In 2019, 57 million people worldwide were living with dementia, a figure expected to soar to 153 million by 2050. A recent Lancet Commission report suggests that nearly half of dementia cases could be prevented or delayed by addressing 14 modifiable risk factors, including impaired vision.
The report’s authors recommend that vision-loss screening and treatment be universally available. But are these recommendations warranted? What is the evidence? What is the potential mechanism? And what are the potential implications for clinical practice?
Worldwide, the prevalence of avoidable vision loss and blindness in adults aged 50 years or older is estimated to hover around 13%.
“There is now overwhelming evidence that vision impairment in later life is associated with more rapid cognitive decline and an increased risk of dementia,” said Joshua Ehrlich, MD, MPH, associate professor in ophthalmology and visual sciences, the Institute for Social Research at the University of Michigan, Ann Arbor.
The evidence includes a meta-analysis of 14 prospective cohort studies with roughly 6.2 million older adults who were cognitively intact at baseline. Over the course of up to 14 years, 171,888 developed dementia. Vision loss was associated with a pooled relative risk (RR) for dementia of 1.47.
A separate meta-analysis also identified an increased risk for dementia (RR, 1.38) with visual loss. When broken down into different eye conditions, an increased dementia risk was associated with cataracts and diabetic retinopathy but not with glaucoma or age-related macular degeneration.
A US study that followed roughly 3000 older adults with cataracts and normal cognition at baseline for more than 20 years found that those who had cataract extraction had significantly reduced risk for dementia compared with those who did not have cataract extraction (hazard ratio, 0.71), after controlling for age, race, APOE genotype, education, smoking, and an extensive list of comorbidities.
Causation or Coincidence?
The mechanisms behind these associations might be related to underlying illness, such as diabetes, which is a risk factor for dementia; vision loss itself, as might be suggested by a possible effect of cataract surgery; or shared neuropathologic processes in the retina and the brain.
A longitudinal study from Korea that included roughly 6 million adults showed that dementia risk increased with severity of visual loss, which supports the hypothesis that vision loss in itself might be causal or that there is a dose-response effect to a shared causal factor.
“Work is still needed to sort out” exactly how visual deficits may raise dementia risk, although several hypotheses exist, Dr. Ehrlich said.
For example, “decreased input to the brain via the visual pathways may directly induce brain changes. Also, consequences of vision loss, like social isolation, physical inactivity, and depression, are themselves risk factors for dementia and may explain the pathways through which vision impairment increases risk,” he said.
Is the link causal? “We’ll never know definitively because we can’t randomize people to not get cataract surgery versus getting cataract surgery, because we know that improving vision improves quality of life, so we’d never want to do that. But the new evidence that’s come in over the last 5 years or so is pretty promising,” said Esme Fuller-Thomson, PhD, director of the Institute for Life Course and Aging and professor, Department of Family and Community Medicine and Faculty of Nursing, at the University of Toronto, Ontario, Canada.
She noted that results of two studies that have looked at this “seem to indicate that those who have cataract surgery are not nearly at as high risk of dementia as those who have cataracts but don’t have the surgery. That’s leaning towards causality.”
A study published in July suggests that cataracts increase dementia risk through vascular and non–Alzheimer’s disease mechanisms.