[Electrophysiology of juvenile muscular atrophy of unilateral upper limb (Hirayama's disease)].

To examine the validity of electrodiagnostic tests to help diagnosis and to evaluate disease activity of juvenile atrophy of unilateral upper limb (Hirayama's disease), we reviewed the results of nerve conduction studies, motor unit number estimates, electromyography, and somatosensory evoked potentials in consecutive 38 patients. There was selective involvement of the C8-T1 myotomes, in which motor unit loss was more prominent in the abductor digiti minimi than in the abductor pollicis brevis muscles, and F wave persistency was decreased out of proportion to the motor unit loss. These findings may help diagnosis. With respect to the disease activity and treatment, denervation potentials in the flexor carpi radialis muscle were more frequent in patients with shorter disease duration than those with longer duration. Median F wave persistency was greater in patients treated with neck brace than in patients without the treatment. These parameters can monitor the disease activity and efficacy of treatment.