Myopathies
Myopathies
Definition
Myopathies are diseases of skeletal muscle that are not caused by nerve disorders. These diseases cause the skeletal or voluntary muscles to become weak or shrunken (atrophied).
Description
There are many different types of myopathies. Some are inherited, some inflammatory, and some caused by endocrine or metabolic problems. Myopathies usually are not fatal. Typically they cause muscle weakness and movement problems. The shoulders and thigh muscles are usually, but not always, affected earlier than the muscles of the hands and feet. Most myopathies are degenerative, meaning they become more pronounced over time. Some weaknesses are transitory. Only rarely do individuals become dependent on a wheelchair. However, muscular dystrophy (technically a myopathy) is far more severe. Some types of muscular dystrophy are fatal in early adulthood.
Causes and symptoms
There is great variety among myopathies, but what they all share are effects on the skeletal muscles. The main causes of myopathies are genetic, inflammatory (caused by infection), endocrine (hormonal), and metabolic (errors in how cells function). Often the cause of the myopathy is not known (idiopathic disease).
Genetic myopathies
Among their many functions, genes are responsible for overseeing the production of proteins important in maintaining healthy cells. Muscle cells produce thousands of proteins. With each of the inherited myopathies, a genetic defect is linked to a lack of, or defect in, one of the proteins needed for normal muscle cell function.
There are several different kinds of myopathy caused by defective genes:
- central core disease
- centronuclear (myotubular) myopathy
- myotonia congenita
- nemaline myopathy
- paramyotonia congenita
- periodic paralysis (hypokalemic and hyperkalemic forms)
- mitochondrial myopathies
Most, but not all, of these genetic myopathies are inherited through an autosomal dominant pattern of inheritance. In this pattern of inheritance, one copy of each gene comes from each parent. Only one of these two copies needs to have the mutation (change) or defect in order for the child to have the disease. The parent with the defective gene has the disease, and each of this parent's children has a 50 percent chance of inheriting the disease. This percentage is not changed by results of other pregnancies. With this pattern of inheritance, male and female children are equally at risk of developing the disease.
However, for a child to have one type of myotonia congenita and some forms of nemaline myopathy, two defective genes must be inherited—one from each parents. This is called an autosomal recessive pattern of inheritance. Neither parent may have symptoms of the disease, but each carries a recessive defective gene for it. Each child of such parents has a 25 percent chance of inheriting both genes and showing signs of the disease, and a 50 percent chance of inheriting one defective gene from only one parent. If the child has inherited just one defective gene, he or she will be a carrier of the disease and can pass the gene on to his or her offspring, while showing no signs of the disease himself.
A few forms of centronuclear myopathy develop primarily in males. Females who inherit the defective gene are usually carriers without symptoms, like their mothers, but they can pass on the disease to their sons. Mitochondrial myopathies are inherited only through the mother, since sperm do not contain mitochondria.
The major symptoms associated with the genetic myopathies are:
- Central core disease: mild weakness of voluntary muscles, especially in the hips and legs; hip displacement; delays in reaching developmental motor milestones; problems with running, jumping, and climbing stairs develop in childhood.
- Centronuclear myopathy: weakness of voluntary muscles, including those on the face, arms, legs, and trunk; drooping upper eyelids; facial weakness; foot drop; affected muscles almost always lack reflexes.
- Myotonia congenita: voluntary muscles of the arms, legs, and face stiff or slow to relax after contracting (myotonia); stiffness triggered by fatigue, stress, cold, or long rest periods, such as a night's sleep ; stiffness can be relieved by repeated movement of the affected muscles.
- Nemaline myopathy: moderate weakness of voluntary muscles in the arms, legs, and trunk; mild weakness of facial muscles; delays in reaching developmental motor milestones; decreased or absent reflexes in affected muscles; long, narrow face; high-arched palate; jaw projects beyond upper part of the face.
- Paramyotonia congenita: stiffness of voluntary muscles in the face, hands, and forearms; attacks spontaneous or triggered by cold temperatures; stiffness made worse by repeated movement; episodes of stiffness last longer than those seen in myotonia congenita.
- Periodic paralysis: attacks of temporary muscle weakness (muscles work normally between attacks); in the hypokalemic (low potassium) form, attacks triggered by vigorous exercise , heavy meals high in carbohydrates, insulin, stress, alcohol, infection, pregnancy; in the hyperkalemic (high potassium) form, attacks triggered by vigorous exercise, stress, pregnancy, missing a meal, steroid drugs, high potassium intake.
