Incidence of late onset multiple sclerosis in Italy: a population-based study.

Late onset multiple sclerosis (LOMS) represents between 0.6 and 12% of all MS patients. However, little is known on the incidence of LOMS in the general population. Therefore, we aimed to study the annual incidence of LOMS in a population-based cohort. The study was conducted in the province of Catania, Italy. Case ascertainment was conducted retrospectively including all patients aged ≥ 50 years at onset and with the onset between 2005 and 2020. Incidence rates (IR) have been calculated for all the study period, according to sex, age classes and for subperiods. Incidence rate ratios (IRR) have been computed to compare incidence rates. During the study period, 183 patients with LOMS were identified (113 women; 61.8%). The mean age at onset was 55.8 ± 5.4 years and the main phenotype was Relapsing Remitting MS (n = 123; 67.2%). The average annual crude IR was 2.87/100,000 person-years (95% Confidence Intervals, CI 2.31-3.13). IR increased from 2.54/100,000 in 2005-2010 to 3.32/100,000 in 2016-2020, especially in in the age group 60-69 (IRR 3.48; 95%CI 1.41-9.76; p-value 0.002). In conclusion, an increased IR over the time was observed in the age-group 60-69, possibly reflecting an increased age at onset of MS.