Migration and age at onset of multiple sclerosis: some pitfalls of migrant studies.

The aim of this study was to compare age at onset of multiple sclerosis (MS) in North African-born and French-born patients. The migrant group consisted of 246 patients who arrived in France during the period 1960-1965. Among these migrants, 27 (11%) had first symptoms before migration. The French-born group consisted of MS patients of same sex and age at the time of the study as migrants who were randomly selected from a large national sample. After controlling for various biases which could explain differences between migrants and French-born patients, we found no differences in mean age at MS onset between the two groups. Therefore, it is likely that MS was acquired by the same age in migrants as in French-born patients. This finding may constitute an indirect support for the hypothesis that the unknown causative factors of MS are equally frequent whatever the latitude of origin.