People who have cataracts can reduce their risk of developing dementia by about 30 per cent by undergoing surgery to restore their sight, although it’s not clear why
6 December 2021
Older people who have cataract surgery to improve their eyesight are less likely to develop dementia afterwards.
The effect could be because people who lose their eyesight typically spend more time at home, and so get less mental stimulation – or it could be down to a strange effect that cataracts have on the colours that reach the retina at the back of the eye.
Cataracts, which involve the lens of the eye becoming more cloudy with age, are one of the most common causes of vision loss in older people. They can be fixed by surgically removing the lens to replace it with a plastic one.
Sight loss was already known to be a risk factor for developing Alzheimer’s disease and other forms of dementia. Cecilia Lee at the University of Washington in Seattle wondered whether cataract surgery would have a noticeable correlation with dementia incidence.
She and her team took advantage of an ongoing US study that began in the 1990s and that aims to identify risk factors for dementia. They looked at the health records of about 3000 participants who were 65 or older and had either cataracts or glaucoma, another eye condition that is treated with surgery. This was used as a comparison.
Over the next eight years, those who had their cataracts removed had, on average, 71 per cent of the chance of developing dementia as those who had cataracts that went untreated.
As it wasn’t a randomised trial, the study couldn’t prove that cataract surgery protected against dementia – another possible explanation, for instance, is that people are more likely to be recommended surgery if they are healthier to start with. But there was no significant difference in dementia rates between people with glaucoma who did or didn’t have surgery, suggesting that cataract surgery was indeed protective.
The surgery could lower the risk of dementia by enabling “higher-quality sensory input to the retina and therefore improving stimuli to the brain”, says Lee.
Another idea is that because cataracts filter out blue light, this disrupts light sensing by cells at the back of eye that are especially sensitive to blue light and help govern our body clock. This could disturb our normal circadian rhythm, which has previously been linked with dementia.
“Because cataracts affect the overall quality of light that reaches the retina, including the blue light, cataract surgery may enable the reactivation of those cells in a way that is protective against cognitive decline,” says Lee. “There is so much that we do not know yet.”
Journal reference: JAMA Internal Medicine, DOI: 10.1001/jamainternmed.2021.6990
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