BACKGROUND AND PURPOSE: Few data are available on the course of myasthenia gravis (MG) regarding disease severity and stability over time in real-world settings. This study used the French National Health Insurance Database (SNDS) to assess markers of disease severity in patients with MG longitudinally. METHODS: All patients with MG-related claims in the SNDS between 2013 and 2020 were identified. Patients were followed for up to 8 years after the first claim. Intensive care unit (ICU) stays, treatment with intravenous immunoglobulin (IVIg) or plasma exchange (PE), and death were documented throughout the follow-up period. Standardized mortality rates were estimated, and mortality-related variables were identified using a Cox model. RESULTS: In all, 14,459 individuals constituted the full study population, including 6354 incident patients. In the incident population, 2199 (34.6%) were admitted to ICUs at least once, principally during the first year after the index date (N = 1477
23.3%). This proportion decreased progressively to reach 3.0% in the seventh year. A total of 2817 patients received IVIg and 432 PE, again principally in the first year. In the full study population, the standardized mortality rate was 1.08 (95% confidence interval [CI] 1.03-1.13), being lower in men (0.95, 95% CI 0.89-1.02) than in women (1.15, 95% CI 1.07-1.23) and in patients aged >
65 years (1.06, 95% CI 1.01-1.11) than in younger patients (1.50, 95% CI 1.24-1.76). Male gender, older age and higher comorbidity were independently associated with mortality. CONCLUSIONS: A subgroup of patients with MG require ICU admission and rescue therapy with IVIg or PE, indicative of poor disease control. New therapies are needed to improve disease control and reduce disease burden.