Multiple sclerosis in island populations: prevalence in the Bailiwicks of Guernsey and Jersey.

The aim of this study was to establish for the first time the prevalence of multiple sclerosis in the Bailiwicks of Guernsey and Jersey, as representing the most southerly part of the British Isles. All patients with multiple sclerosis in the Channel Islands resident on prevalence day were identified by contacting all medical practices, Multiple Sclerosis, and Action Research for Multiple Sclerosis societies by letter and visits. The crude overall prevalence rates were 113/100,000 (95% confidence interval (95% CI) 90.3-135.7) and 86.7/100,000 (95% CI 63.3-110.0) for the Bailiwicks of Jersey and Guernsey respectively. When standardised to the age and sex structure of a previously reported Northern Ireland population the standardised prevalence ratios were 120.2/100,000 (95% CI 96.0-144.3) for Jersey and 95.6/100,000 (95% CI 69.9-121.3) for the Bailiwick of Guernsey. When compared with recent studies in the northern United Kingdom the prevalence rates for multiple sclerosis in the Channel Islands lend some support to the proposed latitudinal gradient in the British Isles although the standardised prevalence ratio in the Bailiwick of Jersey is similar to those found in recent studies of southern Britain. The standardised prevalence rates of probable and definite multiple sclerosis for the male populations were 37.3/100,000 (95% CI 17.9-56.7) for the Bailiwick of Guernsey and 45.5/100,000 (95% CI 26.3-64.7) for the Bailiwick of Jersey whereas the standardised prevalence rates for the female populations were 97.5/100,000 (95% CI 73.9-143.5) and 139.5/100,000 (95% CI 112.6-181.2) respectively. Thus there is a striking and unexplained 43% higher prevalence of probable and definite multiple sclerosis in the female population of Jersey compared with that of the Bailiwick of Guernsey. This seems to be due to an unusually low prevalence of the disease among the female population of the Bailiwick of Guernsey compared with that of the United Kingdom mainland.