Fatigue trends in and the diagnosis of myasthenia gravis by frequency analysis of EMG interference patterns

Twenty patients with myasthenia gravis were studied. Needle interference patterns at maximal isometric contractions were recorded from the biceps brachii muscles. Each recording lasted for 30 seconds and induced som fatigue. The EMG signals were transformed into power spectra and were analyzed for differences between control and myasthenic fatigue trends and were tested for the power of the frequency variables to classify unknown subjects. Both groups showed a similar averaged spectra for the first 5 seconds. Thereafter, the controls manifested continuous increase in power, and a power peak frequency shift toward low frequencies. The myasthenics showed an initial increase throughout the frequency ranges; however, later, there was a marked decrease in power and their peak frequency shifted toward the lower frequencies. These fatigue trends differed significantly from one another. Discriminant analysis correctly classified 83% of the subjects. This technique may be helpful in the diagnosis of myasthenia gravis.