It is believed vision and hearing impairment increase dementia risk through pathways of increased social isolation and loneliness, leading to increased cognitive load from poor sensory encoding and neurodegeneration and cortical reorganization from sensory deprivation. Photo: Getty Images. Click image to enlarge.

Neurodegeneration, cognitive decline and diagnosed cases of dementia have many interrelated factors that influence development. Researchers suggest that one potentially modifiable risk factor is visual impairment, which has not yet been studied in terms of multiple, objectively measured impairments, like distance and near visual acuity and contrast sensitivity, in a nationally representative sample.

Researchers from Johns Hopkins, the University of Michigan and Duke University did just that, compiling a population-based analysis in the National Health and Aging Trends Study, which gathers nationally representative data on Medicare beneficiaries over age 65. Included in this particular investigation were 2,767 adults. The study was published last week in JAMA Ophthalmology.

The prevalence of vision impairment in those aged 71 or older was 32.2%. The researchers calculated a metric called the population-attributable fraction (PAF)—a way of linking outcomes with exposure to a risk factor—to describe the extent of dementia that can be attributed to having at least one visual impairment. They determined that the PAF was 19.0% overall, with dementia cases most strongly attributable to contrast sensitivity impairment (15.0%), followed by near acuity (9.7%) and distance acuity (4.9%). PAFs from at least one impairment were highest in those age 71 to 79 (24.3%), women (26.8%) and non-Hispanic white subjects (22.3%).

More generally, the authors of the study argue that their results indicate about one in five cases of prevalent dementia in those aged 71 or older could have been avoided if their visual impairment had been addressed or treated. What’s more, the paper also suggests that studies underestimate the impact of vision impairment on dementia if it’s defined simply by distance visual acuity. This paper’s estimates are up to ninefold higher than those made from distance acuity alone.

“It’s interesting that the authors concluded that addressing vision would limit the rate of dementia,” comments Christopher Kuc, OD, of Newtown Square, PA, a clinician with expertise in neurodegenerative disorders. “Certainly, addressing cataracts and vision correction will help with function, but the root cause of the dementia is what is causing the visual impairment through atrophy of the RGC/visual centers in the brain. By addressing the refractive error and contrast sensitivity issues presented with cataracts, you don’t eliminate the underlying disease.” He’s of the opinion that this would only improve function for those already in advanced cognitive decline. “It’s also noteworthy that hearing was also estimated at 19%—sort of a similar number tied to other manifestations of sensory decline.”

In their paper, the researchers explain the importance of prioritizing contrast sensitivity impact on dementia, as this had the highest PAF. This type of impairment can occur in various eye diseases but may be addressed in cataract extraction, which decreases dementia risk by nearly 30% in older adults, they say. Nonreversible cases, such as with geographic atrophy, may be addressed with vision rehabilitation.

Overall, these data argue for the inclusion of sensory impairments as potentially modifiable risk factors for dementia. As the authors expound, “since vision impairments, including visual acuity and contrast sensitivity impairments, often result from common treatable conditions that may be reversible, such as uncorrected refractive errors and cataracts, our analyses provide additional support to address vision impairments globally, where up to 90% are estimated to be preventable or correctable.”¹

Additionally, they consider that, “while not establishing a cause-and-effect relationship, these findings support considering vision health in dementia prevention strategies aimed at reducing modifiable risk factors.”¹

However, an invited commentary also published by JAMA Ophthalmology articulates certain limitations to the study at hand. The original study’s estimate of 19.0% of prevalent dementia cases were attributed to at least one vision impairment, which is comparable to the PAF of hearing loss; however, the older population used in this study (aged 71 or older) does not reflect a previous meta-analysis done (aged 50 or older), potentially explaining the large difference seen with the much lower 4.7% reported in said meta-analysis.

What’s more, the PAF from at least one impairment was larger in the group of 71- to 79-year-olds than those 90 or older, with only 4.9% in this older age bracket, or less than 140 patients. They used a weighted number, but it should be similar to the unweighted one, with this potentially decreasing the ability to test for statistical significance.

Although this is something to consider, the commentary authors echo that “the findings provide unique value regarding the burden of dementia associated with vision impairment. Vision impairment deserves scrutiny as an intervention priority for the prevention of dementia in older adults.”²