% of cases. The spleen, an organ that helps to fight bacterial infections, is particularly vulnerable to the effects of sickling. Sickle cells can impede blood flow through the spleen, causing organ damage, which usually results in loss of spleen function by late childhood. The spleen can also become enlarged due to blockages and/or increased activity of the spleen. Rapid enlargement of the spleen may be a sign of another complication called splenic sequestration, which occurs mostly in young children and can be life-threatening. Widespread sickling in the spleen prevents adequate blood flow from the organ, removing increasing volumes of blood from the circulation and leading to accompanying signs of severe anemia.

Painful events

Painful events, also known as vaso-occlusive events, are a hallmark symptom of sickle cell disease. The frequency and duration of the pain can vary tremendously from person to person and over an individual's life cycle. Painful events are the most common cause of hospitalizations in sickle cell disease. However, only a small portion of individuals with sickle cell disease experience frequent and severe painful events. Most painful events can be managed at home. Pain results when small blood vessel blockages prevent oxygen from reaching tissues. Pain can affect any area of the body, although the extremities, chest, abdomen, and bones are frequently affected sites. There is some evidence that cold temperatures or infection can trigger a painful event, but most events occur for unknown reasons. The hand-foot syndrome, or dactylitis, is a particular type of painful event. Most common in toddlers, dactylitis results in pain and swelling in the hands and feet, sometimes accompanied by a fever.

Anemia

Sickle cells have a high turnover rate leading to a deficit of red blood cells in the bloodstream. Common symptoms of anemia include fatigue, paleness, and a shortness of breath. A particularly severe form of anemia—aplastic anemia—occurs following infection with parvovirus. Parvovirus causes extensive destruction of the bone marrow, bringing production of new red blood cells to a halt. Bone marrow production resumes after seven to 10 days; however, given the short lives of sickle cells, even a brief shut-down in red blood cell production can cause a rapid decline in hemoglobin concentrations.

Delayed growth

The energy demands of the bone marrow for red blood cell production compete with the demands of a growing body. Children with sickle cell anemia may have delayed growth and reach puberty at a later age than normal. By early adulthood, they catch up on growth and attain normal height; however, weight typically remains below average.

Stroke

Children with sickle cell disease have a significantly elevated risk of having a stroke, which can be one of the most concerning complications of sickle cell disease. Approximately 11% of individuals with sickle cell disease will have a recognizable stroke by the age of 20. Magnetic resonance imaging studies have found that 17% of children with sickle cell anemia have evidence of a previous stroke or clinically 'silent' stroke-like events called transient ischemic events. Stroke in sickle cell disease is usually caused by a blockage of a blood vessel, but about one fourth of the time may be caused by a hemorrhage (or rupture) of a blood vessel.

Strokes result in compromised delivery of oxygen to an area of the brain. The consequences of stroke can range from life-threatening, to severe physical or cognitive impairments, to apparent or subtle learning disabilities, to undetectable effects. Common stroke symptoms include weakness or numbness that affects one side of the body, sudden behavioral changes, loss of vision, confusion, loss of speech or the ability to understand spoken words, dizziness, headache, seizures, vomiting, or even coma.

Approximately two-thirds of the children who have a stroke will have at least one more. Transfusions have been shown to decrease the incidence of a second stroke. A recent study showed that children at highest risk to experience a first stroke were 10 times more likely to stroke if untreated when compared to high-risk children treated with chronic blood transfusion therapy. High-risk children were identified using transcranial doppler ultrasound technology to detect individuals with increased blood flow speeds due to constricted intracranial blood vessels.

As of 2003, researchers are investigating various techniques for helping children with memory loss related to strokes caused by sickle cell disease.

Acute chest syndrome

Acute chest syndrome (ACS) is a leading cause of death for individuals with sickle cell disease, and recurrent attacks can lead to permanent lung damage. Therefore rapid diagnosis and treatment is of great importance. ACS can occur at any age and is similar but distinct from pneumonia. Affected persons may experience fever, cough, chest pain, and shortness of breath. ACS seems to have multiple causes including infection, sickling in the small blood vessels of the lungs, fat embolisms to the lungs, or a combination of factors.

Priapism

Males with sickle cell anemia may experience priapism, a condition characterized by a persistent and painful erection of the penis. Due to blood vessel blockage by sickle cells, blood is trapped in the tissue of the penis. Priapism may be short in duration or it may be prolonged. Priapism can be triggered by low oxygen (hypoxemia), alcohol consumption, or sexual intercourse. Since priapism can be extremely painful and result in damage to this tissue causing impotence, rapid treatment is essential.

Kidney disease

The environment in the kidney is particularly prone to damage from sickle cells. Signs of kidney damage can include blood in the urine, incontinence, and enlarged kidneys. Adults with sickle cell disease often experience insufficient functioning of the kidneys, which can progress to kidney failure in a small percentage of adults with sickle cell disease.

Jaundice and gallstones

Jaundice is indicated by a yellow tone in the skin and eyes, and alone it is not a health concern. Jaundice may occur if bilirubin levels increase, which can occur with high levels of red blood cell destruction. Bilirubin is the final product of hemoglobin degradation, and is typically removed from the bloodstream by the liver. Therefore, jaundice can also be a sign of a poorly functioning liver, which may also be evidenced by an enlarged liver. Increased bilirubin also leads to increased chance for gallstones in children with sickle cell disease. Treatment, which may include removal of the gall bladder, may be selected if the gallstones start causing symptoms.

Retinopathy

The blood vessels that supply oxygen to the retina—the tissue at the back of the eye—may be blocked by sickle cells, leading to a condition called retinopathy. This is one of the only complications that is actually more common in SC disease as compared to SS disease. Retinopathy can be identified through regular ophthalmology evaluations and effectively treated in order to avoid damage to vision.

Joint problems

Avascular necrosis of the hip and shoulder joints, in which bone damage occurs due to compromised blood flow due to sickling, can occur later in childhood. This complication can affect an individual's physical abilities and result in substantial pain.

Diagnosis

Inheritance of sickle cell disease or trait cannot be prevented, but it may be predicted. Screening is recommended for individuals in high-risk populations. In the United States, African Americans and Latino Americans have the highest risk of having the disease or trait. Sickle cell is also common among individuals of Mediterranean, Middle Eastern, and Eastern Indian descent.

A complete blood count (CBC) will describe several aspects of an individual's blood cells. A person with sickle cell disease will have a lower than normal hemoglobin level, together with other characteristic red blood cell abnormalities. A hemoglobin electrophoresis is a test that can help identify the types and quantities of hemoglobin made by an individual. This test uses an electric field applied across a slab of gellike material. Hemoglobins migrate through this gel at various rates and to specific locations, depending on their size, shape, and electrical charge. Although sickle hemoglobin (Hb S) and regular adult hemoglobin (called Hb A) differ by only one amino acid, they can be clearly separated using hemoglobin electrophoresis. Isoelectric focusing and high-performance liquid chromatography (HPLC) use similar principles to separate hemoglobins and can be used instead of or in various combinations with hemoglobin electrophoresis to determine the types of hemoglobin present.

Another test called the sickledex can help confirm the presence of sickle hemoglobin, although this test cannot provide accurate or reliable diagnosis when used alone. When Hb S is present, but there is an absence or only a trace of Hb A, sickle cell anemia is a likely diagnosis. Additional beta globin DNA testing, which looks directly at the beta globin gene, can be performed to help confirm the diagnosis and establish the exact genetic type of sickle cell disease. CBC and hemoglobin electrophoresis are also typically used to diagnose sickle cell trait and various other types of beta globin traits.

Diagnosis of sickle cell disease can occur under various circumstances. If an individual has symptoms that are suggestive of this diagnosis, the above-described screening tests can be performed followed by DNA testing, if indicated. Screening at birth using HPLC or a related technique offers the opportunity for early intervention. More than 40 states include sickle cell screening as part of the usual battery of blood tests done for newborns. This allows for early identification and treatment. Hemoglobin trait screening is recommended for any individual of a high-risk ethnic background who may be considering having children. When both members of a couple are found to have sickle cell trait, or other related hemoglobin traits, they can receive genetic counseling regarding the risk of sickle cell disease in their future children and various testing options.

Sickle cell disease can be identified before birth through the use of prenatal diagnosis. Chorionic villus sampling (CVS) can be offered as early as 10 weeks of pregnancy and involves removing a sample of the placenta made by the baby and testing the cells. CVS carries a risk of causing a miscarriage that is between one-half to one percent.

Amniocentesis is generally offered between 15 and 22 weeks of pregnancy, but can sometimes be offered earlier. Two to three tablespoons of the fluid surrounding the baby is removed. This fluid contains fetal cells that can be tested. This test carries a risk of causing a miscarriage, which is not greater than one percent. Pregnant woman and couples may choose prenatal testing in order to prepare for the birth of a baby that may have sickle cell disease. Alternately, knowing the diagnosis during pregnancy allows for the option of pregnancy termination.

Preimplantation genetic diagnosis (PGD) is a relatively new technique that involves in-vitro fertilization followed by genetic testing of one cell from each developing embryo. Only the embryos unaffected by sickle cell disease are transferred back into the uterus. PGD is currently available on a research basis only, and is relatively expensive.

Treatment

There are several practices intended to prevent some of the symptoms and complications of sickle cell disease. These include preventative antibiotics, good hydration, immunizations, and access to comprehensive care. Maintaining good health through adequate nutrition, avoiding stresses and infection, and getting proper rest is also important. Following these guidelines is intended to improve the health of individuals with sickle cell disease.

Penicillin

Infants are typically started on a course of penicillin that extends from infancy to age six. Use of this antibiotic is meant to ward off potentially fatal infections. Infections at any age are treated aggressively with antibiotics. Vaccines for common infections, such as pneumococcal pneumonia, are also recommended.

Pain management

Pain is one of the primary symptoms of sickle cell anemia, and controlling it is an important concern. The methods necessary for pain control are based on individual factors. Some people can gain adequate pain control through over-the-counter oral painkillers (analgesics ). Other individuals, or painful events, may require stronger methods that can include administration of narcotics. Alternative therapies may be useful in avoiding or controlling pain, including relaxation, hydration, avoiding extremes of temperature, and the application of local warmth.

Blood transfusions

Blood transfusions are not usually given on a regular basis but are used to treat individuals with frequent and severe painful events, severe anemia, and other emergencies. In some cases blood transfusions are given as a preventative measure, for example to treat spleen enlargement or prevent a second stroke (or a first stroke in an individual shown to be at high risk).

Regular blood transfusions have the potential to decrease formation of hemoglobin S, and reduce associated symptoms. However, there are limitations and risks associated with regular blood transfusions, including the risk of blood-borne infection and sensitization to proteins in the transfused blood that can make future transfusions very difficult. Most importantly, chronic blood transfusions can lead to iron overload. The body tends to store excess iron, such as that received through transfusions, in various organs. Over time, this iron storage can cause damage to various tissues and organs, such as the heart and endocrine organs.

Some of this damage can be prevented by the administration of a medication called desferoxamine that helps the body to eliminate excess iron through the urine. Alternately, some individuals receive a new, non-standard treatment called erythrocytapheresis. This involves the automated removal of sickle cells and is used in conjunction with a reduced number of regular transfusions. This treatment helps to reduce iron overload.

Hydroxyurea

Emphasis is being placed on developing drugs that treat sickle cell anemia directly. The most promising of these drugs in the late 1990s is hydroxyurea, a drug that was originally designed for anticancer treatment. Hydroxyurea has been shown to reduce the frequency of painful crises and acute chest syndrome in adults, and to lessen the need for blood transfusions. Hydroxyurea seems to work by inducing a higher production of fetal hemoglobin. The major side effects of the drug include decreased production of platelets, red blood cells, and certain white blood cells. The effects of long-term hydroxyurea treatment are unknown; however, a nine-year follow-up study of 299 adults with frequent painful crises reported in 2003 that taking hydroxyurea was associated with a 40% reduction in mortality.

Bone marrow transplantation

Bone marrow transplantation has been shown to cure sickle cell anemia in some cases. This treatment is reserved primarily for severely affected children with a healthy donor whose marrow proteins match those of the recipient, namely a brother or sister who has inherited the same tissue type. Indications for a bone marrow transplant are stroke, recurrent acute chest syndrome, and chronic unrelieved pain.

Bone marrow transplantations tend to be the most successful in children; adults have a higher rate of transplant rejection and other complications. There is approximately a 10% fatality rate associated with bone marrow transplants done for sickle cell disease. Survivors face potential long-term complications, such as chronic graft-versus-host disease (an immunemediated attack by the donor marrow against the recipient's tissues), infertility, and development of some forms of cancer. A relatively recent advance in transplantation involves the use of donor stem cells obtained from cord blood, the blood from the placenta that is otherwise discarded following the birth of a new baby. Cord blood cells, as opposed to fully mature bone marrow cells, appear to be less likely to result in graft-versus-host disease in the recipient. This increases the safety and efficacy of the transplant procedure.

Surgery

Certain surgical interventions are utilized in the treatment of specific sickle cell-related complications. Removal of a dysfunctioning gallbladder or spleen can often lead to improvements in health. Investigations are currently underway to establish the efficacy of hip coring surgery, in which a portion of affected bone is removed to treat avascular necrosis of the hip. The hope is that this may provide an effective treatment to alleviate some pain and restore function in the affected hip.

Gene research

Replacing the gene that produces the defective hemoglobin in sickle cell disease patients with one that makes normal hemoglobin may be a possible treatment due to recent research. According to a 1998 report in Science, researchers studied the blood cells from people who carry the sickle cell gene. By using an enzyme called a ribosome, the study was able to alter sickle cells into normal cells. The ribosome cut out the mutated instructions in the cells' genetic pattern and replaced them with the correct instructions. Researchers hope that this will allow the cells to make normal hemoglobin—leading to the ultimate treatment for those with sickle cell disease.

In late 2001, genetic scientists reported that they had designed a gene that might lead to a future treatment of sickle cell anemia. Although the gene had not been tested in humans, early results showed that the injected gene protected cells from sickling. As of 2003,

KEY TERMS

Amino acid— Organic compounds that form the building blocks of protein. There are 20 types of amino acids (eight are "essential amino acids" which the body cannot make and must therefore be obtained from food).

Anemia— A blood condition in which the level of hemoglobin or the number of red blood cells falls below normal values. Common symptoms include paleness, fatigue, and shortness of breath.

Bilirubin— A yellow pigment that is the end result of hemoglobin breakdown. This pigment is metabolized in the liver and excreted from the body through the bile. Bloodstream levels are normally low; however, extensive red cell destruction leads to excessive bilirubin formation and jaundice.

Bone marrow— A spongy tissue located in the hollow centers of certain bones, such as the skull and hip bones. Bone marrow is the site of blood cell generation.

Bone marrow transplantation— A medical procedure used to treat some diseases that arise from defective blood cell formation in the bone marrow. Healthy bone marrow is extracted from a donor to replace the marrow in an ailing individual. Proteins on the surface of bone marrow cells must be identical or very closely matched between a donor and the recipient.

