Age-related cataract, cataract surgery and subsequent mortality: a systematic review and meta-analysis

Methodological quality of the review: Medium confidence

Author: Song E, Sun H, Xu Y, Ma Y, Zhu H, Pan CW.

Region: United States of America (USA), Australia, Netherlands, China, India

Sector: cataract surgery, age-related cataract

Sub-sector: Risk, mortality

Type of cataract: Age-related cataract

Equity focus: None specified

Quantitative synthesis method: Meta-analysis

Qualitative synthesis method: Not applicable

Background:

Age-related cataract seems to increase the risk of death, however the relationship of different cataract subtypes and mortality remains unclear. Changes in lens may reflect the status of systemic health of human beings but the supporting evidences are not well summarized yet.

Research objectives:

Authors aimed to determine the relationship of age-related cataract, cataract surgery and long-term mortality by pooling the results of population-based studies.

Main findings:

In total, authors included 10 population-based studies including 39,659 individuals at baseline reporting the associations of any subtypes of cataract with all-cause mortality from 6 countries. Four studies were conducted in the USA, two in Australia, and one each in Italy, Netherlands, China and India. In most of the included studies, mortality data were reviewed and retrieved from official records or hospital medical records. All studies adjusted for age in the multivariate analysis; however, diabetes – the most important confounding factor – was not adjusted for in one study. All studies gave loss to follow-up rates and described sampling methods in the baseline examinations, albeit in varying degrees. However, only 6 studies outlined specific exclusion criteria and provided information on non-responders. Conflicts of interest were not reported in all included studies.

Authors reported that the presence of any cataract including cataract surgery was significantly associated with a higher risk of death (pooled HR: 1.43, 95% CI, 1.21, 2.02; P,0.001; I2 = 64.2%). In the meta-analysis of nine study findings, adults with nuclear cataract were at higher risks of mortality (pooled HR:1.55, 95% CI, 1.17, 2.05; P = 0.002; I2 = 89.2%). In the meta-analysis of eight study findings, cortical cataract was associated with higher risks of mortality (pooled HR: 1.26, 95% CI, 1.12, 1.42; P,0.001, I2 = 29.7%). In the meta-analysis of six study findings, posterior subcapsular cataract (PSC) was associated with higher risks of mortality (pooled HR: 1.37, 95% CI, 1.04, 1.80; P = 0.03; I2 = 67.3%). The association between cataract surgery and mortality was marginally non-significant by pooling eight study findings (pooled HR: 1.27, 95% CI, 0.97, 1.66; P = 0.08; I2 = 76.6%).

Based on these findings, authors concluded that all subtypes of age-related cataract were associated with an increased mortality, with nuclear cataract having the strongest association among the three cataract subtypes. However, cataract surgery was not significantly related to mortality. These findings indicated that changes in lens may serve as markers for ageing and systemic health in general population.

Methodology:

Inclusion criteria consisted of population-based studies, reporting any subtypes of age-related cataract or cataract surgery as an independent variable and all-cause mortality as the outcome measure.

Authors searched the electronic databases of PubMed and Embase for relevant papers reporting the longitudinal association of age-related cataract, cataract surgery with all-cause mortality published up to March 2014. Titles and abstracts were reviewed by two reviewers independently. Furthermore, authors included studies only if the summary estimates were relative risk or hazards ratios with 95% confidence interval were reported. Authors summarized the measures of association as HR for all the studies.

Authors excluded studies if they were not population-based, did not have standardized cataract grading, or were published in languages other than English. If one study reported two results at different follow-up periods, the result with a longer follow-up period was included in the analysis. The study quality was assessed using the tool described by Sanderson et al. The variables examined included the methods for selecting study participants, methods for measuring exposure (age-related cataract or cataract surgery) and outcome variable (all-cause mortality), design-specific sources of bias, methods for controlling confounding, statistical methods and conflict of interest.

Authors meta-analyzed the fully adjusted study-specific risk estimates under random effects model, accounting for both within- and between-study variability. All-cause mortality was treated as the outcome measure while nuclear, cortical, PSC cataract or cataract surgery were analyzed as the independent variable. Statistical heterogeneity among studies was evaluated using I2 Statistic. Publication bias was assessed using the Egger regression asymmetry test and the Begg’s test.

Applicability/external validity:

Authors note that this systematic review and meta-analysis confirmed a longitudinal associations of all age-related cataract subtypes but not cataract surgery with increased mortality. The association was strongest for nuclear cataract. Clinically, ophthalmologists and clinicians should be aware that lens imaging may be a useful tool for the assessment of ageing of the whole body.

Geographic focus:

Authors did not discuss the applicability of findings to low- and middle-income countries.

Quality assessment:

Overall, there is medium confidence in the conclusions about the effects of this review as some important limitations were identified. Although authors used appropriate methods to pool data for analysis, methods used to identify studies for inclusion were not thorough to ensure that publication was avoided. In addition, language bias was not avoided as authors included studies written in English only. It is not clear from the review if two authors independently extracted data and assessed the quality of included studies avoiding biases. Nevertheless, authors appropriately reported limitations of the review and included studies.