Background Hirayama disease is a juvenile muscular atrophy of the distal upper extremities and affects mainly young males.The present study aimed to investigate the neuroelectrophysiological characteristics of Hirayama disease.Methods We retrospectively analyzed the neural conduction velocity (NCV) parameters and needle-electrode electromyograms (EMG) of 14 patients with Hirayama disease.According to the clinical features of the patients,NCV was performed on affected upper-limb including median nerves and ulnar nerves,while EMG was selectively performed on upper and lower extremities,sternocleidomast and thoracic paraspinal muscles.Results The median nerves of all affected upper limbs of patients with Hirayama disease had normal conduction velocities and compound motor action potentials (CMAPs).The ulnar nerves of all affected upper limbs also had normal conduction velocities.Of the 16 measured ulnar nerves of the affected upper limbs,eight had normal CMAPS,while the other eight showed CMAPs below the normal value by ＞20％.All patients had neurogenic injury on the affected side in muscles innervated by anterior horn cells at the lower cervical region (C7-8,T1).Four patients had unilateral upper-limb muscle neurogenic injury on the affected side.Seven patients had bilateral upper-limb muscle neurogenic injury,while only two patients experienced bilateral upper-limb muscle atrophy / weakness.The other three patients showed extensive neurogenic injury (unilateral upper-limb muscle atrophy/weakness in one patient,bilateral symptoms in the other two patients).Conclusions Electromyographic examination showed that the majority of Hirayama disease patients exhibited characteristic segmental injury in the anterior horn of the lower cervical region,while a few patients exhibited extensive neurogenic injury.These data suggest that the actual influence of Hirayama disease may be more extensive than indicated by the clinical presentations.