Electrical and mechanical responses in the platysma and in the adductor pollicis muscle: in patients with myasthenia gravis.

Electrical and mechanical muscular responses to single and repetitive stimuli were recorded in 24 patients with myasthenia gravis. Findings in the platysma were compared with those in m. adductor pollicis (ADP). In the platysma, but not in the ADP, electrical and mechanical responses to single stimuli were often lower than normal, and could be normalised after tetanus and by endrophonium. The decrement of electrical and mechanical responses to repetitive stimuli was two to three times greater in the platysma than in the ADP; post-tetanic facilitation of the action potential was four times greater. The staircase phenomenon was abnormal in the platysma in patients with moderate and severe myasthenia, and also in the ADP in some patients without decrement in the action potential. Edrophonium was more effective in alleviating decrement in the platysma than in the ADP. In the platysma, block of neuromuscular transmission could account for most abnormalities. The finding in some patients of an abnormal staircase after correction for block of fibres indicates a lesion in excitation-contraction coupling. In six patients only the platysma showed abnormalities, in 10 patients abnormalities were more pronounced in the platysma than in the ADP, and in three patients more pronounced in the ADP than in the platysma; in five patients the platysma and the ADP were equally affected.