Dementia is a syndrome characterised by progressive deterioration of cognitive function, most commonly of memory, but other domains such as language, praxis, visual perception and most notably executive function are also often affected. Most of the causes of this syndrome are progressive, but not invariably so. As cognitive function worsens, there is increasing interference with the patients’ daily activities leading to loss of independence and eventually for some the need for nursing home care. The patients usually survive 5–10 years.

Dementia is common and already places a tremendous burden, not only on patients and their carers, but also on society, a burden that will increase as life expectancy increases. Because of these worrying facts and the development of the first symptomatic treatments, dementia is of growing interest to medical professionals and the public. Furthermore, because disease modifying treatments may be on the horizon, it is ever more important to understand the pathophysiology of the different causes and types of dementia, and to make the diagnosis early—it will probably be easier to stop the damage than to undo it.

EPIDEMIOLOGY

• Dementia is rare in young and middle age but after the age of 50 years it becomes more and more common. At age 60–65 years, approximately 1% of the population is affected, rising to 10–35% in those over 85 years of age. It is more common in men than in women over the age of 80 years (fig 1).

• Of the patients with late onset dementia (⩾65 years), about half have Alzheimer’s disease, 16% vascular dementia, 30% other forms of dementia such as dementia with Lewy bodies and frontotemporal lobar degeneration (fig 2).

• In younger patients, Alzheimer’s disease is relatively less common but it is still the most prevalent cause, while frontotemporal dementia, alcohol related dementia and dementia secondary to …