Polydactyly

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Polydactyly or polydactylism (from Ancient Greek p???? (polus) “many” + d??t???? (daktulos) “finger”), also known as hyperdactyly, is a congenital physical anomaly consisting of supernumerary fingers or toes. When each hand or foot has six digits, it is sometimes called sexdactyly, hexadactyly, or hexadactylism.

The extra digit is usually a small piece of soft tissue; occasionally it may contain bone without joints; rarely it may be a complete, functioning digit. The extra digit is most common on the ulnar (little finger) side of the hand, less common on the radial (thumb) side, and very rarely within the middle three digits. These are respectively known as postaxial (little finger), central (ring, middle, index fingers), and preaxial (thumb) polydactyly. The extra digit is most commonly an abnormal fork in an existing digit, or it may rarely originate at the wrist as a normal digit does.

Polydactyly can occur by itself, or more commonly, as one feature of a syndrome of congenital anomalies. When it occurs by itself, it is associated with autosomal dominant mutations in single genes, i.e. it is not a multifactorial trait.[1] But mutation in a variety of genes can give rise to polydactyly. Typically the mutated gene is involved in developmental patterning, and a syndrome of congenital anomalies results, of which polydactyly is one feature.

The condition has an incidence of 1 in every 500 live births.[2] Postaxial hand polydactyly is a common isolated disorder in African black and African American children, and autosomal dominant transmission is suspected. Postaxial polydactyly is approximately 10 times more frequent in blacks than in whites and is more frequent in male children. In contrast, postaxial polydactyly seen in white children is usually syndromic and associated with an autosomal recessive transmission. One study by Finley et al. combined data from Jefferson County, Alabama and Uppsala County, Sweden. This study showed incidence of all types of polydactyly to be 2.3 per 1000 in white males, 0.6 per 1000 in white females, 13.5 per 1000 in black males, and 11.1 per 1000 in black females.[3]

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