Electrodiagnostic abnormalities in patients with multiple sclerosis.

Prospective electrodiagnostic studies were performed in 26 subjects with definite multiple sclerosis (MS), ranging from mildly to severely affected, to evaluate possible lower motor neuron involvement. Ten of these subjects also underwent single fiber electromyography (SFEMG) examination. Nerve conduction studies (NCS) were normal with no evidence of peripheral or entrapment neuropathies. Abnormal late responses with increased amplitude and chronodispersion of F-waves were found in 9 subjects. Abnormal recruitment patterns with decreased numbers of motor unit action potentials (MUAPs) of normal amplitude, phasicity, and firing rate were found in 9 subjects. Five of them also presented spontaneous electromyographic (EMG) activity in several muscles with no specific myotomal or peripheral nerve distribution. SFEMG showed increased jitter with no blocking in 4 of the 10 subjects studied. Thus, EMG abnormalities were present in many MS patients with no concurrent peripheral or entrapment neuropathies. Electromyographers must be aware of these abnormalities when performing routine EMG examinations in these patients.