- Mitochondrial myopathies: symptoms vary quite widely with the form of the disease and may include progressive weakness of the eye muscles (ocular myopathy), weakness of the arms and legs, or multisystem problems primarily involving the brain and muscles.
Endocrine-related myopathies
In some cases, myopathies can be caused by a malfunctioning endocrine gland that produces either too much or too little of the chemical messengers called hormones. Hormones travel through the bloodstream. One of their many functions is to help regulate muscle activity. Problems in producing hormones can lead to muscle weakness.
Hyperthyroid myopathy and hypothyroid myopathy affect different muscles in different ways. Hyperthyroid myopathy occurs when the thyroid gland produces too much of the hormone thyroxine, leading to muscle weakness, some muscle wasting in hips and shoulders, and, sometimes, problems with eye muscles. The hypothyroid type of myopathy occurs when too little hormone is produced, leading to stiffness, cramps, and weakness of arm and leg muscles.
Inflammatory myopathies
Some myopathies are caused by inflammation. Inflammation is a protective response of injured tissues characterized by redness, increased heat, swelling, and/or pain in the affected area. Examples of this type of myopathy include dermatomyositis , polymyositis, and myositis ossificans.
Dermatomyositis is a disease of the connective tissue that also involves weak, tender, inflamed muscles. Muscle tissue loss may be so severe that the individual may be unable to walk. Skin inflammation is also present. The cause of dermatomyositis is as of 2004 unknown, but viral infection and antibiotic use are associated with the condition. In some cases, dermatomyositis is associated with rheumatologic disease or cancer . Polymyositis involves inflammation of many muscles, usually accompanied by deformity, swelling, sleeplessness, pain, sweating, and tension. It, too, may be associated with cancer. Myositis ossificans is a rare inherited disease in which muscle tissue is replaced by bone, beginning in childhood.
Muscular dystrophies
While considered a separate group of diseases, the muscular dystrophies also involve muscle wasting and can be described as myopathies. Symptoms of muscular dystrophy (MD) diseases usually appear during childhood and adolescence . These are genetic disorders that result in defects in the production of specific proteins. The forms of muscular dystrophy differ according to the way they are inherited, the age at which symptoms begin, the muscles they affect, and how fast they progress.
Demographics
Myopathies are not common. About 14 percent of myopathies are inherited. Worldwide the rate of inflammatory myopathies is about five to ten individuals per 100,000. These myopathies are more often seen in women. MD is found in about 63 of every 1 million individuals, but the rates vary widely depending on the type of MD. The most common type is Duchenne MD, affecting one in every 3,300 boys. Other more common types of MD are Becker's, myotonic dystrophy , limb-girdle MD, and facioscapulohumeral MD. MD is more common in boys. The rate of metabolic and endocrine myopathies was, as of 2004, not known.
When to call the doctor
Parents should let the doctor know as soon as possible if there is a family history of muscle weakness or muscle wasting disease. Otherwise, they should contact their pediatrician if the child is showing any signs of delayed or abnormal growth or unexplained muscle weakness.
Diagnosis
Early diagnosis of myopathy is important in order to provide the best care possible. An experienced physician can diagnose a myopathy by evaluating a child's medical history and by performing a thorough physical examination. Diagnostic tests can help differentiate among the different types of myopathies, as well as between myopathy and other neuromuscular disorders. If the doctor suspects a genetic myopathy, a thorough family history will also be taken. Genetic tests are available for a few myopathies.
Diagnostic tests the doctor may order include: measurements of potassium, (K) creatine kinase,(CK) lactic dehydrogenase (LDH) and pyruvate kinase (PK) and certain antibodies in the blood; muscle tissue biopsy; and electromyogram (EMG).
