Late-onset myasthenia not on the increase: a nationwide register study in Denmark, 1996–2009 E. G. Pedersen a,b , J. Hallas c , K. Hansen d , P. E. H. Jensen e and D. Gaist a,b a Department of Neurology, Odense University Hospital, Odense; b Institute of Clinical Research, Faculty of Health Sciences, University of Southern Denmark (SDU), Odense; c Institute of Public Health, Clinical Pharmacology Unit, SDU, Odense; d Department of Neurology, Rigshospitalet, Copenhagen University Hospital, Copenhagen; and e Neuroimmunology Laboratory, DMSC, Department of Neurology, Rigshospitalet, Copenhagen University Hospital, Copenhagen Denmark Keywords: epidemiology, incidence, myasthenia Received 25 May 2012 Accepted 5 July 2012 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 nation- wide registers. Patients with myasthenia were classified into early onset (<50 years old) and late onset (50+ years). We calculated incidence rates (IRs) and correspond- ing 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 the early-onset group (70.5%), but not 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 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 compared with women, and occurred with a stable incidence in the 14-year study period. Therefore, we speculate whether previ- ous reports of a rise in late-onset MG reflected a non-biological phenomenon, that is, a gradual improvement in the diagnosis of MG in this age group in previous years. Introduction Acquired myasthenia gravis (MG) is an autoimmune disorder involving the neuromuscular junction that causes weakness and fatiguability. Acetylcholine receptor antibodies (AChRab) can be measured in roughly 85% of patients with MG, although antibody positivity varies by MG type ranging from 85% in generalized MG to 50% in ocular MG [1]. A distinc- tion is made between early- and late-onset MG owing to some variation of characteristics of patients in these groups. Late-onset MG has been reported to be on the rise in recent studies [2–6]. This rise in late-onset MG incidence was believed to represent a true biolog- ical phenomenon. However, previous underdiagnosis of MG in older people may be an alternative explana- tion for these findings [7]. We speculated that more recent data on MG incidence trends might shed more light on the issue, especially if derived from a setting where underdiagnosis is less likely to occur. In Den- mark, access to the tax-financed healthcare system is independent of income, and the AChRab test has been widely adopted in clinical practice over the past three decades and can be ordered not only by special- ists but also by general practitioners. Also, we believe that the diagnostic awareness of MG in the elderly has improved in Denmark. Therefore, we conducted the present nationwide study using population-based registers in Denmark to investigate recent trends in late-onset MG incidence. Correspondence: D. Gaist, Department of Neurology, Odense Uni- versity Hospital, Sdr Boulevard 29, 5000 Odense C, Denmark (tel.: +45 6541 2485; fax: +45 6541 3389; e-mail: dgaist@health.sdu.dk). © 2012 The Author(s) European Journal of Neurology © 2012 EFNS 1 European Journal of Neurology 2012 doi:10.1111/j.1468-1331.2012.03850.x