A spinal fluid test may detect Alzheimer's disease decades before it causes dementia, U.S. researchers report.

Their findings suggest the process leading to Alzheimer's disease begins in early adulthood and is well under way by middle age. Therefore, treatment may need to begin long before patients show signs of the disease.

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VA Puget Sound researcher Elaine R. Peskind, MD, and colleagues tested the spinal fluid of 184 adult volunteers aged 21 to 88. They looked for a protein called beta-amyloid 42 -- an essential ingredient in the plaques that form in the brains of those with Alzheimer's.

Peskind's team reasoned that when beta-amyloid 42 levels get low, it is a sign the protein is precipitating into brain plaques.

Sure enough, people carrying the ApoE4 gene -- which predisposes a person to Alzheimer's disease -- had lower levels of beta-amyloid 42 than people not carrying the Alzheimer's gene. In these Alzheimer's-prone people, levels of this protein drop sharply, starting in their 50s.

"These findings have implications for the preclinical diagnosis of Alzheimer's disease, as well as for treatment," Peskind and colleagues conclude. "Therapeutic strategies aimed at prevention of Alzheimer's disease may need to be applied in early midlife, or even younger ages, to have maximal effect."

The bad news is that the disease process may already have gone too far for effective treatment once symptoms of early Alzheimer's appear in elderly people.

While the study’s findings are provocative, there's a lot more work to do. It remains to be shown whether young people with declining beta-amyloid 42 really do go on to get Alzheimer's.

"Not all of these persons will go on to develop Alzheimer's disease," predicts Roger N. Rosenberg, MD, at University of Texas Southwestern Medical Center, in an editorial accompanying the Peskind study.

The study appears in the July issue of the Archives of Neurology.

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By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCES: Peskind, E.R. Archives of Neurology, July 2006; vol 63: pp 936-939. Rosenberg, R.N. Archives of Neurology, July 2006; vol 63 pp: 926-928.