Introduction

Although epidemiology has undoubtedly played a fundamental role in the study of multiple sclerosis (MS) by providing some etiologic clues, a definitive basis for the conclusive resolution of its enigma is still lacking. In fact, despite considerable scientific effort and innumerable descriptive surveys, much of the variation in the distribution of MS found in different parts of the world and at different times may reflect, at least in part, methodologic differences in surveys, especially in case ascertainment and selection. Nevertheless, the scientific data that have emerged from the epidemiologic studies with more rigorous methodology have provided us with some consistent findings.

Well-conducted descriptive studies have shown that MS has an uneven geographic distribution and a changing incidence over time, at least in several areas of the world. A predilection for Caucasians, often dissimilar in their ethnic and cultural histories, together with variations in frequency among and within countries, strongly suggest an etiologic role for both genetic and environmental factors.

The importance of racial and ethnic factors has emerged from prevalence surveys and has been supported by incidence studies. The disease is more frequent in most countries of northern Europe and, as Davenport observed in 1922, the distribution in North American may reflect the pattern of migration and settlement from Scandinavian countries. Conversely, MS is very rare among Africans and Asians and seems to be less common among the white people of Southern and Central America than other Caucasian populations. It is also clear that the disease is more frequent among Caucasians than in other ethnic groups living in the same geographic areas.

The varying susceptibility to the disease according to race appears to emphasize the importance of genetic factors, which has been confirmed by the increased risk of MS in close blood relatives and in monozygotic twins of patients with …