The Ahmed shunt versus the Baerveldt shunt for refractory glaucoma: a meta-analysis

Methodological quality of the review: Medium confidence


Authors: Wang S, Gao X, Qian N


Region: USA, Canada and Singapore


Sector: Refractory glaucoma


Sub-sector: Treatment

Equity focus: None specified

Review type: Other review

Quantitative synthesis method: Meta-analysis

Qualitative synthesis method: Not applicable


In the past two decades, several types of glaucoma drainage implants have been developed. Two of the most commonly implanted aqueous drainage devices are the Ahmed glaucoma valve (AGV) and the Baerveldt implant. The AGV device incorporates a one-way valve to prevent postoperative hypotony and shallow anterior chambers. However, several studies reported that it was associated with high rates of encapsulation and inadequate intraocular pressure reduction (IOPR), often requiring postoperative glaucoma medications


The purpose of this study was to compare the efficacy and tolerability of the Ahmed glaucoma valve (AGV) implant and the Baerveldt implant for the treatment of refractory glaucoma.

Main findings:

The authors included six studies published between 2004 and 2013 in the present meta-analysis. These studies were conducted in various countries: four of them were from the United States, one was from Canada, and one was from Singapore. All studies were attributed a score of over 50% on the quality assessment.

A total of 929 patients from six studies were included. The weighted mean differences of the IOPR between the AGV implant and the Baerveldt implant were 1.58 [95 % confidence interval (CI): −2.99 to 6.15] at six months, −1.01 (95 % CI: −3.40 to 1.98) at 12 months, −0.54 (95 % CI: −4.89 to 3.82) at 24 months, and −0.47 (95 % CI: −3.29 to 2.35) at 36 months. No significant difference was detected between the two groups at any point in time. The pooled ORs comparing the AGV implant with the Baerveldt implant were 0.51 (95 % CI: 0.33 to 0.80) for the complete success rate and 0.67 (95 % CI: 0.50 to 0.91) for qualified success rate. The Baerveldt implant was associated with a reduction in glaucoma medication at −0.51 (95 % CI: −0.90 to −0.12). There were no significant differences between the AGV implant and the Baerveldt implant on the rates of adverse events.


The authors conducted a search of PubMed, ISI Web of Science, EMBASE and the Cochrane Library to identify published literature. The authors also search Google Scholar and the websites of professional associations for relevant articles. Thereafter, references of relevant studies were reviewed. Searches were conducted from inception to October 2015 without any restrictions.

The inclusion criteria were: study type (comparative studies); population (patients with refractory glaucoma (for example, failed trabeculectomy, neovascular and uveitic glaucoma, and traumatic glaucoma)); intervention (AGV implants versus Baerveldt implants); outcome variables (evaluation of at least one of the outcomes of interest mentioned below).

The following exclusion criteria were used: follow-up period less than six months;  trials with a small sample size, (n <10); pediatric patient population; abstracts from conferences, full texts without raw data, duplicate publications, letters, and reviews.

In the case of duplicate publications, the most recent series were included in this analysis. Primary outcome measure was IOPR.

Two authors independently extracted data and assessed the methodological quality of include studies. The later used a system intended for both randomized and non-randomized studies. This system was comprised of 27 items distributed among five subscales: reporting (10 items), external validity (three items), bias (seven items), confounding (six items), and power (one item). Good quality refers to a quality score not lower than 50%.

The outcome measure was assessed on an intent-to-treat basis. The weighted mean differences (WMDs) with 95 % confidence intervals (CIs) and the odds ratios (ORs) with 95 % CIs were used to compare continuous and dichotomous variables, respectively. The random-effects model was used to obtain a conservative estimate of the effect of different clinical characteristics among study groups and the variations in sample sizes among the studies. Statistical heterogeneity between studies was evaluated using Cochran’s Q statistic and the P-value, and the I2 statistic was used to assess the quantity of heterogeneity. The authors performed sensitivity analysis to evaluate the effect of the methodological characteristics of controlled clinical trials: trial design. To detect publication biases, the Begg and Egger measures were calculated.

Applicability/external validity:

This meta-analysis suggests primarily that the Baerveldt and Ahmed implants are comparable in IOPR outcomes. However, it is not possible to discuss the generalizability of findings of this review due to the limited number of studies included in the review.

Geographic focus

The authors noted that the majority of included subjects were Caucasian; only one study, consisting of 24 eyes, was from Singapore.

Summary of quality assessment:

Medium confidence was attributed in this review as important limitations were identified. Although the authors used appropriate methods to identify studies, it is unclear if selection bias was avoided in the review whilst screening studies for inclusion. In addition, the authors did not report the risk of bias for each criterion assessed.

Publication source:

Wang S, Gao X, Qian N (2016) The Ahmed shunt versus the Baerveldt shunt for refractory glaucoma: a meta-analysis. BMC Ophthalmol. 2016; 16: 83.