Optimal Strategy for Repetitive Nerve Stimulation in the Diagnosis of Myasthenia Gravis

Abstract

Aim: 

This study aimed to evaluate the impact of various technical factors on the diagnostic utility of repetitive nerve stimulation (RNS) and propose a standardized electrodiagnostic protocol for myasthenia gravis (MG).

Materials and Methods 

RNS recordings from 67 patients with MG, who demonstrated a ≥10% decrement in the abductor digiti minimi (ADM), nasalis, or orbicularis oculi (OOc) muscles, were retrospectively analyzed.

Results: 

Abnormal RNS responses were detected in the nasalis muscle in 59 patients (88.1%) and in the ADM muscle in 27 patients (40.3%). In six (9%) patients, additional recordings from the OOc muscle were required to confirm the diagnosis. In two patients (3%), an abnormal decrement was observed exclusively in the ADM muscle. Stimulation at 3 Hz and 5 Hz elicited decrements more effectively than 2 Hz; however, no significant difference was found between 3 Hz and 5 Hz trains. The highest sensitivity was achieved with post-exercise (PE) recordings at 3 and 4 min, yielding 35.8% at both time points for the ADM muscle and 78.7% and 70.2% for the nasalis muscle, respectively. Moreover, when 3-Hz RNS recordings obtained at baseline and at 3 and 4 min PE from all three muscles were analyzed together, abnormal RNS responses were identified in 65 of 67 patients (97%).

Conclusion: 

To improve diagnostic yield, increasing the number of muscles examined – particularly facial muscles – appears to be a key strategy. Accordingly, we recommend performing 3-Hz stimulation trains for each muscle, including one at baseline and two additional trains at 3 and 4 min following exercise.