Globin— One of the component protein molecules found in hemoglobin. Normal adult hemoglobin has a pair each of alpha-globin and beta-globin molecules.

Heme— The iron-containing molecule in hemoglobin that serves as the site for oxygen binding.

Hemoglobin— Protein-iron compound in the blood that carries oxygen to the cells and carries carbon dioxide away from the cells.

Hemoglobin A— Normal adult hemoglobin that contains a heme molecule, two alpha-globin molecules, and two beta-globin molecules.

Hemoglobin electrophoresis— A laboratory test that separates molecules based on their size, shape, or electrical charge.

Hemoglobin S— Hemoglobin produced in association with the sickle cell trait; the beta-globin molecules of hemoglobin S are defective.

Hemolytic— Referring to the destruction of the cell membranes of red blood cells, resulting in the release of hemoglobin from the damaged cell.

Hydroxyurea— A drug that has been shown to induce production of fetal hemoglobin. Fetal hemoglobin has a pair of gamma-globin molecules in place of the typical beta-globins of adult hemoglobin. Higher-than-normal levels of fetal hemoglobin can prevent sickling from occurring.

Impotence— The inability to have a penile erection, which can be due to tissue damage resulting from sickling within the penis (priapism).

Iron overload— A side effect of frequent blood transfusions in which the body accumulates abnormally high levels of iron. Iron deposits can form in organs, particularly the heart, and cause life-threatening damage.

Jaundice— Yellowing of the skin or eyes due to excess of bilirubin in the blood.

Magnetic resonance imaging (MRI)— A technique that employs magnetic fields and radio waves to create detailed images of internal body structures and organs, including the brain.

Meniscocytosis— Another word for sickle cell disease.

Mutation— A permanent change in the genetic material that may alter a trait or characteristic of an individual, or manifest as disease, and can be transmitted to offspring.

Narcotics— Strong, prescription medication that can be effective in treating pain, but have the potential to be habit-forming if their use is not supervised correctly.

Nucleic acid— The cellular molecules DNA and RNA that act as coded instructions for the production of proteins and are copied for transmission of inherited traits.

Ophthalmology— The medical specialty of vision and the eye.

Placenta— The organ responsible for oxygen and nutrition exchange between a pregnant mother and her developing baby.

Red blood cell— Hemoglobin-containing blood cells that transport oxygen from the lungs to tissues. In the tissues, the red blood cells exchange their oxygen for carbon dioxide, which is brought back to the lungs to be exhaled.

Screening— Process through which carriers of a trait may be identified within a population.

Sickle cell— A red blood cell that has assumed an elongated shape due to the presence of hemoglobin S.

experiments in gene therapy for sickle cell disease have been carried out in mice, using lentiviral vectors to transfer the corrective gene into the mouse's stem cells. This technique, however, has not yet been attempted in human subjects as of late 2003.

Psychosocial support

As in any lifelong, chronic disease, comprehensive care is important. Assistance with the emotional, social, family-planning, economic, vocational, and other consequences of sickle cell disease can enable affected individuals to better access and benefit from their medical care.

Prognosis

Sickle cell disease is characteristically variable between and within affected individuals. Predicting the course of the disorder based solely on genes is not possible. Several factors aside from genetic inheritance determine the prognosis for affected individuals, including the frequency, severity, and nature of specific complications in any given individual. The availability and access of comprehensive medical care also plays an important role in preventing and treating serious, acute complications, which cause the majority of sickle cell-related deaths. For those individuals who do not experience such acute events, life expectancy is probably substantially greater than the average for all people with sickle cell disease. The impact of recent medical advances supports the hypothesis that current life expectancies may be significantly greater than those estimated in the early 1990s. At that time, individuals with SS disease lived to the early- to mid-40s, and those with SC disease lived into the upper 50s on average. As of 2003, the life expectancy of persons with SS is over 50. With early detection and comprehensive medical care, most people with sickle cell disease are in fairly good health most of the time. Most individuals can be expected to live well into adulthood, enjoying an improved quality of life including the ability to choose a variety of education, career, and family-planning options for themselves.

Resources

BOOKS

Beers, Mark H., MD, and Robert Berkow, MD., editors. "Anemias Caused by Excessive Hemolysis: Sickle Cell Diseases." Section 11, Chapter 127 In The Merck Manual of Diagnosis and Therapy. Whitehouse Station, NJ: Merck Research Laboratories, 2004.

Beers, Mark H., MD, and Robert Berkow, MD., editors. "Pregnancy Complicated by Disease: Hemoglobinopathies." Section 18, Chapter 251 In The Merck Manual of Diagnosis and Therapy. Whitehouse Station, NJ: Merck Research Laboratories, 2004.

PERIODICALS

Davies, S. C., and A. Gilmore. "The Role of Hydroxyurea in the Management of Sickle Cell Disease." Blood Reviews 17 (June 2003): 99-109.

Egbert Maikler, Virginia, et al. "Children's and Adolescents' Use of Diaries for Sickle Cell Pain." Journal of the Society of Pediatric Nurses 6, no. 4 (October—December 2001): 161-169.

Harris, Leslie. "Living Well with Sickle Cell." Essence September 1999: 58.

Nienhuis,A. W., H. Hanawa, N. Sawai, et al. "Development of Gene Therapy for Hemoglobin Disorders." Annals of the New York Academy of Science 996 (May 2003): 101-111.

Seppa, N. "Gene Therapy for Sickle-cell Disease?" Science News 160, no. 24 (December 15, 2001): 372.

Steinberg, M. H., F. Barton, O. Castro, et al. "Effect of Hydroxyurea on Mortality and Morbidity in Adult Sickle Cell Anemia: Risks and Benefits up to 9 Years of Treatment." Journal of the American Medical Association 289 (April 2, 2003): 1645-1651.

Winrow, N., and E. R. Melhem. "Sickle Cell Disease and Stroke in a Pediatric Population. Evidence-Based Diagnostic Evaluation." Neuroimaging Clinics of North America 13 (May 2003): 185-196.

Yerys, B. E., D. A. White, C. F. Salorio, et al. "Memory Strategy Training in Children with Cerebral Infarcts Related to Sickle Cell Disease." Journal of Pediatric Hematology and Oncology 25 (June 2003): 495-498.

ORGANIZATIONS

Mayo Foundation for Medical Education and Research. 〈http://www.mayohealth.org〉.

Sickle Cell Disease Association of America. 200 Corporate Point, Suite 495, Culver City, CA 90230-7633. (310) 216-6363. (800) 421-8453. 〈http://sicklecelldisease.org/〉.

Sickle Cell Disease Program, Division of Blood Diseases and Resources. National Heart, Lung, and Blood Institute. II Rockledge Centre, 6701 Rockledge Dr. MSC 7950, Bethesda, MD 20892-7950. (301) 435-0055.

Gale Encyclopedia of Medicine, 3rd ed. Bojanowski, Jennifer; Frey, Rebecca

Sickle Cell Anemia

views updated Jun 08 2018

Sickle cell anemia

Definition

Sickle cell anemia, which is also known as meniscocytosis or sicklemia, is an inherited blood disorder that arises from a gene mutation. As a result, affected hemoglobin molecules have a tendency to stick to one another, forming abnormal strands of hemoglobin within the red blood cells. The cells that contain these strands become stiff and elongated—sickle-shaped.

Because sickle cell anemia is characterized by the rapid loss of red blood cells as they enter the circulation, it is classified as a hemolytic anemia, "hemolytic" referring to the destruction of the cell membrane of red blood cells, resulting in the release of hemoglobin.

Description

Sickle-shaped cells die much more rapidly than normal red blood cells and the body cannot create replacements fast enough. Anemia develops due to the chronic shortage of red blood cells. Further complications arise because sickle cells do not fit well through small blood vessels, and can become trapped. The trapped sickle cells form blockages that prevent oxygenated blood from reaching associated tissues and organs. The damaged tissues and organs cause considerable pain and can lead to serious complications, including stroke and an impaired immune system. Sickle cell anemia primarily affects people with African, Mediterranean, Middle Eastern, and Indian ancestry. In the United States, one in 12 African Americans are carriers. An additional 72,000 Americans have sickle cell anemia, meaning they have inherited the trait from both parents. Among African Americans, approximately one in every 500 babies is diagnosed with sickle cell anemia. Hispanic Americans are also heavily affected; sickle cell anemia occurs in one of every 1,000-1,400 births. Worldwide, it has been estimated that 250,000 children are born each year with sickle cell anemia.

Hemoglobin structure

Normal hemoglobin is composed of a heme molecule and two pairs of proteins called globins. Humans have the genes to create six different types of globins—alpha, beta, gamma, delta, epsilon, and zeta—but do not use all of them at once. The type of genes expressed depends upon the stage of development: embryonic, fetal, or adult. Virtually all of the hemoglobin produced in humans from ages 2-3 months and onward contains a pair of alpha-globin and beta-globin molecules.

Sickle cell hemoglobin

A change, or mutation, in a gene can alter the formation or function of its product. In the case of sickle cell hemoglobin, the gene that carries the blueprint for beta-globin has a tiny alteration that makes it different from the normal gene. This mutation affects a single nucleic acid along the entire DNA strand that makes up the beta-globin gene. (Nucleic acids are the chemicals that make up deoxyribonucleic acid [DNA].) Specifically, the nucleic acid adenine is replaced by a different nucleic acid called thymine.

Because of this seemingly slight mutation, called a point mutation, the finished beta-globin molecule has a single amino acid substitution: valine occupies the spot normally taken by glutamic acid. (Amino acids are the building blocks of all proteins.) This substitution is incorporated into the beta-globin molecule—and eventually returning in a hemoglobin molecule—that does not function normally.

Normal hemoglobin, referred to as hemoglobin A, transports oxygen from the lungs to tissues throughout the body. In the smallest blood vessels, the hemoglobin exchanges the oxygen for carbon dioxide, which it carries back to the lungs for removal from the body. The defective hemoglobin, designated hemoglobin S, can also transport oxygen. However, once the oxygen is released, hemoglobin S molecules have an abnormal tendency to clump together. Aggregated hemoglobin molecules form strands within red blood cells, which then lose their usual shape and flexibility.

The rate at which hemoglobin S aggregation and cell sickling occurs depends on many factors, such as the blood flow rate and the concentration of hemoglobin in the blood cells. If the blood flows at a normal rate, hemoglobin S is reoxygenated in the lungs before it has a chance to aggregate. The concentration of hemoglobin within red blood cells is influenced by an individual's hydration level—that is, the amount of water contained in the cells. If a person becomes dehydrated, hemoglobin becomes more concentrated in the red blood cells. In this situation, hemoglobin S has a greater tendency to clump together and induce sickle cell formation.

Sickle cell anemia

Genes are inherited in pairs, one copy from each parent. Therefore, each person has two copies of the gene that makes beta-globin. As long as a person inherits one normal beta-globin gene, the body can produce sufficient quantities of normal beta-globin. A person who inherits a copy of each of the normal and abnormal beta-globin genes is referred to as a carrier of the sickle cell trait. Generally, carriers do not have symptoms, but their red blood cells contain some hemoglobin S.

A child who inherits the sickle cell trait from both parents—a 25% possibility if both parents are carriers—will develop sickle cell anemia. These cells have a decreased life span in comparison to normal red blood cells. Normal red blood cells survive for approximately 120 days in the bloodstream; sickle cells last only 10-12 days. As a result, the bloodstream is chronically short of red blood cells and the affected individual develops anemia.

The sickle cells can create other complications. Due to their shape, they do not fit well through small blood vessels. As an aggravating factor, the outside surfaces of sickle cells may have altered chemical properties that increase

the cell's "stickiness." These sticky sickle cells are more likely to adhere to the inside surfaces of small blood vessels as well as to other blood cells. As a result of the sickle cells' shape and stickiness, blockages occasionally form in small blood vessels. Such blockages prevent oxygenated blood from reaching areas where it is needed, causing extreme pain as well as organ and tissue damage.

The severity of the symptoms cannot be predicted based solely on the person's genetic inheritance. Some individuals with sickle cell anemia develop health- or life-threatening problems in infancy but others may have only mild symptoms throughout their lives. For example, genetic factors, such as the continued production of fetal hemoglobin after birth can modify the course of the disease. Fetal hemoglobin contains gamma-globin in place of beta-globin; if enough of it is produced, the potential interactions between hemoglobin S molecules are reduced.

Affected populations

Worldwide, millions of people carry the sickle cell trait. Individuals whose ancestors lived in sub-Saharan Africa, the Middle East, India, or the Mediterranean region are the most likely to have the trait. The areas of the world associated with the sickle cell trait are also strongly affected by malaria , a disease caused by blood-borne parasites transmitted through mosquito bites. According to a widely accepted theory, the genetic mutation associated with the sickle cell trait occurred thousands of years ago. Coincidentally, this mutation increased the likelihood that carriers would survive malaria outbreaks. Survivors then passed the mutation on to their offspring, and the trait became established throughout areas where malaria was common.

Causes & symptoms

Symptoms typically appear during the first year or two of life. However, some individuals do not develop symptoms until adulthood and may not be aware that they have the genetic inheritance for sickle cell anemia.

Anemia

Sickle cells have a high turnover rate, and there is an ongoing deficit of red blood cells in the bloodstream. Common symptoms of anemia include fatigue , paleness, and shortness of breath. A particularly severe form of anemia—aplastic anemia—occurs following infection with parvovirus. Though temporary, parvovirus infection causes extensive destruction of the bone marrow, bringing production of new red blood cells to a halt. Bone marrow production resumes after 7–10 days, but given the short lives of sickle cells, even a brief shutdown in red blood cell production can cause a major decline in hemoglobin concentrations. This event is called "aplastic crisis."

Painful crises

Painful crises, also known as vasoocclusive crises, are a primary symptom of sickle cell anemia in children and adults. The pain may be caused by small blood vessel blockages that prevent oxygen from reaching tissues. An alternate explanation, particularly with regard to bone pain, is that blood is shunted away from the bone marrow but through some mechanism other than blockage by sickle cells.

These crises are unpredictable and can affect any area of the body, although the chest, abdomen, and bones are frequently affected sites. There is some evidence that cold temperatures or infection can trigger a painful crisis, but most crises occur for unknown reasons. The frequency and duration of the pain can vary tremendously. Crises may be separated by more than a year or possibly only by weeks, and they can last from hours to weeks.

The hand-foot syndrome is a particular type of painful crisis, and is often the first sign of sickle cell anemia in an infant. Common symptoms include pain and swelling in the hands and feet, possibly accompanied by a fever . Hand-foot syndrome typically occurs only during the first four years of life, with the greatest incidence at one year.

