Many aspects of aging correlate with one another, even those with quite different underlying mechanisms and proximate causes. The various forms of root cause damage that result in the aging process, as well as their downstream consequences, all interact with one another. So whether or not any specific correlation teaches us anything about the way in which aging works under the hood is very dependent on the details. That loss of vision and hearing correlate with dementia risk is known, but the relative contribution of different mechanisms is up for debate. How much is due to similar biochemical mechanisms of damage in nervous system tissues, and how much is due to loss of sensory stimulation, for example.
Recent studies reported that the incidence of dementia in western countries may be declining whereas low and middle income countries are predicted to have the largest increase in incident dementia. This discrepancy is suggested to be due to the differences in the effective management of cardiovascular disease, hypertension, and diabetes in these regions. However, even though early recognition of risk factors for dementia is of utmost priority, the sporadic incidence of dementia is a challenge in preventive medicine.
Previous studies have suggested that an association exists between vision loss and cognitive impairment. Low vision and blindness are commonly seen in the older population as the risk of developing cataract; age-related macular degeneration and glaucoma increase with advancing age. Despite the body of evidence, there are few reports regarding causal relationships or direct association between low vision and dementia. A clear understanding of this association may facilitate the development of strategies for reducing the burden of cognitive impairment.
The National Health Insurance Service (NHIS) database has recently become accessible to researchers in Korea. This large-scale database permits the identification of the longitudinal incidence of diseases and allows for the analysis of the association between diseases and health conditions. Using the six levels of disability of the Korean rating system, an individual with low vision can be classified depending on his/her visual acuity. Employing the disability grade used in Korea, we were able to investigate the impact of low vision on incident dementia by using a nationwide population-based cohort that includes over 1.5 million Koreans.
Statistical analysis showed that subjects with more severe visual impairments have a higher risk of dementia, Alzheimer's disease, and vascular dementia after adjusting for compounding variables. The hazard ratios (HRs) of dementia increased significantly as visual acuity worsened: 1.444 for visual acuity (VA) < 1.0, 1.734 for VA < 0.3, 1.727 for VA < 0.1, and 1.991 for visual loss. Baseline visual loss and visual impairment were positively associated with the risk of dementia, Alzheimer's disease, and vascular dementia. From the results of this nationwide population-based cohort study, we suggest that there is a significant increase in the incidence of dementia in subjects with low vision.