Abstract

ABSTRACT:We found that stimulating the ulnar nerve below the elbow at 50 pulses per second (pps) and recording from the abductor digiti minimi in the normal subject produced facilitation first and then rapid fatigue. The peak facilitation occurred at an average of 4.2 seconds and there was an averageof 25.9% increase in amplitude from the initial response. We also found that the normal muscle will fatigue approximately linearly to an average of 69.0% of maximal response in 15 seconds and to an average of 36.1% of the maximal response at 30 seconds. Thus, the pattern recorded in the normal muscle at a tetanizing rate was similar to that reported in the literature for patients with the myasthenic syndrome. We hope that the recognition that there is a similar pattern will help electromyographers to evaluate myasthenic-syndrome-like recordings. Moreover, by a review of the literature, we have reached the conclusion that stimulating at some rate between 10 and 25 pps should differentiate the normal person, the myasthenia gravis patient, and the myasthenic syndrome patient. The normal subject will facilitate and show no fatigue. The patient with myasthenia gravis will show only fatigue. The patient with the myasthenic syndrome will facilitate first and then fatigue. An alternate method to differentiate the three entities lies in stimulating at two different rates. Stimulation at 1 ppswill differentiate the normal person from the two other entities. The normal subject will not fatigue, the patients with myasthenia gravis and myasthenic syndrome will show a fatigue pattern. Next, the patient who showed fatigue at 1 pps should be stimulated at a rate greater than 10 pps. This will differentiate myasthenia gravis and myasthenic syndrome. The former will only fatigue, while the latter will facilitate and then fatigue.