11th December 2021, Dr Chee L Khoo
Cataract is a leading cause of blindness worldwide, affecting more than 35 million people globally and causing blindness in approximately 20 million. Visual impairment is an important dementia risk. Addressing sensory loss from visual impairment that affects older adults may be a potentially modifiable risk factor for dementia in late life (1,2) Because sensory impairments and dementia are both strongly associated with aging the link 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 (3). Does sight restoring cataract surgery reduce the risk of developing dementia?
In a population-based cohort study using data from Taiwan National Health Insurance Research Database, 491 226 subjects aged 70 or older with first-time diagnosis of cataract were screened (4). Patients with cataracts who underwent surgery were matched with patients with cataract that did not undergo surgery. Incidence rate and hazard ratio of dementia first diagnosed after more than 365 days after cataract surgery were compared between the cataract surgery and cataract diagnosis cohorts. Patients were matched for age, sex and co-morbidities including diabetes and hypertension.
The incidence rate of dementia was 22.40 per 1000 person-years in the cataract surgery cohort and 28.87 per 1000 person-years in the cataract diagnosis cohort. The rate of dementia was significantly lower in the cataract surgery group (hazard ratio 0.77, 95% confidence interval 0.75-0.79, P < 0.001). Female gender (P < 0.001) and a shorter interval between the date of first coding of a cataract diagnosis and the date of cataract surgery (P = 0.009) were significantly associated with a lower incidence rate of dementia.
Is it possible that eye surgery itself was responsible for the effects on dementia risk? The comparator in this Taiwanese study were patients with cataracts who did not undergo surgery. A recent study sought to answer that question. In a prospective, longitudinal cohort study, data from the Adult Changes in Thought (ACT) study were used to determine whether cataract extraction is associated with reduced risk of dementia among older adults (5). The ACT is an ongoing, population-based cohort of randomly selected, cognitively normal members of Kaiser Permanente Washington. The incidence rate of dementia amongst patients who underwent cataract extraction were compared with those who underwent glaucoma surgery which was not sight restoring.
3038 participants who received a diagnosis of cataract before the diagnosis of dementia were followed up. The mean age of the participants were 74 years old and the mean follow up was 5.1 years. 46% of the participants underwent cataract surgery. Like the Taiwanese study, cataract extraction was significantly associated with a 29% reduction in dementia risk (HR, 0.71; 95% CI, 0.62-0.83; P < .001). This finding of lower risk was stronger during the first 5 years following cataract surgery (HR, 0.68; 95% CI, 0.56-0.81; P < .001) compared with later years (HR, 0.76; 95% CI, 0.63-0.92; P = .02).
In comparison, 728 participants received a diagnosis of glaucoma. 105 of them underwent glaucoma surgery. Glaucoma surgery was not associated with a decreased risk of dementia (HR, 1.08; 95% CI, 0.75-1.56; P = .68).
Associations between cataract extraction and dementia development have been conflicting. In a cross-sectional study of 2764 Japanese participants, the cataract surgery group (n = 668 [24.2%]) had a lower odds ratio for mild cognitive impairment than the group without cataract surgery (n = 2096 [75.8%]) (odds ratio, 0.78; 95% CI, 0.64-0.96; P = .02), but no difference was found for dementia (odds ratio, 1.10; 95% CI, 0.75-1.62; P = .64) (6).
Why?
Sensory impairment may contribute to social isolation and decreased cognitive stimulation, which may increase the risk of dementia (1,7). However, cataract extraction could appear to have a protective association owing to healthy patient bias, in which participants who underwent cataract surgery were healthier and at lower risk of dementia.
Low vision from cataract may impair performance on vision-dependent screening tests for dementia and scores may improve after cataract surgery owing to better vision.
Visual impairment may lead to psychosocial difficulties, withdrawal from social interactions, and reduction in activity or exercise, all of which are associated with cognitive decline (1,8). Cataract-related visual impairment may also decrease neuronal input, potentially accelerating neurodegeneration or magnifying the effect of neurodegeneration through cortical atrophy. The visual cortex undergoes structural changes with vision loss (9,10). For patients with neovascular age-related macular degeneration, vision loss was associated with visual cortex atrophy during a 5-year follow-up (11) and an increase in grey matter volume has been observed after cataract surgery (12). Compensation for visual input deficit may increase cognitive load and exacerbate cognitive decline (13)
In southwest Sydney, the public waiting list for cataract surgery is ~18 months. Based on the above studies, this long wait is contributing to the incidence of dementia amongst our patients in southwest Sydney.
References:
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