Quantitative EMG findings at different force levels in patients with myasthenia gravis

Quantitative electromyography, comprising manual motor unit potential analysis at weak effort and turns/amplitude analysis at 30% of maximum force were performed in the brachial biceps muscle of 17 patients with myasthenia gravis (MG). Findings simulating myopathic and less often neurogenic changes were observed in seven out of 17 patients, suggesting that the myasthenic muscle may have random block of muscle fibres, functional block of the main part or whole motor units or a changed recruitment pattern. In addition, turns/amplitude analysis was performed at low force levels (10% of maximum force) both before and after a provocative manoeuvre consisting of a maximum effort sustained for 1 min, and at maximum force at the beginning and at the end of the provocation itself. Turns/amplitude analysis at low force failed to differentiate MG patients from controls. The relative change of ratio of turns to mean amplitude calculated from 10% to maximum force (at the beginning of the provocative manoeuvre) was increased in seven out of eight patients compared to the controls. This last finding seems to be useful in differentiating MG patients from patients with myopathy.