Increasing incidence of multiple sclerosis in women in Northern Finland

Background: The geographical distribution of multiple sclerosis (MS) means that prevalence rates increase with latitude north or south of the equator. Temporally, a tendency for increased incidences of MS has been observed over the past two decades. Objectives: Since epidemiological studies of MS in areas close to the Arctic Circle are rare, we evaluated the incidence and prevalence of MS in Northern Ostrobothnia by means of a retrospective cohort study covering the period 1992-2007. Methods: Patients with a definite clinical diagnosis of MS based on the Poser criteria and the early McDonald criteria of 2001 were identified in the region of Northern Ostrobothnia (population 386,972) and the incidence was calculated at 1-year time intervals, both overall and by gender. Results: The overall prevalence was 103/100,000 (95% CI, 93-113), with a female/male ratio of 2.17. The mean overall incidence was 6.3/100,000 (95% CI, 5.2-7.2). The incidence shows a tendency to increase over the 16-year period due to a pronounced rise in the female incidence. Conclusions: Our results show a high prevalence of MS in Northern Ostrobothnia and a disproportional increase in the female MS incidence. These recent epidemiological features may be associated with environmental risk factors such as a vitamin D deficit, low life-long UV radiation and the high-latitude geographical location.