Enlarged spleen and infections

Sickle cells can impede blood flow through the spleen and cause organ damage. In infants and young children, the spleen is usually enlarged. After repeated incidence of blood vessel blockage, the spleen usually atrophies by late childhood. Damage to the spleen can have a negative impact on the immune system, leaving individuals with sickle cell anemia more vulnerable to infections . Infants and young children are particularly prone to life-threatening infections.

Anemia can also impair the immune system, because stem cells—the precursors of all blood cells—are earmarked for red blood cell production rather than white blood cell production. White blood cells form the cornerstone of the immune system within the bloodstream.

Delayed growth

The energy demands of the bone marrow for red blood cell production compete with the demands of a growing body. Children with sickle cell anemia have delayed growth and reach puberty at a later age than normal. By early adulthood, they catch up on growth and attain normal height, but their weight typically remains below average.

Stroke

Blockage of blood vessels in the brain can have particularly harsh consequences and can be fatal. When areas of the brain are deprived of oxygen, control of the associated functions may be lost. Sometimes this loss is permanent. Common stroke symptoms include weakness or numbness that affects one side of the body, sudden loss of vision, confusion, loss of speech or the ability to understand spoken words, and dizziness . Children between the ages of 1 and 15 have a 30% risk of suffering a stroke. Approximately two-thirds of the children who have a stroke will have at least one more; those who survive typically suffer severe learning disabilities. As of 2003, researchers are investigating various techniques for helping children with memory loss related to strokes caused by sickle cell disease.

Acute chest syndrome

Acute chest syndrome can occur at any age, and is caused by sickle cells blocking the small blood vessels of the lungs. This blockage is complicated by accompanying problems such as infection and pooling of blood in the lungs. Affected persons experience fever, cough , chest pain, and shortness of breath. Recurrent attacks can lead to permanent lung damage.

Diagnosis

Sickle cell anemia is suspected based on an individual's ethnic or racial background, and on the symptoms of anemia. A blood count reveals the presence of anemia, and a sickle cell test reveals the presence of the sickle cell trait.

To confirm a diagnosis of the sickle cell trait or sickle cell anemia, another laboratory test called gel electrophoresis is performed. This test uses an electric field applied across a slab of gel-like material to separate protein molecules based on their size, shape, or electrical charge. Although hemoglobin S (sickle) and hemoglobin A (normal) differ by only one amino acid, they can be clearly separated using gel electrophoresis. If both types of hemoglobin are identified, the individual is a carrier of the sickle cell trait; if only hemoglobin S is present, the person most likely has sickle cell anemia.

The gel electrophoresis test is also used as a screening method for identifying the sickle cell trait in newborns. More than 40 states screen newborns in order to identify carriers and individuals who have inherited the trait from both parents.

Treatment

In general, treatment of sickle cell anemia relies on conventional medicine. However, alternative therapies may be useful in pain control.

Massage

The daily pain caused by sickle cell disease has been shown to be managed by massage. A pilot study whose results were published in 1999 indicated that those who received massage reported less perception of pain than those who were part of a relaxation control group during the research. Massage is recommended as a complementary treatment in the management of the chronic disease.

Pain diaries

A 2001 study revealed that diaries kept by children and adolescents could help the patients and their families better manage sickle cell pain from home. If children (who are old enough to read and write) can record pain episodes, they have better recall and provide improved documentation for physicians and parents so they can relate pain episodes to possible causes.

Acupuncture

Acupuncture may relieve some of the pain caused by sickle cell disease. For longer-lasting results, acupuncturists indicate that the treatment works with the body's subtle energies by manipulating the "chi" to remove blockages and allow the body to heal itself. Acupuncture uses extremely thin needles that are inserted into various areas of the body, with placement depending on the patient's condition. Each treatment usually takes 20-45 minutes.

Diet

While the pain of sickle cell disease ranges from acute to chronic, simple alterations to the diet are one way to help those who endure the illness. Foods like horseradish, cassava, yams, corn, bamboo shoots, sweet potatoes, and lima beans contain cyanogenic glucosides, or natural plant compounds that are recommended additions to the diet. These natural plant compounds interact with bacteria in the large intestine and aid the body in producing a type of hemoglobin that can effectively carry oxygen through blood cells—possibly leading to less pain.

Allopathic treatment

Early identification of sickle cell anemia can prevent many problems. The highest death rates occur during the first year of life due to infection, aplastic anemia, and acute chest syndrome. If anticipated, steps can be taken to avert these crises. With regard to long-term treatment, prevention of complications remains a main goal. Sickle cell anemia cannot be cured—other than through a risky bone marrow transplant—but treatments are available for symptoms.

Pain management

Pain is one of the primary symptoms of sickle cell anemia, and controlling it is an important concern. The methods necessary for pain control are based on individual factors. Some people can gain adequate pain control through over-the-counter oral painkillers (analgesics), local application of heat, and rest. Others need stronger methods, which can include administration of narcotics.

Blood transfusions

Blood transfusions are usually not given on a regular basis but are used to treat painful crises, severe anemia, and other emergencies. In some cases, such as treating spleen enlargement or preventing stroke from recurring, blood transfusions are given as a preventative measure. Regular blood transfusions have the potential to decrease formation of hemoglobin S and reduce associated symptoms.

Drugs

Infants are typically started on a course of penicillin that extends from infancy to age six. This treatment is meant to ward off potentially fatal infections. Infections at any age are treated aggressively with antibiotics. Vaccines for common infections, such as pneumococcal pneumonia , are administered when possible.

Emphasis is being placed on developing drugs that treat sickle cell anemia directly. The most promising of these drugs in the late 1990s is hydroxyurea, a drug that was originally designed for anticancer treatment. Hydroxyurea has been shown to reduce the frequency of painful crises and acute chest syndrome in adults, and to lessen the need for blood transfusions. Hydroxyurea seems to work by inducing a higher production of fetal hemoglobin. The major side effects of the drug include decreased production of platelets, red blood cells, and certain white blood cells. The effects of long-term hydroxyurea treatment are unknown; however, a nine-year follow-up study of 299 adults with frequent painful crises reported in 2003 that taking hydroxyurea was associated with a 40% reduction in mortality.

Bone marrow transplantation

Bone marrow transplantation has been shown to cure sickle cell anemia in severely affected children. Indications for a bone marrow transplant are stroke, recurrent acute chest syndrome, and chronic unrelieved pain. Bone marrow transplants tend to be the most successful in children; adults have a higher rate of transplant rejection and other complications.

Gene research

Replacing the gene that produces the defective hemoglobin in sickle cell disease patients with one that makes normal hemoglobin may be a possible treatment due to recent research. According to a 1998 report in Science, researchers studied the blood cells from people who carry the sickle cell gene. By using an enzyme called a ribosome, the study was able to alter sickle cells into normal cells. The ribosome cut out the mutated instructions in the cells' genetic pattern and replaced them with the correct instructions. Researchers hope that this gene therapy will allow the cells to make normal hemoglobin—leading to the ultimate treatment for those with sickle cell disease.

In late 2001 genetic scientists reported that they had designed a gene that might lead to a future treatment of sickle cell anemia. Although the gene had not tested in humans, early results showed that the injected gene protected cells from sickling. As of 2003, experiments in gene therapy for sickle cell disease have been carried out in mice, using lentiviral vectors to transfer the corrective gene into the mouse's stem cells. This technique, however, has not yet been attempted in human subjects as of late 2003.

Expected results

Several factors aside from genetic inheritance determine the prognosis for affected individuals. Therefore, predicting the course of the disorder based solely on genes is not possible. In general, given proper medical care, persons with sickle cell anemia are in fairly good health most of the time. The life expectancy for these individuals has steadily increased over the last 30 years, and many are now surviving past the age of 50. In the United States, the average life expectancy for men with sickle cell anemia is 42 years; for women, it is 48 years. The most common causes of death are infections, lung disease, the blocking of a blood vessel supplying a vital organ, and kidney failure. Pregnant women with sickle cell disease are particularly vulnerable to infection, most often pneumonia or urinary tract infections.

Prevention

The sickle cell trait is a genetically linked, inherited condition. Inheritance cannot be prevented but may be predicted. Screening is recommended for individuals in high-risk populations; in the United States, African Americans, and Hispanic Americans have the highest risk of being carriers.

Screening at birth offers the opportunity for early intervention; more than 40 states include sickle cell screening as part of the usual battery of blood tests done for newborns. Pregnant women and couples planning to have children may also wish to be screened to determine their carrier status. Carriers have a 50% chance of passing the trait to their offspring. Children born to two carriers have a 25% chance of inheriting the trait from both parents and having sickle cell anemia. Carriers may consider genetic counseling to assess any risks to their offspring. The sickle cell trait can also be identified through prenatal testing, specifically through use of amniotic fluid testing or chorionic villus sampling.

By maintaining a good diet, staying well hydrated with plenty of fluids, exercising regularly, and getting enough sleep those with sickle cell disease may help their bodies remain strong and ward off fatigue and dehydration.

Resources

BOOKS

"Anemias Caused by Excessive Hemolysis: Sickle Cell Diseases." Section 11, Chapter 127 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 2002.

Beutler, Ernest. The Sickle Cell Diseases and Related Disorders. Williams Hematology, edited by Ernest Beutler, et al. 5th ed. New York: McGraw-Hill, 1995.

Bloom, Miriam. Understanding Sickle Cell Disease. Jackson, MS: University Press of Mississippi, 1995.

The Editors of Time-Life Books. Sickle Cell Anemia. The Medical Advisor: The Complete Guide to Alternative & Conventional Treatments, Richmond, VA: Time-Life Inc., 1996.

Embury, Stephen H., et al., eds. Sickle Cell Disease: Basic Principles and Clinical Practice. New York: Raven Press, 1994.

"Pregnancy Complicated by Disease: Hemoglobinopathies." Section 18, Chapter 251 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 2002.

PERIODICALS

Davies, S. C., and A. Gilmore. "The Role of Hydroxyurea in the Management of Sickle Cell Disease." Blood Reviews 17 (June 2003): 99–109.

Egbert Maikler, Virginia, et al." Children's and Adolescents' Use of Diaries for Sickle Cell Pain." Journal of the Society of Pediatric Nurses 6. no. 4 (October – December 2001): 161 – 169.

Harris, Leslie. "Living Well with Sickle Cell." Essence (September 1999): 58.

Nienhuis, A. W., H. Hanawa, N. Sawai, et al. "Development of Gene Therapy for Hemoglobin Disorders." Annals of the New York Academy of Science 996 (May 2003): 101–111.

Seppa, N. "Gene Therapy for Sickle-cell Disease?." Science News 160, no. 24 (December 15, 2001): 372.

"Sickle Cell Pain Relieved by Massage." Massage Magazine (June 30, 1999): 52.

Steinberg, M. H., F. Barton, O. Castro, et al. "Effect of Hydroxyurea on Mortality and Morbidity in Adult Sickle Cell Anemia: Risks and Benefits up to 9 Years of Treatment." Journal of the American Medical Association 289 (April 2, 2003): 1645–1651.

Winrow, N., and E. R. Melhem. "Sickle Cell Disease and Stroke in a Pediatric Population. Evidence-Based Diagnostic Evaluation." Neuroimaging Clinics of North America 13 (May 2003): 185–196.

Yerys, B. E., D. A. White, C. F. Salorio, et al. "Memory Strategy Training in Children with Cerebral Infarcts Related to Sickle Cell Disease." Journal of Pediatric Hematology and Oncology 25 (June 2003): 495–498.

ORGANIZATIONS

Mayo Foundation for Medical Education and Research. <http://www.mayohealth.org>.

Sickle Cell Disease Association of America. 200 Corporate Point, Suite 495, Culver City, CA 90230-7633. (310) 216-6363. (800) 421-8453. <http://sicklecelldisease.org/>.

Sickle Cell Disease Program, Division of Blood Diseases and Resources. National Heart, Lung, and Blood Institute. II Rockledge Centre, 6701 Rockledge Dr. MSC 7950, Bethesda, MD 20892-7950. (301) 435-0055.

Beth Kapes

Teresa Norris

Rebecca J. Frey, PhD

Gale Encyclopedia of Alternative Medicine Kapes, Beth; Norris, Teresa; Frey, Rebecca

Sickle Cell Anemia

views updated May 23 2018

Sickle Cell Anemia

Hemoglobin structure

Sickle cell hemoglobin

Enlarged spleen and infections

Delayed growth

Stroke

Acute chest syndrome

Hemoglobin structure

Normal hemoglobin is composed of a heme molecule and two pairs of proteins called globins. Humans have the genes to create six different types of globins— alpha, beta, gamma, delta, epsilon, and zeta—but do not use all of them at once. Which genes are expressed depends on the stage of development: embryonic, fetal, or adult. Virtually all of the hemoglobin produced in humans from ages two to three months onward contains a pair of alpha-globin and beta-globin molecules.

Sickle cell hemoglobin

A change, or mutation, in a gene can alter the formation or function of its product. In the case of sickle cell hemoglobin, the gene that carries the blueprint for beta-globin has a minute alteration that makes it different

from the normal gene. This mutation affects a single nucleic acid along the entire DNA strand that makes up the beta-globin gene. (Nucleic acids are the chemicals that make up deoxyribonucleic acid, known more familiarly as DNA.) Specifically, the nucleic acid, adenine, is replaced by a different nucleic acid called thymine.

Because of this seemingly slight mutation, called a point mutation, the finished beta-globin molecule has an amino acid substitution: valine occupies the spot normally taken by glutamic acid. (Amino acids are the building blocks of all proteins.) This substitution creates a beta-globin molecule—and eventually a hemoglobin molecule—that does not function normally.

Normal hemoglobin, referred to as hemoglobin A, transports oxygen from the lungs to tissues throughout the body. In the smallest blood vessels, the hemoglobin exchanges the oxygen for carbon dioxide, which it carries back to the lungs for removal from the body. The defective hemoglobin, designated hemoglobin S, can also transport oxygen. However, once the oxygen is released, hemoglobin S molecules have an abnormal tendency to clump together. Aggregated hemoglobin molecules form strands within red blood cells, which then lose their usual shape and flexibility.

The rate at which hemoglobin S aggregation and cell sickling occur depends on many factors, such as the blood flow rate and the concentration of hemoglobin in the blood cells. If the blood flows at a normal rate, hemoglobin S is reoxygenated in the lungs before it has a chance to aggregate. The concentration of hemoglobin within red blood cells is influenced by an individual’s hydration level—that is the amount water contained in the cells. If a person becomes dehydrated, hemoglobin becomes more concentrated in the red blood cells. In this situation, hemoglobin S has a greater tendency to clump together and induce sickle cell formation.

Sickle cell anemia

Genes are inherited in pairs, one copy from each parent. Therefore, each person has two copies of the gene that makes beta-globin. As long as a person inherits one normal beta-globin gene, the body can produce sufficient quantities of normal beta-globin. A person who inherits a copy each of the normal and abnormal beta-globin genes is referred to as a carrier of the sickle cell trait. Generally, carriers do not have symptoms, but their red blood cells contain some hemoglobin S.

