Smoking and risk of age-related cataract: A meta-analysis

Methodological quality of the review: Low confidence

Author: Ye J, He J, Wang C, Wu H, Shi X, Zhang H, Xie J, Lee SY.

Region: North America, Europe, Australia, Asia and Africa

Sector: Age-related cataract

Sub-sector: Smoking

Type of cataract: Age-related cataract

Equity focus: None specified

Review type: Effectiveness review

Quantitative synthesis method: Meta-analysis

Qualitative synthesis methods: Not applicable

Background        

Smoking is a well-known risk factor for a number of diseases. Tobacco smoke contains hundreds of chemicals including nicotine, free radicals and carbon monoxide which can increase oxidative stress and as a result have an important role in the pathogenesis of age-related cataract. Authors noted the lack of quantitative evidence to date that investigates and establishes the association between smoking and age-related cataract.

Research objectives

To evaluate through meta-analysis the relationship between smoking and age-related cataract (ARC).

Main findings

The systematic review included 13 prospective cohort and eight case-control studies, from North America, Australia, Europe, Asia and Africa, looking at the association of smoking and ARC.

From the prospective cohort studies, eight were included in the analysis of the association between past smoking and the risk of age-related cataract. Ten studies analysed the association between current smoking and the risk of age-related cataract. It was found that the association was stronger with current smokers. It was noted that statistically significant heterogeneity existed among the eight studies evaluating past smoking, but not among the ten studies looking at current smoking and risk of ARC.

Eleven studies reported results for ever smoking and found a strong association between ever smoking with risk of nuclear cataract. Five studies evaluated the association between ever smoking and cortical cataract and no association was reported.

From the case-control studies, eight studies were included in the analysis of the association between age-related cataract risk. Ever smoking was associated strongly with increased risk of cataract as was past and current smoking.

The authors concluded that ‘The review findings suggest that smoking is associated with an increased risk of age-related cataract, especially nuclear cataract. Current smokers are at a higher risk of age-related cataract than past smokers. No association was found between smoking and cortical cataract was found.’

Authors noted that for some of the data synthesis, there was a significant amount of heterogeneity among the studies, which was not explained through a meta-regression analysis that assessed the effect of publication year, conducted area, study design, primary outcome and sample size on heterogeneity. This indicated a need for ‘consensus definitions’ for age-related cataract and subtypes in future research conducted. Future research was needed to confirm these findings and resolve remaining problems.

Methodology

Studies were selected if the following criteria was met: (1) case-control or cohort study published as an original article, (2) papers reported in English between 1980 and August 2011, (3) estimation of the relationship between active smoking and the risk of ARC expressed as odds ratio (OR) or relative risk (RR) with their corresponding 95% intervals and (4) adjustment made for potential risks, at least age, or sufficient information to allow authors to compute them.

The authors conducted a search of literature on databases including MEDLINE, PUBMED, Web of Science, and the Cochrane Library up to 2011. They also checked reference lists of included studies. Authors did not report contacting authors of included studies for additional information and unpublished reports were not considered. Information from the included studies was extracted independently by two reviewers, although it was not clear whether studies were selected by one or two reviewers.

Authors conducted a meta-analysis using OR as a measure of the relative risk for all studies, and the RR estimated were log-transformed. Data from each study was pooled by the use of a random-effect model. Sensitivity analyses were performed and meta-regression was used to assess heterogeneity.

Applicability/external validity

The authors did not explicitly discuss how generalizable the results were, although they reported that studies came from hospital-based and population-based studies; and also that studies limited to non-generalizable patients were excluded.

Geographic focus

The authors included studies from Europe, North America, Australia, Asia and two studies from Africa. It was not clear which countries are included from Africa and whether low- and middle-income countries were included in the review.

Quality assessment

This review was awarded low confidence in the conclusions about the effects as major limitations were identified. The systematic review was based on a partially comprehensive search as language and publication bias was not avoided. Authors did not state if two reviewers independently conducted the process of selecting studies; additionally, sensible criteria were not used to address bias of studies included.  Authors acknowledged that, among other limitations of this systematic review, publication bias may have been induced by not considering unpublished papers for inclusion in the review.

In terms of data synthesis, the outcome measure of cataract was not consistent across the studies and authors acknowledged the lack of homogeneity in the reported outcomes. However, the likelihood of bias within the included studies was not clearly addressed.