Congenital deficiency of the extrinsic extensor mechanism of the hand.

In a 16-year-old girl, congenital extensor deficiency of the hand with absence of true clasping of the thumb was transmitted by an autosomal dominant mechanism of inheritance. The age of presentation was more advanced than usual. This case is the basis for proposing an hypothesis that the condition is a manifestation of a failure of innervation to develop in the extensor muscles of the forearm.