Temperature dependence of multiple sclerosis mortality rates in the United States.

BACKGROUND: It is well known that multiple sclerosis (MS) patients are very sensitive to heat events. However, how MS patients respond to the significant temperature difference between the high- and low-latitude regions is not understood.

OBJECTIVE: The goal is to identify the primary factor responsible for the fact that MS mortality rates of the United States is more than three times higher in the northern states than in the southern states.

METHODS: Correlation coefficients between the age-adjusted mortality rate of MS as the underlying cause of death and the state average temperature, altitude, latitude, duration of sunshine hours, and solar radiation in the 48 contiguous states were compared.

RESULTS: MS mortality rates correlate significantly and inversely with temperatures in the 48 states (correlation coefficient r = -0.812 and significance p = 0.00). Durations of sunshine hours and solar radiation do not correlate significantly with MS mortality rates ( r = -0.245, -0.14, and p = 0.101, 0.342, respectively).

CONCLUSION: High environmental temperature is the primary reason for the low MS mortality rates and likely the low MS prevalence in low-latitude regions. Implication of the study result is that benefits of long-term heat acclimation through gradual and prolonged exposure to environmental heat for MS patients may be greatly underappreciated.