NEW YORK, Apr 11 (Reuters Health) - People diagnosed with dementia may not live as long with the disorder as previous studies have suggested, new findings reveal.
　　Only half of a group of 821 adults older than 65 years with dementia lived more than 3.3 years after symptoms began, compared with previous estimates of 5 to 9.3 years, according to the report in the April 12th issue of The New England Journal of Medicine.

The findings are important for patients, families and healthcare providers in light of the aging US population and the increasing number of people being diagnosed with dementia, particularly Alzheimer's disease, Dr. Claudia H. Kawas from the University of California, Irvine, and Dr. Ron Brookmeyer of Johns Hopkins University, Baltimore, Maryland, write in an accompanying editorial.

"In the next 50 years, the prevalence of Alzheimer's disease in the United States is projected nearly to quadruple--which means that 1 in every 45 Americans will be afflicted with the disease," the editorialists write.

In their report, researchers led by Dr. Christina Wolfson from McGill University in Montreal, Canada, explain that most studies have calculated survival from the time a person enters a study. However, patients with the most rapidly progressing form of dementia may die before entering a study and therefore are omitted from the analysis. This phenomenon, known as length bias, can skew estimates of survival.

Wolfson's team recommends that survival be estimated from the onset of disease, rather than from the beginning of a study.

According to the report, 48% of patients had been diagnosed with probable Alzheimer's disease, 31% with possible Alzheimer's, and 21% with vascular dementia--a type of brain damage caused by blood vessel disease. The patients' average age was 84 years.

Before the investigators took length bias into account, half of all patients lived nearly 7 years. Life expectancy declined to just over 3 years, however, when the researchers included in their calculations patients with severe disease who had died before the study began.

"Median survival after the onset of dementia is much shorter than has previously been estimated," Wolfson and colleagues conclude.

But Kawas and Brookmeyer caution that underestimating survival of patients with dementia can affect how public funds are allocated to assist these patients. They also note that it is difficult to calculate a date of onset, since dementia is usually gradual and insidious.

Furthermore, most patients in the current study were elderly, and "the main determinant of survival and mortality in elderly populations, regardless of the presence of dementia, is age," the editorial states.

From ReutersHealth