Explaining multiple sclerosis prevalence by ultraviolet exposure: a geospatial analysis

BACKGROUND: Epidemiologic studies have shown a positive correlation of multiple sclerosis (MS) prevalence with latitude. However, there has not been a causal association found. Increased dietary intake and increased serum levels of vitamin D showed to be protective for the development of MS. Ultraviolet (UV) radiation plays an important role in vitamin D synthesis and could potentially explain both latitude differences in MS prevalence and the low levels of vitamin D in individuals with MS. OBJECTIVE: To evaluate the relationship between UV radiation and MS prevalence using geospatial analysis. METHODS: Geospatial analysis was performed on North American regions and separately for the continental United States. The correlation of UV radiation (measured as UV index [UVI]) versus MS prevalence and UV radiation versus case-control ratios was calculated. In addition, the relative risk (RR) of MS was determined for regions/states with low UV radiation exposure. RESULTS: Case-control ratios by US state and MS prevalence by North American region showed a strong negative (inverse) correlation with UVI (R = -0.72 and -0.86, respectively). The RR for the five highest risk states/lowest UVI versus the five lowest risk states/highest UVI was increased (RR = 1.8-5.4). The RR for MS, when comparing North American regions with lowest and highest UVI, was 3.78 and within US regions was 1.52. CONCLUSION: This analysis suggests a strong association between UV radiation and MS distribution, and an increase in risk for MS in those areas with a low UVI.