Analysis of decrement patterns in distal and proximal muscles on repetitive nerve stimulation test in myasthenia gravis

SUMMARY Introduction: Myasthenia gravis (MG) is an autoimmune disorder causing impaired neuromuscular transmission. One of the most commonly used diagnostic methods is the repetitive nerve stimulation (RNS) test. This study investigated the sensitivity of RNS in different muscle groups, focusing particularly on the diagnostic contribution of the anterior digastric and extensor indicis proprius (EIP) muscles, which have been rarely studied in the literature. Materials and Methods: Between January 1, 2024, and July 1, 2025, 154 patients referred to the EMG laboratory of Bezmialem Vakif University with suspected MG were evaluated. Forty-three patients were diagnosed with MG. Neurological examination, antibody testing, thymic imaging, MGFA classification, and MGCS scoring were performed. RNS was applied to orbicularis oculi (OOC), nasalis, anterior digastric, EIP, abductor digiti minimi (ADM), and abductor pollicis brevis (APB) muscles. Single-fiber EMG was performed in selected RNS-negative cases. Results: The highest positivity rate in RNS was observed in the OOC muscle, followed by the nasalis and anterior digastric muscles. Among distal muscles, the most prominent decrement was found in the EIP muscle. In some patients negative for OOC and nasalis, the anterior digastric muscle yielded positive results, highlighting its diagnostic value. RNS-positive patients had higher MGCS scores and were in more advanced MGFA stages. A correlation was observed between antibody positivity and facial muscle decrements, and RNS positivity was more frequent in patients with thymoma or thymic hyperplasia. Conclusion: While OOC muscle showed the highest sensitivity in RNS for MG diagnosis, anterior digastric and EIP muscles provided significant additional diagnostic yield. Including these muscles may improve sensitivity, especially in cases negative in classical muscle testing. The association between RNS positivity and clinical severity suggests prognostic as well as diagnostic value. Larger multicenter studies are needed to determine whether routine evaluation of these muscles should be recommended. Keywords: Myasthenia gravis, repetitive nerve stimulation, decrement, anterior digastric muscle, extensor indicis proprius muscle