We thank Dr Sekhar and Associates for their interest in our article. Failure in the Tube Versus Trabeculectomy (TVT) Study was prospectively defined as inadequate intraocular pressure (IOP) reduction (IOP > 21 mm Hg or not reduced by 20%), persistent hypotony (IOP ≤ 5 mm Hg), reoperation for glaucoma, or loss of light perception vision. The cumulative probability of failure at 3 years using Kaplan-Meier survival analysis was 15.1% in the tube group and 30.7% in the trabeculectomy group ( P = .010; HR = 2.2; 95% CI = 1.2-4.1). Dr Sekhar and associates suggest that “the greater failure in the trabeculectomy group is attributable to greater number of repeat surgeries in the trabeculectomy group than in the tube group.” In fact, greater numbers of failures were observed in the trabeculectomy group for each of the failure criteria (not just reoperations for glaucoma). Although the rate of glaucoma reoperations was not statistically different between the 2 treatment groups, the overall failure rate was significantly higher in the trabeculectomy group compared to the tube group when all reasons for treatment failure were taken into consideration.
Because the surgeon was not masked to the treatment assignment and the decision to reoperate was left to the surgeon’s discretion, a potential bias existed in the decision to reoperate for glaucoma. We explored for this reoperation bias by comparing the IOPs between the 2 treatment groups at the time of reoperation. If surgeons had a higher threshold for reoperating on patients who underwent tube shunt surgery, one would expect that the IOP level at the time of additional glaucoma surgery would have been higher in the tube group than the trabeculectomy group. The opposite was actually observed. The IOP (mean ± SD) was 21.5 ± 6.6 mm Hg in the tube group and 27.9 ± 9.6 mm Hg in the trabeculectomy group at the time of reoperation ( P = .12). Additionally, the number of glaucoma medications (mean ± SD) was 3.4 ± 0.5 in the tube group and 2.9 ± 0.8 in the trabeculectomy group prior to reoperation ( P = .17). These observations strongly suggest that there was not a reoperation bias in the study. We do not believe that a higher rate of qualified success in the tube group in any way suggests a bias against reoperating for glaucoma. It simply reflects that adjunctive medical therapy is more frequently used after tube shunt surgery to achieve a desired level of IOP, and this is entirely consistent with our clinical experience.
The rates of complete success, qualified success, and failure were presented in Table 3. The rates of failure and complete success were statistically compared between the tube group and the trabeculectomy group because this information is of great clinical relevance. Comparing the rates of qualified success requires that the proportion of patients with qualified success be compared with those who were categorized as a complete success or failure in each treatment group. It makes little clinical sense to combine failures and complete successes together, and this is the reason why this P value was not included in the table.
We agree with Dr Sekhar and associates that “the final verdict on the supremacy of tube shunt to trabeculectomy is still open.” They have correctly indicated that mean IOP levels and rates of serious complications were similar in the tube group and the trabeculectomy group after 3 years of follow-up. This prompted our conclusions that “the TVT Study does not demonstrate clear superiority of one glaucoma operation over the other,” and “additional follow-up is needed to fully assess the risks and benefits of tube surgery and trabeculectomy with MMC [mitomycin C] in similar patient groups.”