Different Muscles, Lower Cutoff Values: Does it Truly Improve the Diagnostic Sensitivity of Repetitive Nerve Stimulation in Myasthenia Gravis?

Abstract

ABSTRACT Context: This study evaluated the diagnostic accuracy of repetitive nerve stimulation (RNS) compared with single-fiber electromyography (SF-EMG) in generalized myasthenia gravis (MG). Aims: The aim of the study was to determine the diagnostic sensitivity of RNS in various muscles and assess the effect of varying decrement cutoff values on diagnostic efficacy. Setting and Design: This was a prospective study of 34 confirmed generalized MG patients. Subjects and Methods: RNS was performed on five muscles. Clinical features, autoantibody presence, and SF-EMG jitter rates recorded from the frontalis muscle were analyzed. Patients were classified according to the MG Foundation of America classification. Statistical Analysis: SPSS version 23 was used for descriptive statistics, comparisons of categorical and quantitative data, and correlation analysis, with the significance level set at P < 0.05. Results: Among the patients (58.8% males and mean age = 59.47 ± 13.37 years), SF-EMG detected increased jitter in 91.2%, whereas RNS identified a significant decrease of 50% across at least one muscle. The decremental response was >10% in the abductor digiti minimi (ADM), trapezius, nasalis, abductor pollicis brevis, and extensor indicis proprius muscles in 5.9%, 26.5%, 29.4%, 11.8%, and 29.4% of patients, respectively. A sensitivity improvement at lower decrement values was observed only in the ADM muscle. A significant correlation was found between decreases in the trapezius muscle and seropositivity. Conclusions: Optimal muscle selection improves RNS diagnostic accuracy in MGs. The study recommends incorporating the spinal accessory and radial nerves in RNS due to their high sensitivity, although modifying the decrement cutoff value has a limited overall effect.