Sensory loss is associated with increased risk for dementia. As one of the 12 addressable risk factors identified by the Lancet Commission on dementia prevention, hearing loss is the best established sensory-related risk factor [1]. But there is growing evidence to suggest that vision loss, especially when combined with hearing loss, also contributes to dementia risk [2; 3]. Similar to the research showing a protective role for hearing aids, a recent study provides evidence that correcting age-related vision impairment may also reduce the risk for dementia in this population [4].
There are multiple mechanisms by which sensory loss, such as vision loss, is thought to be associated with dementia risk [2]. The loss of sensory input into the brain may result in the atrophy? of those parts of the brain, leading to faster brain volume loss. Individuals with sensory impairments are more likely to withdraw from social events, and may be less able to safely engage in healthy behaviors, such as exercise, without assistance. There is also overlap between risk factors for dementia and sensory loss disorders, such as age.
Meta-analyses of observational studies have found that visual impairment is associated with around a 40% increased risk for dementia [5; 6]. However, visual impairment can stem from a variety of different underlying causes, and these analyses suggest that eye diseases do not all impact dementia risk to the same degree. Furthermore, eye diseases vary in their ability to be treated or corrected. There is considerable variability in the findings across individual studies evaluating the relationship between eye disease and dementia, which may stem from differences in the ways the studies were conducted. Nevertheless, meta-analyses of multiple studies can indicate which eye diseases have the strongest level of evidence linking them to dementia risk.
A cataract is the clouding of the lens in the eye, which results in blurry vision, and is the leading cause of blindness worldwide [7]. A meta-analysis of observational studies found that having cataracts is associated with a 17% increased risk for dementia [5]. There is evidence to suggest that this association is partially mediated by the relationship between cataracts and other conditions known to be associated with dementia risk, such as stroke, heart disease, diabetes, and depression [8]. However, the relative risk is higher for those with cataracts plus these other conditions, compared to any of these conditions alone.
Glaucoma is the second leading cause of blindness worldwide, and involves progressive damage to the optic nerve, which connects the eye to the brain [9]. Multiple meta-analyses indicate that there is no clear association between a glaucoma diagnosis and dementia risk, although modest associations have been seen in individual studies [5; 10]. This may be reflective of the heterogenous nature of glaucoma, as there are multiple types, which may be differentially associated with dementia. Disease severity of included participants may also play a role, as those with mild disease may not experience a level of vision loss that significantly impairs their ability to carry out daily activities. Additionally, since the thinning of the optic nerve is found in both glaucoma and Alzheimer’s disease, a glaucoma diagnosis made using this criterion could be misleading [11]. Therefore, the potential link between glaucoma and dementia remains unclear.
An important difference between vison loss stemming from cataracts and glaucoma is that the former is correctable, while the latter is not. Current medications and surgery options can slow the progression of glaucoma, but cannot restore lost vision [9].
A recent study including over 3000 participants found that relative to individuals with cataracts who were not treated, those who underwent lens replacement surgery had a 30% reduction in dementia risk [4]. No signficiant reduction in dementia risk was seen in individuals who underwent surgery for glaucoma. The different outcomes for the two conditions may stem from glaucoma surgery not restoring vision and that glaucoma has a weaker association with dementia.
Part of why cataracts appear to show a stronger association with dementia relative to many other eye diseases may be related to their effect on circadian rhythms. This 24-hour rhythmic cycle regulates many physiological functions, such as the sleep-wake cycle. Disruption of these rhythms can promote inflammation, and lead to a variety of negative health effects [12]. Circadian rhythm disruptions are common in dementia patients, and may accelerate cognitive decline. Blue light-sensitive cells in the eye are important for the proper maintenance of circadian rhythms. The yellowing of the lens associated with cataract formation reduces the level of blue light that gets into the eye [4]. Therefore, cataract surgery may benefit by restoring both vision and circadian rhythms.
Be sure to undergo annual vision exams so that possible signs of eye disease or vision loss can be detected as early as possible, and to keep this modifiable dementia risk factor at bay.
Betsy Mills, PhD, is a member of the ADDF's Aging and Alzheimer's Prevention program. She critically evaluates the scientific evidence regarding prospective therapies to promote brain health and/or prevent Alzheimer's disease, and contributes to CognitiveVitality.org. Dr. Mills came to the ADDF from the University of Michigan, where she served as the grant writing manager for a clinical laboratory specializing in neuroautoimmune diseases. She also completed a Postdoctoral fellowship at the University of Michigan, where she worked to uncover genes that could promote retina regeneration. She earned her doctorate in neuroscience at Johns Hopkins University School of Medicine, where she studied the role of glial cells in the optic nerve, and their contribution to neurodegeneration in glaucoma. She obtained her bachelor's degree in biology from the College of the Holy Cross. Dr. Mills has a strong passion for community outreach, and has served as program presenter with the Michigan Great Lakes Chapter of the Alzheimer's Association to promote dementia awareness.
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