A child who inherits the sickle cell trait from both parents—a 25% possibility if both parents are carriers—will develop sickle cell anemia. Sickle cell anemia is characterized by the formation of stiff and elongated red blood cells, called sickle cells. These cells have a decreased life span in comparison to normal red blood cells. Normal red blood cells survive for approximately 120 days in the bloodstream; sickle cells last only 10 to 12 days. As a result, the bloodstream is chronically short of red blood cells and the affected individual develops anemia.

The sickle cells can create other complications. Due to their shape, they do not fit well through small blood vessels. As an aggravating factor, the outside surfaces of sickle cells may have altered chemical properties that increase the cell’s stickiness. These sticky sickle cells are more likely to adhere to the inside surfaces of small blood vessels, as well as to other blood cells. As a result of the sickle cells” shape and stickiness, blockages occasionally form in small blood vessels. Such blockages prevent oxygenated blood from reaching areas where it is needed, causing extreme pain, as well as organ and tissue damage.

However, the severity of the symptoms cannot be predicted based solely on the genetic inheritance. Some individuals with sickle cell anemia develop health- or life-threatening problems in infancy, but others may have only mild symptoms throughout their lives. For example, genetic factors, such as the continued production of fetal hemoglobin after birth, can modify the course of the disease. Fetal hemoglobin contains gamma-globin in place of beta-globin; if enough of it is produced, the potential interactions between hemoglobin S molecules are reduced.

Affected populations

Worldwide, millions of people carry the sickle cell trait. Individuals whose ancestors lived in sub-Saharan Africa, the Middle East, India, or the Mediterranean region are the most likely to have the trait. The areas of the world associated with the sickle cell trait are also strongly affected by malaria, a disease caused by blood-borne parasites transmitted through mosquito bites. According to a widely accepted theory, the genetic mutation associated with the sickle cell trait occurred thousands of years ago. Coincidentally, this mutation increased the likelihood that carriers would survive malaria outbreaks. Survivors then passed the mutation on to their offspring, and the trait became established throughout areas where malaria was common.

Although modern medicine offers drug therapies for malaria, the sickle cell trait endures. Approximately two million Americans are carriers of the sickle cell trait. Individuals who have African ancestry are particularly affected; one in 12 African Americans is a carrier. People with Mediterranean, Middle Eastern, and Indian ancestry are also highly affected. An additional 72,000 Americans have sickle cell anemia, meaning they have inherited the trait from both parents. Among African Americans, approximately one in every 500 babies is diagnosed with sickle cell anemia. Hispanic Americans are also heavily affected; sickle cell anemia occurs in one of every 1,000 to 1,400 births. Worldwide, it has been estimated that 250,000 children are born each year with sickle cell anemia.

Causes and symptoms

Sickle cell anemia results from an inheritance of the sickle cell trait—that is, a defective beta-globin gene—from each parent. Due to this inheritance, hemoglobin S is produced. This hemoglobin has a tendency to aggregate and form strands, thereby deforming the red blood cells in which it is contained. The deformed, short-lived red blood cells cause effects throughout the body.

Symptoms typically appear during the first year or two of life, if the diagnosis has not been made at or before birth. However, some individuals do not develop symptoms until adulthood and may not be aware that they have the genetic inheritance for sickle cell anemia.

Anemia

Sickle cells have a high turnover rate, and there is a deficit of red blood cells in the bloodstream. Common symptoms of anemia include fatigue, paleness, and a shortness of breath. A particularly severe form of anemia—aplastic anemia—occurs following infection with parvovirus. Parvovirus causes extensive destruction of the bone marrow, bringing production of new red blood cells to a halt. Bone marrow production resumes after 7 to 10 days; however, given the short lives of sickle cells, even a brief shut-down in red blood cell production can cause a precipitous decline in hemoglobin concentrations. This is called aplastic crisis.

Painful crises

Painful crises, also known as vaso-occlusive crises, are a primary symptom of sickle cell anemia in children and adults. The pain may be caused by small blood vessel blockages that prevent oxygen from reaching tissues. An alternate explanation, particularly with regard to bone pain, is that blood is shunted away from the bone marrow but through some other mechanism than blockage by sickle cells.

These crises are unpredictable, and can affect any area of the body, although the chest, abdomen, and bones are frequently affected sites. There is some evidence that cold temperatures or infection can trigger a painful crisis, but most crises occur for unknown reasons. The frequency and duration of the pain can vary tremendously. Crises may be separated by more than a year or possibly only by weeks, and they can last from hours to weeks.

The hand-foot syndrome is a particular type of painful crisis, and is often the first sign of sickle cell anemia in an infant. Common symptoms include pain and swelling in the hands and feet, possibly accompanied by a fever. Hand-foot syndrome typically occurs only during the first four years of life, with the greatest incidence at one year.

Enlarged spleen and infections

Sickle cells can impede blood flow through the spleen and cause organ damage. In infants and young children, the spleen is usually enlarged. After repeated incidence of blood vessel blockage, the spleen usually atrophies by late childhood. Damage to the spleen can have a negative impact on the immune system, leaving individuals with sickle cell anemia more vulnerable to infections. Infants and young children are particularly prone to life-threatening infections.

Anemia can also impair the immune system, because stem cells—the precursors of all blood cells—are earmarked for red blood cell production rather than white blood cell production. White blood cells form the cornerstone of the immune system within the bloodstream.

Delayed growth

The energy demands of the bone marrow for red blood cell production compete with the demands of a growing body. Children with sickle cell anemia have delayed growth and reach puberty at a later age than normal. By early adulthood, they catch up on growth and attain normal height; however, weight typically remains below average.

Stroke

Blockage of blood vessels in the brain can have particularly harsh consequences and can be fatal. When areas of the brain are deprived of oxygen, control of the associated functions may be lost. Sometimes this loss is permanent. Common stroke symptoms include weakness or numbness that affects one side of the body, sudden loss of vision, confusion, loss of speech or the ability to understand spoken words, and dizziness. Children between the ages of 1 to 15 years are at the highest risk of suffering a stroke. Approximately two-thirds of the children who have a stroke will have at least one more.

Acute chest syndrome

Acute chest syndrome can occur at any age, and is caused by sickle cells blocking the small blood vessels of the lungs. This blockage is complicated by accompanying problems such as infection and pooling of blood in the lungs. Affected persons experience fever, cough, chest pain, and shortness of breath. Recurrent attacks can lead to permanent lung damage.

Diagnosis

Sickle cell anemia is suspected based on an individual’s ethnic or racial background, and on the symptoms of anemia. A blood count reveals the anemia and the presence of sickle cells in blood samples is easily confirmed by microscopic examination. A sickle cell test can reveal the presence of the sickle cell trait.

The sickle cell test involves mixing equal amounts of blood and a 2% solution of sodium bisulfite. Under these circumstances, hemoglobin exists in its deoxy-genated state. If hemoglobin S is present, the red blood cells are transformed into the characteristic sickle shape. This transformation is observed with a microscope, and quantified by expressing the number of sickle cells per 1,000 cells as a percentage. The sickle cell test confirms that an individual has the sickle cell trait, but it does not provide a definitive diagnosis for sickle cell anemia.

To confirm a diagnosis of the sickle cell trait or sickle cell anemia, another laboratory test called gel electrophoresis is performed. This test uses an electric field applied across a slab of gel-like material to separate protein molecules based on their size, shape, or electrical charge. Although hemoglobin S (sickle) and hemoglobin A (normal) differ by only one amino acid, they can be clearly separated using gel electrophoresis. If both types of hemoglobin are identified, the individual is a carrier of the sickle cell trait; if only hemoglobin S is present, the person most likely has sickle cell anemia.

The gel electrophoresis test is also used as a screening method for identifying the sickle cell trait in newborns.

More than 40 states in the United States screen newborns in order to identify carriers and individuals who have inherited the trait from both parents. Physicians and researchers also recommend that individuals likely to be exposed to low oxygen tensions (e.g., pilots, divers) undergo screening tests for sickle-cell trait, as studies have shown that those with sickle-cell trait are often less able to cope with low oxygen levels than individuals with normal hemoglobin.

Treatment

Early identification of sickle cell anemia can prevent many problems. The highest death rates occur during the first year of life due to infection, aplastic anemia, and acute chest syndrome. If anticipated, steps can be taken to avert these crises. With regard to long-term treatment, prevention of complications remains a main goal. Sickle cell anemia cannot be cured—other than through a risky bone marrow transplant—but treatments are available for symptoms.

Pain management

Pain is one of the primary symptoms of sickle cell anemia, and controlling it is an important concern. The methods necessary for pain control are based on individual factors. Some people can gain adequate pain control through over-the-counter oral painkillers (analgesics), local application of heat, and rest. Others need stronger methods, which can include administration of narcotics.

Blood transfusions

Blood transfusions are usually not given on a regular basis but are used to treat painful crises, severe anemia, and other emergencies. In some cases, such as treating spleen enlargement or preventing stroke from recurring, blood transfusions are given as a preventa-tive measure. Regular blood transfusions have the potential to decrease formation of hemoglobin S, and reduce associated symptoms. However, regular blood transfusions introduce a set of complications, primarily iron loading, risk of infection, and sensitiza-tion to proteins in the transfused blood.

Drugs

Infants are typically started on a course of penicillin that extends from infancy to age six years. This treatment is meant to ward off potentially fatal infections. Infections at any age are treated aggressively with antibiotics. Vaccines for common infections, such as pneu-mococcal pneumonia, are administered when possible.

Emphasis is being placed on developing drugs that treat sickle cell anemia directly. The most promising of these drugs is hydroxyurea, a drug that was originally designed for anticancer treatment. Hydroxyurea has been shown to reduce the frequency of painful crises and acute chest syndrome in adults, and to lessen the need for blood transfusions. Hydroxyurea seems to work by inducing a higher production of fetal hemoglobin. The major side effects of the drug include decreased production of platelets, red blood cells, and certain white blood cells. The effects of long-term hydroxyurea treatment are unknown.

Bone marrow transplantation

Bone marrow transplantation has been shown to cure sickle cell anemia in severely affected children. Indications for a bone marrow transplant are stroke, recurrent acute chest syndrome, and chronic unrelieved pain. Bone marrow transplants tend to be the most successful in children; adults have a higher rate of transplant rejection and other complications.

The procedure requires a healthy donor whose marrow proteins match those of the recipient. Typically, siblings have the greatest likelihood of having matched marrow. Given this restriction, fewer than 20% of sickle cell anemia individuals may be candidates. The percentage is reduced when factors such as general health and acceptable risk are considered. The procedure is risky for the recipient. There is approximately a 10% fatality rate associated with bone marrow transplants done for sickle cell anemia treatment. Survivors face potential long-term complications, such as chronic graft versus host disease (an immune-mediated attack by the donor marrow against the recipient’s tissues), infertility, and development of some forms of cancer.

Alternative treatment

In general, treatment of sickle cell anemia relies on conventional medicine. However, alternative therapies may be useful in pain control. Relaxation, application of local warmth, and adequate hydration may supplement the conventional therapy. Further, maintaining good health through adequate nutrition, avoiding stresses and infection, and getting proper rest help prevent some complications.

Prognosis

Several factors aside from genetic inheritance determine the prognosis for affected individuals. Therefore, predicting the course of the disorder based solely on genes is not possible. In general, given proper medical care, individuals with sickle cell anemia are in fairly good health most of the time. The life expectancy for these individuals has increased over the last 35 years, and many survive well into their 40s or beyond. In the United States, the average life span for men with sickle cell anemia is 40 to 44 years; for women, it is 46 to 50 years.

KEY TERMS

Amino acid —An organic compound whose molecules contain both an amino group (-NH₂) and a carboxyl group (-COOH). One of the building blocks of a protein.

Anemia —A condition in which the level of hemoglobin falls below normal values due to a shortage of mature red blood cells. Common symptoms include pallor, fatigue, and shortness of breath.

Bilirubin —A yellow pigment that is the end result of hemoglobin degradation. Bilirubin is cleared from the blood by action of liver enzymes and excreted from the body.

Bone marrow —A spongy tissue located in the hollow centers of certain bones, such as the skull and hip bones. Bone marrow is the site of blood cell generation.

Bone marrow transplantation —A medical procedure in which normal bone marrow is transferred from a healthy donor to an ailing recipient. An illness that prevents production of normal blood cells— such as sickle cell anemia—may be treated with a bone marrow transplant.

Gel electrophoresis —A laboratory test that separates molecules based on their size, shape, or electrical charge.

Globin —One of the component protein molecules found in hemoglobin. Normal adult hemoglobin has a pair each of alpha-globin and beta-globin molecules.

Heme —The iron-containing molecule in hemoglobin that serves as the site for oxygen binding.

Hemoglobin —The red pigment found within red blood cells that enables them to transport oxygen throughout the body. Hemoglobin is a large molecule composed of five component molecules: a heme molecule and two pairs of globin molecules.

Hemoglobin A —Normal adult hemoglobin which contains a heme molecule, two alpha-globin molecules, and two beta-globin molecules.

Hemoglobin S —Hemoglobin that is produced in association with the sickle cell trait; the beta-globin molecules of hemoglobin S are defective.

Hydroxyurea —A drug that has been shown to induce production of fetal hemoglobin. Fetal hemoglobin has a pair of gamma-globin molecules in place of the typical beta-globins of adult hemoglobin. Higher-than-normal levels of fetal hemoglobin can prevent sickling from occurring.

Iron loading —A side effect of frequent transfusions in which the body accumulates abnormally high levels of iron. Iron deposits can form in organs, particularly the heart, and cause life-threatening damage.

Jaundice —A condition characterized by higher-than-normal levels of bilirubin in the bloodstream and an accompanying yellowing of the skin and eyes.

Mutation —A change in a gene’s DNA. Whether a mutation is harmful is determined by the effect on the product for which the gene codes.

Nucleic acid —A type of chemical that is used as a component for building DNA. The nucleic acids found in DNA are adenine, thymine, guanine, and cytosine.

Red blood cell —Hemoglobin-containing blood cells that transport oxygen from the lungs to tissues. In the tissues, the red blood cells exchange their oxygen for carbon dioxide, which is brought back to the lungs to be exhaled.

Screening —Process through which carriers of a trait may be identified within a population.

Sickle cell —A red blood cell that has assumed a elongated shape due to the presence of hemoglobin S.

Sickle cell test —A blood test that identifies and quantifies sickle cells in the bloodstream.

Prevention

The sickle cell trait is a genetically linked, inherited condition. Inheritance cannot be prevented, but it may be predicted. Screening is recommended for individuals in high-risk populations; in the United States, African Americans and Hispanic Americans have the highest risk of being carriers.

Screening at birth offers the opportunity for early intervention; more than 40 states include sickle cell screening as part of the usual battery of blood tests done for newborns. Pregnant women and couples planning to have children may also wish to be screened to determine their carrier status. Carriers have a 50% chance of passing the trait to their offspring. Children born to two carriers have a 25% chance of inheriting the trait from both parents and having sickle cell anemia. Carriers may consider genetic counseling to assess any risks to their offspring. The sickle cell trait can also be identified through prenatal testing; specifically through use of amniotic fluid testing or chorionic villus sampling.

