Checkpoint: RNS and Blink Reflex: Deeper Understanding for the NCS Technologist-Dr. Sankar Bandyopadhyay Presentation

Dr. Sankar Bandyopadhyay Presentation

Pdf Summary

This presentation reviews repetitive nerve stimulation (RNS) for evaluating neuromuscular junction disorders, focusing on technique, interpretation, and common pitfalls. RNS is used primarily to assess myasthenia gravis (MG), Lambert-Eaton myasthenic syndrome (LEMS), and botulism. Reported sensitivity for MG is about 70% overall and 50% in ocular MG.<br /><br />Testing may involve the ulnar nerve with the abductor digiti minimi or first dorsal interosseous, spinal accessory nerve with trapezius, facial nerve with nasalis, or trigeminal nerve with masseter. Key technical steps include considering withholding pyridostigmine, warming the limb, immobilizing the limb and recording muscle, and using supramaximal stimulation. Slow RNS is generally performed at 2–3 Hz. A one-minute isometric exercise or high-frequency stimulation can assess post-tetanic exhaustion; a 10-second exercise can test for post-exercise facilitation.<br /><br />A decrement is typically defined as a 10% or greater reduction in compound muscle action potential (CMAP) amplitude or area. MG commonly produces a decrement during slow stimulation or after prolonged exercise, with recovery or only mild facilitation after brief exercise. LEMS often shows a low baseline CMAP, decrement, and marked post-exercise facilitation. Botulism may also produce a low baseline CMAP and facilitation, though findings vary; facilitation may persist for several minutes and may be absent in severe cases. Normal studies generally show no decrement.<br /><br />Common sources of error include acetylcholinesterase inhibitors, inadequate warming, selecting an unsuitable muscle, failure to achieve supramaximal stimulation, and inadequate immobilization. These factors can compromise test reliability and interpretation.

Keywords

repetitive nerve stimulation

neuromuscular junction disorders

myasthenia gravis

Lambert-Eaton myasthenic syndrome

botulism

compound muscle action potential

post-exercise facilitation

supramaximal stimulation

decrement response

electromyography

EMG

post-tetanic exhaustion

electrodiagnostic testing