Clinicopathological profiling and prognosis of idiopathic inflammatory myopathy overlapping with myasthenia gravis: a retrospective cohort study.

Background: The clinicopathological spectrum of idiopathic inflammatory myopathy (IIM) overlapping with myasthenia gravis (MG) remains poorly defined. This study aimed to investigate the clinicopathological characteristics and outcomes of patients with IIM-MG to establish early diagnostic clues for the timely recognition of this unique subgroup and optimize therapeutic strategies.

Methods: The clinical, serological, and histopathological data of 682 consecutive patients with IIM diagnosed between 2016 and 2024 were retrospectively analyzed. Seven (1.03%) IIM-MG cases were identified.

Results: Six patients exhibited simultaneous onset of IIM and MG, while one developed MG 2 years post-IIM diagnosis. Two patients were classified as dermatomyositis, and the other five as necrotizing autoimmune myopathy. Notably, four patients experienced dyspnea without interstitial lung disease, of whom three required mechanical ventilation. Three patients had thymomas, two had ocular symptoms and only one had myocarditis. The mean creatine kinase (CK) level was 887.69 ± 247.37 U/L. None of the patients tested positive for myositis-specific antibodies (MSAs), while all were positive for the anti-acetylcholine receptor antibody (AChR-Ab). Muscle biopsies showed CD3+ T cell infiltration and major histocompatibility complex-I expression in all cases. All patients were treated with glucocorticoids plus other immunotherapies. Five patients showed marked improvement, of whom one experienced relapse, and two patients died within 3 months due to severe infection.

Conclusions: IIM-MG is a rare overlap syndrome characterized by dyspnea, mild CK elevation, anti-AChR-Ab seropositivity, and T-cell-mediated damage in muscle. Early MG screening in MSA-negative patients with IIM is critical. Although immunotherapy shows efficacy, infection-related mortality remains a concern.