Most GDDs have been developed in a virtual publication vacuum, with little available data to substantiate manufacturers’ claims for flow performance or biocompatibility (Prata et al. 1995). Clinical data are largely restricted to uncontrolled retrospective case series (Krawczyk 1995) with variable follow-up and differing definitions of surgical success. Evaluation is further complicated by the heterogeneity of inclusion criteria. Series include a variable proportion of complex cases, such as neovascular glaucoma, in which there is a higher risk of filtration failure.

Overall success rates, in terms of IOP control, appear similar between devices (Table 5.2), with a reasonably high proportion of cases achieving a final IOP in the target range at 1 year after surgery. Half to two-thirds of these cases still require glaucoma medications, however, target IOPs in the low teens (≤16 mmHg) may be more realistic in terms of preventing disease progression than commonly adopted target levels (≤21 or 22 mmHg).

Another important caveat concerns attrition rates, or continued increments in the proportion of filtration failures with lengthening postoperative follow-up. Again, evaluation is difficult, with few series including either long-term data or survival analysis (Mills et al. 1996) (Table 5.2). Mills et al. reported a 10 % failure rate per postoperative year in a series including longer-term follow-up for single and double plate Molteno tubes. Extrapolating from this would suggest that many GDDs have a functional lifespan of less than 5 years before failure though fibrous encapsulation. However, the 5-year data from the Tube vs. Trabeculectomy (TVT) study found a 10 % failure rate per year for the first 3 years, and an average of 5 % per year for the subsequent 2 years, which may indicate better long-term results with less complex cases (Gedde et al. 2012).