Treatment
As of 2004, there was no cure for many myopathies. Treatment depends on the specific type of myopathy the person has and is aimed at controlling symptoms. Specific treatment approaches for specific forms of myopathies are as follows:
- periodic paralysis: medication and dietary changes
- hyperthyroid or hypothyroid myopathy: treatment of the underlying thyroid abnormality
- myositis ossificans: medication to help prevent abnormal bone formation, but there is no cure following onset
- central core disease: no treatment
- nemaline myopathy: no treatment
- centronuclear (myotubular) myopathy: no treatment
- paramyotonia congenita: treatment often unnecessary
- myotonia congenita: drug treatment (if necessary), but drugs do not affect the underlying disease, and attacks may still occur
General treatments aim at supporting the individual's functioning and independence. Physical therapy can help preserve or increase strength and flexibility in muscles. Ankle and wrist braces can support weakened limbs. Occupational therapy is used to develop tools and techniques to compensate for loss of strength and dexterity. A speech-language pathologist can provide retraining for weakness in the muscles controlling speech and swallowing.
Prognosis
The prognosis for patients with myopathy depends on the type and severity of the individual's disease. In most cases, the myopathy symptoms can be successfully treated, but in others, the disease can be fatal in childhood or adolescence.
KEY TERMS
Electrooculography (EOG) —A diagnostic test that records the electrical activity of the muscles that control eye movement.
Hyperkalemia —An abnormally high level of potassium in the blood.
Hypokalemia —A condition characterized by a deficiency of potassium in the blood.
Inflammation —Pain, redness, swelling, and heat that develop in response to tissue irritation or injury. It usually is caused by the immune system's response to the body's contact with a foreign substance, such as an allergen or pathogen.
Mitochondria —Spherical or rod-shaped structures of the cell. Mitochondria contain genetic material (DNA and RNA) and are responsible for converting food to energy.
Voluntary muscles —Muscles that can be moved by conscious thought.
Muscular dystrophy is generally a more serious disease than many other types of myopathies. Duchenne's MD is usually fatal by the late teens; Becker's MD is less serious and may not be fatal until the 50s.
Prevention
As of 2004 there is no way to prevent the genetic mutations that cause myopathies, nor are there ways to prevent metabolic and endocrine failures that result in myopathies. Inflammatory myopathies often occur as a result of exposure to viruses or drugs, but it is almost impossible to predict their development.
Parental concerns
Individuals with known myopathies who wish to become parents may want to seek genetic counseling before attempting to have children.
See also Muscular dystrophy; Myotonic dystrophy.
Resources
BOOKS
Barnes, P. R. J., et al. Myopathies in Clinical Practice. Oxford, UK: Isis Medical Media, 2003.
The Official Patient's Sourcebook on Mitochondrial Myopathies. San Diego, CA: Icon Group International, 2002.
ORGANIZATIONS
Muscular Dystrophy Association. 3300 East Sunrise Dr., Tucson, AZ 85718. Web site: <ww.mdausa.org>.
National Organization for Rare Disorders Inc. 55 Kenosia Ave, PO Box 1968, Danbury, CT 06813–1968. Web site: <www.rarediseases.org>.
WEB SITES
"NINDS Myopathy Information Page." National Institute of Neurological Disorders and Stroke, January 6, 2005. Available online at <www.ninds.nih.gov/health_and_medical/disorders/myopathy.htm> (accessed January 13, 2005).
Tish Davidson, A.M.
Carol A. Turkington
Myopathy
Myopathy
Definition
Myopathy is a general term referring to any skeletal muscle disease or neuromuscular disorder. Myopathy can be acquired or inherited, and can occur at birth or later in life. Myopathies can result from endocrine disorders, metabolic disorders, muscle infection or inflammation, drugs, and mutations in genes.
Description
Skeletal muscle diseases, or myopathies, are disorders with structural changes or functional impairment of the muscle. These conditions can be differentiated from other diseases of the motor unit by characteristic clinical and laboratory findings. The main symptom is muscle weakness that can be either intermittent or persistent. Different myopathy types exist with different associated causes. The main types include congenital myopathy, muscular dystrophy , inflammatory myopathy , and drug-induced myopathy.
Congenital myopathy (CM) is a term used for muscle disorders present at birth. According to this definition, the CMs could include hundreds of distinct neuromuscular syndromes and disorders. In general, this disease causes loss of muscle tone and muscle weakness in infancy and delayed motor skills, such as walking, later in childhood. Four distinct disorders are classified as CMs: central core disease, nemaline rod myopathy, centronuclear (myotubular) myopathy, and multicore myopathy.
Muscular dystrophy (MD) refers to a group of genetic diseases characterized by progressive weakness and degeneration of the skeletal or voluntary muscles that control movement. The muscles of the heart and some other involuntary muscles are also affected in some forms of MD, and a few forms involve other organs as well. The major forms of MD include myotonic, Duchenne, Becker, limbgirdle, facioscapulohumeral, congenital, oculopharyngeal, distal, and Emery-Dreifuss.
