For a MUP train, segments of MUPs generated by single muscle fibers (SF MUP segments) are found and jitter is measured between pairs of these segments. This paper presents an automatic method to estimate jitter from trains of motor unit potentials (MUPs), for both SFE and CNE records. As CNEs are larger, voltage contributions from individual fibers of a MU are more difficult to detect, making jitter measurement more difficult. SFEs are expensive and reused, implying a potential risk of patient infection so, they are being gradually substituted by safer, disposable, concentric needle electrodes (CNEs).
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Traditionally, jitter has been measured using single fiber electrodes (SFEs) and a difficult and subjective manual technique. This variability is called jitter and is increased in pathological processes that affect the neuromuscular junctions or terminal axonal segments of MUs. In an active motor unit (MU), the time intervals between the firings of its muscle fibers vary across successive MU activations. This can complicate the detection or exclusion of abnormal NMT. Jitter is abnormal after injections of botulinum toxin, even in muscles remote from the injection site, and can persist for 6 mo or more. With SFEMG, these can be identified by increased FD however, FD cannot be measured with CNE, and conventional EMG should be performed in muscles with increased jitter to detect neurogenic or myogenic abnormalities. However, jitter is also seen in ongoing reinnervation and some myopathic conditions. CNE and SFEMG have similar and very high sensitivity in detecting increased jitter, as in myasthenia gravis and other myasthenic conditions. In CNE recordings, spikes with shoulders or rising phases that are not parallel are produced by summation of SFAPS these should be excluded and reference values for CNE jitter should be used. Pitfalls due to unstable triggers and inconstant firing rates during voluntary activation and subliminal stimulation during axon stimulation should be identified and avoided. With activation by axon stimulation, jitter is measured between the stimulus and individual SFAPs. With voluntary activation, jitter is measured between two SFAPs with acceptable amplitude and rise time. SFEMG records action potentials from single muscle fibers (SFAPs), which permits measuring fiber density (FD), a sensitive measure of reinnervation, and jitter, a sensitive measure of abnormal neuromuscular transmission (NMT).
This monograph contains descriptions of the single-fiber electromyography (SFEMG) method and of the more recently implemented method of recording jitter with concentric needle electrodes (CNE). The activity of reinnervated muscle could maintain neonatal MHC and repress new NMJs development. Jitter measurement must be avoided in chronic denervated muscles, regardless of FPs' presence.
For those, higher jitter may be due to the persistence of atrophic fibers expressing neonatal myosin heavy chain (MHC) and immaturity of NMJ composting instead of the overspread of immature AChRs. Conversely, the neuromuscular junctions (NMJs) assemble may be repressed by the already reinnervated muscles. The muscles with FPs were associated with the immature spread of acetylcholine receptors (AChRs) throughout the muscle membrane. No correlation was found between jitter and motor unit action potential amplitude. In muscles without FPs, the mean jitter was abnormal in 78.9% for voluntary activation and 68.4% for electrical activation. In muscles with fibrillation potentials (FPs), the mean jitter was abnormal in all cases, and impulse blocking was frequent (53.4 to 92.3%). Mean jitter was abnormal in 87.5% (mean 49.2 µs) and 81.25% (mean 36.8 µs), for voluntary and electrical activation. Measurements were done in chronic denervated muscles by voluntary and electrical activation using a concentric needle electrode. To measure the jitter parameters in muscles with denervation/reinnervation in 32 chronic radiculopathy cases.
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