2025 Workshop Handout Bundle-W117 Mastering RNS NCS

W117 Mastering RNS NCS - Punga Eriksson

Pdf Summary

This article by Dr. Anna Punga and Dr. Annika Eriksson provides a comprehensive guide to mastering nerve conduction studies (NCS) and repetitive nerve stimulation (RNS), two critical neurophysiological techniques used in diagnosing neuromuscular disorders. 

<strong>Nerve Conduction Studies (NCS):</strong>  
NCS assess peripheral nerve function by electrically stimulating nerves and recording responses, measuring parameters such as conduction velocity, amplitude, and latency. Key technical factors include precise electrode placement—over motor endplates for motor studies—and maintaining limb temperature above 32°C to avoid variability. Supramaximal stimulation ensures all nerve fibers are activated, preventing false results. Sensory conduction studies require even greater care due to low signal amplitudes and susceptibility to noise and temperature effects. Clinical interpretation relies heavily on reference values adjusted for age, height, and sex, and it is important to consider both sensory and motor results together to differentiate true pathology from artifacts.

<strong>Repetitive Nerve Stimulation (RNS):</strong>  
RNS is a specialized motor NCS used to evaluate neuromuscular transmission, especially in conditions like myasthenia gravis (MG) and Lambert-Eaton Myasthenic Syndrome (LEMS). It involves delivering repeated stimuli (typically at 3 Hz) to a motor nerve and analyzing the resulting compound muscle action potentials (CMAPs) for characteristic decrements. Muscle selection is guided by clinical presentation, with the deltoid and trapezius muscles often providing high sensitivity in generalized MG. Supramaximal stimulation and consistent electrode positioning are essential to avoid false negatives. RNS interpretation depends on careful analysis of CMAP amplitude and area, checking for technical artifacts, and contextualizing findings with clinical symptoms. A decrement of 6–10% between the first and fourth responses typically indicates abnormal neuromuscular transmission.

<strong>Clinical Integration:</strong>  
Despite advances in antibody testing, RNS remains a first-line test in seronegative MG and, combined with NCS, is indispensable for distinguishing neuromuscular transmission disorders from neuropathies. The authors stress that examiner experience, thorough technique, and contextual interpretation are fundamental for reliable, clinically meaningful results.

In summary, adherence to meticulous methodology—accurate electrode placement, temperature control, supramaximal stimulation—and interpretation grounded in clinical context underpin successful and reliable use of NCS and RNS in diagnosing neuromuscular diseases.

Keywords

Nerve Conduction Studies

ncs

Repetitive Nerve Stimulation

Neuromuscular Disorders

Electrode Placement

Supramaximal Stimulation

Myasthenia Gravis

Lambert-Eaton Myasthenic Syndrome

Compound Muscle Action Potentials

Neuromuscular Transmission

Clinical Neurophysiology