Today, scientists are continuing their research into new treatments for sickle cell anemia. Some of the research includes the creation of new drugs that increase blood levels of fetal hemoglobin and new techniques within gene therapy.

Resources

BOOKS

Nussbaum, Robert L., Roderick R. McInnes, and Huntington F. Willard. Genetics in Medicine. Philadelphia, PA: Saunders, 2001.

Platt, Allan F. Hope and Destiny: The Patient’s and Parent’s Guide to Sickle Cell Disease and Sickle Cell Trait. Roscoe, IL: Hilton Publ. Co., 2002.

Rimoin, David L. Emery and Rimoin’s Principles and Practice of Medical Genetics. London, UK, New York: Churchill Livingstone, 2002.

PERIODICALS

Anie K.A., A. Steptoe, D.H. Bevan. “Sickle cell disease: Pain, Coping and Quality of Life in a Study of Adults.” Br J Health Psychol. Sep;7 (2002):331-344.

Kar B.C. “Clinical Profile of Sickle Cell Trait.” J Assoc Physicians India. Nov;50 (2002):1368-71.

Thomas V.J., L.M. Taylor. “The Psychosocial Experience of People with Sickle Cell Disease and its Impact on Quality of Life: Qualitative Findings from Focus Groups.” Br J Health Psychol. Sep;7 (2002):345-363.

Fixler J., L. Styles. “Sickle Cell Disease.” Pediatr Clin North Am. Dec;49(6)(2002):1193-210.

Dorn-Beineke, A., T. Frietsch. “Sickle Cell Disease— Pathophysiology, Clinical and Diagnostic implications.” Clin Chem Lab Med. Nov;40 (11)(2002):1075-84.

OTHER

Sickle Cell Disease Association of America, Inc. “Break the Sickle Cycle,” <http://www.sicklecelldisease.org/>. (accessed December 2, 2006).

Julia Barrett

The Gale Encyclopedia of Science

Sickle Cell Anemia

views updated Jun 11 2018

Sickle cell anemia

Sickle cell anemia is an inherited blood disorder that arises from a single amino acid substitution in one of the component proteins of hemoglobin. The component protein, or globin, that contains the substitution is defective. Hemoglobin molecules constructed with such proteins have a tendency to stick to one another, forming strands of hemoglobin within the red blood cells. The cells that contain these strands become stiff and elongated—that is, sickle shaped.

Sickle-shaped cells—also called sickle cells—die much more rapidly than normal red blood cells, and the body cannot create replacements fast enough. Anemia develops due to the chronic shortage of red blood cells. Further complications arise because sickle cells do not fit well through small blood vessels, and can become trapped. The trapped sickle cells form blockages that prevent oxygenated blood from reaching associated tissues and organs. Considerable pain results in addition to damage to the tissues and organs. This damage can lead to serious complications, including stroke and an impaired immune system . Sickle cell anemia primarily affects people with African, Mediterranean, Middle Eastern, and Indian ancestry. In the United States, African Americans are particularly affected.

Hemoglobin structure

Normal hemoglobin is composed of a heme molecule and two pairs of proteins called globins. Humans have the genes to create six different types of globins—alpha, beta, gamma, delta, epsilon, and zeta—but do not use all of them at once. Which genes are expressed depends on the stage of development: embryonic, fetal, or adult. Virtually all of the hemoglobin produced in humans from ages 2-3 months onward contains a pair of alpha-globin and beta-globin molecules.

Sickle cell hemoglobin

A change, or mutation , in a gene can alter the formation or function of its product. In the case of sickle cell hemoglobin, the gene that carries the blueprint for beta-globin has a minute alteration that makes it different from the normal gene. This mutation affects a single nucleic acid along the entire DNA strand that makes up the beta-globin gene. (Nucleic acids are the chemicals that make up deoxyribonucleic acid, known more familiarly as DNA.) Specifically, the nucleic acid, adenine, is replaced by a different nucleic acid called thymine.

Because of this seemingly slight mutation, called a point mutation, the finished beta-globin molecule has an amino acid substitution: valine occupies the spot normally taken by glutamic acid. (Amino acids are the building blocks of all proteins.) This substitution creates a beta-globin molecule—and eventually a hemoglobin molecule—that does not function normally.

Normal hemoglobin, referred to as hemoglobin A, transports oxygen from the lungs to tissues throughout the body. In the smallest blood vessels, the hemoglobin exchanges the oxygen for carbon dioxide , which it carries back to the lungs for removal from the body. The defective hemoglobin, designated hemoglobin S, can also transport oxygen. However, once the oxygen is released, hemoglobin S molecules have an abnormal tendency to clump together. Aggregated hemoglobin molecules form strands within red blood cells, which then lose their usual shape and flexibility.

The rate at which hemoglobin S aggregation and cell sickling occur depends on many factors, such as the blood flow rate and the concentration of hemoglobin in the blood cells. If the blood flows at a normal rate, hemoglobin S is reoxygenated in the lungs before it has a chance to aggregate. The concentration of hemoglobin within red blood cells is influenced by an individual's hydration level—that is the amount water contained in the cells. If a person becomes dehydrated, hemoglobin becomes more concentrated in the red blood cells. In this situation, hemoglobin S has a greater tendency to clump together and induce sickle cell formation.

Sickle cell anemia

Genes are inherited in pairs, one copy from each parent. Therefore, each person has two copies of the gene that makes beta-globin. As long as a person inherits one normal beta-globin gene, the body can produce sufficient quantities of normal beta-globin. A person who inherits a copy each of the normal and abnormal beta-globin genes is referred to as a carrier of the sickle cell trait. Generally, carriers do not have symptoms, but their red blood cells contain some hemoglobin S.

A child who inherits the sickle cell trait from both parents—a 25% possibility if both parents are carriers—will develop sickle cell anemia. Sickle cell anemia is characterized by the formation of stiff and elongated red blood cells, called sickle cells. These cells have a decreased life span in comparison to normal red blood cells. Normal red blood cells survive for approximately 120 days in the bloodstream; sickle cells last only 10-12 days. As a result, the bloodstream is chronically short of red blood cells and the affected individual develops anemia.

The sickle cells can create other complications. Due to their shape, they do not fit well through small blood vessels. As an aggravating factor, the outside surfaces of sickle cells may have altered chemical properties that increase the cell's "stickiness." These sticky sickle cells are more likely to adhere to the inside surfaces of small blood vessels, as well as to other blood cells. As a result of the sickle cells' shape and stickiness, blockages occasionally form in small blood vessels. Such blockages prevent oxygenated blood from reaching areas where it is needed, causing extreme pain, as well as organ and tissue damage.

However, the severity of the symptoms cannot be predicted based solely on the genetic inheritance. Some individuals with sickle cell anemia develop health- or life-threatening problems in infancy, but others may have only mild symptoms throughout their lives. For example, genetic factors, such as the continued production of fetal hemoglobin after birth , can modify the course of the disease . Fetal hemoglobin contains gamma-globin in place of beta-globin; if enough of it is produced, the potential interactions between hemoglobin S molecules are reduced.

Affected populations

Worldwide, millions of people carry the sickle cell trait. Individuals whose ancestors lived in sub-SaharanAfrica , the Middle East, India, or the Mediterranean region are the most likely to have the trait. The areas of the world associated with the sickle cell trait are also strongly affected by malaria , a disease caused by blood-borne parasites transmitted through mosquito bites. According to a widely accepted theory, the genetic mutation associated with the sickle cell trait occurred thousands of years ago. Coincidentally, this mutation increased the likelihood that carriers would survive malaria outbreaks. Survivors then passed the mutation on to their offspring, and the trait became established throughout areas where malaria was common.

Although modern medicine offers drug therapies for malaria, the sickle cell trait endures. Approximately two million Americans are carriers of the sickle cell trait. Individuals who have African ancestry are particularly affected; one in 12 African Americans are carriers. An additional 72,000 Americans have sickle cell anemia, meaning they have inherited the trait from both parents. Among African Americans, approximately one in every 500 babies is diagnosed with sickle cell anemia. Hispanic Americans are also heavily affected; sickle cell anemia occurs in one of every 1,000-1,400 births. Worldwide, it has been estimated that 250,000 children are born each year with sickle cell anemia.

Causes and symptoms

Sickle cell anemia results from an inheritance of the sickle cell trait—that is, a defective beta-globin gene—from each parent. Due to this inheritance, hemoglobin S is produced. This hemoglobin has a tendency to aggregate and form strands, thereby deforming the red blood cells in which it is contained. The deformed, short-lived red blood cells cause effects throughout the body.

Symptoms typically appear during the first year or two of life, if the diagnosis has not been made at or before birth. However, some individuals do not develop symptoms until adulthood and may not be aware that they have the genetic inheritance for sickle cell anemia.

Anemia

Sickle cells have a high turnover rate, and there is a deficit of red blood cells in the bloodstream. Common symptoms of anemia include fatigue, paleness, and a shortness of breath. A particularly severe form of anemia—aplastic anemia—occurs following infection with parvovirus. Parvovirus causes extensive destruction of the bone marrow, bringing production of new red blood cells to a halt. Bone marrow production resumes after 7-10 days; however, given the short lives of sickle cells, even a brief shut-down in red blood cell production can cause a precipitous decline in hemoglobin concentrations. This is called "aplastic crisis."

Painful crises

Painful crises, also known as vaso-occlusive crises, are a primary symptom of sickle cell anemia in children and adults. The pain may be caused by small blood vessel blockages that prevent oxygen from reaching tissues. An alternate explanation, particularly with regard to bone pain, is that blood is shunted away from the bone marrow but through some other mechanism than blockage by sickle cells.

These crises are unpredictable, and can affect any area of the body, although the chest, abdomen, and bones are frequently affected sites. There is some evidence that cold temperatures or infection can trigger a painful crisis, but most crises occur for unknown reasons. The frequency and duration of the pain can vary tremendously. Crises may be separated by more than a year or possibly only by weeks, and they can last from hours to weeks.

The hand-foot syndrome is a particular type of painful crisis, and is often the first sign of sickle cell anemia in an infant. Common symptoms include pain and swelling in the hands and feet, possibly accompanied by a fever. Hand-foot syndrome typically occurs only during the first four years of life, with the greatest incidence at one year.

Enlarged spleen and infections

Sickle cells can impede blood flow through the spleen and cause organ damage. In infants and young children, the spleen is usually enlarged. After repeated incidence of blood vessel blockage, the spleen usually atrophies by late childhood. Damage to the spleen can have a negative impact on the immune system, leaving individuals with sickle cell anemia more vulnerable to infections. Infants and young children are particularly prone to life-threatening infections.

Anemia can also impair the immune system, because stem cells—the precursors of all blood cells—are earmarked for red blood cell production rather than white blood cell production. White blood cells form the cornerstone of the immune system within the bloodstream.

Delayed growth

The energy demands of the bone marrow for red blood cell production compete with the demands of a growing body. Children with sickle cell anemia have delayed growth and reach puberty at a later age than normal. By early adulthood, they catch up on growth and attain normal height; however, weight typically remains below average.

Stroke

Blockage of blood vessels in the brain can have particularly harsh consequences and can be fatal. When areas of the brain are deprived of oxygen, control of the associated functions may be lost. Sometimes this loss is permanent. Common stroke symptoms include weakness or numbness that affects one side of the body, sudden loss of vision , confusion, loss of speech or the ability to understand spoken words, and dizziness. Children between the ages of 1-15 are at the highest risk of suffering a stroke. Approximately two-thirds of the children who have a stroke will have at least one more.

Acute chest syndrome

Acute chest syndrome can occur at any age, and is caused by sickle cells blocking the small blood vessels of the lungs. This blockage is complicated by accompanying problems such as infection and pooling of blood in the lungs. Affected persons experience fever, cough, chest pain, and shortness of breath. Recurrent attacks can lead to permanent lung damage.

Diagnosis

Sickle cell anemia is suspected based on an individ ual's ethnic or racial background, and on the symptoms of anemia. A blood count reveals the anemia and the presence of sickle cells in blood samples is easily confirmed by microscopic examination. A sickle cell test can reveal the presence of the sickle cell trait.

The sickle cell test involves mixing equal amounts of blood and a 2% solution of sodium bisulfite. Under these circumstances, hemoglobin exists in its deoxygenated state. If hemoglobin S is present, the red blood cells are transformed into the characteristic sickle shape. This transformation is observed with a microscope , and quantified by expressing the number of sickle cells per 1,000 cells as a percentage. The sickle cell test confirms that an individual has the sickle cell trait, but it does not provide a definitive diagnosis for sickle cell anemia.

To confirm a diagnosis of the sickle cell trait or sickle cell anemia, another laboratory test called gel electrophoresis is performed. This test uses an electric field applied across a slab of gel-like material to separate protein molecules based on their size, shape, or electrical charge. Although hemoglobin S (sickle) and hemoglobin A (normal) differ by only one amino acid, they can be clearly separated using gel electrophoresis. If both types of hemoglobin are identified, the individual is a carrier of the sickle cell trait; if only hemoglobin S is present, the person most likely has sickle cell anemia.

The gel electrophoresis test is also used as a screening method for identifying the sickle cell trait in newborns.

More than 40 states screen newborns in order to identify carriers and individuals who have inherited the trait from both parents. Physicians and researchers also recommend that individuals likely to be exposed to low oxygen tensions (e.g. pilots, divers) undergo screening tests for sickle-cell trait, as studies have shown that those with sickle-cell trait are often less able to cope with low oxygen levels than individuals with normal hemoglobin.

Treatment

Early identification of sickle cell anemia can prevent many problems. The highest death rates occur during the first year of life due to infection, aplastic anemia, and acute chest syndrome. If anticipated, steps can be taken to avert these crises. With regard to long-term treatment, prevention of complications remains a main goal. Sickle cell anemia cannot be cured—other than through a risky bone marrow transplant—but treatments are available for symptoms.

Pain management

Pain is one of the primary symptoms of sickle cell anemia, and controlling it is an important concern. The methods necessary for pain control are based on individual factors. Some people can gain adequate pain control through over-the-counter oral painkillers (analgesics), local application of heat , and rest. Others need stronger methods, which can include administration of narcotics.

Blood transfusions

Blood transfusions are usually not given on a regular basis but are used to treat painful crises, severe anemia, and other emergencies. In some cases, such as treating spleen enlargement or preventing stroke from recurring, blood transfusions are given as a preventative measure. Regular blood transfusions have the potential to decrease formation of hemoglobin S, and reduce associated symptoms. However, regular blood transfusions introduce a set of complications, primarily iron loading, risk of infection, and sensitization to proteins in the transfused blood.

