While Alzheimer's disease is the commonest form of dementia, other damage to brain, whether by impaired blood circulation, injury or tumor, can also cause dementia. The brain does not function normally and deteriorates causing other serious health problems.

In previous studies survival rates after the start of dementia were shown to be from 5 to 9.3 years. These studies did not, however, consider patients who died before they could be included in a study and did not accurately measure the onset of dementia.

The scientists who conducted this new study took these factors into consideration and came up with new results. Previously studies of people with dementia showed that they had a probable life expectancy that ranged from 5 to 9.3 years after they contracted the disease. But these studies probably overestimated the survival rates since they did not take in consideration those patients who died quickly after onset of dementia, and they did not accurately determine exactly when dementia actually began, a difficult undertaking.

The scientists who conducted this study used data from the Canadian Study of Health and Aging. They tried to calculate the exact onset of dementia in 821 people over 65 years old randomly selected from 10,263 volunteers included in the Canadian study. The actual age of onset was calculated using proven clinical methods. Then these patients were followed to determine how long they survived after onset of dementia.

The scientists also looked for factors that would predict survival.

After a careful statistical analysis it was determined that the estimated average survival rate for all the subjects was 3.3 years, much lower than previous studies had shown. The differences in survival rates between men and women were negligible.

Patients with a high probability of having Alzheimer's disease tended to survive less long, about 3.1 years. Diagnosis of probable Alzheimer's disease was based on clinical examination criteria established by the National Institute of Neurological and Communicative Disorders and Stroke, and the Alzheimer's Disease and Related Disorders Association.

The scientists also showed that the level of education had no predictive effect on survival rates. Those people who contracted dementia at a younger age tended to survive longer. The scientists pointed out that this survival rate is, in fact, about the same as that of older people with other serious diseases such as congestive heart failure.