The epidemiology of Huntington's disease in Northern Ireland.

A survey of Huntington's disease (HD) in Northern Ireland, with a population of 1.5 million, has shown a 1991 prevalence rate of 6.4/100,000. Virtually complete ascertainment was achieved, enabling prevalence rate estimates and age statistics to be calculated over the last 20 years. The prevalence rate is similar to the high prevalence rates of HD found in most European populations, suggesting the presence of either one extremely ancient or a number of separate mutational origins, resulting in a uniform European HD prevalence. The ages at diagnosis and duration of the disease are similar to previous studies, suggesting a consistent effect of the HD gene in all families. Estimates of heterozygote frequency (HF), direct and indirect mutation rate, fertility, and genetic fitness (W) were made. Reliable HF estimates gave values between 10 and 11 x 10(-5). The direct and indirect mutation rates were 0.32 x 10(-6) and 1.05 x 10(-6) respectively. W was increased in the affected HD population but decreased in the at risk population. Fertility in HD is not reduced, but it appears that at risk patients have actively limited their family size. Factors responsible include, among others, the fear of developing HD and genetic counselling of families. This is the first published epidemiological survey to include ascertainment data in a population both before and after isolation of the HD gene, and with the diagnosis in virtually all patients confirmed by DNA mutation testing.