TUESDAY, April 19 (HealthDay News) -- The first new guidelines in 27 years for the diagnosis of Alzheimer's disease could double the number of Americans defined as having the brain-robbing illness.

The guidelines, issued Tuesday by the Alzheimer's Association and the U.S. National Institute of Aging, differ in two important ways from the last recommendations, which have been in use since 1984.

First, Alzheimer's is now being recognized as a continuum of stages: Alzheimer's itself with clear symptoms; mild cognitive impairment (MCI) with mild symptoms; and also the "preclinical" stage, when there are no symptoms but when recognizable brain changes may already be occurring.

Second, the new guidelines incorporate the use of so-called "biomarkers" -- such as the levels of certain proteins in blood or spinal fluid -- to diagnose the disease and assess its progress, but almost exclusively for research purposes only.

Still, the authors of the guidelines emphasized that these revisions are unlikely to change what happens in doctors' offices when diagnosing Alzheimer's or its precursors.

"It will not change practice," said Dr. Guy M. McKhann, one of the guideline authors, at a Monday press conference.

MCI will, however, become a new diagnosis. And that could mean that the number of people considered to be on the new Alzheimer's continuum could double, said Marilyn Albert, another author, director of the division of cognitive neuroscience at Johns Hopkins. But how MCI is determined won't change.

The new U.S. National Institute on Aging/Alzheimer's Association Diagnostic Guidelines for Alzheimer's Disease now recognize three clear stages of Alzheimer's disease.

The first and most severe is Alzheimer's dementia, when patients are clearly cognitively and functionally impaired. This is to be characterized now not just by memory loss but also visual, spatial and judgment problems.

The new guidelines also make a clearer distinction between Alzheimer's dementia and vascular dementia (such as that caused by stroke), McKhann said. The diagnosis will still be made by a doctor, with help from someone who knows the patient and perhaps the patient him- or herself, but biomarkers may be called in "to augment our certainty about the diagnosis," said McKhann, a professor of neurology and neuroscience at Johns Hopkins University School of Medicine in Baltimore.

Another stage, MCI, can represent an earlier phase of dementia and consists of modest impairments, primarily in memory, which can be a harbinger of full-blown Alzheimer's years down the road. In the research arena, investigators will be working towards standardizing biomarkers which indicate, for example, the presence of amyloid protein or nerve damage in the brain.

But for now, how diagnoses are made "will be extremely similar to what's been used in the last 10 years," said Albert, who added that "a very large number" of individuals with MCI do go on to develop Alzheimer's.

"Older adults with MCI progress to dementia at a higher rate than those with no impairment, but progression is not inevitable," according to the Alzheimer's Association's online overview of mild cognitive impairment.

"Not everyone diagnosed with MCI goes on to develop Alzheimer's," the association noted.

The preclinical category was formulated for research purposes only, namely to study biomarkers that may be present in the blood or cerebrospinal fluid or evident on different imaging tests that would indicate the build-up of amyloid plaque or damage to nerve cells.

"The main conceptual point was to define Alzheimer's on the basis of the underlying brain changes rather than just requiring clinical symptoms," said Dr. Reisa A. Sperling, a neurologist at Brigham and Women's Hospital and associate professor of neurology at Harvard Medical School in Boston. "We thought our best chance for disease-modifying therapy was to detect evidence of the disease and intervene much earlier."

As in cancer and diabetes, McKhann pointed out, if you're trying therapies "only in people who have advanced dementia, the chances of them working is not very great."

"We're worried that there could be drugs around now that could be beneficial but that we could be using them too late in the disease course," added Albert.

The new guidelines, summarized William Thies, chief medical and scientific officer of the Alzheimer's Association, "will result in little change in current clinical practice of medicine as applied to Alzheimer's disease. . . . [However] the new criteria are really extending the range of our ability to investigate this disease and eventually to find treatments that will be so necessary to avoid the epidemic of Alzheimer's that we see facing us."

The new guidelines appear in four papers in Alzheimer's & Dementia: The Journal of the Alzheimer's Association.

More information

The Alzheimer's Association has more about mild cognitive impairment (MCI).