The initial database search totalled 122 articles: 31 from AgeLine; 33 from APA PsychInfo and 58 from CINAHL Complete. After the exclusion of 46 duplicates, 76 articles remained and when the inclusion and exclusion criteria were applied, a further 26 articles were removed from the review. Of these, 10 were not empirical studies, 4 used samples of care professionals, 2 focused on assessment and intervention and 11 had insufficient focus on cognitive decline. Of the remaining 50 articles, 30 were selected to be included in this review based on the entire text. The entire scoping review data collection process is summarised in Fig. 1.
Of the final 30 studies selected based on the entire text, 20 employed a longitudinal design, 7 were cross-sectional and 3 used a combined approach. The longitudinal studies in this review include papers using secondary data and cohort studies. This review includes studies conducted in the United States, United Kingdom, Singapore, Iran, China, Malaysia, Germany, Australia, Hong Kong, Japan, Israel and one study used data from a survey taken in six low and middle-income countries: China, India, Ghana, Mexico, Russia, and South Africa [24].
Measures of vision and cognitive health
There was substantial heterogeneity of visual and cognitive assessment between studies. In many studies, visual impairment was defined as poor visual acuity (n = 21). The most common measure of vision was the Logarithm of the Minimum Angle of Resolution (LogMAR), which was employed in fourteen of the studies. However, there was variability between studies as to what LogMAR score constituted visual impairment, with one study classifying anything above 1.0LogMAR as impaired and others using 3.0LogMAR or 0.48LogMAR as the threshold for impairment. Other less common measures of visual acuity included the Snellen chart, the Jaeger chart, and the Bailey-Lovie chart. Seven studies relied on self-report as measures of vision, often using Likert scales or questionnaires to quantify the extent of the visual impairment. Contrast sensitivity was measured in 4 of the included studies [10, 26, 29, 30], using the Pelli-Robson chart. In three of these studies, contrast sensitivity was measured in conjunction with both visual acuity and stereo acuity [26, 29, 30]. However, stereo acuity was measured using three separate tools: the Frisby Stereo Test, Stereo Fly Test and by measuring the smallest depth disparity identified. Visual field defects were measured for in one study [23], using a Zeiss Humphrey Field Analyzer II 750i. Two studies used a diagnosis of ocular disease such as glaucoma, cataracts, or lens disorders as a measure of visual impairment [9, 22]. Overall, there was vast heterogeneity in the conceptualisation and operationalisation of visual impairment across the included studies. In cases where the same type of impairment was measured and the same tests were administered, the threshold of what was considered impaired vision differed between studies. Therefore, further meta-analyses are required to summarise findings from studies which have the same definition of visual impairment to improve the validity of the findings. Moreover, further research is needed to elucidate the effects of different visual deficits on cognition. These studies should employ multiple measures of vision to investigate the effects of different aspects of the visual system, such as contrast sensitivity, stereo acuity, and specific ocular diseases.
A variety of cognitive domains were measured across the studies including language, executive functioning, memory, attention, perception, and visuospatial skills. The Mini-Mental State Exam (MMSE), and its adapted versions (e.g., MMSE-blind for the visually impaired, 3MS, Singapore-adapted MMSE) was used to assess cognitive health in ten of the thirty included studies. Other cognitive tests such as the Montreal Cognitive Assessment (n = 3), the Abbreviated Mental Test (n = 3) and Telephone Interview for Cognitive Status (n = 4) were commonly employed measures of cognitive health. However, these are tests of general cognition, leaving a gap in the literature as to whether general cognition or particular cognitive domains are affected by visual impairment. Eleven studies used diagnosis of dementia or Alzheimer’s Disease to define cognitive impairment. However, there was heterogeneity between studies as to the diagnostic criteria used, including the International Classification of Diseases (ICD: versions 9 and 10) and Clinical Dementia Rating (CDR). This method of measuring cognitive impairment relies on practitioner opinion which may differ between physicians. It also fails to account for cognitive impairment which is too mild or different in clinical manifestation to be considered dementia. Two studies [12, 29] defined cognitive health as the frequency of participation in cognitively stimulating activities such as reading, solving crosswords, and playing board games. Whilst participation in cognitively stimulating activities is associated with improved cognitive functioning [29], it is not a direct measure of cognitive status. Therefore, it is with caution that participation in cognitively stimulating activities is taken to correlate with cognitive health. A wide variety of measures of cognition were used across the studies, assessing a range of domain-general and domain-specific factors. Further research is needed to disentangle the relationship between visual impairment and specific aspects of cognition, and how impairments in specific aspects of vision affect different cognitive domains.
