WS3.7. Repetitive Nerve Stimulation Testing. Issue 8 (August 2021)

• Record Type: Journal Article
• Authors: Sanders, Donald B.
• Is Part Of: Clinical neurophysiology. Volume 132:Issue 8(2021)
• Page Start: e56
• Publication Date: 2021-08
• Subjects: Neurophysiology -- Periodicals; Electroencephalography -- Periodicals; Electromyography -- Periodicals; Neurology -- Periodicals; 612.8
• DOI: 10.1016/j.clinph.2021.02.087
• Languages: English
• ISSNs: 1388-2457

Abstract

Abstract : Repetitive nerve stimulation (RNS) is the most frequently used electrodiagnostic test to diagnose neuromuscular junction disorders. Although seemingly simple, attention to technique is essential to avoid technical errors. Stimulation at low rates (2–5 Hz) (LF-RNS) depletes acetylcholine stores at diseased motor endplates, causing failure of neuromuscular transmission. A decrement of at least 10% to trains of 2-3 Hz stimulation at rest or immediately after a period of maximal muscle contraction ("activation") is the usual criterion of abnormality; 5% can be used if technical artifacts can be excluded. Take care to deliver adequate stimulation and prevent movement of the tested muscle throughout nerve stimulation; peripheral muscles should be kept warm during testing. RNS is reportedly abnormal in up to 90% of generalized myasthenia gravis (MG) patients and in up to 60% of ocular myasthenia – sensitivity is strongly dependent on the muscles selected for testing. RNS is more often abnormal in proximal, facial and clinically weak muscles; even so, RNS is frequently normal in mild or purely ocular disease. For maximal diagnostic yield, test several muscles, including those that are weak. The characteristic findings in Lambert-Eaton myasthenia (LEM) are: small CMAPs at rest, a decrement to LF-RNS, and marked facilitation. In MG, activation for 30–60 seconds is used to unmask a mild decrement. In LEM, activation is used to demonstrate facilitation; the simplest procedure is to elicit a single CMAP from the rested muscle, have the patient contract the muscle for 10 seconds, then immediately elicit a single CMAP. Facilitation of at least 60% is evidence of a pre-synaptic disorder, but may not be seen in all muscles. A decrement in at least one hand muscle is the most frequent RNS abnormality in LEM and may be seen even when CMAPs are not small and diagnostic facilitation cannot be demonstrated. In MG the decrement is usually greatest in the 4th or 5th response and is less during subsequent responses, producing a so-called ''U-shaped" pattern. A continually progressive decrement is usually seen in LEM. The pattern in MuSK MG resembles that in LEM.