Polydactyly, postaxial, type B

Postaxial polydactyly, type B is the most common type of polydactyly. The vestigial sixth finger is attached by a narrow neurovascular pedicle to the lateral aspect of the hand or foot at the level of the metacarpal-phalangeal joint or the metatarsal-phalangeal joint. The occurrence of this type of polydactyly varies among racial groups, by sex and sidedness. Postaxial polydactyly, type A is a fully developed extra digit on the lateral aspect of the hand or foot with a bifid fifth or sixth metacarpal/metatarsal and is much less common.

In a malformations surveillance program, the frequency in racial groups, sex ratio and the frequency of other anomalies can be established.

Five hundred forty-five affected infants were identified from 1972 to 2012 in the surveillance of 289,365 liveborn and stillborn infants and elective terminations because of fetal anomalies detected prenatally. Postaxial polydactyly, type B was an isolated anomaly in 95% of the affected newborns. There were more affected males than females. Black infants were affected more often than White infants: 0.91/100 vs. 0.035/100 infants. The dangling extra digit was much more common in the hands than in the feet.

Postaxial polydactyly, type B is almost always an isolated, mild malformation with no medical significance. Postaxial polydactyly, types B and A occurred in several infants, suggesting that either the underlying mutation(s) can cause both types of postaxial polydactyly or that some affected infants have more than one mutation. Autosomal dominant inheritance with variable expressivity is postulated.