Author: Nagarajan N, Assi L, Varadaraj V, et al.
Geographical coverage: Global
Sector: Burden of disease
Equity focus: Older adults
Study population: People aged 50+
Review type: Other review
Quantitative synthesis method: Narrative synthesis
Qualitative synthesis method: Not applicable
Background: There has been increasing epidemiological research examining the association between vision impairment (VI) and cognitive impairment and how poor vision may be a modifiable risk factor for cognitive decline. Both cognitive impairment and VI are expected to affect an increasing number of people over time. Prior work has suggested that cognition and vision are associated, and while there are shared risk factors (neuropathological/vascular), there is also longitudinal evidence that VI is associated with cognitive changes – although the mechanism has not been fully identified.
Objectives: To synthesise the published literature on the association of VI with cognitive decline, cognitive impairment or dementia, to aid the development of interventions and guide public policies pertaining to the relationship between vision and cognition.
Overall, most of the included studies examining the vision-cognition relationship report that VI is associated with more cognitive decline, cognitive impairment or dementia among older adults.
A total of 110 articles were included in the review, encompassing 9,799,329 participants (range: 112-7 210,535 per study), with a mean age of 73.0 years, (range: 50.0-93.1). Of the 110 studies included, 53 were cross-sectional, 43 were longitudinal and 14 had a case-control study design. The range of follow-up time for the longitudinal studies was 2 months to 10 years. Of the 110 studies included, 51 reported findings from participants enrolled in population-based studies. The studies included participants from over 17 different countries, 30 studies (27%) from the USA, followed by 25 studies from Europe (23%) including the UK, Germany, Ireland, Finland, Switzerland, France and the Netherlands. Ninety of the studies were published between 2009 and 2020. All papers provided a description of sampling methods. Sixteen studies were included which were either conference abstracts or short oral presentations. Of the included studies, 17 received a rating of strong, 70 moderate and 23 weak.
Of the 110 studies included, 91 found a significant positive association between VI and cognitive decline, cognitive impairment or dementia, and 13 studies found no significant association. There were six studies that were inconclusive. Of the 91 studies that found a significant association, 77 used objective methods to assess their vision or cognitive outcome. Of the 43 longitudinal studies, 35 found a significant association between VI and cognitive decline, cognitive impairment or dementia. The most commonly presented statistical measures were ORs and HRs. The random 10% of the study sample that was separately extracted by an independent author was found to be similar to elements from the primary extraction.
Authors note that this synthesis supports the need for additional research to understand the mechanisms underlying the association between VI and cognitive impairment and to test interventions that mitigate the cognitive consequences of VI.
Inclusion criteria consisted of cross-sectional and longitudinal studies reporting a measure of association between visual function and cognitive impairment; they were included if they had ≥100 participants aged ≥50 years (mean) at baseline. Reasons for exclusion of studies were: (1) outcome measure was not vision or cognition; (2) association between vision and cognition was not explored; (3) sample size <100; (4) publication not in English; (5) mean ages <50 years; (6) no cognitive measure; (7) no vision measure; and (8) outcome was not part of inclusion criteria.
An academic librarian searched: Ovid Medline, EMBASE, Cochrane Library and PubMed from inception to March 2020, and developed a search strategy that combined controlled vocabulary and keywords related to geriatrics, cognition and vision. Searches were limited to human studies published in English. Conference and poster abstracts, and short oral presentations were also included. Three teams of two reviewers each worked independently and in duplicate to screen titles, abstracts and full-text articles to determine inclusion. Disagreements were adjudicated by a member of the other study team.
Data was extracted from the included publications by one author, and another extracted data from a random sample of 10% to compare results. Any discrepancies were adjudicated by a third author. The methodological quality of included studies was assessed by one author using the Effective Public Health Practice Project Quality Assessment Tool and the global quality ratings and findings were summarised qualitatively.
Applicability/external validity: The authors point out constraints with the design of the studies included, which could potentially affect the applicability of the findings.
Geographic focus: Participants were drawn from 17 distinct countries, with at least one being a low-to-middle-income country (China). However, the authors did not take into account potential variations in the findings across different geographical contexts.
Summary of quality assessment:
There were a number of limitations to the approaches used to identify, include and critically appraise studies. There was no reference to the reference sections of included articles being reviewed and the search was limited to material published in English. In addition, full details of the measures that informed the quality assessments of individual studies were not included. While the approaches to the narrative synthesis of the data were generally robust, only 10% of the included articles were subject to data extraction by more than one author, and there was limited consideration of the extent to which the inclusion of studies of different levels of quality might have informed the published results. For these reasons, we have low confidence in the findings of this review.
Nagarajan N, Assi L, Varadaraj V, et al. Vision impairment and cognitive decline among older adults: a systematic review. BMJ Open 2022;12:e047929. doi:10.1136/ bmjopen-2020-047929