Upper motor neuron dysfunction reduces the EMG evidence of myasthenia gravis.

Hyposensitivity to the action of nondepolarizing muscle relaxants has been frequently observed in patients with upper motor neuron dysfunction and in patients treated with antiepileptic drugs. To ascertain whether the same conditions are also characterized by a reduced sensitivity to the effects of anti-acetylcholine-receptor antibodies occurring in myasthenia gravis, bilateral investigations of neuromuscular transmission were conducted on proximal and distal muscles of a myasthenic patients with a long history of hemiparesis and epilepsy. It was observed that electromyographic evidence of myasthenia gravis was absent in distal muscles (and low in proximal ones of the paretic side. Even though the influence of antiepileptic treatment could not be assessed with certainty in this patient, indirect evidence suggests that hyposensitivity to anti-acetylcholine-receptor antibodies is an unlikely consequence of neuromuscular changes induced by anticonvulsants.