Temporal trends in multiple sclerosis prevalence and incidence in a large population.

Objective: We sought to better understand the reasons for increasing prevalence of multiple sclerosis (MS) by studying prevalence in relation to incidence, mortality rates, sex ratio, and geographic distribution of cases.

Methods: We identified MS cases from 1996 to 2013 in Ontario, Canada, by applying a validated algorithm to health administrative data. We calculated age- and sex-standardized prevalence and incidence rates for the province and by census division. Incidence and prevalence sex ratios for women to men were computed.

Results: The prevalence of MS increased by 69% from 1.57 (95% confidence interval [CI]: 1.54-1.59) per 1,000 in 1996 (n = 12,155) to 2.65 (95% CI: 2.62-2.68) in 2013 (n = 28,192). Incidence remained relatively stable except for a spike in 2010, followed by a subsequent decline in 2011-2013, particularly among young people and men. Mortality decreased by 33% from 26.7 (95% CI: 23.5-30.3) per 1,000 to 18.0 (95% CI: 16.4-19.8) per 1,000. The incidence sex ratio was stable from 1996 to 2009, then declined in 2010, with partial rebound by 2013. MS prevalence and incidence showed no consistent association with latitude.

Conclusion: In this large, population-based MS cohort, we found stable incidence and increasing prevalence of MS; the latter largely reflected declining mortality. A spike in incidence in 2010 among younger patients and men at a time of widespread media coverage of MS suggests that these groups may be vulnerable to delayed diagnosis. We did not find a latitudinal gradient; however, most Ontarians live between the 42nd and 46th parallels, reducing our ability to detect an effect of latitude.