Myopathic changes detected by quantitative electromyography in patients with MuSK and AChR positive myasthenia gravis: A single-center cross sectional study

Background and objectives. Myasthenia gravis (MG) is an autoimmune neuromuscular disorder with distinct serological subtypes that may exhibit different clinical presentations and electrophysiological patterns. This study aimed to assess subtle myopathic changes and their correlation with disease severity using quantitative electromyography (QEMG) among MG patients with different antibody profiles. Materials and methods. A cross-sectional observational study was conducted on 59 MG patients at Mansoura University Hospital. Patients were classified as AChR-positive (n = 44, 74.6%), MuSK-positive (n = 6, 10.2%), or double seronegative (n = 9, 15.3%). Clinical severity was evaluated using MGFA classification, MRC scale, MGCS, and MGADL. All patients underwent thymic CT, repetitive nerve stimulation, and QEMG of orbicularis oculi, biceps brachii, and tibialis anterior muscles using MULTIMUPS and CLOUD methods. Results. MuSK-positive patients showed the most severe phenotype, with higher MGCS scores (17.0 [16.2–20.0] vs. 12.0 [9.2–16.2] in AChR-positive and 10.0 [8.0–14.0] in seronegative, p = 0.0276) and more axial weakness (66.7% vs. 18.2%, p = 0.032). QEMG revealed significantly shorter motor unit potential durations and lower amplitudes in MuSK-positive patients across all tested muscles. In orbicularis oculi, amplitudes were lower (237.0 μV, p = 0.0332), with similar abnormalities in biceps brachii (duration p = 0.041) and tibialis anterior (duration p = 0.0161, amplitude p = 0.0382). CLOUD analysis demonstrated significant abnormalities in NSS/sec vs. activity (p = 0.003) and Amp/Turn (p = 0.012). Strong correlations were found between QEMG parameters and clinical severity, particularly tibialis anterior duration with MRC score (r = 0.511, p < 0.0001) and MGCS score (r = −0.546, p < 0.0001). Conclusions. MuSK-positive patients exhibit more severe clinical and electrophysiological abnormalities than other subtypes. QEMG correlates strongly with validated clinical severity scales and may serve as an objective biomarker for disease assessment and monitoring, supporting its potential role in guiding tailored therapeutic strategies.