A systematic review of risk factors for cataract in type 2 diabetes

Methodological quality of the review: High confidence

Author: Drinkwater JJ, Davis WA, Davis T.

Region: United States of America (USA), United Kingdom (UK), Taiwan, Sweden, Iran, India and Canada

Sector: Biomedical

Subsector: Risk factors

Equity focus: Not stated

Review type: Other review

Quantitative synthesis method: Narrative synthesis

Qualitative synthesis method: Not applicable

Background:
Type 2 diabetes (T2D) is a risk factor for cataract development. With T2D prevalence increasing, the burden of cataract‐associated vision loss will also increase.

Objectives:
Authors aimed to characterise cataract diabetes‐specific risk factors to assist prevention and management strategies.

Main findings:
Of 5,255 publications identified, 19 from 13 study populations were included in this review. These 19 publications reported studies in 13 different populations; five articles from the United Kingdom Prospective Diabetes and three from a population study in Taiwan were included as risk factors and outcomes differed between publications. Five were conducted in the US, three in the UK, and one each in Taiwan, Sweden, Iran and India, and one multi‐centre study was conducted over two countries (the US and Canada). The overall risk of bias was low. There was between‐study variability. Age and glycemic control were consistently associated with cataract development in T2D, but blood pressure, diabetes duration, sex, and aspirin use were not. Serum lipids and smoking remain possible risk factors, but available data is inconclusive. Glycaemia is the only consistent modifiable risk factor among a range of candidate variables. Due to the lack of consistency of the available evidence, and since mortality associated with T2D is declining with the likelihood of increased cataract‐associated vision loss, additional well‐conducted longitudinal studies are needed to identify modifiable risk factors that could prevent or delay cataract formation.

Methodology:
Authors conducted a search on Embase, PubMed and Medline databases for potentially relevant studies. The references cited in all the eligible published studies were reviewed to identify articles that had been missed in the initial search but which met the inclusion criteria.
Inclusion criteria consisted of longitudinal studies written in English, where diabetes was defined, the criteria for determining cataract status were described, and people with T2D who developed a cataract were compared with people with T2D who did not. A minimum sample size of 100 participants was applied to minimise publication bias. To ensure reasonable quality, the statistical analyses used in observational studies were required to include, as a minimum, a model adjusted for age and sex. The summary measures used by the authors were those reported in individual studies. These included relative risk, odds ratios and hazard ratios.
The risk of bias was assessed using the Cochrane handbook guidelines. The quality of observational studies was assessed using the Newcastle Ottawa Quality Assessment Form for Cohort Studies. The level of evidence from RCTs was assessed using GRADE. Although authors planned to conduct a meta-analysis, this was not appropriate.

Applicability/external validity:
Authors note that applicability of findings may be limited due to heterogeneity of included studies, such as variation in the definition of cataract, outcomes measured and due to the different glycemic thresholds used for diabetes diagnosis.

Geographic focus:
Authors do not discuss the applicability of findings to low and middle income countries.

Summary of quality assessment:
Overall, there is high confidence in the conclusions about the effects of this study, as authors used rigorous methods to conduct the review.