Association of visual impairments and cognitive decline
To varying degrees, 24 of the studies reported an association between visual impairment and poorer cognitive functioning. Davis-Kershaw et al. (2018) [5] reported a significant association between self-rated moderate and poor vision and dementia risk, with a two-fold and four-fold increase, respectively. These findings are supported by a recent longitudinal study which reported a significant association between visual acuity and dementia onset at a six-year follow up. Moderate and severe visual acuity impairment were associated with an increased risk of dementia, with severe impairment corresponding to the highest risk of dementia [16]. These findings persisted after adjustment for lifestyle factors, pre-existing health conditions and demographic characteristics.
Poor visual acuity has also been associated with poor scores on the Montreal Cognitive Assessment adapted for the visually impaired (MoCA-Blind) in an urban-dwelling sample of Malaysian elders [7]. The study found a weak to moderate correlation between cognitive functioning and visual impairment; the relationship did not remain significant after adjustment for years of education. In a study of a multi-ethnic Asian population, participants with visual impairment were more likely to score poorly on the Abbreviated Mental Test of cognitive function (AMT) at baseline [17]. During the 6-year follow up, participants with a decline in AMT scores were significantly more likely to have poor visual acuity at baseline. These findings persisted after adjustment for demographic, education level and various co-morbidities. These findings are supported by Luo et al. (2018) [19], who reported an increased prevalence of professionally assessed ‘severe’ and ‘extremely severe’ dementia in participants with poor visual acuity. These findings persisted after adjustment for sociodemographic factors.
Two of the studies used data from longitudinal women’s health cohort studies [15, 27]. ‘Moderate’ and ‘worse’ visual acuity scores were significantly associated with poorer performance on various measures of cognition including the East Boston Memory Test (immediate and delayed) and the Digit Span Backwards Test in a midlife-older female sample [15]. However, the severity of visual impairment and the rate of cognitive decline was not significantly related to the severity of visual impairment. These findings are supported by Tran et al. (2020) [27] who reported an increased incidence of dementia or mild cognitive impairment in participants with visual impairment, assessed using objective measures of visual acuity. However, no such association was found between cognitive impairment and self-reported visual impairment, assessed using questionnaires. Individual analysis of common causes of vision loss: age related macular degeneration, cataract, glaucoma, and diabetic retinopathy were not associated with significant increase in dementia risk.
A longitudinal study of two cohorts (UK Biobank and EPIC-Norfolk study participants) reported double the likelihood of developing dementia in participants with moderate-severe visual impairment, compared to participants with no visual impairment [18]. The relationship between visual and mild cognitive impairments was not significant in either cohort. After sensitivity analyses, excluding participants with less than 5 years of data or self-reported poor health, the main results were attenuated indicating a potential reverse causation or bidirectionality. In a study of participants from low- and middle-income countries, near visual impairment but not far visual impairment was associated with mild cognitive impairment, assessed using the National Institute on Aging-Alzheimer’s Association criteria [24]. Co-occurring near and far visual impairment was also significantly associated with MCI but the independent association of near and far vision requires further research to examine the underlying mechanism influencing these findings.
Visual acuity, contrast sensitivity and stereo acuity impairments have also been identified as potential risk factors for cognitive decline. All three measures of vision were significantly associated with poorer scores on the Modified Mini-Mental State Examination (3MS), compared to participants with no visual impairment [26]. None of the measures of vision impairment were associated with poorer performance on the Digit Symbol Test and when evaluated as a continuous variable, only contrast sensitivity was associated with annual reductions in 3MS scores. These results suggest visual impairment may be connected to cognitive decline, but the effects may vary based on different measures and facets of vision and cognition.
Regular participation in cognitively stimulating activities is associated with a reduced risk of developing cognitive impairment [29]. Visual acuity, contrast sensitivity and stereo acuity impairments were reported to correlate with a lower frequency of engagement in cognitively stimulating activities such as community event attendance, reading or writing, and handcrafting. Longitudinally, the amount and rate of decline in participation in cognitively stimulating activities was not significantly higher for participants with poor visual acuity and contrast sensitivity and the frequency of participation decreased for participants, regardless of visual status. Higher participation at baseline was associated with superior cognitive functioning, but did not have protective effects against cognitive decline, longitudinally.
