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Objectives: To determine the prevalence of dementia in people under the age of 65 in a large catchment area, and use these figures to estimate the number of younger people affected by dementia in the UK.
Design: Epidemiological catchment area prevalence survey.
Setting: The London boroughs of Kensington and Chelsea, Westminster, and Hillingdon with a total population of 567 500 people.
Participants: All residents of the catchment area with dementia, where the illness began before the age of 65 years. Participants were notified to the study by medical and care professionals. The diagnosis and age of onset was established from all available health and social care records. In total, 227 people were identified, of whom 185 fulfilled the inclusion criteria of having a dementia which started before their 65th birthday.
Main outcome measures: Diagnosis of dementia and differential diagnosis of the cause of the dementia.
Results: The prevalence of dementia in those aged 30–64 was 54.0 per 100 000 (95% CI 45.1 to 64.1 per 100 000). For those aged 45–64 years, the prevalence was 98.1 per 100 000 (95% CI 81.1 to 118.0 per 100 000). From the age of 35 onwards, the prevalence of dementia approximately doubled with each 5 year increase in age. Extrapolating these figures nationally suggests that there are 18 319 (15 296–21 758) people with dementia under the age of 65 in the UK.
Conclusions: The study confirms previous "guestimates" of the number of younger people affected by dementia in UK. The prevalence figures generated are robust, and are supported by other smaller and targeted prevalence surveys. The prevalence figures provided by this study will allow health planners to accurately estimate need and plan services.