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Up to one in five prevalent dementia cases may be attributable to impaired vision, a cross-sectional analysis suggested.

In a study of nearly 2,800 older Americans, the population attributable fraction (PAF) of prevalent dementia from at least one vision impairment was 19% (95% CI 8.2-29.7), reported Jason Smith, ScM, of Johns Hopkins University Bloomberg School of Public Health in Baltimore, and co-authors.

PAFs -- the upper limits of dementia attributable to vision impairment, assuming causality -- varied across contrast sensitivity, near visual acuity, and distance visual acuity, the researchers said in .

Contrast sensitivity impairment had the strongest attributable PAF (15%, 95% CI 6.6-23.6), followed by near acuity (9.7%, 95% CI 2.6-17.0), and distance acuity (4.9%, 95% CI 0.1-9.9) impairments.

"In a nationally representative sample of older U.S. adults, up to 19% of dementia prevalence might have been avoided through prevention and treatment of vision impairments that are largely correctable," Smith and colleagues wrote.

"Contrast sensitivity was associated with the largest PAF, indicating the total impact of vision impairment on dementia should be quantified using measures beyond just distance visual acuity," they continued. "Overall, while not establishing a cause-and-effect relationship, these findings support considering vision health in dementia prevention strategies aimed at reducing modifiable risk factors."

Sensory loss has emerged as an important modifiable dementia risk factor in recent years. A prospective study showed that cataract surgery was tied to a nearly 30% lower risk of dementia in older adults, suggesting that cataract patients may benefit from higher sensory input. In an autopsy study, hearing impairment was linked with neuropathological hallmarks of dementia.

Recent research also has suggested that nearly 100,000 dementia cases in the U.S. may have been prevented by improving vision.

In 2024, the Lancet Commission on dementia prevention, intervention, and care added vision loss to its list of key modifiable risk factors tied to cognitive impairment. Hearing loss was added to the commission's list in 2020.

"Vision impairment deserves scrutiny as an intervention priority for the prevention of dementia in older adults," wrote Mingguang He, MD, PhD, and Xianwen Shang, PhD, both of the Hong Kong Polytechnic University in China, in an .

"With an unweighted population attributable fraction of 22.2%, hearing loss was ranked at the top of the 12 modifiable risk factors for the global burden of dementia in the 2020 report of the Lancet dementia commission," He and Shang observed.

"Using data from a nationally representative study of older U.S. individuals, Smith et al. estimated that 19% of prevalent dementia cases were attributed to at least one vision impairment. This is comparable to the population attributable fraction of hearing loss," they stated.

But the PAF estimated in this study is much larger than the 4.7% reported in a , the editorialists pointed out. This might be due to age: "The higher prevalence of vision impairment among the older population might partly explain why there was a larger burden of dementia due to vision impairment in the current study's population," they noted.

Smith and co-authors based their analysis on the of Medicare beneficiaries. They included 2,767 community-dwelling adults eligible for vision and cognitive testing in 2021. All participants were ages 71 or older, and 54.7% were women.

The prevalence of at least one vision impairment in the study was 32.2%, most often near acuity or contrast sensitivity impairment. A total of 11.3% of participants had dementia.

Participants who had impairment to either near acuity, distance acuity, or contrast sensitivity were considered to have at least one vision impairment. Vision tests collected objective, presenting (everyday conditions, with eyeglasses or contact lenses if needed) binocular measures.

Population attributable fractions from at least one impairment were highest among participants ages 71 to 79 (24.3%, 95% CI 6.6-41.8), women (26.8%, 95% CI 12.2-39.9), and non-Hispanic white (22.3%, 95% CI 9.6-34.5) subgroups. PAFs from at least one vision impairment and from contrast sensitivity were similar across levels of education.

The study provided a snapshot of visual impairment and dementia among U.S. older adults at one point in time. It could not account for the timing of vision impairment relative to dementia diagnoses and competing risks.

"Although the study is limited by its cross-sectional design and small sample size, the findings provide unique value regarding the burden of dementia associated with vision impairment," the editorialists noted.

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