Exposure of the eyes to near-horizon sunshine may be a trigger for multiple sclerosis

Multiple sclerosis (MS) incidence is higher among those who live at high latitudes before adulthood. This is usually attributed to lower levels of Vitamin D, caused by lower UV levels. However direct damage of the optic nerve by near-horizon sunshine is a possible alternative explanation.

Historical reports of MS from European populations in well characterised geographic locations where the numbers of cases and the target population were reported were collected, and the distribution of MS prevalence was calculated. Total UV, visible and infra-red exposure over a year as a function of latitude, and the fraction of time the Sun spends near the horizon as a function of latitude were calculated from geometric considerations, and were compared with the observed prevalence of MS.

The observed distribution of MS prevalence fits well with the relative time that the Sun spends within 3 degrees and 8 degrees of the horizon, as calculated geometrically and summed over a year. Correlation with total UV exposure (without consideration of weather or shielding by clothing or buildings) was less satisfactory.

I suggest that direct solar damage to the optic nerve may be a trigger for MS.