Drugs

Infants are typically started on a course of penicillin that extends from infancy to age six. This treatment is meant to ward off potentially fatal infections. Infections at any age are treated aggressively with antibiotics . Vaccines for common infections, such as pneumococcal pneumonia , are administered when possible.

Emphasis is being placed on developing drugs that treat sickle cell anemia directly. The most promising of these drugs is hydroxyurea, a drug that was originally designed for anticancer treatment. Hydroxyurea has been shown to reduce the frequency of painful crises and acute chest syndrome in adults, and to lessen the need for blood transfusions. Hydroxyurea seems to work by inducing a higher production of fetal hemoglobin. The major side effects of the drug include decreased production of platelets, red blood cells, and certain white blood cells. The effects of long-term hydroxyurea treatment are unknown.

Bone marrow transplantation

Bone marrow transplantation has been shown to cure sickle cell anemia in severely affected children. Indications for a bone marrow transplant are stroke, recurrent acute chest syndrome, and chronic unrelieved pain. Bone marrow transplants tend to be the most successful in children; adults have a higher rate of transplant rejection and other complications.

The procedure requires a healthy donor whose marrow proteins match those of the recipient. Typically, siblings have the greatest likelihood of having matched marrow. Given this restriction, fewer than 20% of sickle cell anemia individuals may be candidates. The percentage is reduced when factors such as general health and acceptable risk are considered. The procedure is risky for the recipient. There is approximately a 10% fatality rate associated with bone marrow transplants done for sickle cell anemia treatment. Survivors face potential long-term complications, such as chronic graft versus host disease (an immune-mediated attack by the donor marrow against the recipient's tissues), infertility , and development of some forms of cancer .

Alternative treatment

In general, treatment of sickle cell anemia relies on conventional medicine. However, alternative therapies may be useful in pain control. Relaxation, application of local warmth, and adequate hydration may supplement the conventional therapy. Further, maintaining good health through adequate nutrition , avoiding stresses and infection, and getting proper rest help prevent some complications.

Prognosis

Several factors aside from genetic inheritance determine the prognosis for affected individuals. Therefore, predicting the course of the disorder based solely on genes is not possible. In general, given proper medical care, individuals with sickle cell anemia are in fairly good health most of the time. The life expectancy for these individuals has increased over the last 30 years, and many survive well into their 40s or beyond. In the United States, the average life span for men with sickle cell anemia is 40-44 years; for women, it is 46-50 years.

Prevention

The sickle cell trait is a genetically linked, inherited condition. Inheritance cannot be prevented, but it may be predicted. Screening is recommended for individuals in high-risk populations; in the United States, African Americans and Hispanic Americans have the highest risk of being carriers.

Screening at birth offers the opportunity for early intervention; more than 40 states include sickle cell screening as part of the usual battery of blood tests done for newborns. Pregnant women and couples planning to have children may also wish to be screened to determine their carrier status. Carriers have a 50% chance of passing the trait to their offspring. Children born to two carriers have a 25% chance of inheriting the trait from both parents and having sickle cell anemia. Carriers may consider genetic counseling to assess any risks to their offspring. The sickle cell trait can also be identified through prenatal testing; specifically through use of amniotic fluid testing or chorionic villus sampling.

Resources

books

Nussbaum, Robert L, Roderick R. McInnes, and Huntington F. Willard. Genetics in Medicine. Philadelphia: Saunders, 2001.

Rimoin, David L. Emery and Rimoin's Principles and Practice of Medical Genetics. London; New York: Churchill Livingstone, 2002.

periodicals

Anie K.A., A. Steptoe, D.H. Bevan. "Sickle Cell Disease: Pain, Coping and Quality of Life in a Study of Adults." Br J Health Psychol. Sep: 7 (2002):331-344.

Kar, B.C. "Clinical Profile of Sickle Cell Trait." J Assoc Physicians India. Nov: 50 (2002): 1368-71.

Thomas V.J., L.M. Taylor. "The Psychosocial Experience of People with Sickle Cell Disease and its Impact on Quality of Life: Qualitative Findings from Focus Groups." Br J Health Psychol. Sep: 7 (2002): 345-363.

Fixler J., L. Styles. "Sickle Cell Disease." Pediatr Clin North Am. Dec: 49 (6)(2002): 1193-210.

Dorn-Beineke, A., T. Frietsch. "Sickle Cell Disease—Pathophysiology, Clinical and Diagnostic implications." Clin Chem Lab Med. Nov: 40 (11)(2002): 1075-84.

organizations

Sickle Cell Disease Association of America [cited March 2003]. <http://sicklecelldisease.org/>.

Julia Barrett

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Amino acid: —An organic compound whose molecules contain both an amino group (-NH2) and a carboxyl group (-COOH). One of the building blocks of a protein.
Anemia: —A condition in which the level of hemoglobin falls below normal values due to a shortage of mature red blood cells. Common symptoms include pallor, fatigue, and shortness of breath.
Bilirubin: —A yellow pigment that is the end result of hemoglobin degradation. Bilirubin is cleared from the blood by action of liver enzymes and excreted from the body.
Bone marrow: —A spongy tissue located in the hollow centers of certain bones, such as the skull and hip bones. Bone marrow is the site of blood cell generation.
Bone marrow transplantation: —A medical procedure in which normal bone marrow is transferred from a healthy donor to an ailing recipient. An illness that prevents production of normal blood cells—such as sickle cell anemia—may be treated with a bone marrow transplant.
Gel electrophoresis: —A laboratory test that separates molecules based on their size, shape, or electrical charge.
Globin: —One of the component protein molecules found in hemoglobin. Normal adult hemoglobin has a pair each of alpha-globin and beta-globin molecules.
Heme: —The iron-containing molecule in hemoglobin that serves as the site for oxygen binding.
Hemoglobin: —The red pigment found within red blood cells that enables them to transport oxygen throughout the body. Hemoglobin is a large molecule composed of five component molecules: a heme molecule and two pairs of globin molecules.
Hemoglobin A: —Normal adult hemoglobin which contains a heme molecule, two alpha-globin molecules, and two beta-globin molecules.
Hemoglobin S: —Hemoglobin that is produced in association with the sickle cell trait; the beta-globin molecules of hemoglobin S are defective.
Hydroxyurea: —A drug that has been shown to induce production of fetal hemoglobin. Fetal hemoglobin has a pair of gamma-globin molecules in place of the typical beta-globins of adult hemoglobin. Higher-than-normal levels of fetal hemoglobin can prevent sickling from occurring.
Iron loading: —A side effect of frequent transfusions in which the body accumulates abnormally high levels of iron. Iron deposits can form in organs, particularly the heart, and cause life-threatening damage.
Jaundice: —A condition characterized by higher-than-normal levels of bilirubin in the bloodstream and an accompanying yellowing of the skin and eyes.
Mutation: —A change in a gene's DNA. Whether a mutation is harmful is determined by the effect on the product for which the gene codes.
Nucleic acid: —A type of chemical that is used as a component for building DNA. The nucleic acids found in DNA are adenine, thymine, guanine, and cytosine.
Red blood cell: —Hemoglobin-containing blood cells that transport oxygen from the lungs to tissues. In the tissues, the red blood cells exchange their oxygen for carbon dioxide, which is brought back to the lungs to be exhaled.
Screening: —Process through which carriers of a trait may be identified within a population.
Sickle cell: —A red blood cell that has assumed a elongated shape due to the presence of hemoglobin S.
Sickle cell test: —A blood test that identifies and quantifies sickle cells in the bloodstream.

The Gale Encyclopedia of Science

Sickle Cell Anemia

views updated Jun 11 2018

Sickle cell anemia

Definition

Sickle cell anemia, also called sickle cell disease (SS disease), is an inherited condition caused by having abnormal hemoglobin, the protein that carries oxygen in the blood. People with sickle cell anaemia have sickle hemoglobin (HbS) which is different from the normal hemoglobin (HbA).

Description

Children with sickle cell anemia produce two abnormal hemoglobin proteins (inheriting one from each parent), which makes their red blood cells easily destructible while giving them a sickle-like shape. Since the red blood cells do not have a normal shape, their circulation in the small blood vessels is impaired as well as the function of the abnormal hemoglobin (HbS) which can no longer carry oxygen with maximum efficiency.

Transmission

Sickle cell anemia is usually inherited from parents who are carriers, who have the sickle cell trait—a milder form of sickle cell anemia, or one abnormal hemoglobin.

Demographics

Sickle cell anemia and sickle cell trait are found mainly in people whose families come from Africa, the Caribbean, the Eastern Mediterranean, Middle East, and Asia. In the United States, sickle cell anemia affects some 72,000 people. The families of most of the people affected come from Africa. The disease occurs in about one in every 600 African-American births and in one in every 1,000 to 1,400 Hispanic-American births. Some 2 million Americans carry the sickle cell trait and about one in 12 African Americans have the trait.

Causes and symptoms

Sickle cell anemia is caused by an error in the gene that signals the body how to make hemoglobin. The defective gene tells the body to make the abnormal hemoglobin HbS instead of the normal HbA, and this results in deformed red blood cells. The error in the hemoglobin gene is due to a genetic mutation that occurred many thousands of years ago in people living in Africa, the Mediterranean basin, the Middle East, and India. A deadly form of malaria was very common at that time, and research has shown that in areas where malaria was endemic, children who inherited one HbS gene and who, therefore, carried the sickle cell trait, had a survival advantage because, unlike the children who had normal HbA genes, they survived malaria. They grew up, had their own children, and passed on the gene for HbS.

Symptoms or complications associated with sickle cell anemia usually start after the age of four to six months and can include all or some of the following:

anemia, caused by low amounts of red blood cells in the bloodstream, resulting in insufficient oxygen delivery to tissues and organs
vaso-occlusive pain , meaning severe episodes of pain in the arms, legs, or back, due to impaired blood circulation in the blood vessels
chest pain and fever with coughing
dactylitis, or hand-foot syndrome, with painful swelling of the bones in the hands or feet of young children
aplastic crises, during which the body stops making new red blood cells causing severe anemia usually following an infection with the parovirus B19, which causes fifth disease
priapism, a painful and prolonged erection
stroke , usually causing sudden weakness of one side of the body
acute splenic sequestration, with pooling of blood causing a sudden enlargement of the spleen
jaundice, a yellowing of the skin and white of the eyes
frequent infection with certain bacteria, particularly pneumococcus and salmonella, which are due to auto-infarction of the spleen or death of the spleen due to poor blood flow

When to call the doctor

Children suffering from sickle anemia have episodes during which they suddenly become unwell or complain of severe abdominal or chest pain, headache , stiffness of the neck or drowsiness. Parents should know that a child having a sickle cell crisis requires urgent hospital treatment. They should also call a doctor if the child has a temperature above 101°F (38.5°C).

Diagnosis

The diagnosis of sickle cell anemia is established during the newborn screen testing that is performed in the nursery at time of birth. For children who are not tested, an electrophoresis test of the blood can detect the abnormal hemoglobin of sickle cell anemia. This test measures the speed at which a molecule moves in a gel and can detect abnormal hemoglobin HbS.

Treatment

Treatment usually includes frequent monitoring of red blood counts, antibiotics for infections, transfusions for aplastic crises and splenic sequestration when required, and oxygen as well as respiratory support for chest syndrome. Some patients with severe symptoms receive regular blood transfusions to prevent crises and/or other complications such as stroke and organ damage.

Children with sickle cell disorders are at risk of developing severe infections, and penicillin is usually prescribed to prevent dangerous pneumococcal infections.

Sickle cell pain can be managed with a variety of measures including the following:

warmth, to increase the blood flow, by massaging and rubbing and by heat from hot water bottles and deep heat creams
bandaging to support the painful region
rest
getting the child to relax, by deep breathing exercises and distracting the attention, and by other psychological methods
use of pain-killing medicines (analgesics )

Analgesics should only be given as recommended by the treating physician. The gentlest analgesic usually prescribed is paracetamol, given three times a day (62.5 mgm under 12 months; 125 mgm 1–4 years; 250 mgm 4–10 years; 500 mgm 10–14 years; and 1 gm 15 years upwards). The next gentlest is codeine phosphate, given four times a day, at 1–2 mgm for every kilogram of body weight.

Bone marrow transplantation has been shown to provide a cure for severely affected children with sickle cell disease, but the procedure is not entirely without risk. In addition, the marrow must come from a healthy matched sibling donor and only about 18 percent of children with sickle cell anemia are likely to have a matched sibling.

Alternative treatment

Research contributed a great deal about sickle cell anemia from 1970 to the early 2000s concerning what causes it, how it affects the patient, and how to treat it. Scientists were as of 2004 starting to be successful at developing drugs that prevent the symptoms of sickle cell anemia and procedures that they hope should eventually provide a cure. Drug research is focused on identifying drugs, such as hydroxyurea, that can increase the level of fetal hemoglobin in the blood. Fetal hemoglobin is a form of hemoglobin that all humans produce before birth, but most stop producing it after birth. It has been observed that some children with sickle cell anemia continue to produce large amounts of fetal hemoglobin after birth, and studies have shown that these children have less severe cases of the disease. Fetal hemoglobin seems to prevent "sickling" of red cells, and cells containing fetal hemoglobin tend to survive longer in the bloodstream. Butyrate, a substance widely used as a food additive, was also being investigated as of 2004 as an agent that may increase fetal hemoglobin production.

Nutritional concerns

Thirst and dehydration caused by not drinking enough, even if thirst is not felt, are known to trigger sickle pain. Parents should accordingly monitor fluid intake closely.

Children with sickle cell anemia are anemic to various degrees. Most of the time they feel quite well, but if the anemia gets worse, they may feel very tired. Folic acid , a vitamin found in fruit and vegetables, supports making blood and anemic children especially need it to prevent them from becoming run down.

Prognosis

Sickle cell anemia is an inherited disease and lasts a lifetime.

Prevention

Both sickle cell trait and sickle cell anemia are inherited. Therefore, parents can pass it to their offspring. It is important for parents to get tested. If one partner has sickle cell trait and the other does not, their children each have a 50 percent chance of having the sickle cell trait, and a 50 percent chance of having normal hemoglobin. If one parent has sickle cell trait it is extremely important that the other parent be tested. If both parents have sickle cell trait, there is a 25 percent chance that the child will have normal hemoglobin, a 50 percent chance that the child will have the sickle cell trait, and a 25 percent chance the child will have sickle cell disease. If both parents have sickle cell trait and want to know whether the unborn child has sickle cell anemia, testing can be performed as early as the tenth week of pregnancy. If the results are normal, the parents can be reassured. If the tests show that the baby will be affected, the parents can be better prepared and they can make an informed decision concerning the pregnancy.

Parental concerns

Parents should be aware that children with sickle cell anemia are also at increased risk of infection, especially from the Streptococcus pneumonia and H. influenzae bacteria.

Sickle cell anemia does not affect intelligence . Children with sickle cell disorders can almost always attend school and participate fully in normal activities. The child's teacher and the school principal should know of the diagnosis and understand the limitations sickle cell anemia can impose on a child, for instance the need for frequent drinks and easy access to the bathroom, and the triggering of pain by over-exertion or cold.

