Hirayama disease presenting as isolated triceps atrophy.

We describe an atypical case of Hirayama disease (HD) presenting as unilateral triceps atrophy. HD is juvenile muscular atrophy involving predominantly the seventh and eighth cervical, and the first thoracic spinal segments of the distal upper extremities. In contrast to typical HD, our patient presented with isolated triceps atrophy, innervated by the sixth and seventh cervical segments. We discuss the differences with previously reported HD and the mechanisms involved in the development of HD.