Triphalangeal Thumb, Nonopposable

Clinical Features

Swanson and Brown (1962) described a family in which 30 persons in 5 generations had 5 triphalangeal digits of each hand and apparently lacked a true thumb. The 'thumb' could not be opposed. No associated internal malformations were detected. Triphalangeal thumb of this type occurs in some cases of the Holt-Oram syndrome (142900), although the thumb, when present, was opposable in the family I reported. Whereas in the cases of Swanson and Brown (1962) the metacarpal of the triphalangeal thumb had only a distal epiphysis, as is normal for metacarpals 2-5, the first metacarpal in the Holt-Oram syndrome shows both a proximal and a distal epiphysis. In 3 of the affected persons, Swanson and Brown (1962) found polydactyly.

Pierce (1974) described nonopposable triphalangeal thumbs in 3 sibs.

Warm et al. (1988) described a family in which 11 members in 4 generations had bilateral nonopposable triphalangeal fingerlike 'thumb.' One person had the defect only unilaterally; the contralateral biphalangeal nailless 'thumb' was hypoplastic. Polydactyly was noted in 2 members but the type could not be determined. Since 3 of the 30 affected persons in the family reported by Swanson and Brown (1962) had polydactyly as well, this may be an integral although variable feature of the disorder. Polydactyly with opposable triphalangeal thumb is probably a distinct disorder (174500).

Inheritance

Qazi and Kassner (1988) suggested that isolated TPT of the nonopposable type is usually autosomal dominant, whereas isolated TPT of the opposable type is more likely to be unilateral and sporadic. Isolated TPT is bilateral in about 90% of cases; whereas bilateral TPT is almost always familial, unilateral TPT is usually sporadic.