Methodological quality of the review: Low confidence

Author: Wang X, Tang L, Gao L, Yang Y, Cao D, Li Y

Region: Asia, Australia, United States of America (USA)

Sector: Diabetic retinopathy

Sub-sector: Myopia

Equity focus: None specified

Review type: Other review

Quantitative synthesis method: Meta-analysis

Qualitative synthesis method: Not applicable

Background:

Myopia may have protective effects against diabetic retinopathy (DR). However, the data from epidemiologic studies are inconsistent.

Objectives:

To examine the association between myopia and DR by conducting a meta-analysis.

Main findings:

A total of nine studies were included in the systematic review, including six population-based studies (four cross-sectional studies and two cohort studies) and three clinic-based cross-sectional studies. Among the seven cross-sectional studies, five were conducted in Asia, one was conducted in Australia and one was conducted in USA. Among the two cohort studies, one was conducted in USA, and one was conducted in China. The association of myopic spherical equivalent (SE) with DR was reported in four population-based cross-sectional studies, one cohort study and two clinic-based cross-sectional studies. When authors combined the estimated effects based on these studies, myopic SE was significantly associated with a decreased risk for DR (pooled odds ratio (OR) 0.80; 95% confidence interval (CI), 0.67–0.95; P = 0.011; I² = 36.7%). Excluding the clinic-based study, authors observed a borderline significant association between myopic SE and DR in a meta-analysis of the five population-based studies (pooled OR, 0.83; 95% CI, 0.69–0.99; P = 0.043; I² = 53.0%).

Authors reported that no significant associations were observed between each diopter decrease in SE toward myopia and DR (pooled OR, 1.01; 95% CI,0.97–1.06; P = 0.597; I² = 91.50%). In addition, authors observed that each millimeter increase in axial elongation (AL) was significantly associated with a decreased risk of DR (pooled OR, 0.79; 95% CI, 0.73–0.86; P = 0.000; I²=57.6%, P = 0.038).

Sensitivity analysis showed that the Beijing Eye Study influenced the pooled OR. After excluding the Beijing Eye Study, the pooled OR was 0.81 (95% CI, 0.74–0.89), with no evidence of heterogeneity (I² = 17.4%; P = 0.304).

Considering that two clinic-based studies were included in the above analysis, authors also performed a meta-analysis using the four population-based studies alone (three cross-sectional studies and 1 cohort study). The pooled OR reported by the authors was 0.79 (95% CI, 0.72–0.87; P = 0.000), and significant heterogeneity was observed (I2 = 70.5%; P = 0.017). Authors noted that after excluding the Beijing Eye Study, the pooled OR was 0.81 (95% CI, 0.74–0.90; P = 0.000), with low heterogeneity (I2 = 39.7%; P = 0.190).

Authors noted that the meta-analysis suggests that individuals with myopia exhibit a decreased risk of developing DR or vision-threatening diabetic retinopathy. An increased AL plays a critical role in this protective effect.

Methodology: 

Studies were included if they met the following criteria: (i) explored the associations among myopia, AL and DR; (ii) used DR as an outcome measure, which was assessed based on fundus photographs according to standardized protocols, such as the Early Treatment Diabetic Retinopathy Study (ETDRS) or the Airlie House classification system; and (iii) reported a measure of the association either as an odds ratio (OR) or a hazard ratio (HR) with a 95% confidence interval (CI) or allowed for the calculation of such metrics from the raw data presented in the article. Authors excluded (i) studies published in non-English languages and (ii) studies without a clear threshold definition of myopia or lacking fundus photography results according to standardized protocols.

Authors conducted a search of PubMed and EMBASE to identify all relevant population-based and clinic-based studies published up to March 2015. In addition, authors also searched reference lists of all identified studies as part of the search strategy. Two authors independently screened studies for inclusion.

Authors extracted data of included studies, including reference details and methodology. Quality of each included study was assessed using the tool described by Sanderson and colleagues.

Authors performed a meta-analysis using Stata software. The fully adjusted study-specific ORs were combined to estimate the pooled OR and 95% CI. Statistical heterogeneity among study was evaluated suing the I² statistic. Where a moderate or high heterogeneity was observed authors used a random-effects mode, otherwise a fixed-effects model was used.  Heterogeneity caused by the study design was addressed by performing a meta-analysis separately for all studies combined and for population-based studies alone. Authors evaluated publication bias using the Egger regression asymmetry test and Begg’s test.

Applicability/external validity:

Authors report that individuals with myopia exhibit a decreased risk of developing DR or VTDR. An increased AL plays a critical role in this protective effect.

Geographic focus:

Authors did not discuss the applicability of findings to low- and middle- income countries. Authors did not conduct through searches of the literature to ensure that all relevant studies were identified for inclusion in the review.

Summary of quality assessment:

Overall, there is low confidence in the conclusions about the effects of this study as major concerns were identified. In addition, authors did not employ rigorous methods at the data extraction stage to ensure biases were avoided.