Temporal trends in the incidence of multiple sclerosis: a systematic review

Abstract

Background: Multiple sclerosis (MS) has been traditionally considered to be more frequent in women and in regions more distant from the equator. However, recent reports suggest that the latitude gradient could be disappearing and that the female-to-male ratio among patients with MS has increased in the last decades. We have conducted a systematic review of incidence studies of MS to assess the overall incidence of MS and explore possible changes in the latitude gradient and the female-to-male ratio over time.

Methods: Systematic review of incidence studies of MS published in Medline between 1966 and February 2007. Age- and sex-specific incidence rates were collected from eligible publications. We computed age-adjusted rates using the world population as standard, and assessed differences in rates according to latitude and period of case ascertainment. Additionally, we evaluated the association between period of case ascertainment and the female-to-male ratio.

Results: The overall incidence rate of MS was 3.6 cases per 100,000 person-years (95% CI 3.0, 4.2) in women and 2.0 (95% CI 1.5, 2.4) in men. Higher latitude was associated with higher MS incidence, though this latitude gradient was attenuated after 1980, apparently due to increased incidence of MS in lower latitudes. The female-to-male ratio in MS incidence increased over time, from an estimated 1.4 in 1955 to 2.3 in 2000.

Conclusion: The latitude gradient present in older incidence studies of multiple sclerosis (MS) is decreasing. The female-to-male MS ratio has increased in the last five decades.

Figure 1 Age-adjusted incidence rate of multiple sclerosis by latitude in 38 different incidence studies Each observation corresponds to the incidence in one study. Size of each observation is proportional to the inverse of its variance.

Figure 2 Sex (female-to-male) incidence rate ratio of multiple sclerosis by study year and region Study period was defined as the mean of the first and last year of the case ascertainment for that particular study. Only regions with information from three or more periods of time are included.