[Juvenile monomelic amyotrophy: Hirayama disease]

We present three patients with unilateral upper limb weakness (with muscular atrophy)-two of them with distal and one with proximal localization. The disease onset was between 18th end 35-th year of life; the disease course was biphasic (i.e. progressive within first 1 to 3 years, and stabilized during following 4-24 years). The laboratory investigations permitted to diagnose juvenile monomelic amyotrophy, an entity that is very rare outside Japan. Electromyography revealed neurogenic involvement with spinal features also in clinically unaffected muscles. We suggest that these results may support the hypothesis of this disease being a benign variant of spinal muscular atrophy.