Congenital clasped thumb describes an anomaly which is characterized by a fixed thumb into the palm at the metacarpophalangeal joint in one or both hands. The incidence and genetic background are unknown. A study of Weckesser et al. showed that boys are twice as often affected with congenital clasped thumb compared to girls. The anomaly is in most cases bilateral (present in both hands). A congenital clasped thumb can be an isolated anomaly, but can also be attributed to several syndromes. Causes The thumb contains five groups of muscle and/or tendons: Extensor tendons (to stretch the thumb) Flexor muscles/tendons (to bend the thumb) Abductor muscles/tendons (to move the thumb outwards) Adductor muscle (to move the thumb inwards) Opposing muscles (to move the thumb opposite the small finger) In order for the thumb to maintain a normal position, a strict balance between these groups is required. Weak or absent extensors and/or abductors (the extensor pollicis brevis tendon, the extensor pollicis longus tendon or, rarely, the abductor pollicis longus tendon), can cause a disbalance, leading to an abnormal position of the thumb: congenital clasped thumb. There is also the possibility that two tendons are affected simultaneously. The following tendon deviations can induce congenital clasped thumb: The flexor tendons are too short (the thumb is drawn into the palm) The abductor tendons are hypoplastic or absent The extensor tendons are hypoplastic or absent Furthermore, a tight thumb web space (the area between thumb and index finger) can contribute to congenital clasped thumb. The thumb cannot be properly abducted (moved outwards), if the web space is too tight. To summarize, the causes of congenital clasped thumbs may vary between patients and can sometimes be a combination of the preceding components. Treatment should be tailored to all occurring components in order to achieve good results. Diagnosis Diagnosing the congenital clasped thumb is difficult in the first three to four months of life, as it is normal when the thumb is clutched into the palm in these first months. Diagnoses that cause the same flexion or adduction abnormalities of the thumb are: Congenital clasped thumb Congenital Trigger thumb (flexion of the interphalangeal joint) - Trigger finger Spasticity: overstimulation of muscles Syndrome associated flexion-adduction of the thumb: Freeman–Sheldon syndrome (a congenital, heritable affection of the face, the hands, the feet and some joints) Distal arthrogryposis MASA syndrome X-linked hydrocephalus Adducted thumb syndrome Waardenburg syndrome Whistling face syndrome