Today's research materials report on a solid correlation between cataract surgery to restore vision and lower risk of later dementia. This provides support for the view that a reduced flow of sensory information to the brain accelerates the onset of neurodegeneration and loss of function with age. This is quite distinct from the usual set of underlying biochemical processes that are investigation in connection with cognitive decline and dementia: the accumulation of molecular waste in the brain; the chronic inflammation of brain tissue; the loss of mitochondrial function; the dysfunction of the vascular system leading to lesser delivery of oxygen and nutrients to an energy-hungry tissue; and so forth. In addition to all of these, laid atop the foundation of a failing biology, there is good evidence for a "use it or lose it" view of the aging brain.
In the past, researchers have found correlation between age-related retinal degeneration (and consequent vision loss) and dementia, and between and age-related deafness and dementia. There is considerable discussion in the research community regarding the causes of the latter association. Is it that brain networks thrive on input, and deafness reduces that input, or is it that common processes of aging damage both the brain and the sensory hair cells of the inner ear? Looking at the surgical reversal of cataracts sidesteps the question of common age-related mechanisms, and supports the view that there is a causal relationship between lack of input to the brain and the pace of progression towards neurodegeneration and loss of cognitive function.
The Adult Changes in Thought (ACT) study is a long-standing, Seattle-based observational study of more than 5,000 participants older than 65. Based on the longitudinal data of over 3,000 ACT study participants, researchers have now found that subjects who underwent cataract surgery had nearly 30% lower risk of developing dementia from any cause compared with those who did not. This lowered risk persisted for at least a decade after surgery. Cataract surgery was also associated with lower risk of Alzheimer disease dementia specifically. The mechanisms by which cataract surgery and lessened dementia risk are associated was not determined in this study. Researchers hypothesize that people may be getting higher quality sensory input after cataract surgery, which might have a beneficial effect in reducing the risk of dementia.
Twenty percent of adults older than 65 years in the United States experience significant sensory impairment, such as vision or hearing loss, even with correction. Addressing sensory loss that affects a substantial portion of older adults may be a potentially modifiable risk factor for dementia in late life. Because sensory impairments and dementia are both strongly associated with aging, more knowledge about the association between sensory impairment and dementia may have important implications for individual and global public health, particularly if interventions to improve sensory function reduce dementia risk.
Visual impairment is an important dementia risk. Cataract is a leading cause of blindness worldwide, affecting more than 35 million people globally and causing blindness in approximately 20 million. Cataract affects most older adults at risk of dementia. However, there are conflicting results regarding the association between cataract extraction and cognitive impairment or dementia.
We hypothesized that older adults with cataract who undergo cataract extraction may have a lower risk of developing dementia compared with participants who do not undergo cataract surgery or participants who undergo other eye procedures that do not restore vision, such as glaucoma surgery. Previous studies exploring this association have been limited by small sample sizes, cross-sectional designs, and varying qualities of dementia assessment. More importantly, these studies have failed to account for healthy patient bias (i.e. when surgery is more likely in healthier individuals with the same cataract severity).
In total, 3038 participants were included, with mean age at first cataract diagnosis of 74.4 years. Based on 23,554 person-years of follow-up, cataract extraction was associated with significantly reduced risk (hazard ratio, 0.71) of dementia compared with participants without surgery after controlling for years of education, self-reported White race, and smoking history and stratifying by apolipoprotein E genotype, sex, and age group at cataract diagnosis. Similar results were obtained in marginal structural models after adjusting for an extensive list of potential confounders. Glaucoma surgery did not have a significant association with dementia risk. Similar results were found with the development of Alzheimer's disease dementia.