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

The relationship between migration and the age at onset of multiple sclerosis (MS) is examined for 246 patients born in North Africa and French-born controls matched in a ration of 3:1 patients born in North Africa. The 246 migrants arrived between 1960 and 1965 or during the Algerian war of independence. 65% were from Algeria and 35% came from Morocco and Tunisia. A sample of 152 migrants showed that 85.5% were of European origin and 14.5% were Arabs or Berbers. The results show that for 89% of migrants the onset of disease appeared after migration (mean interval between migration and onset of 15 years). 11% reported MS either in the process of migration or before. The mean interval between migration and onset of MS was about 10 years for patients older than 20 years. There was no difference in mean time interval for those 10-14 years and 15-19 years (pre- and postpuberty). The comparison of migrants with their French counterparts by age and sex showed more self-reports of disability for migrants; 28% of migrants needed walking aids 28% were restricted to a wheelchair and 44% had no walking handicap. The French controls showed 28% with walking aids 22% in wheelchairs and 49% unaffected. Other differences were among patients older than 30 years where mean age of MS onset was significantly older in migrants (45.5 years) than controls (36.6 years) at p < .001. The explanation is that 48 patients had migrated when older than 30 years of which 21 had MS before migration or before 30 years and had been excluded in the migration group but not the control group. When all 48 MS migrant patients are compared with the corresponding French controls the mean age at disease onset is similar (37.0 years for migrants vs. 36.6 years for controls). Avoidance of erroneous interpretation must take into account age at migration age at onset of disease and country of residence at time of disease onset. Bias also enters in through the reluctance of diseased patients to migrate. Another caveat is the migrants tend to have limited access to the health care system. In the present study biases are limited. The implication is that age of MS onsent is independent of place of birth and migration if the environmental risk due to promoting factors is the same in both places. Because the prior generational genetic background of North African migrants was French the similarities do not reflect distinct genetic factors.