Risk of cataract in smokers: A meta-analysis of observational studies

Author: Beltrán-Zambrano E, García-Lozada D, Ibáñez-Pinilla E.

Geographical coverage: The USA, France, Australia, China, Finland, Japan, Scotland, India, Nigeria, Greece, Korea, Myanmar, Singapore, and Taiwan.

Sector: Risk of cataracts

Sub-sector: Smoking

Equity focus: Not reported

Study population: Patients with cataract

Review type: Effectiveness review

Quantitative synthesis method: Meta-analysis

Qualitative synthesis method: Not applicable

Background: Cataracts are the major cause of global blindness and are mainly reported in low- and middle-income countries. Ageing is the most significant risk factor; however, environmental exposures, particularly smoking, have also emerged as modifiable contributors. Smoking has been linked to accelerated lens opacification and oxidative damage, and may indirectly contribute to nutritional imbalances, all of which can increase the risk of cataract. Global tobacco use statistics further highlight the importance of examining its impact on ocular health. Therefore, understanding this association is vital to inform public health strategies and prevent avoidable vision loss.

Objective: To compare cataract risk in smokers and ex-smokers.

Main findings: The review included 43 studies. Of these, 13 were cohort, 12 were case-control, and the remaining 18 were cross-sectional by design. The included studies were conducted in the USA, France, Australia, China, Finland, Japan, Scotland, India, Nigeria, Greece, Korea, Myanmar, Singapore, and Taiwan. All 13 cohort studies had a low risk of bias; however, the methodological quality of case-control and cross-sectional studies varied.

Across all study designs, a significant association was found between current smoking and an increased risk of cataracts: cohort studies (odds ratio, OR: 1.41; 95% confidence interval [CI]: 1.24–1.60), case-control studies (OR: 1.45; 95% CI: 1.08–1.96), and cross-sectional studies (OR: 1.21; 95% CI: 1.09–1.34). Among ex-smokers, the association was less consistent; a significant risk was found in case-control (OR: 1.51) and cross-sectional studies (OR: 1.23) but not in cohort studies (OR: 1.18, 95% CI: 0.95–1.46).

Nuclear cataracts demonstrated the strongest and most consistent association with smoking across all study types. Current smokers had significantly higher odds of nuclear cataracts in cohort (OR: 1.71; 95% CI: 1.47–1.98), case-control (OR: 1.79; 95% CI: 1.43–2.25), and cross-sectional studies (OR: 1.45; 95% CI: 1.27–1.65). Ex-smokers also showed elevated risk in case-control (OR: 1.57) and cross-sectional (OR: 1.21) studies, though not in cohort studies (OR: 1.20, 95% CI: 0.95–1.54).

The evidence for an association between smoking and cortical cataracts was weaker. Only cross-sectional studies reported a statistically significant but modest increase in risk (OR: 1.13, 95% CI: 1.01–1.27), while cohort and case-control studies found no significant association. No clear relationship was observed among ex-smokers.

Results for posterior subcapsular cataracts were mixed. Only case-control studies showed a significant association for current smokers (OR: 1.89, 95% CI: 1.17–3.03), while cohort and cross-sectional studies did not find a statistically significant risk. Among ex-smokers, no consistent association was observed across study designs.

Overall, the findings indicated a clear, positive association between smoking and cataract risk, especially for the nuclear type. The association is strongest in current smokers, with weaker and less consistent evidence among ex-smokers.

Methodology:

The searches were conducted in Medline, Embase, and Lilacs until January 2017 to identify observational studies published in English or Spanish that reported the association between smoking cigarettes and any type of clinically diagnosed cataract. In addition, grey literature databases (OpenGrey, The Electronic Theses Online Service, Tesis Doctorales en Red) and bibliographic references were searched to identify additional relevant publications.

Two reviewers independently screened the articles and extracted the relevant data. The quality of the included cohort and case-control studies was assessed using the Newcastle-Ottawa Scale, while cross-sectional studies were evaluated using the 11-item AHRQ (Agency for Healthcare Research and Quality) checklist. The findings were synthesised using meta-analysis. Heterogeneity was assessed using I² statistics, and publication bias using a funnel plot and Egger’s test. A meta-regression was performed to explore potential sources of heterogeneity between studies.

Applicability/external validity: The review did not explicitly discuss the external validity or applicability of its findings.

Geographic focus: The review did not apply any geographical limits. The included studies were conducted in the USA, France, Australia, China, Finland, Japan, Scotland, India, Nigeria, Greece, Korea, Myanmar, Singapore, and Taiwan.

Summary of quality assessment: Overall, there is medium confidence in the review’s conclusion. The searches were comprehensive. Inclusion and exclusion criteria were clearly defined, and two reviewers independently screened the articles and extracted data. The study quality was assessed using established tools. Characteristics of included studies were well-documented, meta-analyses were appropriately performed, and heterogeneity was addressed. However, the search was limited to identifying articles published in the English and Spanish languages only, and the review did not provide a list of excluded studies.