Rate of Autoimmune Diseases in the 10 Years Preceding Myasthenia Gravis Diagnosis: A Nationwide Cohort Study in South Korea

BACKGROUND AND OBJECTIVES: Patients with myasthenia gravis (MG) are frequently associated with other autoimmune diseases. However, the temporal relationship between autoimmune diseases and MG remains unclear. This study aimed to evaluate the rates and temporal patterns of autoimmune diseases during the 10 years preceding MG diagnosis. METHODS: This retrospective population-based cohort study analyzed claims data from the Korean National Health Insurance Service between 2010 and 2021 and included individuals aged ≥20 years. MG was defined using the International Classification of Diseases, 10th Revision (ICD-10) code (G70.0) and Rare Intractable Disease registration (V012). Each MG case was matched with 10 controls by age, sex, and index year. Outcomes were autoimmune thyroid disease (AITD), systemic lupus erythematosus (SLE), seropositive rheumatoid arthritis (SRA), Sjögren syndrome (SjS), psoriasis, type 1 diabetes mellitus (T1DM), Crohn disease, and ulcerative colitis occurring during the 10 years before the index year. Rate ratios (RRs) and 95% CIs were estimated using Poisson regression models for 0-2, 2-5, and 5-10 years before the index year. RESULTS: A total of 8,355 patients with MG and 83,550 matched controls were included. The mean age (SD) was 53.7 (16.0) years, and 44.1% were male. Patients with MG had a higher rate of any autoimmune disease during 10 years before diagnosis than controls (RR 2.12, 95% CI 1.97-2.28), with the highest rates in 0-2 years before diagnosis (RR 4.27, 95% CI 3.78-4.83). SLE (RR 4.87, 95% CI 2.71-8.77), SjS (RR 4.75, 95% CI 3.01-7.50), AITD (RR 3.66, 95% CI 3.29-4.07), and SRA (RR 2.45, 95% CI 1.89-3.19) showed the strongest associations. Psoriasis (RR 1.68, 95% CI 1.25-2.25) and T1DM (RR 2.03, 95% CI 1.52-2.71) were associated with MG only within 0-2 years before diagnosis, whereas Crohn disease and ulcerative colitis were not associated. DISCUSSION: Autoimmune diseases were more frequent in the years preceding MG diagnosis than in controls, particularly within 2 years before diagnosis. Strong associations were observed for SLE, SjS, AITD, and SRA. However, these findings should be interpreted with caution because of limitations of administrative claims data.