These guidelines express the consensus of international experts on the current status of Single Fiber EMG (SFEMG) and the measurement of neuromuscular jitter with concentric needle electrodes (CNE–CN jitter).1 The authors describe methods for evaluating fiber density (FD) and define the criteria for acceptable signals (spikes with amplitude >200 µV and rise time <300 µs, constant shape at consecutive discharges and absence of notches or inflections). They report on procedures for jitter studies during voluntary contraction and nerve stimulation, as well as on the differences between CN1 and SFEMG2 recordings. Then, the authors review the application of jitter analysis in neurological conditions such as myasthenia gravis (MG), polyneuropathy, motor neuron disease, cervical radiculopathy, multifocal motor neuropathy, Guillain-Barré syndrome, myopathy, botulism and Lambert-Eaton syndrome (LEMS). In addition, suggestions for jitter measuring in paediatric population are provided. The authors specifically focus on sensitivity and specificity of jitter measurements in MG and discuss the options for serial jitter studies in MG and LEMS: in the former jitter is a sensitive measure of disease severity and has a potential role as a biomarker whilst in the latter it gives a similar amount of information compared to CMAP studies but being technically more complicated to perform. In the last part other possibile applications of SFEMG are reported, namely single axon conduction studies (which has better sensitivity than conventional tests), reflex studies and responses to transcranial cortical stimulation. Finally, the document includes a review of possible pitfalls regarding measurement of jitter, and recommendations to avoid misinterpretation of the recording signals.
Key Points
-Fiber density can be measured only with SFEMG, features of action potentials produced by single muscle fibers are spikes with amplitude >200 microV and rise time <300 micros, and a constant shape at consecutive discharges without notches or inflections.
-Jitter studies can be based on voluntary contraction or on electrical stimulation. It should be measured in a minimum of 20 potential pairs with voluntary contraction and a minimum of 30 endplates with nerve stimulation.
-10 kHz low-pass filter is set equal for both CN and SFEMG jitter studies, however the former require a high-pass filter setting of 1 kHz (compared to 500 Hz for SFEMG) as a consequence of its larger recording area.
-Jitter measurement is a sensitive diagnostic tool for MG. Accuracy of diagnosis improves when an increased number of muscles is analysed.
-Numerous pitfalls can affect jitter analysis, careful attention and useful tricks can avoid misinterpretation of recording data.
References
1) Sanders DB, Arimura K, Cui L, Ertaş M, Farrugia ME, Gilchrist J, Kouyoumdjian JA, Padua L, Pitt M, Stålberg E. Guidelines for single fiber EMG. Clin Neurophysiol. 2019. pii: S1388-2457(19)30128-2. doi: 10.1016/j.clinph.2019.04.005.
2) Stålberg EV, Sanders DB. Jitter recordings with concentric needle electrodes. Muscle Nerve 2009;40:331–9.
2) Ekstedt J, Nilsson G, Stålberg E. Calculation of the electromyographic jitter. J Neurol Neurosurg Psychiatry 1974;37:526–39.