EDINBURGH, July 5 -- When patients leave the hospital after a lacunar stroke, they leave carrying a familiar prescription: aspirin. A study published Thursday in Circulation suggests that prescription has spent decades addressing a cause that was never actually there.

Researchers at the University of Edinburgh found that lacunar ischemic stroke, which accounts for roughly a quarter of all ischemic strokes in the United Kingdom, is not driven by fatty deposits building up inside arteries, as has long been assumed. The real mechanism appears to be damage and abnormal widening in the brain's own small blood vessels, deep within the tissue.

The study, led by Professor Joanna Wardlaw at Edinburgh's Institute for Neuroscience and Cardiovascul...