An Eye on the Brain: Insights from the Eye Determinants of Cognition Study

Blindness and visual impairment are leading causes of disability and reduced quality of life in older adults, affecting more than 11% of people aged 50 years and older in the general population [1]. The global burden will only grow as populations around the world age [1,2]. Visual impairment has also been identified as a risk factor for cognitive decline, another leading cause of disability, dependency and early mortality in older adults [3-5]. There are a number of potential underlying mechanisms that could link visual function to decline in cognition. The common cause theory postulates that systemic disease processes such as diabetes mellitus or hypertension could damage both the retina and the brain resulting in visual and cognitive impairment, respectively. Similarly, behavioral or social factors could contribute to both higher likelihood of uncorrected refractive error or cataract and poorer performance on cognitive testing. On the other hand, the sensory deprivation theory states that loss of vision function could lead to declining engagement in physical and social activities, and depression, which in turn could increase risk of cognitive decline. Understanding the connection between visual impairment and cognitive decline may help identify high risk populations and inform screening and prevention strategies. 

The Eye Determinants of Cognition (EyeDOC) study was designed to look at relationships of vision and eye health with cognitive decline, using a thorough battery of vision function and eye health measures, and cognitive performance data spanning a 7-year period in a biracial, community-based cohort of older adults [6]. Studies that have examined vision and cognition relationships have generally assessed only Distance Visual Acuity (DVA) - a single measure of vision function that may not be closely linked to physical or social functioning [7-11]. Unlike these studies, the EyeDOC Study measured several aspects of visual function including presenting DVA, corrected DVA and contrast sensitivity. Presenting DVA is the visual acuity that the participants have using their regular glasses/lenses, if any. It is a function of both vision loss due to disease or refractive error as well as under-correction for refractive error potentially due to poor access to vision care or irregular visits to an eye doctor. Corrected DVA, on the other hand, is the optimal visual acuity that participants could achieve after perfect correction of any refractive error. Contrast sensitivity is the participants’ ability to detect subtle differences in shading and patterns. It is important in detecting objects with ill-defined outlines and discriminating objects or details from their background, such as the ability to find a black mobile phone on a dark countertop. Contrast sensitivity has a large impact on daily activities like reading speed and navigating dimly lit spaces, and remains relatively unaffected by uncorrected refractive error [12,13]. In addition to more expansive vision measures, the EyeDOC Study benefitted from a thorough assessment of cognitive status via a comprehensive battery of tests that spanned several cognitive domains, a feature that is not available in many studies. 

Results from the EyeDOC Study suggested that worse visual function was associated with greater rates of concurrent decline in global cognition, as well as in the domains of memory and executive function, independent of well-known risk factors for cognitive decline such as diabetes mellitus and hypertension. This pattern of associations was consistent in all the three measures of visual function. The associations between each measure of visual function and cognitive decline may be linked to different mechanistic pathways. Weaker associations in corrected DVA, as compared to presenting DVA, suggest that part of the relationship may arise from unmet vision care needs and social factors that drive them. Hence, treating residual refractive error and encouraging older adults into vision care may be a modifiable risk factor for cognitive decline in older adults. Beyond refractive error, contrast sensitivity, which cannot be corrected in most cases, was also associated with greater decline in cognition. Vascular damage related to underlying diseases such as hypertension and diabetes mellitus can result in damage to the optic nerve that leads to loss of contrast sensitivity, and both diseases are known risk factors for cognitive decline [14-16]. However, the association persisted even after accounting for diabetes mellitus and hypertension, though the possibility of residual and/or unmeasured confounding remains but it was only observed in one of the two community/race groups that were studied - the Washington County/White group. 

The results speak to important contextual factors that drive relationships between vision function and cognition. Washington County, Maryland and Jackson, Mississippi represent two distinct communities that differ by geographic location, state level health policy, urbanicity, as well as culture. These factors may influence educational quality and availability, opportunity for social interactions, access to outdoor physical activities and the risk of depression. Community differences in these potential mediating factors could influence relationships between vision function and cognitive decline. For example, in a community with low access to outdoor physical activities, the loss of vision function may not do much to further limit an individual’s physical activity. Research has demonstrated a link between high area-deprivation-index and high disability prevalence in USA, where disability was defined as having limitations in vision, hearing, cognitive, ambulatory, self-care, or independent living [17]. Clearly, additional research is needed into community and individual level factors that may play key roles in ameliorating or exacerbating the impact of vision loss on daily activities, emotional well-being, and quality of life. 

While the EyeDOC study examined three aspects of visual function (presenting DVA, corrected DVA and contrast sensitivity), other domains of vision function such as color vision and visual field were not assessed. Visual acuity and color vision have been shown to be correlated. However, the severity of color vision loss in relation to visual acuity differs depending on the cause of visual impairment [18]. In addition, at least for certain etiologies, such as glaucoma and optic nerve disease, visual field tests may be superior to visual acuity in determining functional vision loss [19]. Hence, further studies investigating the association between visual impairment and cognitive decline should expand on the EyeDOC study results, including an even wider range of vision function tests and additional follow-up to better assess the temporality of the relationship between aspects of vision function loss and cognitive decline. Longer follow-up and repeated measures of vision function and cognition over time could elucidate the remaining mechanistic question: is visual impairment the key factor, or are underlying systemic diseases associated with changes in both the retina and brain responsible for the observed associations [20]. 

In conclusion, results from the EyeDOC study support a functional link between visual impairment and cognitive decline in older adults which, given the high prevalence of vision impairment in this age group, could have substantial public health implications. Since many common causes of vision loss are preventable or treatable, regular visual screening, as well as appropriate treatment, would likely improve the quality of life and promote healthy aging, potentially including better cognitive health. Older adults may benefit from a more aggressive approach to vision loss by clinical care providers including advocating for frequent eye examinations, more awareness of the problem of uncorrected refractive error and encouraging timely cataract surgery when warranted.

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