Head injuries could be linked to dementia, researchers have suggested.
Scientists in America and Scotland noted that people with Alzheimer's disease and those who had suffered a head injury both had a large number of proteins in their brain.
They examined the brain tissue of 39 people who had recovered from a brain injury and 47 who had never been hurt after they died.
The tissue used was from people living in the west of Scotland.
They discovered that a third of those who had a head injury displayed a large number of proteins, or "abnormalities".
Teams from the Southern General Hospital in Glasgow and the University of Pennsylvania in Philadelphia carried out the study.
Dr William Stewart, who led the team in Glasgow and is co-author of the study, said: "We were really quite astonished to find up to a third had extensive pathology.
"The abnormalities that we saw in the brain is normally seen in people with dementia or very old people but we are seeing this in people in their 20s, 30s, 40s. The proteins are typical of those you might find in Alzheimer's disease."