Incidence and prevalence of multiple sclerosis during eras of evolving diagnostic criteria—a nationwide population-based registry study over five decades

Abstract

<p><strong>Background: </strong>Impact of changing diagnostic criteria for the population-based incidence of multiple sclerosis (MS) has not been investigated.</p><p><strong>Objective: </strong>To assess the effect of changing diagnostic criteria on national MS incidence and prevalence in Finland from 1974 to 2021.</p><p><strong>Methods: </strong>We identified patients with MS (pwMS) through the National MS registry and the national Care Register for Healthcare and divided them into four groups based on the year of MS diagnosis: 1) Schumacher criteria (1974-1982), 2) Poser criteria (1983-2000), 3) Earlier McDonald criteria (2001-2016), and 4) Current McDonald criteria (2017-2021). Age-adjusted incidence and prevalence were calculated.</p><p><strong>Results: </strong>Age-adjusted incidence per 10⁵ person years increased from 3.7 (95% CI 3.5-3.8) during the Schumacher criteria period to 9.2 (95% CI 9.0-9.4) during the earlier McDonald criteria. During the Current McDonald criteria incidence stabilized to 8.6 (95% CI 8.3-9.0). Prevalence increased from 24.3 (95% CI 22.8-25.8) to 241.5 (95% CI 237.3-245.6) per 10⁵ person years.</p><p><strong>Conclusion: </strong>Both incidence and prevalence of MS increased significantly. Incidence showed a sharp increase when entering the twenty-first century, after which it stabilized. Increasing incidence was likely related to incorporation of MRI in the diagnostic criteria. Current diagnostic criteria did not further increase the incidence.</p>