Month of birth in multiple sclerosis with and without longitudinally extensive spinal cord lesions: a study of a Japanese national survey.

OBJECTIVES Month of birth has been associated with the environmental factors for multiple sclerosis (MS). This study aimed to investigate whether individuals with MS had significantly different frequencies of birth in a particular month of the year, and whether month-of-birth patterns were influenced by the longitudinally extensive spinal cord lesions (LESCL) status relative to the general population in the same Japanese birth cohort. METHODS In this study, performed as a part of the fourth Japanese nationwide survey of MS, patients were divided into two groups according to the presence (n=307) or absence (n=906) of LESCLs that were diagnosed on the basis of magnetic resonance imaging findings. The number of births in every month was counted for both groups. Control data were obtained from birth records of the Japanese general population of the median years of birth of each group. Differences in the month-of-birth distributions between the patients and the general population were assessed using the chi-square test. RESULTS In MS patients without LESCLs, significantly more patients were born in January and June, whereas significantly fewer patients were born in May, compared to the general population. The seasonal patterns of birth were not in association with those of ambient ultraviolet (UV) radiation. No significant differences were found for the month-of-birth distributions between MS patients with LESCLs and the general population. CONCLUSIONS A different distribution of month of birth in Japanese MS patients without LESCLs from the general population, but not in those with LESCLs, suggests some role for environmental factors in the pathogenesis of the former group. Environmental factors other than UV radiation should be further elucidated in Japanese and other Asian MS cohorts.