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Government and other scientists are proposing a new way to define Alzheimer’s disease – basing it on biological signs, such as brain changes, rather than memory loss and other symptoms of dementia that are used today.

The move is aimed at improving research, by using more objective criteria like brain scans to pick patients for studies and enroll them sooner in the course of their illness, when treatments may have more chance to help.

But it’s too soon to use these scans and other tests in routine care, because they haven’t been validated for that yet, experts stress. For now, doctors will still rely on the tools they’ve long used to evaluate thinking skills to diagnose most cases.

Regardless of what tests are used to make the diagnosis, the new definition will have a startling effect: Many more people will be considered to have Alzheimer’s, because the biological signs can show up 15 to 20 years before symptoms do.

“The numbers will increase dramatically,” said Dr. Clifford R. Jack Jr., a Mayo Clinic brain imaging specialist. “There are a lot more cognitively normal people who have the pathology in the brain who will now be counted as having Alzheimer’s disease.”

He led a panel of experts, working with the Alzheimer’s Association and the National Institute on Aging, that updated guidelines on the disease, published Tuesday in Alzheimer’s & Dementia: The Journal of the Alzheimer’s Association.

About 50 million people worldwide have dementia, and Alzheimer’s is the most common form. In the U.S., about 5.7 million have Alzheimer’s in its current definition, based on memory problems and other symptoms.

There is no cure – current medicines such as Aricept and Namenda just temporarily ease symptoms. Dozens of hoped-for treatments have failed, and doctors think one reason may be that the studies enrolled patients after too much brain damage had already occurred.

“By the time that you have the diagnosis of the disease, it’s very late,” said Dr. Eliezer Masliah, neuroscience chief at the Institute on Aging.

Another problem: as many as 30 percent of people enrolled in Alzheimer’s studies based on symptoms didn’t actually have the disease – they had other forms of dementia or even other medical conditions. That doesn’t give an accurate picture of whether a potential treatment might help, and the new definition aims to improve patient selection by using brain scans and other tests.

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