Analyzing clinical characteristics of patients with myasthenia gravis through repetitive nerve stimulation testing

Background The diagnosis of Myasthenia gravis (MG) is confirmed by clinical features with the presence of MG‐specific antibodies or the effect of intravenous edrophonium (Tensilon) administration. Diagnosis of MG is difficult in cases with negative serologic and Tensilon testing. Electrophysiological tests can support the diagnosis in such cases. Repetitive nerve stimulation testing (RNST) is one such electrophysiological test, and it is usually performed on several muscles. Aim The probability of RNST‐positivity varies between muscles, so we aimed to elucidate the relationship between muscle‐specific RNST‐positivity and patients’ clinical characteristics. Methods We included 46 patients and examined RNST results for the abductor digiti minimi (ADM), trapezius, and frontalis muscles. We classified muscles as RNST‐positive or RNST‐negative and compared RNST‐positive and RNST‐negative muscles for various clinical characteristics. Results We observed RNST‐positivity in 11 out of 40 (27.5%) ADM muscles, 20 out of 35 (57.1%) trapezius muscles, and 14 out of 29 (48.3%) frontalis muscles. The trapezius muscle's RNST‐positivity rate was the highest in the 21 patients who underwent RNST for all 3 muscles and was significantly higher than the ADM's RNST‐positivity rate. RNST‐positivity in any muscle was significantly associated with higher serum antiacetylcholine receptor antibody (AChR‐Ab) titers. Frontalis muscle RNST‐positivity was significantly associated with higher Quantitative Myasthenia Gravis scores. Conclusion When diagnosing MG, physicians should recognize that the trapezius has the highest RNST‐positivity rate and that patients with low AChR‐Ab titers may exhibit negative RNST results. Furthermore, patients with frontalis muscle RNST‐positivity may present severe symptoms.