Brachydactyly type A1, also known as Farabee type brachydactyly, is a subtype of brachydactyly.
The anomaly is characterized by hypoplasia or aplasia of middle phalanges of the second to fifth digits in hands and feet and proximal phalanges of the thumbs and great toes. The fusion of hypoplastic middle phalanges and distal phalanges also can be seen which is known as terminal symphalangism. There may be a single palmar crease. Abnormalities can occur in metacarpals and metatarsals also and all these findings are typically symmetrical.
Brachydactyly type A1 was the first human genetic condition recognized to have a mendelian pattern of inheritance. It is inherited as an autosomal dominant trait. Some cases occur due to a heterozygous missense mutation in the Indian hedgehog (IHH) gene located on chromosome 2q, which is responsible for cartilage growth, maturation and differentiation.
On radiographs, middle phalanges of fingers and toes and proximal phalanges of thumbs and great toes appear hypoplastic or absent. Epiphysis of affected phalanges may be absent or dysmorphic and prematurely fused. There can be sloping of the distal radius, ulna and/or tibia, hypoplasia or aplasia of ulnar styloid.
History and etymology
This anomaly was first described by Farabee and Drinkwater et al. in 1903.