Thumb hypoplasia is a spectrum of congenital abnormalities of the thumb varying from small defects to complete absence of the thumb.[1] It can be isolated, when only the thumb is affected, and in 60% of the cases[2] it is associated with radial dysplasia[1] (or radial club, radius dysplasia, longitudinal radial deficiency). Radial dysplasia is the condition in which the forearm bone and the soft tissues on the thumb side are underdeveloped or absent.[3]
In an embryo the upper extremities develop from week four of the gestation.[1] During the fifth to eighth week the thumb will further develop.[4] In this period something goes wrong with the growth of the thumb but the exact cause of thumb hypoplasia is unknown.[1] One out of every 100,000 live births shows thumb hypoplasia.[2] In more than 50% of the cases both hands are affected, otherwise mainly the right hand is affected.[1][2]
About 86% of the children with hypoplastic thumb have associated abnormalities.[1][2] Embryological hand development occurs simultaneously with growth and development of the cardiovascular, neurologic and hematopoietic systems.[2] Thumb hypoplasia has been described in 30 syndromes wherein those abnormalities have been seen. A syndrome is a combination of three or more abnormalities. Examples of syndromes with an hypoplastic thumb are Holt–Oram syndrome, VACTERL association[1] and thrombocytopenia absent radius (TAR syndrome).[2]