2023 Workshop Handout Bundle-SFEMG and Jitter Measurement During Voluntary Muscle Contraction

SFEMG and Jitter Measurement During Voluntary Muscle Contraction

Pdf Summary

Dr. Anna Punga's presentation focuses on Single-Fiber Electromyography (SFEMG) for detecting neuromuscular transmission failures. SFEMG measures jitter, or the variation in the time interval between action potentials within a motor unit, and is sensitive enough to detect subclinical neuromuscular defects even in unaffected muscles. Initially developed by Ekstedt and Stålberg in Uppsala, Sweden, SFEMG has become a global standard for diagnosing conditions like Myasthenia Gravis (MG), Lambert-Eaton Myasthenic Syndrome (LEMS), and congenital myasthenic syndromes (CMS). <br /><br />Jitter can be measured during voluntary muscle contraction or via electrical stimulation. Two main electrodes are used for these measurements: the original SFEMG electrodes and concentric needle electrodes (CNE). SFEMG is considered more sensitive, particularly in facial muscles like the orbicularis oculi, which are often used due to their high sensitivity for detecting abnormal jitter in MG. Different muscles have different normal jitter values, so muscle-specific reference values are crucial for accurate diagnosis.<br /><br />During SFEMG, the analysis may be affected by medications like acetylcholinesterase inhibitors, which can mask jitter. Therefore, patients are recommended to avoid these medications before tests. Abnormal jitter does not definitively diagnose NMJ disorders; it may also appear in conditions like motor neuron disease and neuropathy. The specificity of SFEMG is lower compared to its sensitivity, indicating a high rate of false positives. <br /><br />The document notes that optimal jitter recording involves clear solitary spikes with a well-defined negative peak. Regular interpretations of jitter might include an MSD (mean-sorted-data difference) for adjustments in firing rate variations. The amplitude and rise-time criteria differ between SFEMG and CNE. Technical pitfalls include avoiding high activation levels and summation spikes, which can blur action potentials.<br /><br />SFEMG is essential for diagnosing neuromuscular transmission failures, but it must be used in conjunction with clinical presentation and other diagnostic tests like nerve conduction studies and repetitive nerve stimulation. The accuracy and predictive value of SFEMG for certain NMJ disorders are also highlighted, underscoring its role in clinical practice.

Keywords

Single-Fiber Electromyography

SFEMG

neuromuscular transmission

jitter

Myasthenia Gravis

Lambert-Eaton Myasthenic Syndrome

congenital myasthenic syndromes

acetylcholinesterase inhibitors

orbicularis oculi

nerve conduction studies

ncs

nerve conduction studies

nerve conduction study

ncs