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4/12/2001

Alzheimer's Disease Shortens Life Span

Kate Traynor

A Canadian study claims that seniors’ remaining life span after the onset of dementia is much lower than has been generally believed.

The study, which was published in the April 12 New England Journal of Medicine, found that half of dementia patients lived 3.3 years after onset of the disease. Life expectancy estimates from other published studies, the research team said, ranged from 5.0 to 9.3 years after symptoms first occur.

In general, the older a patient was when dementia symptoms first appeared, the fewer years of life remained, according to the new study.

Data on life span were obtained from 821 of 1,132 elderly patients who participated in the Canadian Study of Health and Aging and had a diagnosis of vascular dementia, which is a consequence of multiple cerebral infarctions, or possible or probable Alzheimer’s disease. Study participants were evaluated in 1991 and 1992, and the patients or their families were contacted again in 1993 and 1996. Seventy-eight percent of the seniors died during the study or the follow-up period.

The researchers adjusted their life span data to overcome a statistical phenomenon known as "length bias," which can limit study participation to patients with a typical disease course. People whose mental state fails quickly, said the researchers, have less time to qualify for clinical trials than do other patients with dementia and may be underrepresented in such studies.

Before correction of the data for length bias, the seniors’ median life expectancy was 6.6 years after the onset of dementia.

The researchers defined the date of symptom onset as the date that patients first informed a physician about symptoms of dementia or became affected by memory problems. Most research into seniors’ life span after the onset of dementia, the Canadian-based group said, has imprecisely measured survival time starting from the date that patients enroll in a clinical study.

The life expectancy of the study participants, who averaged in their mid-80s at the start of the study, was not compared with age-matched controls. For this reason, the study's results should be interpreted with caution, urged a neurologist and a biostatistics researcher writing in the same issue of the journal. Half of all 85-year-olds in the United States live only five more years, according to the editorial, making it possible that confounding factors contributed to the study participants’ unusually short life expectancy after onset of dementia symptoms.