Tiny stroke can damage brain tissue and up risk of Alzheimer's

Researchers from the University of California, San Diego worked with rats using laser light to clot blood at precise points within small blood vessels that dive from the surface of the brain to penetrate neural tissue.

When they looked at the brains up to a week later, they saw tiny holes reminiscent of the widespread damage often seen when the brains of patients with dementia are examined as a part of an autopsy.

These micro-lesions are too small to be detected with conventional MRI scans, which have a resolution of about a millimetre. Nearly two dozen of these small vessels enter the brain from a square millimetre area of the surface of the brain.

"This data shows us, for the first time, that even a tiny stroke can lead to disability," said Patrick D Lyden, a co-author of the study and chair of the department of neurology at Cedars-Sinai Medical Center in Los Angeles.

"I am afraid that tiny strokes in our patients contribute over the long term to illness such as dementia and Alzheimer's disease," he said, adding that "better tools will be required to tell whether human patients suffer memory effects from the smallest strokes".

"The brain is incredibly dense with vasculature. It was surprising that blocking one small vessel could have a discernable impact on the behaviour of a rat," said Andy Y Shih, lead author of the paper.

To see whether such minute damage could change behaviour, the scientists trained thirsty rats to leap from one platform to another in the dark to get water.

The rats readily jump if they can reach the second platform with a paw or their snout, or stretch farther to touch it with their whiskers.

"The whiskers line up in rows and each one is linked to a specific spot in the brain. By training them to use just one whisker, we were able to distill a behaviour down to a very small part of the brain," Shih said in a statement.

When Shih blocked single microvessels feeding a column of brain cells that respond to signals from the remaining whisker, the rats still crossed to the far platform when the gap was small. But when it widened beyond the reach of their snouts, they quit.

The drug memantine approved by Food and Drug Administration, prescribed to slow one aspect of memory decline associated with Alzheimer's disease, ameliorated these effects.

The study was published in the journal Nature Neuroscience.