The new finding may one day allow doctors to detect and treat patients earlier and perhaps keep them functional for longer.
Researchers at Massachusetts General Hospital and the University of Pennsylvania used magnetic resonance imaging (MRI) scans to measure the thickness of the brain's outer layer in 159 people who were free of memory problems.
Previous studies have linked Alzheimer's disease with characteristic shrinkage in nine different regions of the brain's gray matter, or cerebral cortex. Doctors call this the "Alzheimer's signature."
Researchers say the brain shrinks as it loses nerve cells, or neurons. They aren't entirely sure what causes the nerve cells to die. One theory is that the cells die after they become choked by excess amounts of two kinds of protein -- beta amyloid and tau.
"The neurons degenerating over time are really what we think causes the shrinkage," says researcher Brad Dickerson, MD, an associate professor of neurology at Harvard Medical School and director of the frontotemporal disorders unit at Massachusetts General Hospital. "And that shrinkage in their size is something you can measure with an MRI scan."
Study: Rating the Risk for Alzheimer's
Nineteen people who had the most shrinkage in those regions were rated as high risk for Alzheimer's. Another 116 were rated as average risk, and 24 more were rated as low risk based on their brain measurements.
After three years, people who were classified as high risk were also the most likely to have what Dickerson calls "subtle, but meaningful" deficits in the way they learned and remembered words and solved problems.
About 21% of people in the high-risk group showed those kinds of changes in their thinking and memory, compared to 7% of those in the average-risk group, and none in the low-risk group.
People in the high-risk group were also more likely than those at the average-risk or low-risk groups to have abnormal levels of the protein beta amyloid in their spinal fluid.
Beta amyloid gums up the brain in Alzheimer's disease and is thought to jam the connections between individual nerve cells.
The study is published in the journal Neurology.
More Research Needed
Experts who were not involved in the study called it promising, but preliminary.
"It's not ready for prime time yet," says Adam Rosenblatt, MD, a professor of psychiatry and neurology at Virginia Commonwealth University School of Medicine, in Richmond.
Rosenblatt notes that the researchers followed people in the study for only a short time. So they can't be sure those who developed memory loss and other problems would go on to develop frank Alzheimer's disease.
Doctors are only able to diagnose Alzheimer's disease with certainty at this time by looking at the brain after death.
"It's awfully suggestive that that's what they're observing, but it's not like they followed people to autopsy and looked at their brains and proved they had Alzheimer's," Rosenblatt says.