Despite the introduction of several new incisional glaucoma procedures in recent years, trabeculectomy and glaucoma drainage device surgery remain the most frequently performed glaucoma operations worldwide. Tran et al (1690). have provided an outstanding retrospective case-controlled study comparing the Ahmed Glaucoma Valve (AGV) implant with trabeculectomy with mitomycin C (MMC). The study has 166 patients with primary or secondary open-angle glaucoma, including 78 patients who underwent AGV implantation and 88 patients who underwent trabeculectomy with MMC. All patients had at least three years of follow-up, and the two groups were matched with respect to age, preoperative surgery, preoperative IOP and preoperative medications. Four criteria were used to compare the probability of success between the two groups: (A) IOP ? 21 mmHg and ? 15% reduction from baseline; (B) IOP ? 18 mmHg and ? 20% reduction from baseline; (C) IOP ? 15 mmHg and ? 25% reduction from baseline; and (D) IOP ? 12 mmHg and ? 30% reduction from baseline. All criteria for success also required no loss of light perception vision, no additional glaucoma surgery, and no persistent hypotony (IOP ? 5 mm Hg). The IOP criteria were applied beginning six months postoperatively with or without adjunctive medical therapy, and failure was established by IOP measurements outside the success range on two consecutive visits.
Surgical outcomes are affected by the criteria used to define success
The higher five-year probability of success that was observed with trabeculectomy with MMC compared with AGV implantation became progressively more statistically significant as the success criteria were made more stringent. For criterion A, the difference between the AGV group and the trabeculectomy group was not statistically significant (36% vs. 48%, p = 0.094). However, for criteria B, C and D, the cumulative probabilities of success over five years were 28% vs. 44% (p = 0.024), 19% vs. 40% (p < 0.001), and 9% vs. 22% (p < 0.001) for the AGV group and the trabeculectomy group, respectively. The mean visual acuity score difference from baseline was not significantly different between treatment groups. Twice as many patients in the trabeculectomy group (26 patients) required repeat glaucoma surgery compared with the AGV group (13 patients). More patients in the AGV group (9 patients) had penetrating keratoplasty than in the trabeculectomy group (2 patients).
Trabeculectomy with MMC has a higher probability of success when greater IOP reduction is desired
The results of this study are consistent with randomized prospective studies by Wilson et al. that reported similar success rates with the AGV implant and trabeculectomy when success was defined in a manner similar to criterion A.1,2 This study also illustrates the degree to which surgical outcomes are affected by the criteria used to define success. The authors concluded that trabeculectomy with MMC has a higher probability of success when greater IOP reduction is desired among similar patient groups. The limitations of the study are acknowledged by the authors. There are inherent biases in retrospective studies, although these have been minimized through the matching of patient groups. Additionally, the surgical procedures in this study were not standardized. Both the S-2 and FP-7 models of the AGV were used, and prior studies have suggested that there may be differences in safety and efficacy between these AGV models.3-7 Varying MMC concentrations and exposure times were used during trabeculectomy. Despite these limitations, Tran et al. have delivered valuable information comparing two commonly performed glaucoma procedures.