Factors in the rising prevalence of multiple sclerosis in the north-east of Ireland.
Northern Ireland is recognized as an area of high risk for multiple sclerosis. The original study of Allison and Millar in 1951 found a prevalence of 51/100,000 and mean annual incidence of 2.74/100,000/year. Subsequent studies in 1961, 1986, and 1996 suggested rising prevalence--80, 138, and 168.2/100,000, respectively. In 2004, we surveyed the North-East of Northern Ireland (population 160,446, area 2030 km(2)) using multiple sources of case ascertainment, all satisfying the Poser criteria for definite or probable multiple sclerosis (MS) or the McDonald criteria. From a provisional list of 469 cases, 370 (123 males, 247 females) were identified. The prevalence was 230.6 per 100,000 (95% CI 207.0-255.4) with significantly higher prevalence in females (300.8/100,000) than males (157.0/100,000). Direct standardization to the 1961 Northern Ireland population reduced the overall prevalence rate to 200.5/100,000 (95% CI 193.2-208.0), in females to 270.2/100,000 (95% CI 258.8-282.4) and in males to 131.1/100,000 (95% CI 122.8-139.9). In 1996, incidence had risen to 9.3/100,000/year (14 cases in population of 151,000) with a higher incidence in females (10.3/100,000/year) than males (8.3/100,000/year). Northern Ireland continues to have a rising prevalence of MS. The increase in incidence suggests a true increase in the disease.