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

Authors


Correspondence: D. Gaist, Department of Neurology, Odense University Hospital, Sdr Boulevard 29, 5000 Odense C, Denmark (tel.: +45 6541 2485; fax: +45 6541 3389; e-mail: dgaist@health.sdu.dk).

Abstract

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.

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