Cataract surgery is the most frequently performed surgical procedure in South Korea according to the KNHIS.5 Although there have been several studies that evaluate the relationship between cataract surgery and mortality from other countries, it is uncertain whether findings from other countries are generalizable to the Korean population. Moreover, there has been no published study evaluating the relationship between cataract surgery and mortality in the Korean elderly population using the NHIS-Senior database. Given these strengths of the NHIS database and the need for further understanding of the relationship of cataract surgery with total and cause-specific mortality, we aim to investigate the relationship between cataract surgery and total and cause-specific mortality in the Korean elderly population.
In this nationwide cohort study, Korean elderly population with cataract who underwent cataract surgery showed a decreased hazard of all-cause mortality when compared with those who did not undergo surgery after adjusting for demographics, and systemic and ocular comorbidities. Cataract surgery was associated with a borderline increased hazard of mortality in the unadjusted model (HR, 1.03; 95% CI, 1.01–1.05), but a decreased hazard of mortality in the adjusted model accounting for age and sex (HR, 0.95; 95% CI, 0.94–0.97). This protective association increased after additionally adjusting for systemic and ocular comorbidities, and patients with cataract surgery had a 7% reduced adjusted hazard of mortality when compared with those without surgery. Moreover, the elderly population after cataract surgery had a lower risk of death due to vascular and neurologic conditions than those with cataract who did not undergo cataract surgery. The protective association between cataract surgery and mortality seemed to be affected by age, gender, income, CCI score, and the presence or absence of glaucoma. Patients with cataract who were 85 years of age and older, women, those who had lower income and a severe burden of systemic disease as measured by the CCI, or those without glaucoma revealed the largest reductions in mortality hazards resulting from cataract surgery.
One previous study from the United States population reported that cataract surgery was associated with a decreased all-cause mortality in cataract patients after adjusting for demographics, and systemic and ocular comorbidities.8 Two previously published reports conducted in Western Sydney, Australia also demonstrated that cataract surgery was associated with decreased all-cause mortality in patients with cataract.6,7 In accordance with these results, our results showed that cataract surgery was associated with a decreased all-cause and cause-specific mortality, especially vascular and neurologic causes, after adjusting for demographics, and systemic and ocular comorbidities in the Korean elderly population.
We hypothesized that cataract surgery can be protective against all-cause mortality via the improvement in overall function from decreased fracture and accidents, improved mental health, and increased social and physical activities.8 Both improvements in quality of life and reduction in depressive symptoms after surgery can also make a contribution to the protective association.10,11
Several studies demonstrated that patients report higher scores on cognition assessments after cataract surgery.10,12−14 In one recent study that evaluated the effects of cognitive performance and visual acuity on mortality, impairment in cognitive performance and vision increased the odds for mortality.15 Among cognitive impaired elderly population, impairment in vision predicted nearly a three-fold increased risk of all-cause mortality (HR, 2.74; 95% CI, 2.02–3.70) and nearly a four-fold higher risk of non-cardiovascular disease/non-cancer mortality (HR, 3.72; 95% CI, 2.30-6.00) compared to having neither impairment.15 Our results, showing a protective association between cataract surgery and mortality from neurologic causes, were in line with those studies. This protective relationship increased after fully adjusting for demographics, and systemic and ocular comorbidities, and patients with cataract surgery had almost 30% reduced adjusted hazard of mortality compared with those without cataract surgery. Although we did not investigate the relationship between neurologic diseases and eventual mortality before and after cataract surgery, we suggest that improvement in cognitive performance and vision after cataract surgery plays a crucial role in the decreased mortality from neurologic causes.
In the present study, we demonstrated a protective association between cataract surgery and mortality from vascular causes after adjusting for demographics, systemic and ocular comorbidities. Cardiovascular risk factors which were associated with cataract and cataract surgery include hypertension, diabetes, hypercholesterolemia, and high body mass index (BMI).16–18 Meta-analyses of cohort studies showed that hypertension is related to incident cataract, especially posterior subcapsular cataract.16,17 Cataract surgery is not only more prevalent, but also performed at younger age in a high cardiovascular risk cohort.19 Patients being less than 65 years old with taking hypertension medication showed a higher incidence of cataract surgery, and angina history was associated with a higher incidence of cataract surgery.20,21 Although there have been no studies evaluating the relationship between cataract surgery and mortality from vascular causes, we suggest that cataract surgery can be protective against mortality from vascular causes, possibly via an increased ability to receive routine medical care, to take medications properly, and to maintain physical activities secondary to the vision improvement after cataract surgery. Further study on the inter-relationship among cataract surgery, cardiovascular disease, disease-related mortality is needed to investigate the mechanisms underlying the relationship between cataract surgery and mortality from vascular causes.
In a recent published study that evaluated the relationship between cataract surgery and all-cause and cause-specific mortality in older women with cataract, cataract surgery was associated with increased all-cause and cause-specific mortality (cancer, vascular, pulmonary, infectious, and accidental, conditions) after adjusting for demographics, systemic and ocular comorbidities, alcohol intake, smoking status, BMI, and physical activity.9 In the present study, cataract surgery was associated with an increased cause-specific mortality (infectious and trauma or accidental). Given that cataract related-vision impairment is associated with increased incidence of fall and fracture, and cataract surgery showed a reduced risk of fracture and accidents, protective association can be attributed to reductions in fracture and accidents after cataract surgery.22–26 However, several studies that evaluated interventions for preventing falls in elderly patients reported that the relationship of vision improvement after cataract surgery with reduced accidents and falls is uncertain.27–29 Moreover, our observed increases in the HR might be the result of individually postponing surgery until time point where the HR increases, in addition to the general effects of covariates.9
Our study had some limitations. First, this study was mainly limited by its observational nature. Second, as this study was based on data from a medical insurance claims database, the diagnostic accuracy of cataract cannot be guaranteed. The identification of patients with cataract surgery, systemic comorbidities, and ocular comorbidities, using healthcare claims and Korean Standard Classification of Diseases (KCD) and Korean Electronic Data Interchange (KEDI) codes, might be inaccurate when compared with information obtained from medical charts. Moreover, the NHIS-Senior database cannot provide the information on cataract grading, objective visual acuity, axial length, presence of pseudoexfoliation syndrome, and postoperative inflammation grade. In addition, there was a lack of availability of certain covariates including metabolic profiles, BMI, alcohol intake, smoking status, and physical activity, proposing the need for further studies including various covariates. Finally, we focused only on residents of South Korea. Therefore, the observed findings cannot be generalized to other ethnic groups.
Irrespective of these limitations, this is the first report revealing the significant relationship between cataract surgery and all-cause and cause-specific mortality for the elderly population in South Korea using a nationwide, general population-based database. Another point is that this study used a large sample size of the NHIS-Senior database and selection bias is relatively low as the entire Korean population was enrolled in the same insurance system. In addition, we followed up the subjects until 3 years after the diagnosis of cataract without missing data because of the thorough nature of the NHIS.
In conclusion, we demonstrated that cataract surgery decreased all-cause and cause-specific mortality (vascular and neurologic causes) in the Korean elderly patients with cataract, especially in patients of 85 years of age and older, women, lower income, having a CCI score of 5 or more, and having no glaucoma. Even though cataract surgery group showed lower mortality rates, it does not definitely prove a causal relationship between cataract surgery and decreased mortality, and the mechanisms underlying the relationship between cataract surgery and decreased mortality are unclear. Therefore, further studies evaluating the relationships among the cataract surgery, systemic disease, and disease-specific mortality are needed for improving the selection for patient and cataract surgery timing.