Single fibre electromyography (SFEMG) is a sensitive diagnostic method for neuromuscular transmission disorders,' 2 and its diagnostic yield in myasthenia gravis is significantly higher than that of repetitive nerve stimulation and determination of acetylcholine receptor antibodies.3 In a recent study of 450 myasthenic patients,3 the jitter was abnormal in 95%, when two muscles were studied. In patients with moderate to severe generalised symptoms, abnormal findings were obtained in 100%, and in those with mild generalised weakness in 95%. However, in patients with the ocular form of myasthenia and in those in clinical remission the jitter was only abnormal in 61% and 51%, respectively. A facial muscle (frontalis) had a higher diagnostic yield than limb muscles (extensor digitorum communis), the jitter being more abnormal in the former in 72% of 156 patients compared to 17% with more pronounced abnormality in the latter. This is consistent with earlier observations.1 45 In our experience with a small series of patients with mild generalised disease and patients in

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SUMMARY Denervated muscle fibres were stimulated electrically with needle electrodes introduced close to a recording single fibre electrode. The denervated muscle fibre could be driven with rates up to 100 Hz. The jitter was large at threshold but low at suprathreshold stimulus strength. There was evidence of discrete low threshold sites along the denervated muscle fibre, seen as stepwise latency change on smoothly changing stimulus strength, ephaptic activation from other fibres and also as extra-discharges originating from such sites. fibres by electrical stimulation was quite easily achieved when the stimulus duration was 10 ms or longer. It was possible however to use much shorter pulses down to 50 ,us or even less but then the position of the stimulating electrodes became critical. The pulse duration of 50 ,us or less was preferred in order to minimise the stimulation jitter, that is variability of latency due to the undefined takeoff of the muscle fibre action potential. ResultsOccasionally a spontaneously fibrillating muscle fibre was found by the stimulating electrode and activated in isolation from other fibres. Then its rhythm was disrupted, introducing a compensatory pause following each stimulus induced response.On stimulation at high repetition rates, for example, 10-100 Hz there was a very pronounced progressive increase of the latency, usually associated with a change of the potential shape (decrease of its slopes and amplitude). On double pulse stimulation most fibres were found to have only the subnormal phase of the velocity recovery function,3 but some also exhibited the supernormal phase.When the stimulus strength was near threshold the latency variation in the response was extremely large, sometimes in the range of several thousand ,ts, but when the stimulus was well above threshold the jitter became small, less than 2-5 ,us (fig 1). With increasing stimulus strength new potentials usually entered. Their jitter was large at first but with further increase of stimulus strength it became small. The decrease in jitter was associated with a shortening of latency. Conversely lengthening of latencies and increase in jitter occurred on decreas-305

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