Direct electrical stimulation with paired pulses at varied intervals was used to study the propagation velocity and action potential amplitude recovery functions (VRF and ARF) of single muscle fibers. Following a subnormal period with slowed conduction, most of the muscle fibers tested in healthy subjects showed a period of supernormal propagation velocity starting at 3 to 12 ms, with a peak between about 5 and 15 ms, a mean increase of 7%, and an approximately logarithmic decay toward 1 second. The onset of supernormality was earlier in muscle fibers from patients with muscular dystrophy and significantly delayed in those from denervated muscles. Denervated muscle fibers also had a significantly longer refractory period.
SUMMARY This single fibre EMG study compares the standard method of neuromuscular jitter measurement in voluntarily activated muscle to that by intramuscular electrical stimulation of motor axons in a group of normal subjects. The latter method avoids the interdischarge intervaldependent jitter, as well as a possible failure to recognise split muscle fibres. The mean MCD on axonal stimulation was only 5-2 ps less than in the voluntary activation study and was thus 8% more than theoretically expected for single motor end plates. The difference could be due to an axonal jitter and some other factors. Axonal stimulation has proved to be a relatively easy and reliable method for routine estimation of neuromuscular jitter, provided that the resolution of time measurement is better than 2 ps, so that low jitter due to occasional direct muscle fibre stimulation is not mistaken for a normal reading. The upper normal limits for the extensor digitorum communis muscle suggested by the present study are 40 ps (individual muscle fibres) and 25 ps (mean of 30 muscle fibres).In single fibre electromyography, the jitter of the motor end plates is usually measured during slight voluntary contraction of the muscle. This however, is not always practicable, for example in unco-operative or unconscious patients, young children and even in severely paretic muscles, whether there is an upper motor neuron lesion or very severe myasthenia gravis or weakness due to some other peripheral pathology. Electrical stimulation has been suggested for such cases and has actually been used in research,' but only seldom in clinical practice. Some of the reasons have been lack of relevant experience, the need for a high resolution jittermeter to eliminate the possibility of mistaking the low jitter due to direct muscle fibre stimulation for a normal reading, and unavailability of normal data. The purpose of this work was to collect normal values and compare the technique to the well established method with voluntary activation. Material and methodFifteen normal volunteers participated in the study. Their
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
Electrical microstimulation of motor axons in conjunction with single fiber EMG (SFEMG) is increasingly used to measure the jitter of the motor endplates. This study examines the jitter of the stimulation site on the axon when stimulus strength is at threshold. In the absence of spurious blocking, this was found to be 5 microseconds on the average. With intermittent blocking, however, a mean additional jitter of 40 microseconds was obtained. The latter is considered to result from changing propagation velocity in the muscle fiber due to irregularity of activation rate. In clinical jitter studies, inadvertent threshold stimulation can result in significant error when associated with intermittent blocking.
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