Polydactyly is a congenital condition characterized by the presence of more than five digits on a hand or foot. The presence of an extra finger or toe, often called a sixth finger, is one of the most common congenital hand abnormalities observed at birth. The condition stems from an error during the early stages of fetal limb development. It is estimated to occur in approximately 4 to 12 out of every 10,000 newborns.
Understanding the Different Types of Polydactyly
Polydactyly is classified based on the anatomical location of the extra digit, which helps determine the complexity of the anomaly. The most frequent form is postaxial polydactyly, where the extra digit appears on the side of the little finger (the ulnar side of the hand). This type is common in individuals of African descent and often presents as a rudimentary skin tag or a small bump of soft tissue.
Postaxial duplication is divided into two types. Type A is a well-formed extra digit articulating with the hand bones. Type B is a non-functional, soft-tissue nubbin attached by a thin stalk. The second most common form is preaxial polydactyly, involving the side of the thumb (the radial side of the hand). This duplication often involves the bone structure, meaning the extra digit is more fully developed and may share bone components with the original thumb.
Central polydactyly is the rarest type, involving the duplication of one of the middle three fingers (index, middle, or ring finger). This form is often associated with other hand anomalies, such as syndactyly, where the fingers are fused together. The complexity of the anomaly directly influences the required treatment approach.
The Genetic and Biological Causes
Polydactyly is caused by a disruption of limb bud formation during the fourth to eighth weeks of embryonic development. The hand initially forms as a paddle-like structure that separates into five distinct fingers through programmed cell death. An alteration in the signaling pathways that control this process can lead to the formation of an extra digit.
The condition is often inherited, following an autosomal dominant pattern, meaning only one parent needs to carry the gene mutation for a child to be affected. A primary biological mechanism implicated is the Sonic Hedgehog (SHH) signaling pathway, which is a master regulator of anterior-posterior patterning in the developing limb. The SHH gene is normally expressed in a small region of the limb bud called the zone of polarizing activity.
Polydactyly frequently results from mutations in regulatory sequences, such as the ZPA Regulatory Sequence, that control when and where the SHH signal is activated. An abnormal or ectopic activation of this pathway causes an excess of cells to proliferate, providing the raw material for an additional digit. While many cases are isolated (the polydactyly occurs alone), it can also be part of a larger, syndromic condition involving mutations in genes like GLI3, which is part of the SHH pathway.
Surgical and Non-Surgical Treatment Options
Management of an extra digit depends on its type, location, and whether it contains bone or is merely soft tissue. For the most common postaxial Type B polydactyly, a non-surgical approach is often sufficient, especially when the extra digit is small and rudimentary. This typically involves ligation, where a suture or a vascular clip is placed at the base of the skin tag in the neonatal nursery.
Ligation cuts off the blood supply to the extra tissue, causing it to dry up and fall off naturally within a few weeks, similar to the umbilical cord stump. The goal of intervention is to achieve a hand that functions well and has a typical appearance. The non-surgical method is preferred for non-functional, small digits, as this technique avoids a more invasive operation in a newborn.
More complex types, such as preaxial or central duplications, or postaxial Type A digits containing bone and joint structures, require surgical excision. This surgery is generally performed around one year of age. This timing is considered optimal because the hand is large enough for precise surgery, but the child is not yet aware of the procedure. The surgical goal is not just to remove the extra finger but often involves complex reconstruction of the remaining structures.
Surgeons may need to rearrange or reconnect ligaments, tendons, and joint capsules from the removed digit into the remaining finger to ensure proper function, especially for thumb duplications. Early surgical intervention, typically before the age of three, is recommended to allow for optimal functional development while a child is learning to grasp and manipulate objects. Post-operative care may involve casting and physical therapy to maximize the strength and mobility of the reconstructed hand.