The Bidirectional Relationship between Vision and Cognition: A Systematic Review and Meta-Analysis

TOPIC: Visual impairment (VI) and cognitive impairment (CIM) are prevalent age-related conditions that impose substantial burden on the society. While the bidirectional association of VI and CIM has been hypothesized, findings have been equivocal. Hence, we conduct a systematic review and meta-analysis to examine the bidirectional relationship between VI and CIM. CLINICAL RELEVANCE: 60% risk of CIM has not been well-elucidated in the literature. A bidirectional relationship between CIM and VI may provide opportunities for developing public health strategies for early detection and management of risk factors for both VI and CIM in older people. METHODS: Pubmed, Embase and Cochrane Central registers were systematically searched for observational studies, published from inception until 6 April 2020, in adults aged ? 40 years reporting objectively measured VI, and CIM assessment using clinically validated cognitive screening tests or diagnostic evaluation. Meta-analyses on cross-sectional and longitudinal associations between VI and CIM outcomes (any CIM assessed using screening tests, and clinically diagnosed dementia) were examined. Random effect models were used to generate pooled odds ratios (OR), and 95% confidence interval (CI). Publication bias and heterogeneity were examined using Egger's test, meta-regression, and trim-and-fill methods. RESULTS: Forty studies were included (N=47,913,570). Meta-analyses confirmed that persons with VI were more likely to have CIM, with significantly higher odds [OR (95%CI)] of: (i) any CIM [cross-sectional: 2.38 (1.84-3.07); longitudinal: 1.66 (1.46-1.89)], and (ii) clinically diagnosed dementia [(cross-sectional: 2.43 (1.48-4.01); longitudinal: 2.09 (1.37-3.21)], compared to persons without VI. Significant heterogeneity was partially explained by differences in age, sex and follow-up duration. There was also some evidence that individuals with CIM, relative to cognitively intact persons, were more likely to have VI, with most papers (8/9, 89%) reporting significantly positive associations, however meta-analyses on this association could not be conducted due to insufficient data. CONCLUSIONS: Overall, our work suggests that VI is a risk factor of CIM while further work is needed to confirm the association of CIM as a risk factor for VI. Strategies for early detection and management of both visual and cognitive impairment in older people may minimize individual clinical and public health consequences.