Congenital hemihypertrophy associated with posterior interosseous nerve entrapment.

An entrapment of the posterior interosseous nerve (PIN) associated with congenital hemihypertrophy has not been reported previously. A 63-year-old man noted that since birth his right arm had been considerably larger than his left. Fifteen years before our initial examination, however, he first noted weakness of his right wrist and finger extensors without sensory loss. Physical examination revealed atrophy and weakness of the musculature innervated by the PIN distal to the supinator and the first dorsal interosseous (FDI). Sensation was intact throughout the entire right arm. Electromyographic examination demonstrated membrane instability and decreased activation of voluntary motor units in the radial nerve musculature distal to the supinator and in the FDI. Electrophysiologic evidence suggested an anomalous innervation of the FDI by the PIN. Surgical exploration revealed hypertrophy of the brachioradialis, extensor carpi radialis longus and brevis, and supinator muscles, with atrophy of all muscles distal to the supinator. The supinator muscle appeared to be constricting the PIN and was sectioned. Clinical and electrodiagnostic follow-up at six months and again at four years revealed no further deterioration.