Cervical cord dysfunction during neck flexion in Hirayama's disease.

Neck flexion may play a role in the pathogenesis of Hirayama disease. Upper limb somatosensory evoked potentials were recorded in five patients with Hirayama disease, six patients with ALS, and 14 healthy subjects. Neck flexion caused a significant amplitude decrease of the N13 cervical response only in patients with Hirayama disease. Direct cord compression or microvascular changes can in theory account for this position-related dysfunction.