[Cervical flexion-induced changes of motor evoked potentials by transcranial magnetic stimulation in a patient with Hirayama disease--juvenile muscular atrophy of unilateral upper extremity].

A 16-year-old girl noticed weakness in the left hand in 1990, which gradually progressed over the next 2 years but then stabilized. Her neurologic and electrophysiological signs were compatible with juvenile muscular atrophy of unilateral upper extremity (Hirayama disease). Cine MRIs demonstrated a mild cervical cord atrophy at the C6 spine level together with an engorged epidural vein at the C4-C6 extradural spaces during neck flexion. Transcranial magnetic stimulation over the motor cortex was carried out with a pickup placed on the contralateral abductor brevis muscle. While neck flexion was maintained, the amplitude of the evoked potentials steadily attenuated and the central motor conduction time lengthened as time passed. After 8 minutes, she began to feel some dullness in the left upper limb. These phenomena were reversible and were observed only on the affected limb. The above facts not only supported the presence of a reversible cortico-efferent dysfunction in Hirayama disease, but also appeared to justify the use of neck collars to treat those afflicted with this entity.