The possibility of a bidirectional relationship between cognitive decline and visual impairment was investigated in a recent longitudinal study of a multi-ethnic Asian sample [31]. After adjustment for confounds, the presence of cognitive impairment at baseline was associated with twice the probability of moderate to severe visual impairment and worse visual acuity at the 6-year follow up. Most cases of visual impairment were caused by correctable conditions: cataracts and under-corrected refractive error. The idea that the relationship between cognition and vision may differ across various measures of vision and domains of cognition was investigated by Varadaraj et al. (2021) [30]. Poor measures of visual acuity and stereo acuity were associated with declines in memory and language, whereas poor contrast sensitivity was associated with declines in memory, language, visuospatial ability, and attention.
Six of the studies reported no association between visual impairment and cognitive decline [3, 11, 13, 21, 22, 23]. Bazzazi et al. (2019) [3] investigated the cross-sectional relationship between visual acuity and performance across various vision related and unrelated cognitive domains: auditory information processing speed and flexibility (PASAT), verbal working memory (digit span subtest from WAIS-IV), age related cognitive impairment (MMSE), executive cognitive dysfunction (Clock Drawing Test). Visually impaired participants did not show significant differences in performance on any of the cognitive tests compared to a gender and age matched visually unimpaired control. However, higher education level was associated with better performance on cognitive tests. A ten-year longitudinal study [13] found no significant decline in MMSE scores in visually impaired participants at the five-year or ten-year follow up. These findings persisted after adjustment for confounds, with only age showing an association with cognitive deterioration.
Despite reporting an association between hearing impairment and cognitive decline, three longitudinal studies reported no association between visual impairment and cognitive decline [11, 21, 22]. Ge et al. (2020) reported no significant change in cognition over eight years, as assessed using the Health and Retirement Study version of the Telephone Interview for Cognitive Status [11]. These findings are supported by a similar study which found no association between previously diagnosed visual impairment and onset of dementia, both diagnosed using the ICD-10 diagnostic criteria [22]. No association was found between visual impairment, assessed using a vision testing chart and dementia incidence of any severity, measured by the Japanese Ministry of Health, Labour and Welfare Dementia Scale [21]. The only association with visual impairment was the significantly higher prevalence of day-night reversal symptoms but further research is needed to investigate the validity and mechanism of this finding.
Vision correction
Visual impairment caused by refractive error can usually be corrected with lenses or surgery. The recent finding by Vu et al. (2022) [31] that baseline cognitive impairment was associated with over double the probability of visual impairment, and the fact that most of the participants with visual impairments had easily correctable conditions, emphasises the need for early diagnosis and intervention of visual impairments.
Cao et al. (2020) [4] investigated the longitudinal relationship between visual impairment (near visual impairment, distance visual impairment and co-occurring distance and near visual impairment), visual correction and cognitive function in a middle-to-older-aged population. Participants with no visual impairment showed high cognitive scores at baseline and throughout the study. Near visual impairment, distance visual impairment and co-occurring near and distance visual impairment all showed significantly poorer cognitive status, global cognition, and episodic memory over time than participants without visual impairment, with co-occurring near and distance visual impairment associated with worse cognitive status across all domains. Corrected vision (spectacles or contact lenses) was associated with significantly better cognitive outcomes than participants without corrected vision, even when participants did not have visual impairment.
These findings are consistent with an earlier study which suggested that wearing reading glasses in older age might protect against future cognitive deterioration [25]. Better visual acuity was associated with better cognitive functioning as assessed by the Mini Mental State Examination for the visually impaired (MMSE-Blind) and these finding persisted after adjustment for age, sex, and number of years in education. However, the finding that wearing reading glasses was associated with better cognitive functioning was no longer significant after adjustment for years in education. These findings highlight the importance of regular vision testing and correction of visual impairments. They also implicate corrective lenses as a potential protective measure against cognitive decline, but further research is necessary to authenticate this proposal.
