Myopia and/or longer axial length are protective against diabetic retinopathy: a meta-analysis

Methodological quality of the review: Medium confidence

Author: Yu Fu, Dengfeng Geng, Hua Liu, Huixin Che

Region: USA, China, Singapore, Taiwan, Australia

Sector: Myopia

Subsector: Myopia and diabetic retinopathy

Equity focus: No

Study population: Adults

Type of programme: Hospital based

Review type: Other review

Quantitative synthesis method: Meta-analysis

Qualitative synthesis method: Not applicable

Background: Diabetes mellitus (DM) is one of the major causes of mortality in the world, and its incidence in the middle-aged population is continuously increasing. Diabetic retinopathy (DR), an important cause of visual loss, is one of the most common and devastating microvascular complications of DM. An association was also suggested between ocular risk factors, such as myopia, intraocular pressure, and posterior vitreous detachment and the development and progression of DR. A clearer understanding of the association between myopia and DR would provide insights into the pathophysiology of myopia and DR, and would be clinically important from the point of view of individualised risk management, among others. However, to the best of our knowledge, no systematic approach has yet been taken that quantitatively combines the results of all available studies evaluating the association between myopia and DR.

Objectives: The aim of this study was to evaluate the current evidence of the relationship between myopia, together with its structural and refractive component, and diabetic retinopathy (DR) risk.

Main findings: Eleven articles met the inclusion criteria and were included in this meta-analysis. The 11 studies selected provided a total sample size of 7,804 DM patients. The average study-specific quality score of NOS was 7.18 (range 6-8), indicating that the methodological quality was generally good. Regarding the association between AL or myopia and risk of DR Among these six studies, the ORs for the association varied from 0.48 to 0.93 across studies. Overall, a longer AL (per millimetre increase) was associated with a significantly decreased risk of DR (combined OR, 0.75; 95% CI, 0.65-0.86; P<0.001). Moderate heterogeneity was observed (P=0.037, I²=57.8%). Second, the association between myopia (myopia versus non-myopia) and DR was calculated by including six studies. It also showed a lower risk of DR (combined OR, 0.70; 95% CI, 0.58-0.85; P<0.001). A greater degree of myopic RE (per D decrease) was associated with a significantly decreased risk of DR (combined OR, 0.89; 95% CI, 0.85-0.93; P<0.001). Heterogeneity was rarely observed (P=0.754, I²=0%).

Authors concluded that both myopic refraction and longer AL are associated with a lower risk of DR. They also note that further studies are needed to determine exact mechanisms underpinning the protective effect of myopia against DR.

Methodology:

Inclusion criteria consisted of: 1) studies designed as case-control; 2) evaluated AL or refractive error (RE) with DR; 3) the studies reported the unadjusted or multivariable adjusted odds ratio (OR) with a corresponding 95% confidence interval (CI) or other available data for estimating OR (95% CI). If two or more studies shared the same cases or control subjects, the one with most information was included.

A systematic search was performed in the following databases up to April 2015: PubMed (Medline), Embase, Web of Science in English, the Chinese National Knowledge Infrastructure (http://www.cnki.net/), VIP (http://www.cqvip.com/) and Wanfang (http://www.wanfangdata.com.cn) in Chinese. The references of articles identified for inclusion were also hand searched to identify other relevant studies. The DR group consisted of patients with non-proliferative diabetic retinopathy (NPDR) and patients with proliferative diabetic retinopathy (PDR). Subjects were considered to have NPDR based on established features, such as microaneurysms, hard exudates and retinal haemorrhages. The stages of literature search, data extraction and quality assessment were performed independently by two reviewers (Y.F. and D.G.). Any disagreement was resolved via discussion and consensus. The assessment of methodological quality of the included studies was determined using the Newcastle-Ottawa Scale (NOS) (Stang 2010). The OR and 95% CIs were used to assess the strength of associations between AL or myopia and the risk of DR. The pooled ORs and 95% Cis were calculated for the following three models: AL (per millimetre increase) and DR; myopia (myopia versus non myopia) and DR; and RE (per D decrease) and DR. Heterogeneity was assessed using the Cochran Q and I² statistics. For the Q statistic, a P value <0.10 was considered statistically significant for heterogeneity.

Applicability/external validity: Regarding external validity, the authors reported some limitations to consider: first, the results of this meta-analysis were limited by the small number of included studies, so more studies are needed to confirm the results of this analysis. Second, the included studies were not equally designed. Third, the duration of diabetes and the type of diabetes may have influence on the association between myopia and DR risk.

Geographic focus: Not discussed.

Summary of quality assessment:

Medium confidence was attributed to the conclusions about the effects of this study. Authors used rigorous methods to analyse findings, screen studies for inclusion, extract data and appraise included studies. However, authors did not search for unpublished studies and did not contact authors/experts as part of the search strategy.