PRIMARY MUSCLE DISEASE
In general, the EMG in primary muscle disease of any origin is similar and making distinctions between, say, hereditary and acquired myopathy is difficult. Motor unit potentials are small and spiky and the recruitment pattern becomes full with just a small contraction. In inflammatory muscle disease where there is active degeneration of muscle fibres, fibrillations may be seen, but this is not universal or specific. In muscle diseases which show large variation in fibre diameter, such as the muscular dystrophies, there may be large motor units on a background of small spiky units, presumably arising from large diameter fibres. In cases of Lambert-Eaton myasthenic syndrome (LEMS), the EMG can look myopathic because so many muscle fibres have neuromuscular block; the EMG becomes more normal if the patient is able to maintain a voluntary contraction. In any case where a myopathic EMG is found, LEMS should be considered and should be investigated by measuring the amplitude of a compound muscle action potential evoked by nerve stimulation before and after exercise.