Cambridge: People with dementia survive an average of four and a half years after diagnosis, with age, sex, and existing disability all having an influence on life expectancy, finds a study published on www.bmj.com today.

The authors hope that these estimates will be of value to patients, carers, service providers, and policy-makers.

The number of people affected by dementia is estimated to double every 20 years to 81 million by 2040. Dementia is known to be associated with increased risk of death, but no estimate exists for actual survival with dementia in England and Wales. There is also considerable uncertainty about what influences survival.

So researchers set out to describe overall survival for people with dementia and to examine the association between factors which could affect survival.

The study involved over 13,000 individuals aged 65 years and above who were taking part in a population based study in England and Wales. Participants were assessed for dementia at regular intervals over a 14-year period 1991 to 2005.

Factors known to have an association with mortality, such as age, sex and marital status, accommodation type, education level, social class, self-reported health and disability were also recorded.

438 individuals developed dementia over the study period, of which 356 (81%) died.

Age, sex, and disability before onset all influenced survival independently.

There was nearly seven years difference in survival between the youngest and the oldest people with dementia (10.7 years for those aged 65-69 and 3.8 years for those aged 90 or over).

Average survival time from dementia onset to death was 4.1 years for men and 4.6 years for women.

There was around a three year reduction in survival between the most and least disabled at onset, suggesting that the frailer individuals are at higher risk, even after age is taken into account.

However, living in the community or residential home, marital status, and self-reported health were not associated with survival once other factors were taken into account.

Those with higher education had slightly shorter survival than those with lower education, but this did not reach statistical significance. Social class also showed no pattern

Knowing which factors influence the length of survival after onset of dementia is important, say the authors. These findings will be of value to patients, carers, service providers and policy-makers.

An accompanying editorial urges doctors to pay as much attention to strengths and retained abilities as they do deficits, dysfunction and disease when planning care and support for people with dementia.