[Symptomatological and electrophysiological study on cold paresis in juvenile muscular atrophy of distal upper extremity (Hirayama's disease)].

Patients with juvenile muscular atrophy of distal upper extremity (Hirayama's disease) often show marked weakness of the fingers occurring with exposure to cold. We term this phenomenon cold paresis. We conducted an original test to induce cold paresis (Cold Paresis Inducement Test) in 11 patients of this disease and 10 normal controls. Cold paresis was induced in 9 of 11 patients, but was not induced in the 2 patients who had the disease longer than 20 years and in all normal controls. We examined the electromyogram of abductor digiti minimi during 5 Hz and 20 Hz rate of ulnar nerve stimulation at cooling. The patients in whom cold paresis was induced exhibited a waning of amplitude of compound muscle action potential (M wave) during 20 Hz stimulation. This waning was aggravated by intravenous administration of anticholinesterase (edrohponium). We found a remarkable conduction delay of M waveform at the waning by means of waveform analysis. These results suggest that cold paresis may be caused by a conduction block of the muscle fiber membrane in re-innervating muscles after active denervation.