Functional impairment
Five studies discussed the functional limitations associated with cognitive decline and related vision loss. A recent study investigated the effects of visual and hearing impairments on physical functioning as assessed by the NHATS late-life disability scale [2]. Co-occurring hearing and vision impairment was associated with a higher prevalence of functional difficulties compared to participants with single sensory impairment (SSI) and no sensory impairment (NSI). This was true of all three domains used to assess physical functioning: mobility limitations, self-care limitations and household activities, with the prevalence in participants with DSI being 1.45, 1.41 and 1.54 times higher than participants with NSI, respectively. Clinically assessed dementia or Alzheimer’s disease was also independently associated with functional limitations, particularly with mobility impairment. The combination of DSI and dementia was most strongly associated with all physical limitations, compared to participants with NSI and no dementia, and DSI but no dementia.
Vision-related functional difficulties have been reported to increase the probability of co-occurring dementia or moderate cognitive impairment no dementia (CIND) [9]. Visual acuity, the effect of vision on completing daily tasks (vision-specific functioning) and independent movement (vision-specific mobility) were independently linked with an 1.5-2-fold increased probability of also having clinically determined cognitive decline, as assessed by the Montreal Cognitive Assessment and the Singapore-adapted Mini-Mental State Exam among other cognitive tests. Interventions with the aim of improving visual health were suggested as a preventative measure against cognitive and physical decline [12]. Severe self-reported vision impairment was associated with physical functioning decline in the female group and total sample group (assessed by frequency of participation in physical activities, e.g., walking, cycling gardening) and cognitive decline in all groups (assessed by frequency of participation in cognitive activities, e.g., reading, writing, crosswords).
Despite reporting no association between visual field loss and cognitive impairment, Mudie et al. (2018) also investigated the effects of glaucomatous visual field loss and functional decline. Measures of functioning included grip strength testing, self-reported functioning questionnaires, geriatric depression testing, communication functioning testing, emotional and social functioning, driving limitations and Activities of Daily Living Questionnaires. Participants with DSI or only visual field loss reported difficulty driving due to their visual impairment: 83.3% and 85.7%, respectively. DSI was also associated with communication functioning scores, with 26.8% of participants with DSI reporting communication difficulties, compared with 6.7% of visually impaired participants. There was no significant difference between grip test scores between groups. These findings demonstrate the relationship between sensory impairment and functional decline, with DSI posing the greatest risk to everyday functioning.
Dual sensory impairment
Eleven studies investigated co-occurring vision and hearing loss, compared to SSI. Articles which did not assess the independent association of vision loss and cognitive impairment were rejected at the full-text level. Sensory impairments are often comorbid with cognitive decline and dementia, prompting research into the independent and combined risk of DSI on cognitive health. However, the methodology and design vary drastically between studies and the results are mixed.
Self-reported vision, hearing and combined sensory impairment have been identified as predictors of cognitive decline. In a recent longitudinal study, participants scored lower in the domains of delayed recall, verbal fluency, and processing speed at the 8-year follow up compared to baseline testing [6]. Poor visual acuity, low speech-reception-threshold scores and DSI also showed an independent association with increased incidence of all-cause dementia at follow up (median follow-up 11.1 years). The results remained significant after the removal of participants who developed dementia within one-year post-baseline [14]. Hearing, visual and DSI have also been associated with an increased incidence of probable dementia and CIND. Compared to participants with no sensory impairment, visually impaired participants showed a 35% and 25% increased likelihood of developing CIND or probable dementia, respectively [20]. These results are supported by the finding that hearing, visual and olfactory impairments are all independently associated with cognitive impairment. Impaired contrast sensitivity was significantly associated with poor performance on the Mini-Mental State Examination (MMSE) or a history of dementia or Alzheimer’s Disease during follow-up [10].
The co-occurrence of hearing and visual impairment has been shown to be the greatest threat to cognitive health, compared to SSI. Several studies have shown hearing loss and DSI, but not vision loss, to be a risk factor for cognitive decline [11, 21, 22]. In contrast, a cross sectional study of 250,752 Chinese participants found visual impairment and DSI to be associated with an increased risk of severe to extremely severe dementia [19]. No such association was found between vision loss and moderate dementia, and hearing loss was not associated with cognitive decline at any severity level.
There is also evidence to suggest no association between visual, hearing and DSI and cognitive impairment [13, 23]. Poor visual acuity and hearing were found to have no independent or dual association with a decline in MMSE scores in a longitudinal study of Australian older adults [13]. These findings are supported by another study which defined visual impairment as glaucomatous visual field loss [23]. No significant association was found between visual field loss and cognitive impairment, as measured by the MMSE-Blind. The diverse nature of the results suggests that the association between sensory impairment and cognitive decline is sensitive to the modality of assessment.