Inflammatory myopathies (IM) are a group of muscle diseases involving the inflammation and degeneration of skeletal muscle tissues. They are thought to be autoimmune disorders. In IMs, inflammatory cells surround, invade, and destroy normal muscle fibers as though they were defective or foreign to the body. This eventually results in discernible muscle weakness. This muscle weakness is usually symmetrical and develops slowly over weeks to months or even years. The IMs include dermatomyositis , polymyositis , and inclusion body myositis .
Drug induced myopathy (DIM) is a muscle disease caused by toxic substances that produce muscle damage. The toxic substances may act directly on muscle cells, but muscle damage can also be secondary to electrolyte disturbances, excessive energy requirements, or the inadequate delivery of oxygen and nutrients due to muscle compression. Drug use may also result in development of an immunologic reaction directed against the muscle. Muscle damage can be generalized or local, as occurs when a drug is injected into a muscle.
Demographics
Worldwide, CMs account for about 14% of all myopathies. Central core disease accounts for 16% of cases; nemaline rod myopathy for 20%; centronuclear myopathy for 14%; and multicore myopathy for 10%. Prevalence of MD is higher in males. In the United States, Duchenne and Becker MD occur in approximately one in 3,300 boys. Overall incidence of MD is about 63 per one million people.
Worldwide incidence of IM is about five to 10 per 100,000 people. These disorders are more common in women. Incidence and prevalence of DIM are unknown. Myopathy caused by corticosteroids is the most common disorder, and it is more common in women.
Causes and symptoms
CMs and MDs are caused by a genetic defect. In both conditions, mutations have been identified in genes that encode for muscle proteins. The loss or dysfunction of these proteins presumably leads to the specific morphological feature in the muscle and to clinically noticeable muscle disease. For example, in Becker dystrophy, there is a less-active form of dystrophin (a protein involved in the complex interactions of the muscle membrane and extracellular environment) that may not be effective as a gateway regulator, allowing some leakage of intracellular substances, resulting in the myopathy.
The causes of IM are not known. An autoimmune process is likely, as these conditions are often associated with other autoimmune diseases and because they respond to immunosuppressive medication. Muscle biopsy typically shows changes attributed to destruction by infiltrating lymphocytes (white blood cells).
In DIMs, there are a number of causative agents. Drugs such as lipid-lowering agents (statins, clofibrate and gemfibrizol), agents that cause hypokalemia (diuretics, theophylline, amphotericin B), lithium, succinylcholine, antibiotics (trimethoprim, isoniazid), anticonvulsants (valproic acid, lamotrigine , prolonged propofol infusion), vasopressin, colchicine, episilon, aminocaproic acid, high dose alfa-interferon, and illicit drugs (cocaine, heroin, phencyclidine, amphetamines) are possible myopathy inducers.
Although symptoms depend on the type of myopathy, some generalizations can be made. Skeletal muscle weakness is the hallmark of most myopathies. In most myopathies, weakness occurs primarily in the muscles of the shoulders, upper arms, thighs, and pelvis (proximal muscles). In some cases, the distal muscles of the hands and feet may be involved during advanced stages of the disease. Other typical symptoms of muscle disease include the following:
- muscle aching
- muscle cramping
- muscle pain
- muscle stiffness
- muscle tenderness
- muscle tightness
Initially, individuals may feel fatigued during very light physical activity. Walking and climbing stairs may be difficult because of weakness in the pelvic and leg muscles that stabilize the trunk. Patients often find it difficult to rise from a chair. As the myopathy progresses, there may be muscle wasting.
Diagnosis
Generally, diagnosis involves several outpatient tests to determine the type of myopathy. Sometimes it is necessary to wait until the disease progresses to a point at which the syndrome can be identified.
A blood serum enzyme test measures how much muscle protein is circulating in the blood. Usually, this is helpful only at the early stages of the disease, when the sudden increase of muscle protein in the blood is conspicuous. Antibodies found in the blood might indicate an IM. DNA may be collected to evaluate whether one of the known genetic defects is present.
An electromyogram (EMG) measures the electrical activity of the muscle. It involves placing a tiny needle into the muscle and recording the muscular activity on a monitor (oscilloscope). This helps identify which muscles are weakened.
