Of more than 3,000 older adults with the eye disease, those who had surgery were about 30% less likely to be diagnosed with Alzheimer's in the coming years, researchers found.
The findings cannot prove cataract surgery helps protect against Alzheimer's, said lead researcher Dr. Cecilia Lee. However, it provides strong evidence that that could be the case.
Lee and her colleagues were able to account for numerous other factors that might explain the finding. And even after doing so, cataract surgery was still linked to a reduction in Alzheimer's risk.
"This evidence might be as good as we can get," said Lee, chair of ophthalmology at the University of Washington School of Medicine.
A cataract is a clouding of the eye's lens that can cause symptoms such as blurred vision, difficulty seeing at night, and seeing "halos" around lights. Cataracts are very common among older people -- affecting more than half of Americans by age 80, according to the U.S. National Eye Institute.
Surgery to remove the cataract, and replace it with an artificial lens, can improve vision problems.
Past studies have linked cataracts, as well as other visual impairments, to an increased risk of dementia, including Alzheimer's. That, Lee said, raised the obvious question: Can treating cataracts lower that risk?
It's a tricky question to tackle, however.
"There are so many confounding variables," Lee said, "especially for older people who have other health conditions."
Even if older adults who had cataract surgery show a lower dementia risk, that could be because healthier people are more likely to opt for surgery -- or because those people have better access to health care.
Lee's team addressed those issues by using data on a large group of patients in the Kaiser Permanente health system. All had access to health care, and the researchers had detailed information on their health history.
They focused on 3,038 adults age 65 and older who had cataracts and were free of dementia at the outset. Just under half underwent cataract surgery.
Over an average eight years, 853 patients were diagnosed with dementia, most often Alzheimer's. But the risk was 29% lower among those who'd undergone cataract surgery, compared to those who hadn't.
That reduction was seen after Lee and her team accounted for all the other variables they could -- including whether people had physical disabilities or medical conditions like heart disease, stroke or diabetes. They also factored in body weight, exercise habits, education levels and smoking history -- all of which have been tied to dementia risk.
Why would cataract surgery, and subsequent vision improvement, sway dementia risk? Lee said it's plausible, in part, because vision problems limit older adults' engagement with the world.
"If you can't see well, you may not want to go out and socialize," she said. "Or you may not want to exercise because you're worried about safety."
Like physical exercise, social and mental stimulation are thought to support healthy brain aging.
Another theory, Lee said, relates to blue light. Over time, cataracts can yellow, and that specifically blocks blue light. Certain specialized cells in the eye's retina are very sensitive to blue light, Lee noted, and they have been linked to both sleep cycles and cognition (memory and thinking skills).
Claire Sexton, director of scientific programs and outreach at the Alzheimer's Association, agreed that those theories are plausible.
Given that cataracts are so common, she said, there is great potential in targeting the condition as a risk factor for Alzheimer's.
Sexton pointed to a limitation of the study, however: Most patients were white. She said the findings should be replicated in a diverse group of older adults, to confirm the association holds true for people of color, too.
A bigger-picture message, Sexton said, is that people should be aware that their general health -- including heart health, vision and hearing -- may impact their odds of dementia.
Lee encouraged older adults who are having vision problems to see an ophthalmologist, a physician who diagnoses and treats eye diseases.
The study was published Dec. 6 in JAMA Internal Medicine.
The Alzheimer's Association has more on supporting brain health.
SOURCES: Cecilia Lee, MD, MS, associate professor and chair, ophthalmology, University of Washington School of Medicine, Seattle; Claire Sexton, DPhil, director, scientific programs and outreach, Alzheimer's Association, Chicago; JAMA Internal Medicine, online, Dec. 6, 2021