KEY TERMS

Acute splenic sequestration —Retention of blood in the spleen.

Anemia —A condition in which there is an abnormally low number of red blood cells in the bloodstream. It may be due to loss of blood, an increase in red blood cell destruction, or a decrease in red blood cell production. Major symptoms are paleness, shortness of breath, unusually fast or strong heart beats, and tiredness.

Aplastic anemia —A disorder in which the bone marrow greatly decreases or stops production of blood cells.

Bone marrow —The spongy tissue inside the large bones in the body that is responsible for making the red blood cells, most white blood cells, and platelets.

Dactylitis —Inflammation of the hands or feet.

Endemic —Natural to or characteristic of a particular place, population, or climate.

Hemoglobin —An iron-containing pigment of red blood cells composed of four amino acid chains (alpha, beta, gamma, delta) that delivers oxygen from the lungs to the cells of the body and carries carbon dioxide from the cells to the lungs.

Priapism —A painful, abnormally prolonged penile erection.

Sickle cell trait —Condition that occurs in people who have one of two possible genes responsible for the abnormal hemoglobin of sickle cell anemia. People with this trait may suffer milder symptoms of sickle cell anemia or may have no symptoms. Some scientists believe that the trait actually provides an advantage in tropical environments because the slightly altered shape of the blood cells cause a person to be more resistant to malaria.

See also Anemias.

Resources

BOOKS

Gillie, Oliver. Sickle Cell Disease (Just the Facts). Chicago: Heineman Library, 2004.

Gordon, Melanie A. Let's Talk about Sickle Cell Anemia. New York: Powerkids Press, 2000.

Platt, Alan F., and A. Sacerdote. Hope and Destiny: A Patient's and Parent's Guide to Sickle Cell Disease and Sickle Cell Trait. Roscoe, IL: Hilton Publishing Co., 2003.

PERIODICALS

Fullerton, H. J., et al. "Declining stroke rates in Californian children with sickle cell disease." Blood 104, no. 2 (July 2004): 336–39.

Quinn, C. T., et al. "Survival of children with sickle cell disease." Blood 103, no. 11 (June 2004): 423–27.

Steinberg, M. H. "Therapies to increase fetal hemoglobin in sickle cell disease." Current Hematology Reports 2, no. 2 (March 2003): 95–101.

Zimmerman, S. A., et al. "Sustained long-term hematologic efficacy of hydroxyurea at maximum tolerated dose in children with sickle cell disease." Blood 103, no. 6 (March 2004): 239–45.

ORGANIZATIONS

American Sickle Cell Anemia Association (ASCAA). 10300Carnegie Avenue, Cleveland Clinic EEb18, Cleveland, OH 44106. Web site: <www.ascaa.org>.

Sickle Cell Society. 54 Station Road, London, NW10 4UA, UK. Web site: <www.sicklecellsociety.org>.

WEB SITES

"Sickle Cell Anemia." Medline Plus. Available online at <www.nlm.nih.gov/medlineplus/sicklecellanemia.html> (accessed November 14, 2004).

Monique Laberge, Ph.D.

Gale Encyclopedia of Children's Health: Infancy through Adolescence Laberge, Monique

Sickle Cell Anemia

views updated May 23 2018

Sickle Cell Anemia

SYMPTOMS AND CAUSES

SICKLE CELL ANEMIA AND MALARIA

MODERN-DAY ISSUES

BIBLIOGRAPHY

Sickle cell anemia is a severe and commonly fatal anemia, or failure of oxygen transfer. Its name derives from the fact that the red blood cells (RBCs), the transporters of oxygen, which are normally lozenge shaped, collapse into characteristic sickle-shaped cells under the influence of the disease. These red blood cells have drastically reduced ability to transport oxygen and a reduced life expectancy (10–12 days instead of the normal 120 days). The condition is often punctuated by painful “crises” resulting from the fact that the sickle shaped cells are elongated and relatively stiff and may periodically block capillaries because they clump, or agglutinate. The condition was first described in Western medicine by James Herrick in a series of publications between 1910 and 1912.

SYMPTOMS AND CAUSES

A wide range of symptoms may result, ranging from the weakness and lassitude characteristic of anemia to acute and/or chronic pain. Specific symptoms include shortness of breath, jaundice, poor physical development, small slender bodies, delayed sexual maturity, swelling of hands and feet, impaired mental function, and rheumatism. It may also cause local failure of blood supply, possibly resulting in blindness, strokes, and damage to the heart, brain, lungs, spleen, gastrointestinal tract, and kidneys. In addition, it can cause painful priapism, resulting in damage to the penis. One of its most dangerous consequences is reduced resistance to infection, particularly influenza, pneumonia, and meningitis. Fewer than 10 percent of individuals with sickle cell anemia survive to adulthood, and their evolutionary fitness or reproductive success (compared to average fitness, defined as 1.0) is about 0.1.

Sickle cell anemia was the first genetic disease whose molecular basis was identified. The condition is the result of having two copies (i.e., the homozygote condition) of the HbS allele (variant) of the gene for the production of hemoglobin, a complex molecule of four polypeptide (amino acid) chains. Hemoglobin is found in red blood cells and is the molecule that grabs oxygen from the lungs and transports oxygen in the blood for use in the body. The HbS allele results from a point mutation of the HbA allele, resulting in the substitution of amino acid valine for glutamic acid in position six on the beta chain of the hemoglobin molecule. The codon change is a substitution of GTG for GAG.

HbS is a classic autosomal recessive trait. The alleles are distributed in a normal Mendelian manner such that heterozygote parents (i.e., each with one copy of the allele) will produce homozygote “normals,” sickle cell carriers, and homozygotes with full-fledged sickle cell disease in a ratio of 1:2:1. On average, 25 percent of children born to two heterozygote parents will have the disease. Homozygous individuals are heavily afflicted, whereas heterozygotes possess both types of hemoglobin and function normally unless placed in conditions of high oxygen stress such as heavy labor, athletics, or high-altitude activity. It is now known that the disease is carried by about one in twelve African Americans, resulting in sickle cell anemia in one of 500 births to African Americans, although the rate is gradually declining.

The mutation(s) may have occurred separately several times: in as many as four locations in Africa and an additional one in India or Arabia. However, that interpretation remains controversial. The allele may also have radiated outward from a center in West Africa or, conversely, into Africa from the Middle East. The modern distribution of the allele results in significant part from patterns of trade and travel under Muslim and then Christian dominion of areas of the Middle East and Africa. In the early twenty-first century it occurs with the highest frequency in Central Africa, where 30 percent or more of the population can be carriers, as well as in northeast and northwest India, Turkey, and Saudi Arabia. It also reaches high frequencies in parts of Greece and is found elsewhere in Mediterranean Europe.

SICKLE CELL ANEMIA AND MALARIA

The persistence of such a deleterious allele posed an evolutionary paradox for biological scientists: Why doesn’t the allele simply disappear through natural selection? The solution to the paradox was first addressed by Anthony C. Allison in the 1950s and has since received considerable attention from other scholars, notably F. B. Livingstone. As a result of this attention, the sickle cell allele has emerged as the first and still best understood example of the role of infectious disease and human behavior in natural selection; it remains the model on which interpretation of other genetic disorders such as Tay-Sachs disease and cystic fibrosis is based.

All three are examples of “balanced polymorphisms” in which an allele deleterious in homozygotes is maintained in a population because in the heterozygote form it provides protection from an infectious disease. In the case of the sickle cell trait, the heterozygote HbS-HbA tends to resist malaria, itself one of the most significant debilitating and often fatal diseases affecting human beings throughout history. Malaria has been a major force in human evolution. The sickle cell trait is an example of how rapidly the spread of a disease can result in significant evolutionary change in the human species. Individuals possessing the allele are less likely to harbor the malarial parasite in significant numbers in comparison to homozygote normals. They live longer and are less affected by symptoms of malaria. Heterozygote women may be more fertile; men may also be more fertile because high fevers associated with malaria can reduce spermatogenesis. In malarial areas, both the homozygote HbA and the homozygote HbS are selected against. For every 100 heterozygote survivors in malarial areas, there are only about 88 HbA homozygote survivors and only 14 HbS homozygote survivors. The relative success of the heterozygote results in perpetuation of the HbS allele. (Tay-Sachs disease may be a similar response to tuberculosis, cystic fibrosis, cholera, typhoid, or other intestinal infection.)

The relationship between the sickle cell allele and malaria has been demonstrated sequentially in three ways. First, it has been shown that allowing for some movement of populations and malarial zones, the geographical concordance of HbS (and/or other forms of abnormal protective forms of hemoglobin, including HbC, HbE, HbO, variants of the Duffy blood group, various forms of thalassemia, and the enzyme G6PD, or glucose 6 phosphate dehydrogenase) with malaria is striking and too detailed to reflect mere chance. Figure 1 depicts the broad geographical correlation of the allele and the disease. The detailed correlation in specific locations such as various altitudes adds to the strength of the correlation. Allison (1954), for example, reports that there is no indigenous group in Africa with a high percentage of the S allele in which malaria is not present. It is, for example, rare in highlands and dry areas inhospitable to Anopheles mosquitoes. It is also less common where swidden agriculture is not the norm.

Second, the fitness effects of the HbS allele (as above) have been demonstrated repeatedly. Third, it has been shown that the structure of the sickled red blood cell is less able to support the growth of the protozoan malarial parasite, demonstrating the mechanisms by which heterozygotes are protected against malaria.

The history of the sickle cell allele is bound to that of the spread of malaria, which is in turn bound to the emergence of agricultural economies in the afflicted areas within the last 6,000 years, when falciparum malaria spread in an explosive manner. In West Africa it spread probably in association with the expansion of Bantu farmers into areas previously devoted to hunting and gathering in the last 2,000 to 3,000 years.

Forest clearance associated with farming created the mix of light and shade that in humid climates is highly favorable to Anopheles mosquitoes, which are the most efficient vectors of falciparum malaria. In addition, large populations and the sedentary communities that emerged with farming provided a larger pool of susceptible people, permitting the disease to become endemic. Farming also created layers of highly impenetrable lateritic soils, which in turn led to more standing water where mosquitoes could breed. It also tended to remove or displace other mammals as potential victims of mosquitoes, which then focused the attention of the mosquitoes on human beings.

MODERN-DAY ISSUES

Sickle cell anemia is also in some sense a dietary deficiency disease, at least in the United States. Its effects are mitigated to a significant degree by diets rich in cyanate and thiocyanate, which are typically found in tropical foods such as yams, manioc (cassava), sorghum, and some millets that are rarely eaten in the United States. Symptoms are often worse in the United States than in areas where these foods are more often consumed. The sickle cell trait has been found to be more common in areas of Africa where these foods are eaten, presumably because they reduce the severity of the symptoms, thus changing the balance of selective pressures. Yams, however, also reduce resistance to malaria, often resulting in patterns of yam consumption in Central Africa carefully balanced with seasonal malaria.

In the United States and other areas with the benefits of modern biomedicine, the symptoms of this disease can be alleviated by a therapeutic diet rich in cyanates and antioxidants. It has become possible to identify the presence of the gene by a simple blood test. Prenatal testing of fetuses for the presence of HbS and genetic counseling for heterozygote parents are available. Since 1984 it has been possible to fight HbS with bone marrow transplants from “normal” homozygotes. As of 1995 there has also been a therapy, hydroxyurea, that effectively treats symptoms.

Although often considered a “racial” concomitant of dark skin, sickle cell genetics actually provides one of many pieces of evidence demonstrating that biologically, “races” do not exist. Sickle cell hemoglobin is not limited to individuals of dark skin, and its correlation with dark skin is actually weak. Even in populations where it is endemic, such as in malarial regions of West Africa, no more than 10 to 30 percent of hemoglobin alleles are of the HbS form. The comparable figure in African Americans is no more than 5 percent. In other areas of Africa where malaria is less of a problem, the allele occurs in far lower frequencies or not at all. It does not occur among unrelated dark-skinned populations of Oceania or Australia. That is, the overwhelming majority of dark-skinned people, even those from or derived from West Africa, do not carry the allele. Conversely, the allele is known in malarial areas north of the Mediterranean, from southern Europe to India, among populations normally considered “white.” It is interesting to speculate that if Mediterranean populations from Spain, Portugal, Greece, Italy, or Turkey had been the main colonizers of the New World, and if they had obtained their slaves from some areas of North and South Africa or Australia, the sickle cell trait might be associated with light rather than dark skin.

BIBLIOGRAPHY

Allison, Anthony C. 1954. “Protection Afforded by the Sickle Cell Trait against Malarial Infection.” British Medical Journal 1:290–294.

_____.2004. “Two Lessons from the Interface of Genetics and Medicine.” Genetics 166 (4): 1591–1599.

Livingstone, Frank B. 1958. “Anthropological Implications of Sickle Cell Gene Distribution in West Africa.” American Anthropologist 60 (3): 533–562.

Molnar, Stephen. 2005. Human Variation: Races, Types, and Ethnic Groups, 6th ed. Upper Saddle River, NJ: Pearson Prentice Hall.

Weiss, Kenneth M. 1993. Genetic Variation and Human Disease: Principles and Evolutionary Approaches. Cambridge, U.K.: Cambridge University Press.

Mark Nathan Cohen

Encyclopedia of Race and Racism

Sickle Cell Anemia

views updated May 14 2018

SICKLE CELL ANEMIA

DEFINITION

Sickle cell anemia is an inherited blood disorder in which the body produces c-shaped red blood cells. Because of their shape, these cells may stick to each other or to the sides of blood vessels, and cause serious health disorders.

DESCRIPTION

Blood is made up of many kinds of cells, including red blood cells (RBCs). An important function of RBCs is to carry oxygen from the lungs to cells throughout the body. Red blood cells contain a molecule known as hemoglobin (pronounced HEE-muh-GLOW-bihn) that collects oxygen from the lungs then releases it when the RBC reaches other cells in the body.

Normally, red blood cells have a plump doughnut shape. In some cases, however, a person's body makes red blood cells that are curved with sharp points at the end. They are called sickle cells because they look like a sickle (a long farm tool with a curved blade that is used to cut grain).

Hemoglobin Genes

Sickle cell anemia is caused by defective genes. Genes are chemical units found in all cells that tell cells what functions to perform. For example, RBCs contain genes that tell the cell how to make hemoglobin molecules.

When a gene becomes damaged, the message it carries to the cell is incorrect. A damaged gene for hemoglobin tells the cell to make the wrong kind of hemoglobin. This defective hemoglobin creates a sickle shaped RBC rather than the correct doughnut-shaped RBC.

