Syndactyly, the webbing of two or more fingers or toes, is the most common congenital malformation of the limbs. The unusual term for this disorder derives its name from the Greek words meaning together, "syn," and digits, "dactyl." The most common presentation of this abnormality is a bonding between the second and third toes.

Patients with this condition may exhibit a large degree of variation in the extent of their malformation. Digits can be partially fused or can be fused along their entire length. The fusion can be simple, with the digits connected only by skin, or complex, with shared bones, nerves, blood vessels or nails. Syndactyly is classified into several categories, depending on the severity of the condition: simple, incomplete, complete, complex and complicated.

The condition results from an abnormality in fetal development, usually caused by a genetic defect. It may occur in isolation, but is often one symptom of a multi-symptom syndrome, such as Down syndrome, or, more rarely, other syndromes such as Apert syndrome, Poland syndrome or Holt-Oram syndrome. Another rare cause of the disorder is Fetal Hydantoin Syndrome, a result of the mother's use of the anti-seizure medication hydantoin, commonly known as dilantin, during pregnancy.

The treatment for syndactyly is surgery and is best performed during the first years of life.

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