2025 Workshop Handout Bundle-W27AS RNS

W27AS RNS - Harrison

Pdf Summary

This document, prepared by Dr. Taylor B. Harrison for the American Association of Neuromuscular & Electrodiagnostic Medicine (AANEM), provides an in-depth overview of Repetitive Nerve Stimulation (RNS), a diagnostic tool used to assess neuromuscular junction (NMJ) function. Understanding normal physiology is crucial: a motor unit consists of an anterior horn cell and the muscle fibers it innervates. Nerve depolarization triggers calcium influx at the presynaptic terminal, prompting acetylcholine (ACh) release, which binds to muscle fiber receptors to generate an end plate potential (EPP) necessary for muscle contraction. The “safety factor” describes the margin by which EPP exceeds the threshold needed for muscle activation.<br /><br />In diseases such as myasthenia gravis (MG), autoantibodies reduce the number or function of ACh receptors, decreasing the safety factor and causing a decline in compound muscle action potential (CMAP) amplitude upon repetitive stimulation (a decremental response). Lambert-Eaton myasthenic syndrome (LEMS) involves antibodies against presynaptic calcium channels, reducing ACh release, also lowering safety factor with typical CMAP amplitude increases after brief exercise (post-exercise facilitation). Botulism impairs ACh release via toxin-mediated SNARE protein cleavage, producing similar electrophysiological features.<br /><br />Clinical RNS testing involves delivering supramaximal stimuli at low frequency (2-3 Hz) to stimulate nerve-muscle pairs, commonly the ulnar-abductor digiti minimi or spinal accessory-trapezius. A decrement greater than 10% in CMAP amplitude between the first and fourth stimuli indicates pathology, often seen in MG. Post-exercise facilitation is tested by brief maximal muscle contraction, which can transiently improve NMJ transmission in disorders like MG and LEMS. Fast RNS (30-50 Hz) may be used to observe incremental responses typical in LEMS and botulism.<br /><br />Technical considerations include limb temperature control and ensuring stable electrode placement to avoid false results. Reporting should document stimulation parameters, muscle state, measurement methods, medication status, and include waveform data for review.<br /><br />Overall, RNS is a vital electrodiagnostic tool for diagnosing NMJ disorders, differentiating between pre- and postsynaptic pathologies, and guiding clinical management.

Keywords

Repetitive Nerve Stimulation

Neuromuscular Junction

nmj

Myasthenia Gravis

Lambert-Eaton Myasthenic Syndrome

Botulism

Compound Muscle Action Potential

Acetylcholine Receptors

Electrodiagnostic Testing

Safety Factor

Post-exercise Facilitation