[Single-fibre electromyography]

Single-fibre electromyography (SF-EMG) is the most sensitive electrophysiological test for myasthenia gravis and other neuromuscular junction pathology. It is also useful in the assessment of motor unit morphology in some neuromuscular diseases. A special needle with a 25 microm recording surface is used to record the time-locked potentials of 2 muscle fibers belonging to the same motor unit. Jitter is the variability in the arrival time of action potentials to the recording surface between consecutive discharges. The junctional part of the jitter depends on the amount of Ach released and on the sensitivity of the postsynaptic membrane and is related to the safety margin of neuromuscular transmission. Thus, SF-EMG can detect subclinical impairment of neuromuscular transmission, which cause an increased jitter. However, an increased jitter is not specific of neuromuscular transmission disorders, because it may be due to unstable conduction in motor nerves and muscle fibers. It is the extrajunctional part of the jitter. SFEMG is performed either during slight voluntary contraction (volitional SFEMG) or by axonal stimulation (stimulated SFEMG). Jitter is expressed as the mean of the absolute consecutive differences (MCD) of the latency between the time-locked potentials (volitional SFEMG) or from the stimulus to the negative peak of the potential (stimulated SFEMG). Recording is judged abnormal when more than 10% of the collected pairs are abnormal. On the other hand, single use coaxial needle electrodes which are cheaper could be an alternative for neuromuscular transmission analysis, because the jitter analysed with this type of needle was found highly comparable to that analysed with SF electrode. Moreover, jitter measurements coupled with fiber density estimates provide valuable information on the pathophysiology of the motor units in various neuromuscular disorders.