Changes in Myasthenia Gravis Incidence in Denmark From 1985 to 2021

BACKGROUND AND OBJECTIVES: Previous studies on incident myasthenia gravis (MG) have shown inconsistent results. In this study, we determined the MG incidence rates (IRs) over a 36-year period to detect potential trends in MG incidence over time and for different age groups. METHODS: In this nationwide population-based study from 1985 to 2021, we identified patients aged 18 and older with a primary MG diagnosis in the Danish National Patient Registry. The patients were categorized into 3 distinct groups defined by age at onset: early-onset (≤50 years of age), late-onset (>50 and ≤65 years of age), and very late-onset (>65 years of age). MG IRs were calculated and stratified by sex and time periods in the specified age groups. To increase the validity of the MG IRs, we performed sensitivity analyses from 1995 until 2021, adding ≥2 prescriptions of pyridostigmine and/or acetylcholine receptor seropositivity. RESULTS: We identified 2,110 patients with MG (1,060 women), with a median age of 57.1 (interquartile range IQR 40.2-74.0) years for women and 63.6 (IQR 56.4-74.3) years for men. In the study period, the MG IR was 1.4 (95% CI 1.3-1.4) per 100,000 person-years, which persisted in our sensitivity analyses, with no significant differences between the sexes. Overall, 579 patients (27.4%) were classified as early-onset, 490 (23.2%) as late-onset, and 1,040 (49.3%) as very late-onset. For early-onset MG, the IR was 0.7 (95% CI 0.6-0.7) and remained stable throughout the study period. Inversely, the IRs for late-onset and very late-onset MG increased over time, with corresponding incidence rate ratios (IRRs) of 1.9 (95% CI 1.3-2.6) and 2.2 (95% CI 1.7-2.9), from the first 5 years from 1985 to 1990 until the last 6 years of the study period from 2015 to 2021. DISCUSSION: The overall IR of MG increased from 1985 to 2021, aligning with recent reports from neighboring European countries. The most significant rise occurred during the last 6 years of the study period compared with the first 5 years, with an IRR reaching 1.9 (95% CI 1.6-2.3). This increase in IRs was observed only in the late-onset and very late-onset age groups.