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Preaxial polydactyly (i.e., polydactyly on the radial/tibial side of the hand/foot) covers a heterogeneous group of entities. Four types are defined: (1) thumb/hallux polydactyly (2) triphalangeal thumb (3) polydactyly of index finger and (4) polysyndactyly. Together, they occur in about 1 in 4,000 births. Thumb/hallux polydactyly involves duplication of one or more of the skeletal components of a biphalangeal thumb/hallux. Severity varies from simple broadening of the distal phalanx with slight bifurcation at the tip to full duplication of the thumb/hallux including the first metacarpal/metatarsal. In triphalangeal thumbs (type 2), the thumb is opposable and has a normal metacarpal. This type is often associated with duplication of the hallux. Linkage of a type 2 defect to the 7q36 region has been demonstrated, and sometimes submicroscopic chromosomal deletions can be identified. An historically notable example is the Scipion family in which type 3 preaxial polydactyly (the thumb was present as one or two triphalangeal digits and was sometimes opposable) was transmitted for over two thousand years. In some cases the feet show preaxial polydactyly of the 1st or 2nd toes. Polysyndactyly is classified as preaxial polydactyly type 4. The thumb shows only the mildest degree of duplication, and syndactyly of various degrees affects fingers 3 and 4. The foot malformation is more constant and consists of duplication of part or all of the first or second toes with syndactyly affecting all of the toes. It is possibly the same disorder as that called type I crossed polydactyly. Crossed polydactyly is defined as coexistence of preaxial and postaxial polydactyly with discrepancy in the axes of polydactyly between the hands and feet. Some of these cases have been shown to be caused by mutation in the GLI3 gene. The upper limbs are more often affected than the lower limbs. Biphalangeal thumb duplication or hallux duplication can occur in families with triphalangeal thumb or polysyndactyly propositi. There is a significant excess of males, and right-sidedness is more frequent. Preaxial polydactylies are most often unilateral (80 to 90%). They are about two times more frequent in American Indians than in Caucasians, and thee to four times more frequent in Caucasians than in Blacks. Preaxial polydactyly of hands and/or feet also occurs in a very large group of syndromic entities. |