[Excitability of cervical lower motoneuron in juvenile muscular atrophy of the distal upper limb].

We investigated excitability of cervical lower motoneurons in 18 patients with juvenile muscular atrophy of the distal upper limb (Hirayama's disease), by F wave analysis and electrophysiological estimation of the number of motor units in the abductor pollicis brevis muscles of the affected side. In all the 18 patients, F wave persistencies and numbers of motor units were decreased, whereas amplitudes of single motor unit potentials were increased, compared to those of age-matched normal controls. Patients in a progressive phase showed markedly decreased F wave persistencies, overwhelming the degree of decreases in numbers of motor units. In half of them, F waves were significantly increased in number during neck flexion. On the other hand, patients treated with neck brace and patients having stabilized symptoms with longer duration showed F wave persistence reduction corresponded to the degree of the motor unit loss, and no changes in F wave persistencies in a posture of neck flexion. These results suggest that the disorder is characterized by decreased excitabilities of survived cervical anterior horn cells, as well as denervation and reinnervation, presumably resulting from local compression or circulatory failure by anterior shift of the posterior dural wall during neck flexion.