Sickle Cell Anemia: Words to Know

Anemia:: A condition caused by a decrease in the number of red blood cells in the blood, characterized by fatigue, pale color of the skin, and shortness of breath.
Antibiotic:: A substance derived from bacteria or other organisms that fights the growth of other bacteria or organisms.
Bone marrow:: A spongy tissue in the center of bones where blood cells are produced.
Bone marrow transplantation:: A process by which marrow is removed from the bones of a healthy donor and transferred to the bones of a person's with some kind of blood disorder.
Gel electrophoresis:: A laboratory test that separates different types of molecules from each other.
Hemoglobin:: A molecule found in blood that gives blood its red color. Hemoglobin is responsible for transporting oxygen through the blood stream.
Hydroxyurea:: An experimental drug being tested for use with sickle cell anemia patients.
Red blood cell (RBC):: Blood cells that transport oxygen and carbon dioxide through the blood stream.
Sickle cell:: A red blood cell with an abnormal shape due to the presence of an abnormal form of hemoglobin.

Genes are passed from both parents to their children. A person who receives a damaged hemoglobin gene from just one parent will not get sickle cell anemia. People who have only one defective hemoglobin gene are called carriers. Being a carrier of this particular defective gene may actually increase a person's resistance to malaria (see malaria entry), a dangerous infectious disease. However, if a person receives a defective hemoglobin gene from both parents, he or she will develop sickle cell anemia.

Problems Associated with Sickle Cell Anemia

The presence of sickle cells in the blood can cause many health problems. For instance, sickle cells die more rapidly than normal cells. When this happens, the body often cannot produce new blood cells fast enough to replace the dying ones. Such a loss of RBCs can lead to anemia (pronounced uh-NEE-mee-uh; see anemia entry). Anemia is a disorder caused by an insufficient number of red blood cells.

Sickle cells can also cause health problems because of they tend to stick to each other and to the sides of blood vessels. As they clump or build up they can eventually block the flow of blood through a blood vessel. This blockage limits the flow of blood to cells, keeping them from getting the oxygen they need, and can eventually cause the cells to die.

Blockage can also lead to a stroke. If the clump of cells blocking a vessel breaks loose it may travel to the brain. If the clump blocks blood flow

to the brain it can cause damage to the brain known as a stroke (see stroke entry).

Sickle cell anemia occurs primarily among people with African, Mediterranean, Middle Eastern, and Indian ancestry. Worldwide, about 250,000 children are born each year with sickle cell anemia. About two million Americans are thought to have at least one damaged hemoglobin gene. Approximately 72,000 Americans have two damaged hemoglobin genes and therefore have sickle cell anemia.

In the United States, the condition is most common among African Americans. About 1 in 12 African Americans is a carrier for sickle cell anemia. Hispanic Americans are also heavily affected. About 1 in every 1,000 to 1,400 Hispanic American babies are born with sickle cell anemia.

CAUSES

Sickle cell anemia is caused when a person receives a defective hemoglobin gene from both parents, causing the body to make abnormal red blood cells, which may clump and tend not to live as long as normal red blood cells. A person with sickle cell anemia may become anemic or develop other health problems.

SYMPTOMS

The symptoms of sickle cell anemia usually appear during the first year or two of life. However, some individuals do not develop symptoms until they become adults, and may not be aware for many years that they have the disorder. Some typical symptoms of sickle cell anemia include:

Anemia. Anemia is caused by an inadequate number of red blood cells. It can result in fatigue, paleness, shortness of breath, headache, mild fever, and general ill health.
Painful crises. Pain can strike the patient in any part of the body without notice. These attacks can occur as rarely as once a year or as often as every few weeks. They can also last for a variable period of time, from a few hours to a few weeks. Pain in the hands and feet are sometimes the earliest symptoms of sickle cell anemia in a child.
Enlarged spleen and infections. Sickle-cell blockages can affect any of the body's organs. The organs do not receive the oxygen they need to grow normally. The spleen is especially at risk and may become enlarged or it may die completely. This can weaken the immune system and increase the chance that a patient will develop infections.
Delayed growth. Children with sickle cell anemia usually do not grow as fast as other children. They may also reach puberty (sexual maturity) at a later age.
Stroke. Blockages of blood vessels in the brain are especially dangerous. The brain may not get the oxygen it needs to function normally. When blockages occur, a person may become numb on one side of the body, may lose vision or the ability to speak, and may experience dizziness. Children between the ages of one and fifteen are at the highest risk for having a stroke due to sickle cell anemia.
Acute chest syndrome. Acute chest syndrome is caused by blockage of blood vessels in the lungs. Symptoms of the condition include fever, cough, chest pain, and shortness of breath. The condition can reoccur many times and may cause permanent lung damage.

Other problems caused by blood vessel blockage include kidney damage, enlarged liver, vision problems, and priapism (a condition in which a man experiences repeated and painful erections of the penis not related to sexual arousal; pronounced PREE-uh-piz-um).

DIAGNOSIS

Anemia is easily diagnosed from its symptoms. Once a patient is diagnosed with anemia, a doctor will then try to trace the cause of the disorder. If the person is of African American or other high-risk heritage, sickle cell anemia may be suspected. This diagnosis can be confirmed by at least two laboratory tests. In the first test, a sample of the patient's blood is examined under the microscope where the presence of sickle cells is easy to see.

A doctor can confirm that sickle cells are present with a second test, called gel electrophoresis (pronounced jel ih-LEK-tro-fuh-REE-siss). Gel electrophoresis is a method for distinguishing similar kinds of molecules from each other and will show whether abnormal forms of hemoglobin are present in the blood.

TREATMENT

Sickle cell anemia cannot be cured. However, many of its symptoms can be treated. Its most serious complications can also be prevented. The important factor is to diagnose the disorder as early as possible and begin treatment immediately. Methods used to treat symptoms include:

Pain management. Pain is a common problem with sickle cell anemia. Some patients get the relief they need from over-the-counter medication, such as aspirin and acetaminophen. Others need stronger painkillers. Care givers should be careful giving aspirin to children as it has been linked with development of Reye's syndrome (see Reye's syndrome entry).
Blood transfusions. Blood transfusions are generally used only in extreme situations, such as severe anemia or especially bad episodes of pain.

Drugs. Infants are often treated with antibiotics to prevent infections. (Antibiotics are substances derived from bacteria or other organisms that fight the growth of other bacteria or organisms.) Such treatments may last to the age of six. Research is constantly being conducted to develop drugs for the cure of sickle cell anemia. One promising candidate is hydroxyurea, which seems to reduce pain and acute chest syndrome and can limit the need for blood transfusions in some cases.
Bone marrow transplantation. Bone marrow transplantation is used in only the most severe cases of sickle cell anemia. It is based on the fact that new blood cells are made in the marrow of bones. The marrow is soft tissue found in the center of bones. In a bone marrow transplant, marrow is removed from the bones of a healthy donor. It is then injected into the bones of a person with sickle cell anemia. If the procedure is successful, the donor marrow begins making normal, rather than sickle cell, RBCs. Bone marrow transplantation is a very risky procedure with only limited chances of success.

Alternative Treatment

Sickle cell anemia is best treated by conventional medical techniques. However, alternative treatments may help ease some symptoms of the condition. Relaxation techniques, application of warm compresses, and adequate hydration may increase a patient's comfort. Good nutrition, the avoidance of stress, and proper rest may also help prevent some complications of the disorder.

PROGNOSIS

The average life expectancy for men with sickle cell anemia in the United States is forty-two years. For women, it is forty-eight years. The prognosis for any one individual depends on many factors but in general, patients receiving proper medical care may learn to lead relatively normal lives with the disorder.

PREVENTION

Sickle cell anemia is a genetic disorder. There is no prevention for the disease other than genetic screening. Adults can have tests to find out if they carry the gene for sickle cell anemia. If they find they are carriers they can decide whether or not they want to have children. If they decide to have children, there is a known risk that the children may develop sickle cell anemia.

FOR MORE INFORMATION

Books

Bloom, Miriam. Understanding Sickle Cell Disease. Jackson, MS: University Press of Mississippi, 1995.

Beshore, George W., ed. Sickle Cell Anemia. New York: Franklin Watts, Inc., 1994.

Silverstein, Alvin, Virginia Silverstein, and Laura Silverstein Nunn. Sickle Cell Anemia. Hillside, NJ: Enslow Publishers, Inc., 1997.

Organizations

Sickle Cell Disease Association of America. 200 Corporate Point, Suite 495, Culver City, Ca 90230–7633. (310) 216–6363; (800) 421–8453. http://sicklecelldisease.org.

Sickle Cell Disease Program, Division of Blood Diseases and Resources. National Heart, Lung, and Blood Institute. 11 Rockledge Centre, 6701 Rockledge Dr., MSC 7950, Bethesda, MD 20892–7950. (301) 435–0055.

UXL Complete Health Resource

Sickle Cell Disease

views updated May 08 2018

SICKLE CELL DISEASE

The sickle cell diseases are a group of disorders that have in common the propensity of the red blood cells to become deformed when oxygen tension in the blood is lowered, causing anemia, occlusion of blood vessels by misshapen cells, and various associated clinical consequences, including death. In sickle cell disease, a mutation of the beta-globin gene results in the substitution of valine for glutamic acid in the sixth amino acid of the chain, producing a hemoglobin, designated hemoglobin S, that has less solubility than does normal hemoglobin A. Inheriting one gene for hemoglobin S, together with a normal gene, results in the formation of red cells that contain approximately 40 percent of the abnormal hemoglobin and 60 percent of the normal hemoglobin, an essentially harmless state that is designated as sickle cell trait. But if the gene inherited together with the sickle gene is not normal, then the sickle cell disease may develop. The most common hemoglobin that interacts with sickle hemoglobin is hemoglobin C, and the ß-thalassemia (beta-thalassemia) mutation also interacts with the sickle gene by restricting the formation of normal hemoglobin.

The sickle gene, and genes that interact with it, are common in a number of different populations, but the highest gene frequencies are observed in Africa. The gene is also found in southern Europe, the Middle East, and India. A single dose of the sickle gene provides protection against malaria. Since malaria was a major cause of death in Africa, persons who carried the sickle gene had a survival advantage over those who did not. Thus, the number of persons carrying this mutation has tended to increase generation after generation in areas where malaria was a major killer. Among African Americans, approximately 7.8 percent are carriers of the sickle mutation, that is, they have sickle cell trait; while 2.3 percent have hemoglobin C trait (one copy of the hemoglobin C gene); and0.8 percent have ß-thalassemia trait.

Although a single copy of the hemoglobin S gene is quite harmless, if a person inherits two copies of the hemoglobin S genes, he or she will have sickle cell disease. If one hemoglobin S gene and one hemoglobin C gene are inherited, the patient has hemoglobin S-C disease. Coinheritance of the beta-thalassemia and sickle hemoglobin result in sickle cell thalassemia. Patients with these three disorders have a similar clinical disease. Anemia occurs as a result of the rapid destruction of red blood cells. The red cells may have the shape of sickles, hence the term "sickle cell disease." However, the cells may assume may other forms. The misshapen red cells occlude blood vessels and cause pain and even tissue death.

In small children, one of the great problems incident to sickle cell disease is infections. If these are treated promptly, most children with sickle cell disease survive into adult life. One of the most characteristic manifestations of the disease in adults and older children is "pain crises." These occur at regular intervals, often at a time of stress, and may cause frequent hospitalizations and varying degrees of dependence upon pain-killing drugs. As patients with the sickle cell disease grow older they begin to suffer from the results of accumulated damage in small blood vessels all through the body. Dysfunction of the lungs, kidneys, and heart are common. Strokes may occur. Interruption of the blood supply to bones may result in areas of bone death, particularly in the hips.

Although sickle cell disease is a disorder that has been better understood and studied in more detail than most other disorders, treatment is still very unsatisfactory. Prenatal diagnosis can be carried out quite easily and very reliably, and parents are provided with the option of terminating the pregnancy. Antibiotics and immunization programs have drastically reduced the mortality rate among young children. Transfusion of red blood cells improves the flow properties of blood and may ameliorate the symptoms. Hydroxyurea has been administered to increase the amount of fetal hemoglobin, a hemoglobin that does not interact with sickle hemoglobin. This treatment has met with some success.

The disease is cured by bone marrow transplantation, a procedure with a relatively high risk, even in those patients in whom a match can be found. Ultimately the disease may be treated by putting a normal beta-globin gene into a stem cell of the patient, and then transplanting that patient with his or her own transduced cells, but there are many barriers to implementing such a strategy. Because stem cells do not divide often, they are relatively resistant to many gene-transfer methods. It is not enough to put a normal globin gene in the cell; the abnormal globin gene needs to be inactivated. There is also a tendency for normal human cells to shut off the function of foreign genes that are implanted in them. It is likely that these technical obstacles to gene therapy will be overcome eventually, and that the treatment of this group of diseases will give better results in the future.

Ernest Beutler

(see also: Genes; Genetic Disorders; Hemoglobin; Hemoglobinopathies; Malaria; Medical Genetics )

Encyclopedia of Public Health Beutler, Ernest

Sickle Cell Anemia

views updated May 29 2018

SICKLE CELL ANEMIA

Sickle cell anemia is a genetic disease caused by a single recessive mutation in hemoglobin. Individuals who inherit this recessive gene from both parents exhibit symptoms, while those who inherit only one copy of the gene typically do not exhibit symptoms and are resistant to malaria. About 1 in 12 African Americans worldwide carry the trait, and about 1 in 400 have the disease. Sickling of red blood cells causes them to clump together and impedes the passage of red blood cells in the circulatory system. Painful vaso-occlusive crises are influenced by the frequency of other globin mutations, as well as psychological stress. Thus, many individuals have complications from the disease, whereas others are relatively healthy. The physical signs include slow growth, lethargy, jaundice, anemia, poor feeding, enlargement of liver and spleen, and delay in sexual maturation. Psychosocial symptoms include embarrassment, poor self-esteem, depression, and fear. There is no cure, but there have been significant advances in life expectancy and treatment.

See also:GENOTYPE

Bibliography

Conyard, Shirley, Muthuswamy Krishnamurthy, and Harvey Dosik."Psychological Aspects of Sickle-Cell Anemia in Adolescents." Health and Social Work vol. 5 (1980):20-26.

DeRoin, Dee Ann. "Sickle Cell Anemia." Written for Clinical Reference Systems, 1998, on NBCi [web site]. Available from http://br.nbci.com/lmoid/resource/0,566,-2770,00.html; INTERNET.

National Library of Medicine. "Sickle Cell Disease in Newborns and Infants: A Guide for Parents." In the Wellness Web [web site], 2001. Available from http://wellweb.com/index/qsickle.htm; INTERNET.

Kathryn S.Lemery

Child Development

sickle cell anemia

views updated May 23 2018

sick·le cell a·ne·mi·a / ˈsikəl sel əˈnēmēə/ (also sick·le cell dis·ease) • n. a severe hereditary form of anemia in which a mutated form of hemoglobin distorts the red blood cells into a crescent shape at low oxygen levels. It is commonest among those of African descent.

The Oxford Pocket Dictionary of Current English