Electromyographic assessment of fatiguability and recovery of orbicularis oculi in myasthenia gravis.

Orbicularis oculi fatiguability following maximal voluntary contraction and its recovery after administration of Tensilon (edrophonium chloride) were studied in 13 patients with myasthenia gravis (MG) and 8 normal controls. The number of EMG pulses per sec (C) was calculated using an experimentally manufactured EMG pulse counting system and the mean amplitude of the motor unit potentials per sec (A) was measured using the action potential analysis module (Medelec APA-6 MK-1) during maximal voluntary contraction of the orbicularis oculi muscle. Fatiguability was estimated as percentage change of C, the pre-exercise value being taken as 100%. It was 74.4 +/- 18.2% (mean +/- S.D.) in patients and was significantly different from 110.8 +/- 8.4% found in the controls (p less than 0.01). Recovery following intravenous injection of Tensilon was estimated as percentage change in C and A, the values before Tensilon injection being taken as 100%. The change in EMG pulse per sec (C) was 360.0 +/- 90.0% in MG and 113.0 + 21.2% in controls (p less than 0.005). The mean amplitude of the motor unit potential per sec (A) increased by 180.0 +/- 95.0% in MG and 120.0 +/- 42.0% in controls (p less than 0.01). This technique is thought to be valuable to confirm the diagnosis of MG when routine electrophysiological tests fail to demonstrate abnormalities in neuromuscular transmission.