A muscle tissue biopsy involves surgically removing a very small amount of tissue to be examined under the microscope and analyzed for abnormalities.
Treatment team
A multidisciplinary team is involved in the treatment of myopathy patients. This team may include a neurologist , a rheumatologist, an orthopedic surgeon, a pulmonologist, a cardiologist, an orthopedist, a dermatologist, and a genetic counselor. It can also include physical and occupational therapists.
Treatment
Treatment depends on the cause, and goals are to slow progression of the disease and relieve symptoms. Treatments range from drug therapy for MD and IM to simply avoiding situations that work the muscles too hard. Some physicians recommend that patients keep their weight down (a lighter body demands less work from the muscles) and avoid overexerting their muscles. For MD, the corticosteroids deflazacort and prednisone seem to be the most effective medications. Calcium supplements and antidepressants may be prescribed to counteract the side effects. The IMs are usually treated with drugs that suppress the action of the immune system such as methotrexate, cyclosporine, and azathioprine, all of which have potentially serious side effects. For CM, treatment involves supportive measures to help patients cope with the symptoms.
Recovery and rehabilitation
Physical therapy can prevent weakening in a patient's healthy muscles, however, it cannot restore already weakened muscles. Occupational and respiratory therapy help patients learn how to use special equipment that can improve their quality of life.
Clinical trials
There are numerous open clinical trials for myopathies, including:
- Study and Treatment of Inflammatory Muscle Diseases and Infliximab to Treat Dermatomyositis and Polymyositis, sponsored by National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS)
- Diagnostic Evaluation of Patients with Neuromuscular Disease and Screening Protocol for Patients with Neurological Disorders with Muscle Stiffness, sponsored by National Institute of Neurological Disorders and Stroke (NINDS)
- Physiologic Effects of PRMS & Testosterone in the Debilitated Elderly, sponsored by Department of Veterans Affairs Medical Research Service
- Myositis in Children, sponsored by National Institute of Environmental Health Sciences (NIEHS)
More updated information on clinical trials can be found on the National Institutes of Health clinical trials website at <www.clinicaltrials.gov>.
Prognosis
The prognosis for persons with myopathy varies. Some individuals have a normal life span and little or no disability. In others, however, the disorder may be progressive, severely disabling, life threatening, or fatal. If the underlying cause of the disorder can be treated successfully, the prognosis is usually good. Progressive myopathies that develop later in life usually have a better prognosis than conditions that develop during childhood. Persons with Duchenne MD rarely live beyond their middle to late 20s, and persons with Becker MD may live until middle age.
Special concerns
If the cardiac muscle is affected in later disease stages, abnormal heart rhythms or heart muscle insufficiency (cardiomyopathy) may develop. Cardiomyopathy patients are at risk for congestive heart failure.
When muscles involved in breathing weaken, there may be significant breathing difficulties and increased risk for pneumonia, flu, and other respiratory infections. In severe cases, patients may require a respirator. When swallowing muscles are affected, persons are at increased risk for choking and malnutrition.
Resources
BOOKS
Vinken, Pierre J., et al. Myopathies. New York: Elsevier Health Sciences, 1992.
Askanas, Valerie, et al. Inclusion-Body Myositis and Myopathies. New York: Cambridge University Press, 1998.
PERIODICALS
Mastaglia, F. L., M. Garlepp, B. Phillips, and P. Zilko. "Inflammatory myopathies: clinical, diagnostic and therapeutic aspects." Muscle & Nerve 27 (April 2003): 407–425.
OTHER
"NINDS Myopathy Information Page." National Institute of Neurological Disorders and Stroke. <http://www.ninds.nih.gov/health_and_medical/disorders/myopathy.htm> (March 15, 2004).
"Facts about Duchenne and Becker Muscular Dystrophies (DMD and BMD)." Muscular Dystrophy Association. <http://www.mdausa.org/publications/fa-dmdbmd.html> (March 15, 2004).
ORGANIZATIONS
National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS): National Institutes of Health. Bldg. 31, Rm. 4C05, Bethesda, MD 20892-2350. (301) 496-8188 or (877) 22-NIAMS (226-4267). NIAMSInfo@ mail.nih.gov. <http://www.nih.gov/niams>.
