Exogeneous factors in the aetiology of multiple sclerosis.

Neuroepidemiology has undoubtedly played a fundamental role in the study of multiple sclerosis (MS) by providing some aetiologic clues, although a definitive basis for the conclusive resolution of its enigma is still lacking. Epidemiological and genetic studies have indicated that MS is probably caused by multiple factors, both genetic and environmental, none of which is individually sufficient, which appear to act before adolescence - or possibly later - in genetically susceptible individuals. This unifying hypothesis emphasizes, on the one hand, the role of a genetic-racial susceptibility and the importance of environmental factors and, on the other, a possible aetiologic heterogeneity and lack of specificity of the unknown endogenous and exogeneous agents. In this context, several environmental factors may be involved in the aetiopathogenesis of MS in individuals who are susceptible to the effect of exposure to these factors. Situations or events with biological plausibility, such as childhood or adolescent infectious diseases, exposures to geographic and socio-cultural factors, nutritional habits, hypersensitivity, significant head and spinal trauma, and other factors may contribute, at different times, to the putative acquisition of MS, trigger its onset, and modify its subsequent course. However, additional empirical evidence is needed to clarify the complex interplay of genetic and environmental factors.