Polydactyly is the formal name for the condition where babies are born with extra fingers or toes, derived from the Greek words for “many digits.” This anomaly is one of the most common congenital limb differences observed at birth, affecting the hands or feet. Estimates suggest it occurs in roughly 1 in 500 to 1,000 live births globally, though prevalence varies significantly among different ethnic groups. The presence of an extra digit can range from a small, rudimentary skin tag to a fully formed finger or toe.
The Biological Basis for Extra Digits
The formation of hands and feet occurs early in pregnancy during the limb bud stage of embryonic development. This process requires precise cellular communication to ensure the correct number and pattern of digits are formed along the anterior-posterior axis (thumb side to pinky side). When this process is disrupted, the result can be the duplication of a digit, leading to polydactyly.
Many cases are caused by inherited genetic mutations, often following an autosomal dominant pattern, meaning only one parent needs to carry the gene. The proper development of the limb is tightly regulated by signaling pathways, with the Sonic Hedgehog (Shh) pathway being a well-studied example. The Shh protein acts as a morphogen, secreted from a region of the limb bud called the Zone of Polarizing Activity (ZPA), to control the patterning across the developing hand or foot.
Disruptions in the genes that regulate the Shh pathway, such as mutations in the GLI3 gene, can cause the signaling to be misplaced or overactive, leading to the formation of extra digits. Polydactyly may occur in isolation (non-syndromic), where the extra digit is the only physical anomaly. However, the condition can also be syndromic, meaning it is one feature of a larger genetic disorder, such as Bardet-Biedl syndrome or Ellis-van Creveld syndrome. Differentiating between isolated and syndromic cases is significant for a child’s prognosis, as syndromic polydactyly often involves other health concerns.
Classifying Polydactyly
Physicians categorize extra digits based on anatomical location, which helps determine the complexity and treatment approach. The three main types are named according to their position relative to the main axis of the limb.
The most frequent form is post-axial polydactyly, where the extra digit appears on the small finger or toe side (ulnar or fibular side). This type is generally the least complex and is often observed as a small, rudimentary skin tag connected by a narrow stalk. Conversely, pre-axial polydactyly involves a duplication on the thumb or big toe side (radial or tibial side), often presenting as a more complex duplication.
The third and rarest type is central polydactyly, which involves the duplication of the middle fingers—the index, middle, or ring fingers—or the corresponding toes. Beyond location, the extra digit is also classified structurally into two types. Type A refers to a well-formed, complete digit that includes bone structure and a joint, while Type B is a rudimentary nubbin or skin tag that lacks a complete bone structure. The distinction between a Type A and Type B structure guides the medical management plan.
Diagnosis and Management
Polydactyly is often detected before birth during routine prenatal ultrasounds, which can visualize the developing limbs. Postnatal diagnosis is confirmed by a simple physical examination immediately after birth, followed by X-rays to assess the internal structure of the extra digit. These imaging studies are essential for surgical planning, as they determine if the extra digit shares bone, nerves, or tendons with the adjacent normal digit.
The management approach depends heavily on the extra digit’s location and structural type. For the simpler Type B post-axial polydactyly (the small skin tag), a common procedure is simple ligation, where a suture is tied tightly around the base of the stalk. This cuts off the blood supply, causing the rudimentary tissue to wither and fall off painlessly within a few weeks.
However, there is a risk of a residual bump or a painful nerve ending forming, which may necessitate a secondary procedure. For Type A digits, or any duplication with bone structure, surgical excision and reconstruction are necessary to prevent functional problems and ensure a cosmetic outcome.
The goal of this surgery is to remove the extra part and reconstruct the remaining digit to ensure proper function and appearance. Most surgeons recommend performing this surgery within the first year of life (often between nine and twelve months) to minimize anesthesia risk and ensure treatment is completed before the child begins active grasping or walking. The prognosis for children with isolated polydactyly is generally excellent, with little to no impact on the long-term function of the hand or foot.