Muscular Dystrophy Association. 3300 East Sunrise Drive, Tucson, AZ 85718-3208. (520) 529-2000 or (800) 572-1717; Fax: (520) 529-5300. [email protected]. <http://www.mdausa.org/>.
Greiciane Gaburro Paneto
Iuri Drumond Louro, MD, PhD
Myopathies
Myopathies
Definition
Myopathies are diseases of skeletal muscle which are not caused by nerve disorders. These diseases cause the skeletal or voluntary muscles to become weak or wasted.
Description
There are many different types of myopathies, some of which are inherited, some inflammatory, and some caused by endocrine problems. Myopathies are rare and not usually fatal. Typically, effects are mild, largely causing muscle weakness and movement problems, and many are transitory. Only rarely will patients become dependent on a wheelchair. However, muscular dystrophy (which is technically a form of myopathy) is far more severe. Some types of this disease are fatal in early adulthood.
Causes and symptoms
Myopathies are usually degenerative, but they are sometimes caused by drug side effects, chemical poisoning, or a chronic disorder of the immune system.
Genetic myopathies
Among their many functions, genes are responsible for overseeing the production of proteins important in maintaining healthy cells. Muscle cells produce thousands of proteins. With each of the inherited myopathies, a genetic defect is linked to a lack of, or problem with, one of the proteins needed for normal muscle cell function.
There are several different kinds of myopathy caused by defective genes:
- Central core disease
- Centronuclear (myotubular) myopathy
- Myotonia congenita
- Nemaline myopathy
- Paramyotonia congenita
- Periodic paralysis (hypokalemic and hyperkalemic forms)
- Mitochondrial myopathies.
Most of these genetic myopathies are dominant, which means that a child needs to inherit only one copy of the defective gene from one parent in order to have the disease. The parent with the defective gene also has the disorder, and each of this parent's children has a 50% chance of also inheriting the disease. Male and female children are equally at risk.
However, one form of myotonia congenita and some forms of nemaline myopathy must be inherited from both parents, each of whom carry a recessive defective gene but who do not have symptoms of the disease. Each child of such parents has a 25% chance of inheriting both genes and showing signs of the disease, and a 50% chance of inheriting one defective gene from only one parent. If the child inherited just one defective gene, he or she would be a carrier but would not show signs of the disease.
A few forms of centronuclear myopathy develop primarily in males. Females who inherit the defective gene are usually carriers without symptoms, like their mothers, but they can pass on the disease to their sons. Mitochondrial myopathies are inherited through the mother, since sperm do not contain mitochondria. (Mitochondria play a key role in energy production in the body's cells.)
The major symptoms associated with the genetic myopathies include:
- Central core disease: mild weakness of voluntary muscles, especially in the hips and legs; hip displacement; delays in reaching developmental motor milestones; problems with running, jumping, and climbing stairs develop in childhood
- Centronuclear myopathy: weakness of voluntary muscles including those on the face, arms, legs, and trunk; drooping upper eyelids; facial weakness; foot drop; affected muscles almost always lack reflexes
- Myotonia congenita: voluntary muscles of the arms, legs, and face are stiff or slow to relax after contracting (myotonia); stiffness triggered by fatigue, stress, cold, or long rest periods, such as a night's sleep; stiffness can be relieved by repeated movement of the affected muscles
- Nemaline myopathy: moderate weakness of voluntary muscles in the arms, legs, and trunk; mild weakness of facial muscles; delays in reaching developmental motor milestones; decreased or absent reflexes in affected muscles; long, narrow face; high-arched palate; jaw projects beyond upper part of the face
- Paramyotonia congenita: stiffness (myotonia) of voluntary muscles in the face, hands, and forearms; attacks spontaneous or triggered by cold temperatures; stiffness made worse by repeated movement; episodes of stiffness last longer than those seen in myotonia congenita
- Periodic paralysis: attacks of temporary muscle weakness (muscles work normally between attacks); in the hypokalemic (low calcium) form, attacks triggered by vigorous exercise, heavy meals (high in carbohydrates), insulin, stress, alcohol, infection, pregnancy ; in the hyperkalemic (normal/high calcium) form, attacks triggered by vigorous exercise, stress, pregnancy, missing a meal, steroid drugs, high potassium intake
- Mitochondrial myopathies: symptoms vary quite widely with the form of the disease and may include progressive weakness of the eye muscles (ocular myopathy), weakness of the arms and legs, or multisystem problems primarily involving the brain and muscles.
