Increasing frequency of multiple sclerosis in Padova, Italy: a 30 year epidemiological survey

Since the probability of developing or dying from most neurologic disorders is relatively small, the Poisson distribution is often utilized to establish confidence intervals around rates or ratios for diseases of the nervous system. This report describes a simplified method and provides a table of factors based on the Poisson distribution for calculating confidence intervals around estimates of rates and ratios derived from neuroepidemiologic studies.

Epidemiological studies of multiple sclerosis (MS) have shown the importance of genetic susceptibility factors that are modified by as yet unknown environmental influences. The often-cited interrelation between prevalence and latitude is no longer viable, with the important but unexplained exception of Australia and New Zealand. The standardization and increasing use of uniform diagnostic criteria lend new credibility to the results of MS surveys. More precise criteria are offered for symptoms of disease onset, Devic's syndrome, and progressive disease. The role of magnetic resonance imaging in the diagnosis of MS, and the pitfalls of its abuse, are reviewed. Reports of epidemics of MS are examined and discarded because they are based on the biologically meaningless date of diagnosis or that of onset, rather than when MS was probably acquired, that is, before puberty. The concept of onset-adjusted prevalence suggests that patients who are symptomatic should be included retrospectively in epidemiological surveys, even before they have been formally diagnosed as having MS, but individuals who were symptomatic before moving to a study area should not. The importance of ethnic homogeneity of patients and control subjects cannot be overemphasized in prevalence and risk factor studies. In the latter, close attention must be paid to biological plausibility and to prudent statistical interpretation. The hypothesis of the MS trait describes a systemic, asymptomatic condition that does not affect the nervous system, and may explain the low concordance of the disease in monozygotic twins.

To verify incidence rates and their temporal trend in a homogeneous, ethnically, and genetically distinct population of central Sardinia (the Nuoro province). Intensive epidemiologic studies carried out in Sardinia since the 1970s have suggested that the prevalence and incidence of MS are much higher in this Mediterranean island compared with those found on mainland Italy. The study area had a population of approximately 274, 000 people in the 1991 census. The authors adopted a complete enumerative approach by reviewing all possible sources of case collection available in the investigative area. Based on 469 MS patients, the mean annual incidence for 1955 to 1995 was 4.18 per 100,000 (or 4.3 per 100,000 if age- and sex-adjusted to the European population). The incidence, averaging 1.95 per 100,000 during 1955 to 1959, rose progressively over time, reaching rates of 6.6 in the quinquiennium 1985 to 1989 and 6.4 per 100,000 in 1990 to 1995. On December 31, 1994, the crude prevalence, based on 415 MS patients alive in the study area, was 151.9 per 100,000 (156.6 if adjusted to the European population). These incidence and prevalence rates are the highest to date that have been estimated for a large community in southern Europe, and they constitute some of the highest rates in the world. Based on other surveys, these results reinforce the position of Sardinia as a higher and rising prevalence area for MS compared with other Mediterranean populations. Genetic and social-historic data strengthen the hypothesis of the environmental role and genetic factors among Sardinians in determining the notable difference in MS frequency between Sardinians and other Mediterraneans.