Clinical Features and Outcome in Myasthenia Gravis Patients with Antimuscle-specific Tyrosine Kinase Antibody: A Retrospective Study.

Myasthenia gravis (MG) patients with anti-MuSK (muscle-specific tyrosine kinase) antibody (MuSK-MG) are uncommon. Compare with MG patient with anti-ACh (acetylcholine) receptor antibody (AChR-MG), MuSK-MG exhibit unique features. The aim of this study is to analyze the difference of the clinical characteristics and electrophysiological features between MuSK-MG and AChR-MG. We retrospectively review the medical records of generalized MG patients from 2004 to 2019 at Shin Kong Memorial Wu Ho-Su Hospital. We enrolled 47 MuSK-MG patients (33 females and 14 males), and 48 AChR-MG patient (23 females and 25 males). MuSK-MG patients have a female predominance (70.2% vs. 47.9%, P = 0.027) and no significant difference to AChR-MG in onset age (44.02 ± 15.06 vs. 47.52 ± 15.85, P = 0.273). MuSK-MG are more likely to involve facial (76.6% vs. 16.7%, P < 0.001), bulbar (100% vs. 50%, P < 0.001), neck (55.3% vs. 22.9%, P = 0.001), and respiratory muscles (61.7% vs. 10.4%, P < 0.001). MuSK-MG tent to experience more severe symptoms with MGFA Class III or greater (72.3% vs. 50.0%, P = 0.032). The thymic pathology in MuSK-MG is likely to be normal (55.3% vs. 18.8%) or thymic hyperplasia (44.7% vs. 25%). The overall positive rate for repetitive nerve stimulation (RNS) in MuSK-MG patients is less than AChR-MG patients (66.0% vs. 85.4%, P = 0.027). Further analysis shows the difference is mainly by recording from trapezius muscles (51.1% vs. 79.2%, P = 0.004), whereas there is no difference in nasalis muscles (60% vs. 67.6%, P = 0.544) and abductor digiti minimi muscles (13% vs. 18.4%, P = 0.582). The positive rate for single fiber electromyography (SFEMG) recording from orbicularis oculi is very high in both MuSK-MG and AChR-MG (100% vs. 97.2%, P = 0.289). Both MuSK-MG and AChR-MG patients achieved good outcome after proper treatment (68.1% vs. 75%, P = 0.4455). MuSK-MG patients have a female predominance and more facial, bulbar, neck, and respiratory muscles involvements. The electrodiagnostic features of MuSK-MG are lower positive rate of RNS at trapezius and high positive rate of SFEMG at facial muscles. Although MuSK-MG patients are associated with a more progressive course, proper diagnosis and treatment still can lead to a favorable outcome.