Endocrine-related myopathies
In some cases, myopathies can be caused by a malfunctioning gland (or glands), which produces either too much or too little of the chemical messengers called hormones. Hormones are carried by the blood and one of their many functions is to regulate muscle activity. Problems in producing hormones can lead to muscle weakness.
Hyperthyroid myopathy and hypothyroid myopathy affect different muscles in different ways. Hyperthyroid myopathy occurs when the thyroid gland produces too much thyroxine, leading to muscle weakness, some muscle wasting in hips and shoulders, and, sometimes, problems with eye muscles. The hypothyroid type occurs when too little hormone is produced, leading to stiffness, cramps, and weakness of arm and leg muscles.
Inflammatory myopathies
Some myopathies are inflammatory, leading to inflamed, weakened muscles. Inflammation is a protective response of injured tissues characterized by redness, increased heat, swelling, and/or pain in the affected area. Examples of this type include polymyositis, dermatomyositis, and myositis ossificans.
Dermatomyositis is a disease of the connective tissue that also involves weak, tender, inflamed muscles. In fact, muscle tissue loss may be so severe that the person may be unable to walk. Skin inflammation is also present. The cause is unknown, but viral infection and antibiotics are associated with the condition. In some cases, dermatomyositis is associated with rheumatologic disease or cancer. Polymyositis involves inflammation of many muscles usually accompanied by deformity, swelling, sleeplessness, pain, sweating, and tension. It, too, may be associated with cancer. Myositis ossificans is a rare inherited disease in which muscle tissue is replaced by bone, beginning in childhood.
Muscular dystrophy
While considered to be a separate group of diseases, the muscular dystrophies also technically involve muscle wasting and can be described as myopathies. These relatively rare diseases appear during childhood and adolescence, and are caused by muscle destruction or degeneration. They are a group of genetic disorders caused by problems in the production of key proteins.
The forms of muscular dystrophy (MD) differ according to the way they are inherited, the age of onset, the muscles they affect, and how fast they progress. The most common type is Duchenne MD, affecting one or two in every 10,000 boys. Other types of MD include Becker's, myotonic dystrophy, limb-girdle MD, and facioscapulohumeral MD.
Diagnosis
Early diagnosis of myopathy is important so that the best possible care can be provided as soon as possible. An experienced physician can diagnose a myopathy by evaluating a person's medical history and by performing a thorough physical exam. Diagnostic tests can help differentiate between the different types of myopathy, as well as between myopathy and other neuromuscular disorders. If the doctor suspects a genetic myopathy, a thorough family history will also be taken.
Diagnostic tests the doctor may order include:
- Measurements of potassium in the blood
- Muscle biopsy
- Electromyogram (EMG).
Treatment
Treatment depends on the specific type of myopathy the person has:
- Periodic paralysis: medication and dietary changes
- Hyperthyroid or hypothyroid myopathy: treatment of the underlying thyroid abnormality
- Myositis ossificans: medication may prevent abnormal bone formation, but there is no cure following onset
- Central core disease: no treatment
- Nemaline myopathy: no treatment
- Centronuclear (myotubular) myopathy: no treatment
- Paramyotonia congenita: treatment often unnecessary
- Myotonia congenita: drug treatment (if necessary), but drugs do not affect the underlying disease, and attacks may still occur.
Prognosis
The prognosis for patients with myopathy depends on the type and severity of the individual disease. In most cases, the myopathy can be successfully treated and the patient returned to normal life.
Muscular dystrophy, however, is generally a much more serious condition. Duchenne's MD is usually fatal by the late teens; Becker's MD is less serious and may not be fatal until the 50s.
Resources
ORGANIZATIONS
Muscular Dystrophy Association. 3300 East Sunrise Drive, Tucson, AZ 85718. (800) 572-1717. 〈http://www.mdausa.org〉.
KEY TERMS
Electromyogram (EMG)— A diagnostic test that records the electrical activity of muscles. In the test, small electrodes are placed on or in the skin; the patterns of electrical activity are projected on a screen or over a loudspeaker. This procedure is used to test for muscle disorders, including muscular dystrophy.
Inflammation— A protective response of injured tissues characterized by redness, increased heat, swelling, and/or pain in the affected area.
Voluntary muscles— Muscles producing voluntary movement.