One in five strokes may be caused by a weakening of the tiny arteries in the brain, a new study has found.
The brain damage caused by lacunar strokes, which occur in tiny rather than large arteries, may be caused by a gradual weakening of the artery wall.
It was previously thought this type of stroke was caused only by reduced blood flow to the brain.
Edinburgh University experts believe the weakening occurs in the protective lining of the small arteries.
Known as the blood-brain barrier, the protective lining stops potentially harmful substances getting into the brain.
The research, published in the Annals of Neurology, involved two groups of stroke patients - one with lacunar stroke and one with the large artery type.
Both groups were injected with magnetic dye before having brain scans that showed how the dye travelled through their blood vessels.
In the lacunar patients, more dye leaked out of the blood vessels into the brain than was the case in the other group.
Scientists said it could only have happened if the dye were able to leak through the protective barrier.
Joanna Wardlaw, professor of Applied Neuroimaging at Edinburgh University, said: "This is an important milestone in the understanding of this common type of stroke and dementia.
"We don't know exactly what causes this weakening of the blood-brain barrier - it may be age, blood pressure or inflammation. More research is required, but we hope the results will help in the search for more treatments to this widespread condition."
The Chief Scientist Office, the Wellcome Trust, Chest Heart Stroke Scotland, the Row Fogo Charitable Trust, the Cohen Charitable Trust, and the UK Stroke Association supported the study.