Wearing glasses can ward off dementia.
Experts at Johns Hopkins University in the US have estimated that unfixed vision problems play a role in 19 per cent of cases of the illness.
The new study used medical records from over 2,700 over-70s in the US and found that dementia was more common in those with uncorrected sight problems.
Scientists argue that declining sight lets the brain waste away as it gets less information and adds to dementia symptoms as more effort is diverted from memory and thinking to be able to see instead.
Study author Jason Smith said: "Since vision impairments are common among older adults, the proportion of dementia attributable to them might be sizable.
"Importantly, more than 90 per cent of vision impairments are preventable or correctable.
"Addressing modifiable risk factors is a priority for dementia prevention."