Polydactyly

Definition

Polydactyly is the presence of extra fingers and toes (digits) at birth. Rather than having five fingers per hand or five toes per foot, an infant with polydactyly will have

six or more digits on either the hands or feet that may or may not be fully formed.

Description

Polydactyly, also called supernumerary digits or extra digits, can occur on either the hands or feet and the extent of the digit development is variable. Individuals with polydactyly can have one small extra rudimentary finger or toe on one of their hands or feet or they can have fully formed extra digits on both their hands and feet. Extra digits can be fully formed or rudimentary and may contain no bones at all. Rudimentary digits may only be attached by a thin stalk of skin. Individual are described as having isolated polydactyly if they only have extra digits and have no other obvious malformations. Most of the time, polydactyly occurs as an isolated condition, but it can occur as a symptom of different genetic syndromes. There are more than 100 different genetic syndromes that have polydactyly as one of their findings.

Different experts have different systems for describing polydactyly. The two most common systems describe the location and extent of the polydactyly. Preaxial, central, and postaxial are the terms used to describe the location of the extra digit. If the extra digit is located on the side of the hand by the thumb or on the side of the foot by the big toe, then a person is diagnosed with preaxial polydactyly. Postaxial polydactyly occurs when the extra digit is located on the side of the hand or foot by the fifth digit (the pinky or small toe), and central polydactyly occurs when the extra digit is located in between the thumb and pinky or between the big and little toes.

Type A and type B are used to describe the variable size of the digits in polydactyly. Extra digits can be fully formed with the right number of joints and bones, or they can be poorly formed with the wrong number of joints and no bones (rudimentary digits). Fully formed digits are type A polydactyly and poorly formed or rudimentary digits are type B polydactyly. Thus, a person with a small rudimentary digit on the outside of the pinky is diagnosed with isolated postaxial polydactyly type B.

Genetic profile

Polydactyly is caused by errors that occur during the process of fetal development. There are a number of genes that determine the pattern of hands and feet. Genes are the basic unit of inheritance and provide the instructions for how a body is to form. A difference or error in one of these genes can lead to one of the different types of polydactyly. The exact genes responsible for polydactyly have not been found as of 2005.

The three different types of polydactyly, postaxial, central, and preaxial, appear to have different patterns of inheritance. Postaxial polydactyly is inherited as an autosomal dominant that shows variable expressivity. As of 2005, the inheritance patterns for postaxial and central polydactyly are not well understood.

Genes are made of deoxyribonucleic acid (DNA ) and located on chromosomes. DNA is composed of four units (called bases) that are designated A, T, G, and C. The sequence of the bases contains the instructions for making each of the body's structures. A change in one of the DNA letters making up a gene is a mutation. In some cases, mutations can alter the instructions and lead to malformations.

All individuals pass their chromosomes on to their children, and therefore pass down their DNA instructions (genes). Genes always come in pairs: one pair each from the mother and one from the father. In an autosomal dominant disorder, only one gene needs to be altered for an individual to have the disorder or malformation. If an individual has the gene change (mutation) that causes postaxial polydactyly, there is a 50% chance that the disorder will be passed onto an offspring.

While the gene for polydactyly is passed unchanged from generation to generation, it often manifests itself very differently in different individuals. For example, a father may have the gene for polydactyly and have had a very small nubbin of soft skin on the outer edge of his hand. However, his child with the same gene may have extra fully formed digits on both hands and feet. This difference in how the gene manifests is called variable expression. The causes of variable expression are not well understood.

Most polydactyly occurs as an isolated condition. However, polydactyly can occur as one symptom of other more complicated genetic conditions. Some of the syndromes that polydactyly is a feature of are trisomy 13 syndrome, Meckel-Gruber syndrome , Bardet-Biedl syndrome , and Ellis-van Creveld syndrome .

Trisomy 13 is a rare, usually lethal, genetic syndrome cause by an extra number 13 chromosome . Infants with trisomy 13 have poor growth and multiple birth defects, including cleft lip, heart defects, and defects of the abdominal wall. This syndrome is usually diagnosed at birth.

