Infections and the risk of relapse in multiple sclerosis

Immune dysregulation, likely to be autoimmune and organ‐specific in essence, is a landmark in multiple sclerosis. Not surprisingly, physicians have always been concerned with a possible increase in the risk of multiple sclerosis relapse following an infection. Anecdotal reports have given consistency to this concern.Several epidemiological studies sharing similar features have already addressed this issue. These were observational, prospective studies. Risk periods were defined as encompassing the onset of the clinical infection with the assumption that if a relapse was observed during the risk period, it could be regarded as being associated with the infection. Control periods were made of the non at‐risk periods. The rate of relapses during the risk periods was systematically compared with that of control periods. In the pionneer study of Sibley et al . (1985), 170 patients with clinically definite multiple sclerosis were assessed at monthly intervals for a mean of 5.2 years regarding common viral infections and relapses. Risk periods were chosen as being the interval covering the 2 weeks prior infection to the 5 weeks afterwards. The rate of relapses was found to be 2.8‐fold higher during the at‐risk period by comparison with the control periods. This study provided strong evidence of an association between common viral infections and relapses in multiple sclerosis. However, in a more limited study of 60 patients with benign multiple sclerosis followed over a mean of 31 months (Andersen et al ., 1993), the relative risk of relapses during the risk period by comparison with control periods was only 1.3, i.e. marginally significant ( P = 0.047) when using a time‐window of 4 weeks for the at‐risk period. No significant association was found when using the 7‐week time‐window of Sibley et al. (1985). The results of the two other studies currently available deserve cautious interpretation (Panitch, 1994 …