Polydactyly is a congenital physical aberration in which a person has more than five fingers on his/her hand or if his/her foot has more that five toes each. The term is derived from the Greek words ‘polus’ which means ‘many’ and ‘daktulos’ which means ‘fingers’. The extra digit on either hand or toe may be rudimentary or fully formed with or without bones at all. For rudimentary polydactyly, the extra finger or toe is usually a piece of soft tissue. In rare cases, the extra digit may develop fully and become a functional finger or toe.
Polydactyly is one of the most prevalent inborn hand abnormalities which manifests in 1 out of 500 live births. Experts formed a classification for this congenital condition based on its extent and location. Fully formed digits having a complete number of bones and joints are classified as Type A polydactyly while rudimentary fingers or toes are classified as Type B polydactyly. Based on location, polydactyly can be preaxial, central or postaxial. Preaxial when the extra digit occurs near the thumb or big toe; postaxial when it is located near the pinky or near the smallest digit of the foot; and central when the extra digit arises between the biggest and smallest digits of the limbs. Among Asians, an extra thumb is most likely to arise (preaxial polydactyly) while an extra finger on the small digit side is most common among African-Americans (postaxial polydactyly). Polydactyly may occur as an isolated condition or it may be expressed as a result of an underlying health condition such as Meckel syndrome, trisomy 13 and Grebe type Chondrodysplasia. In most cases however, polydactyly is caused by genetic factors.
Genetic Causes of Polydactyly
The gene that codes for polydactyly is found on one of the short legs of chromosome seven and has an autosomal dominant mode of inheritance. Since it is autosomal, its occurrence among males and females is equal. And because it follows a dominant pattern of inheritance, a child with a polydactyl parent has at least 50% possibility to inherit and show the trait.
One of the genes behind polydactyly is GLI3. It serves its function by providing instructions for making a protein (GLI3 protein) that has the capacity to turn on or to turn off gene expression. During embryonic development, GLI3 protein plays a key role in shaping tissues and organs by interacting with other genes. When a mutation or a change related to the GLI3 gene in chromosome seven occurs, deletion or rearrangement of genetic materials for example, limb development becomes severely affected which can lead to polydactyly at birth.
Another gene that can cause polydactyly is the LMBR1 gene which is also located in chromosome seven. Similar to the case of GLI3 gene, a disruption in the LMBR1 region can alter the expression of a protein called Sonic hedgehog homolog (SHH) which regulates vertebrate organogenesis and growth of digits. Defects in this gene are associated with preaxial polydactyly while abnormalities in GLI3 gene are associated with both preaxial and postaxial polydactyly.[ad#co-1]
Physical examination after birth is the easiest way to diagnose polydactyly. Diagnosis can also be done before childbirth by looking at family histories and by performing several prenatal tests. One of the most commonly used method during pregnancy is a prenatal ultrasound called a sonogram. If polydactyly is seen in the ultrasound, other medical tests may be conducted to determine if the condition is associated with other genetic disorders. In cases where the result of a sonogram becomes unyielding, a method called embryoscopy is also employed. This test involves the the direct visualization of the embryo by inserting a fiber optic scope through the mother’s abdominal wall and uterus. For a more genetic approach and reliable result, amniocentesis is performed. This requires the analysis of amniotic fluid sampled from the mother’s womb primarily to detect chromosomal abnormalities in the fetus.
Treatment for polydactyly depends on the severity of the disorder. X-ray is usually done to determine the extent of polydactyly so as to apply the corresponding treatment for such condition. For isolated cases where no major health consequences are likely to arise, surgery is used to remove the extra digit. For Type B polydactyly, the extra finger or toe can be surgically removed before the newborn is discharged from the hospital. Type A polydactyly on the other hand is more complicated and requires more surgeries compared to the other type and is usually corrected when the child is above one year of age.[ad#afterpost]