Meckel-Gruber syndrome is an autosomal recessive syndrome characterized by poor growth, encephalocele (opening on the spine), postaxial polydactyly, and cystic kidneys. This syndrome is usually diagnosed at birth.

Bardet-Biedl is a rare autosomal recessive genetic syndrome characterized by obesity, postaxial polydactyly, retina (eye) abnormalities, and mental retardation. Often, individuals with this syndrome are not diagnosed until childhood.

Ellis-van Creveld syndrome is a rare autosomal recessive genetic syndrome common in the Amish population. Infants with this syndrome have skeletal abnormalities, short arms and legs, and polydactyly. About half of these infants will also have a heart defect. Most infants with this syndrome are diagnosed shortly after birth or in infancy.

Demographics

Polydactyly is a relatively common birth defect that occurs in all ethnic groups. Postaxial polydactyly is the most common form of polydactyly and accounts for about 80% of cases. It occurs in about one in 2,000 Caucasian births, and is about 10 times more common in individuals with African ancestry. In about 30% of cases of polydactyly, there is a positive family history and about 50% of polydactyly is bilateral (affects both sides of the body). Preaxial polydactyly is the most common form of polydactyly in individuals of Asian ancestry.

Signs and symptoms

The presence of polydactyly is most often noted at birth. While it can be shocking for some parents, the presence of isolated polydactyly poses no immediate health risks for the infant. It is a very common malformation that is easily treated (surgically removed).

Individuals with polydactyly can have one small extra rudimentary finger or toe on one of their hands or feet or they can have fully formed extra digits on both their hands and feet. Extra digits can be fully formed or rudimentary and may contain no bones at all. Rudimentary digits may only be attached by a thin stalk of skin.

Diagnosis

The diagnosis of polydactyly is usually made shortly after birth by physical examination. It can also be diagnosed prenatally or before a baby is born. Polydactyly can sometimes be seen on a prenatal ultrasound (sonogram) as early as 18 weeks gestation. If polydactyly is noticed on a sonogram, other medical tests may be suggested to make sure that the fetus does not have any of the problems associated with other genetic syndromes. A detailed sonogram should be done to measure the limbs and growth of the baby, while an amniocentesis may be offered to do a chromosomal analysis of the fetus. A fetal echocardiograph (heart ultrasound) may be performed to rule out heart abnormalities. In addition, other tests may be done to rule out other syndromes.

Treatment and management

When polydactyly is noted in an infant, a number of the following tests may be done, including:

• A very thorough medical examination to make sure that there are no other birth defects or abnormalities.
• A medical family history to determine if anyone else in the family has had polydactyly or any other syndromes associated with polydactyly.
• X rays to determine the extent of the polydactyly (i.e., whether the digit contains bone or is just skin).
• Laboratory tests, such as chromosome analysis, enzyme assays, and further x rays, may be ordered to determine is a suspected disorder is present.

While the initial diagnosis may shocking to most parents, isolated polydactyly has no health consequences and is usually surgically corrected before the infant leaves the hospital.

Polydactyly is rarely a functional concern and the removal of the extra digit is often for cosmetic purposes or to aid in the wearing of shoes. The age at which surgery is performed depends on the extent and degree of the polydactlyly. A type B, or partially formed, digit that contains no bone may be removed before the infant leaves the hospital. A type A polydactyly is often corrected when the patient is one year of age.

Complex polydactlyly (central polydactyly) may require multiple surgeries to improve function and cosmetic appearance.

Prognosis

The prognosis for isolated polydactyly is excellent. When polydactyly is associated with another genetic syndrome, the prognosis is dependent on the presence of other features of that syndrome. Many of the genetic syndromes that have polydactyly as one of their symptoms are quite serious, and early death and significant disability are possibilities.

Resources

WEBSITES

National Library of Medicine, Medline Plus. (April 11, 2005.) <http://www.nlm.nih.gov/medlineplus/ency/article/003176.htm>.

Kathleen A. Fergus, MS, CGC