Lee CS, Gibbons LE, Lee AY, Yanagihara RT, Blazes MS, Lee ML, McCurry SM, Bowen JD, McCormick WC, Crane PK, Larson EB. Association Between Cataract Extraction and Development of Dementia. JAMA Intern Med. 2022 Feb 1;182(2):134-141. PubMed.

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  1. These four studies help to raise more awareness about the impact of hearing and visual impairments on the development of dementia and cognitive dysfunction. The article by Lee et al. is very interesting as it provides initial evidence that vision loss may be causally related to dementia, since cataract extraction was associated with reduced risk of dementia. More studies need to be done to confirm this finding, with special attention paid to addressing limitations associated with an observational study design and ascertainment of cataract diagnoses and surgery. But the observed findings are intriguing and can motivate further studies to examine whether visual impairment is a causal risk factor for dementia and to test potential interventions to address vision loss.

    I was also struck by the results reported by Liu et al., who showed the heterogeneity in how hearing and vision problem are accounted for during cognitive testing. This is an important descriptive paper because one of the main questions in the field regarding the relationship between hearing and/or vision and cognition, including dementia, is to what degree is performance on cognitive testing influenced by poor hearing and vision, and how that might affect the interpretation of the results. To evaluate the body of evidence more accurately, we need greater consistency in how studies account for participants who have problems with hearing and/or vision at the time of cognitive testing and in what those procedures might be. Hopefully, this paper will stimulate discussion and action around this.

    View all comments by Phillip Hwang

  2. I think it is still unclear what mechanisms are driving the association between sensory impairments and dementia risk. However, if sensory impairments are indeed risk factors for dementia, this would offer an important avenue for intervention. Hearing aids and vision corrections are underutilized, and often interventions occur in more severe stages. There is room to improve aspects of hearing and vision care, as well the accommodations that make it easier for those with impairments to navigate the world.

    But we really aren't quite able to say that sensory impairments cause dementia. There is some question around what cognitive tests are measuring in those with sensory impairment, and whether associations are due to reverse causation or shared risk factors (e.g., genes, vascular disease). As most studies just describe associations in older adults, it's very difficult to tease these issues apart.

    It's important we have more studies aimed at understanding the underlying biology, including those that can test specific mechanisms and different hypotheses. I think Lee et al.’s paper on cataract surgery is a nice attempt to account for some of the potential biases, since they include modeling approaches to reduce bias as well as examine a negative control (glaucoma).

    Overall, I'd say, there is a very intriguing connection between sensory loss and dementia. There is a lot we still don't know about these associations, and a need for further studies. One of the biggest limitations, highlighted by Liu et al., is that many studies don't include both objective sensory measurement and dementia outcomes (including cognitive, imaging, and biomarkers).

    Additionally, few studies include longitudinal sensory measures or measurement spanning the life course. This could be addressed by expanding sensory measures in aging cohorts.

    View all comments by Willa Brenowitz

  3. There is a growing interest in the relationship between sensory impairment and dementia risk. Hearing loss is an established dementia risk factor that has been highlighted as a key modifiable risk factor by both Lancet Commissions on dementia prevention, intervention, and care published in 2017 and 2020. Visual impairment has been studied less extensively. However, two recent meta-analyses suggested an increased risk of all-cause dementia associated with visual impairment (Shang et al., 2021; Kuźma et al., 2021). 

    Another study by Lee and colleagues suggested that cataract extraction is associated with lower dementia risk. This further adds to our understanding of the relationship between eye diseases and dementia risk. Our recent meta-analyses investigating visual impairment, eye diseases, and dementia risk suggested an increased risk of dementia and Alzheimer’s disease associated with cataracts (Kuźma et al., 2021). However, we identified only four prospective studies investigating these associations. I am very pleased to see another study on this topic that addresses key issues, e.g. reverse causation.

    I would like to highlight the systematic review by Liu and colleagues on the association between hearing and vision impairment in cohort studies collecting cognitive data in older adults. I want to emphasize their call for the development of standardized methods to collect cognitive data that consider sensory impairment. Studies involving older people who may have sensory impairment vary greatly in terms of the methodology used, e.g. sample selection, adjustment strategy, and ascertainment of sensory impairment and dementia, which makes comparisons difficult. Harmonization of protocols across cohorts would greatly improve the quality and comparability of studies, and validation of results, ultimately leading to a much better understanding of cognitive changes and dementia.

    Identifying modifiable risk factors for dementia that could inform the development of targeted interventions or help identify those at risk of developing dementia is of great importance given there is currently no cure for dementia. Given that most causes of sensory impairment are treatable or preventable, managing these problems could help improve the quality of life and reduce dementia risk.

    More research is needed to assess the potential to prevent dementia using interventions that reduce sensory impairment. More well-designed studies on dual sensory impairment with objective measurements are also needed as hearing and visual impairment often co-occur. Although there are still a lot of questions to be answered, especially in terms of mechanisms that could explain the sensory-dementia associations, sensory impairment is a very promising target for dementia prevention.

    References:

    Shang X, Zhu Z, Wang W, Ha J, He M. The Association between Vision Impairment and Incidence of Dementia and Cognitive Impairment: A Systematic Review and Meta-analysis. Ophthalmology. 2021 Jan 8; PubMed.

    Kuźma E, Littlejohns TJ, Khawaja AP, Llewellyn DJ, Ukoumunne OC, Thiem U. Visual Impairment, Eye Diseases, and Dementia Risk: A Systematic Review and Meta-Analysis. J Alzheimers Dis. 2021;83(3):1073-1087. PubMed.

    View all comments by Elżbieta Kuźma

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