Methodological quality of the review: Low confidence
Authors: Ramdas WD, Schouten JSAG, Webers CAB
Region: None specified
Sub-sector: Vitamins, prevention
Equity focus: None specified
Review type: Other review
Quantitative synthesis method: Meta-analysis
Qualitative synthesis method: Not applicable
The ubiquitous exposure to vitamins has led to many epidemiological studies for many diseases, including glaucoma. The need to study the occurrence of glaucoma in relation to vitamins stems from the observation that glaucoma is the leading cause of irreversible blindness, that almost half of the glaucoma cases are undiagnosed, and that the prevalence is increasing over time. This makes it relevant to investigate whether vitamins have a preventive effect on the occurrence of glaucoma, therefore may be used as a preventive measure.
The aim of the study is to determine the association of vitamins with glaucoma by performing a systematic review and meta-analyses.
A total of 36 articles were included of which five were eligible for a meta-analysis. Of the studies included in the meta-analysis, three were cross-sectional in design and two were prospective. Two studies included population from Caucasian/Africa background and one each included population from African background, Caucasian and multiracial backgrounds. Regarding the quality rating of each of the included studies, the mean total quality score for all studies was 6.8 (inter-quartile range: 6 to 8), on a scale from 0 to 9.
These included 940 open-angle glaucoma (OAG) cases and 123,697 controls in total and resulted in an odds ratio (OR) [95% confidence interval (CI)] of 0.58 [0.37–0.91] for dietary vitamin A, though heterogeneity was high (I2 = 51%). After omitting studies that contributed significantly to the heterogeneity, the authors reported that the pooled OR [95% CI] was 0.45 [0.30–0.68] for dietary vitamin A on OAG (I2 = 0%). For vitamin B1, C and E, the authors found no significant association with OAG (OR [95% CI]: 0.84 [0.47–1.51]; 0.68 [0.38–1.22]; 0.95 [0.75–1.19]; respectively). However, after addressing heterogeneity, vitamin C showed a protective effect as well. Especially, foods high in these vitamins (such as dark green vegetables) were protective for OAG.
The authors conducted a PubMed and Embase database search for articles published prior to 1 December 2017. The search term used in PubMed included (“vitamin (All Fields) AND glaucoma (All Fields)”). The included entries had to have an available abstract, which had to be either in English or German. Furthermore, investigations had to be performed in humans (or human tissue) and not in animals. Titles and abstracts were scanned to select eligible articles in which the relation between vitamins and glaucoma was studied without any restricting selection criteria. In addition, the authors reviewed references of included studies for further relevant studies.
Studies on the dietary association of nutrition had to use (semi-quantitative) food frequency questionnaires, and/or a detailed interview in case of vitamin supplements use, and had to use clear criteria for defining the type of glaucoma for final inclusion if applicable. The most common forms of glaucoma – primary open-angle glaucoma (POAG), primary angle-closure glaucoma (PACG), normal-tension glaucoma (NTG), and pseudoex foliation (PEX) glaucoma – were considered eligible. Other causes of glaucoma were excluded.
The authors extracted data of included studies including year of publication, study design, sample size, among other things. The authors used the Newcastle-Ottawa Scale to assess the methodological quality of included studies. Results of retrieved studies are presented as Hazard Ratio (HR) or Odds Ratio (OR) with corresponding 95% confidence interval (CI) if provided. Meta-analyses were performed to calculate the summary effect estimate of the relation between the vitamin and glaucoma. Heterogeneity was assessed by calculating the I2.
Dietary intake of vitamin A and C showed a beneficial association with OAG. However, findings on blood levels of vitamins do not show a clear relation with OAG.
The authors did not discuss the applicability of findings to low- and middle-income countries.
Summary of quality assessment:
Low confidence was attributed to this review. It is not clear if the authors avoided biases in the methods used to screen studies for inclusion in the review, or to extract data of included studies. In addition, the authors did not conduct thorough literature searches to ensure all relevant studies were included in the review.