Late‐onset myasthenia not on the increase: a nationwide register study in Denmark, 1996–2009

BACKGROUND: An increase in late-onset myasthenia gravis (MG) has been reported. There are few large population-based studies over longer periods of time reflecting recent developments in MG incidence. METHODS: We identified a nationwide cohort of patients with incident myasthenia in Denmark in 1996-2009. We used a validated algorithm to track subjects based on a combination of diagnosis and prescription (pyridostigmine) data from nationwide registers. Patients with myasthenia were classified into early onset (<50 years old) and late onset (50+ years). We calculated incidence rates (IRs) and corresponding 95% confidence intervals. RESULTS: We identified 693 patients (362 women) with incident MG in the study period corresponding to an IR of 9.2 per million person-years (8.5-9.9). Overall, 207 (29.9%) were classified as early-onset and 486 (70.1%) as late-onset MG. Women predominated in the early-onset group (70.5%), but not in the late-onset group (44.4%). The incidence rate of early-onset MG was 4.2 (3.6-4.8) and late-onset MG 18.9 (17.3-20.7) per million person-years and it did not vary over time in the study period (P-values for trend 0.54 and 0.15, respectively). CONCLUSION: Late-onset MG comprised a large proportion of all incident cases in Denmark, was more common in men than women, and occurred with a stable incidence in the 14-year study period. Therefore, we speculate whether previous reports of a rise in late-onset MG reflect a non-biological phenomenon, that is, a gradual improvement in the diagnosis of MG in this age group in previous years.