No seasonality in the risk of multiple sclerosis in an equatorial country: A case-control ecological study.

BACKGROUND:Multiple sclerosis risk has been shown to have seasonal variations that are more pronounced in higher latitudes. However, this phenomenon has not been adequately studied near the Equator. OBJECTIVE:To explore the risk of multiple sclerosis associated with month, season of birth, and sunlight exposure variables in Colombia. METHODS:In this case-control study, 668 multiple sclerosis cases were matched to 2672 controls by sex and age. Association of multiple sclerosis with each month/season of birth and sunlight exposure variables was estimated with multilevel mixed-effects logistic regression and ecological regression models, respectively. Seasonality in the births of multiple sclerosis was assessed with a non-parametric seasonality test. RESULTS:We found a higher probability of multiple sclerosis in September (0.25; 95% confidence interval (CI) = 0.21-0.31) and lower in March (0.15; 95% CI = 0.10-0.18), which turned non-significant after a multiple comparisons test. Sunlight exposure variables had no significant effect on the risk of MS, and the tests of seasonality in the births of MS did not show significant results. CONCLUSION:Our results show no seasonality in the risk of multiple sclerosis near the Equator, supporting the hypothesis that this phenomenon is latitude dependent.