A Cochrane review included 15 studies with a total of 1153 subjects. Several different procedures were compared. No significant differences were observed between different types of shunts, with the exception of a meta-analysis of two trials comparing Ahmed implant with trabeculectomy where trabeculectomy resulted in lower mean IOPs 11 to 13 months later (mean difference 3.81 mm Hg, 95% CI 1.94 to 5.69 mm Hg).
Comment: The quality of evidence is lowered by poor quality of trials and sparse data in individual comparisons.