By Amy Norton
Known as the blood-brain barrier, it's made up of tightly joined cells that line blood vessels in the brain. They form a filtration system that allows certain essential substances -- such as water and sugar -- into the brain, while keeping potentially damaging substances out.
The new study adds to evidence that leaks in the blood-brain barrier are detectable in Alzheimer's patients.
But it's not clear what it all means.
"They don't know whether this leakage is a result of the disease, or a cause of it," said Dr. Ezriel Kornel, an assistant clinical professor of neurological surgery at Weill Cornell Medical College, in New York City.
It's also unclear exactly what is happening in the leaky areas spotted on patients' brain scans, according to Kornel, who wasn't involved in the study.
In theory, he said, the leaks could be opening the door for toxic substances to enter the brain -- but the study doesn't prove that.
"It's an interesting issue," said David Morgan, director of the Byrd Alzheimer's Institute at the University of South Florida, in Tampa. Morgan also wasn't involved with the current study, but reviewed its findings.
Researchers know that the pathological brain changes associated with Alzheimer's begin at least 15 years before symptoms appear, Morgan explained.
First, there is an abnormal buildup of proteins called amyloid. There are no immediate symptoms because the brain is able to compensate for those protein deposits, Morgan said.
Eventually, though, another type of abnormality appears -- twisted fibers of a protein called tau. Symptoms typically arise not long afterward, according to Morgan.
So, the question -- according to Morgan -- is where in that sequence of events does brain leakage occur?
The findings are based on 16 patients who'd been diagnosed with early Alzheimer's disease, and 17 healthy adults the same age. Walter Backes and colleagues at Maastricht University, in the Netherlands, used a special MRI technique to detect areas of brain leakage in each study participant.
In general, the investigators found that Alzheimer's patients showed more areas of leakage across the brain.
And, the more leakage the study participants had in the brain's gray matter, the worse they did on tests of memory and other mental abilities. (Gray matter basically acts as the brain's information-processing center.)
It's plausible, Morgan said, that a compromised blood-brain barrier could contribute to Alzheimer's -- by allowing certain cells from the bloodstream to "infiltrate" the brain and contribute to inflammation and nerve cell damage, for example.
If that's true, there is no obvious way to intervene.
But both Morgan and Kornel pointed to a possible "silver lining" in the leaky-brain situation. Normally, the blood-brain barrier blocks medications and other systemic therapies from getting into the brain.
"So if Alzheimer's patients do have a leaky blood-brain barrier, in a strange way, that could be a good thing," Morgan said. "Some therapies that are under development might have a better chance of working."
Backes and his colleagues also raise the possibility that MRI scans could help diagnose Alzheimer's early, by detecting leaks.
But Morgan had doubts. For one, he said, the researchers only reported on averages across the two study groups: If only some Alzheimer's patients show excess brain leakage, it would not be a reliable way to detect the disease.
Plus, Morgan said, it's possible that people with other forms of dementia, or other neurological diseases, may also have more leaks in the blood-brain barrier.
The study was published online May 31 in the journal Radiology.