Service planning for people with younger onset dementia (YOD; an onset of symptoms before the age of 65 years) relies on prevalence estimates, with existing models based upon older people. This pilot study investigated the prevalence and causes of YOD in a defined catchment area of Eastern Sydney, Australia.

The study was conducted in three stages: publicity building, case finding, and case validation. A brief structured questionnaire was sent to health professionals in the catchment area asking how many patients with YOD they had seen over the previous 12 months. Memory clinics and hospital records were also searched for YOD patients. Clinicians assigned a Statistical Linkage Key to each patient to prevent double counting, and indicated the cause of dementia. The majority of patients were validated by a review of medical case notes. Prevalence data were calculated for the following age groups: 30–64, 30–44, and 45–64 years.

Two hundred and four potential patients were identified, of which 141 met inclusion criteria. The primary clinical subtypes were alcohol-related dementia (18.4%), Alzheimer's disease (17.7%), vascular dementia (12.8%), and frontotemporal dementia (11.3%). Eighty-eight patients were aged 30 to 64 years on census date and were therefore included in the prevalence calculations. The overall prevalence was 68.2 per 100,000 population at risk for the 30–64-year age group (95% Confidence Interval (CI): 54.9–83.4); 11.6 per 100,000 for the 30–44-year age group (95% CI: 5.3–21.7); and 132.9 per 100,000 for the 45–64 age group (95% CI: 105.8–164.2).

Younger onset dementia affects a significant number of people in Eastern Sydney with a diverse range of clinical types. This prevalence rate is higher than previous reports from the United Kingdom and Japan, with a different distribution of etiologies, which have important implications for service planning for this group.