TUESDAY, May 19, 2015 (HealthDay News) -- Abnormal protein clumps may appear in the brain up to 30 years before people developAlzheimer's disease, a new study estimates, perhaps providing a window of opportunity to intervene.

Scientists have long known that people with Alzheimer's diseaseshow brain "plaques," where pieces of a protein called amyloid abnormally clump together.

The new study, published May 19 in the Journal of the American Medical Association, confirms that brain plaques become increasingly common as people age -- even when memory and thinking are still intact.

However, at all ages, plaques are more common among people with risk factors for Alzheimer's. That includes people who already have milder memory problems, and those who carry a gene variant -- APOE4 -- that boosts risk for Alzheimer's, the most common form of dementia.

But, the study authors estimate those brain plaques may emerge 20 to 30 years before full-blown Alzheimer's symptoms arise.

"The significance of that lies in the possibilities for early intervention," said Dr. Pieter Jelle Visser, who worked on the study. "If we can treatAlzheimer's disease in the early stage, we may prevent the onset of dementia."

But there are caveats, stressed Visser, of Maastricht University in the Netherlands. Possibly the biggest one: There are no established treatments for preventing Alzheimer's in people with evidence of brain plaques.

However, clinical trials testing potential contenders are underway, Visser said.

For example, researchers are testing antibodies and vaccines that encourage the immune system to target amyloid clumps in the brain.

With any such trial, it's critical to recruit the right patients, and the new findings underscore the usefulness of brain imaging to do that, said Dr. Roger Rosenberg, a professor of neurology at the University of Texas Southwestern Medical Center at Dallas.

Having amyloid in the brain does not mean you're doomed to develop Alzheimer's, said Rosenberg, who wrote an editorial published with the study.

"But it's important to have markers that identify people at increased risk," he said. For now, it's useful for clinical trials, Rosenberg said, but if any preventive therapies prove effective, doctors will have to be able to identify people likely to develop the disease.