Vision Impairment May Precede Cognitive Impairment

Vision impairment is associated with an increased risk of dementia and cognitive impairment in patients older than 50, according to research published in Ophthalmology. Investigators concluded that screening and treatment — particularly in low- and middle-income countries — may help reduce the incidence of dementia globally.

According to researchers, sensory impairment plays an important role in the development of dementia, and studies have linked vision impairment with hastened cognitive decline. Further, causes of vision impairment, such as age-related macular degeneration and glaucoma, also have been linked to Alzheimer disease. 

Because most vision impairment cases can be prevented or treated, investigators wanted to explore if identifying and treating impairment was a modifiable risk factor for preventing dementia. The team conducted a literature search. Relative risks (RRs) were pooled and publication bias was evaluated. The team estimated the global burden of dementia associated with vision impairment based on the pooled RR for dementia associated with vision impairment, and the global burden of disease data on dementia and vision impairment prevalence.

These findings revealed that people with vision impairment had a 47% higher risk of developing dementia and a 35% higher risk of developing cognitive impairment than those without impairments. Vision impairment was a significant factor in the global burden of dementia in older adults aged 50 years or older, with those in low-income countries having a higher risk.

Specifically, in the meta-analysis of 14 prospective cohort studies with 6,204,777 participants and 171,394 dementia cases (follow-up range, 4 years to 14 years), the pooled RR (95% confidence interval [CI]) associated with vision impairment was 1.47 (1.36 to 1.60). In the meta-analysis of 12 prospective cohort studies with 45,065 participants and 13,299 cases of cognitive impairment (follow-up range, 3 years to 18 years), the pooled RR (95% CI) was 1.35 (1.28 to 1.41). Stratified analyses show that vision impairment associated with incident dementia and cognitive impairment was similar across vision assessment methods, length of follow-up, and study quality. The global number of people with dementia associated with moderate/severe vision impairment in 2016 was 2.1 million (80% uncertainty interval: 1. 0-3.3), which accounted for 4.7% (2.3% to 7.5%) of the global burden of dementia. Economic inequality was significant for the burden of dementia associated with vision impairment. However, researchers note that the overall quality of evidence was low because of the studies’ observational design. 

Researchers note several other limitations of their study, including that the measures for cognition using visual stimuli might overestimate the association between vision impairment and dementia/cognitive impairment. Secondly, only 2 studies reported data for the association between vision impairment and specific types of dementia. Other limitations include the lack of a meta-analysis of the association between the leading causes of vision impairment and incident dementia and the inclusion of only a small number of cohort studies. Further, investigators concede that RRs for cognitive impairment associated with vision impairment in studies with different follow-up periods may not be comparable. In addition, there may be some overlap in data in 2 of the studies included in the analysis, and prospective cohort studies from only 7 countries were included in the meta-analysis, which may bias the pooled RR used to estimate population attributable risk (PAR). Lastly, the number of cases of dementia associated with vision impairment may have been overestimated because researchers did not weigh the calculation of PAR for communality of risk factors.

Reference

Shang X, Zhu Z, Wang W, et al. The association between vision impairment and incidence of dementia and cognitive impairment: a systematic review and meta-analysis. Ophthalmology. Published online January 7, 2021. doi:10.1016/j.ophtha.2020.12.029.