problems associated with studies in dementia. c SYNDROME AND DISEASE The syndrome of dementia may be caused by various underlying diseases, each characterised by a specific constellation of signs and symptoms in combination with a presumed underlying substrate of neuropathology (fig 1). Alzheimer’s disease (AD) is the most prevalent cause of dementia. It is a neurodegenerative disorder, generally assumed to be caused by neuritic plaques and neurofibrillary tangles accumulating in the brain. The second most prevalent cause of dementia is vascular dementia (VaD), which may be caused by various types of vascular pathology in the brain, such as ‘‘large vessel’’ (large territorial or strategical infarctions) and ‘‘small vessel’’ (lacunes and white matter hyperintensities) disease. Other frequent causes of dementia include frontotemporal lobar degeneration and dementia with Lewy bodies. It is often difficult (if not impossible) to reliably distinguish between subtypes of dementia (we will come back to this subject in the section on methodological issues). Therefore, epidemiological studies often focus on dementia as a whole, sometimes giving separate numbers for the two most important subtypes—AD and VaD. PREVALENCE Prevalence is defined as the proportion of a population that has disease at a specific point in time. Prevalence estimates vary highly between studies. These variations may be due to variations in study population—that is, reflect real differences. For example, age is the most important risk factor for dementia. Differences in age between populations will result in different estimates of prevalence. Alternatively, and just as plausible, is the assumption that differences in prevalence estimates are caused by methodological differences, such as study design and diagnostic procedure. One solution to obtain more certainty about the value of prevalence is to combine data from multiple studies in a meta-analysis. Meta-analyses have two advantages. First, small differences due to methodological differences between studies level out. Secondly, and more importantly, the analysis is based on a far larger sample than a single study could ever realise, resulting in more precise estimates. Pooled estimates of prevalence In 2000, prevalence data from 11 European population based studies were pooled to obtain stable estimates of prevalence of dementia in the elderly (. 65 years).3 Age standardised prevalence was 6.4% for dementia (all causes), 4.4% for AD, and 1.6% for VaD. Prevalence of dementia was higher v2 www.jnnp.com copyright. on December 4, 2021 by guest. Protected by http://jnnp.bmj.com/ J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.2005.082867 on 16 November 2005. Downloaded from in women than in men and nearly doubled with every five year increase in age: 0.8% in the group age 65–69 years and 28.5% at age 90 years and older (fig 2). Of all dementia cases, 54% suffered AD. Prevalence of AD showed the steepest increase with age, from 0.6% in the group age 65–69 years to 22.2% in the group aged 90 years and older. VaD accounted for 16% of cases, and prevalence increased with age from 0.3% (65–69 years) to 5.2% (90+ years). More recently, prevalence rates for dementia were compared among 12 population based European studies. Crude prevalence rates varied between 5.9% (Italy, the Counselice study) and 9.4% (the Netherlands, Rotterdam study).1 Again, an almost exponential increase with age and a female excess—mostly after age 75—was described. Dementia with young onset Most studies on prevalence of dementia focus on subjects aged over 65 years. Although age is well established as its most important risk factor, dementia may also affect people under the age of 65. Few data exist on the prevalence of dementia in younger people. A recent study in the UK was designed to determine the prevalence of dementia in people under the age of 65 in a large catchment area (total population of 567 500 people) and use these figures to estimate the number of younger people affected by dementia in the UK.4 The prevalence of dementia in those aged 30–64 was 54 per 100 000. For those aged 45–64, the prevalence was 98 per 100 000. Like