Discussion
Our study results showed that the implantation of an additional AGV in the eye following a previous failed implantation was effective in controlling IOP. After the second AGV had been implanted, the IOP reduced by 20.8 mm Hg (52.9%), compared with preoperative values. The number of IOP-lowering medications used decreased from 2.8 preoperatively to 1.3 at the final follow-up visit. The success rate of the procedure was 87% and 52% at 1 and >3 years post follow-up, respectively.
To date, several studies have reported the efficacy of additional GDD insertion for patients with a failed initial GDD.[7,10–13] The studies differ in the criteria used to determine success, and the overall rates of success, type of glaucoma, and type of GDDs used. For instance, the success rate after additional GDD implantation ranged from 38.1% to 87.5% and the percentage of neovascular glaucoma ranged from 0% to 37.5%.[7,10–13] Regarding the type of GDD used, 2 studies included eyes with AGV implantation as both the first and second GDD.[12,13] Smith et al[12] investigated the surgical outcome of additional AGV implantation in 21 patients with a preexisting AGV; the mean IOP decrease at 12 months and at final follow-up was 8.0 mm Hg (43%) and 7.9 mm Hg (42%), respectively. Lee et al[13] described a retrospective series of 8 patients who had undergone a second AGV implantation, following a previous AGV implantation failure; the mean IOP reduction was 20.8 mm Hg (61%) at 12 months and 19.3 mm Hg (56%) at the final follow-up. In the present study, the mean reduction in IOP following the second AGV implantation was 24.1 mm Hg (61%) at 12 months and 20.8 mm Hg (53%) at final follow-up, which are comparable with Lee colleagues' study results. To date, this extent of IOP reduction is the highest that is achieved by the second GDD implantation in eyes that have undergone previous GDD surgery. We believe that IOP reduction induced by secondary GDD implantation could be helpful to prevent vision loss and/or relieve ocular symptoms caused by high IOP. Studies using different types of GDDs have also shown an IOP reduction, in the range of 9.9 mm Hg (33.6%) to 12.1 mm Hg (40.6%).[7,10,11]
When an additional procedure is indicated in eyes with a preexisting AGV, the most important factors affecting the choice of treatment are the condition of the conjunctiva and the amount of space available for additional surgery. The implantation of an additional AGV is challenging in eyes that are small in size, inflamed, or have severe conjunctival scarring as a result of previous surgery. In our experience, implanting an AGV with a smaller body plate (FP8) may be a better option than using one with a larger body plate (FP7). Our previous study has reported that FP7 and FP8 AGV implants showed similar outcomes in terms of preservation of vision, IOP reduction, and decrease in the number of glaucoma medications required.[14] In the present study, the FP7 AGV was more commonly used in the initial AGV surgery than the FP8 AGV. However, in the second AGV implantation, FP8 models were predominantly used. In previous studies, only the FP7 model was used in both the initial and second AGV.[12,13] To the best of our knowledge, this is the only published report assessing the suitability of the FP8 AGV for use as a second AGV. FP8 AGV has an advantage owing to its smaller profile than FP7 AGV, thereby enabling operation in a narrow surgical field or a less viable conjunctiva owing to scar formation. The similar IOP reduction in our study results using the FP8 AGV, compared with those of previous studies that used the FP7 AGV, suggests that FP8 AGV implantation, with its smaller profile, may be a useful option for a second GDD surgery.
In our study population, no cases of hypotony, phthisis, valve exposure, or endophthalmitis were found. The most serious complication after the implantation of a second AGV implant was corneal decompensation; this finding is in agreement with previous reports.[7,10–13] In previous studies, the reported rate of corneal complication related to the insertion of a second GDD varied from 5.3% to 43.0%.[7,10–13] In the present study, the incidence of corneal decompensation was 13% after the second AGV implantation. These results suggest that when deciding to perform an additional GDD implantation, the possibility of corneal complications should be considered.
Although postoperative strabismus or ocular movement limitation is a concern when considering implantation of a second GDD, none of the patients in the present study developed these problems. It is worth noting that all our surgeries involved AGV, which is thought to be less likely to cause diplopia than the Baerveldt glaucoma implant.[15–17] In addition, the use of an AGV with a smaller area (FP8) may have contributed to the minimal effect on ocular movement observed.
In our study, previous trabeculectomy was found to be a risk factor for failure. This is consistent with previously published papers that have reported it as a risk factor for surgical failure in AGV implantation.[18] This finding may be explained by the presence of excessive fibrosis and heightened inflammatory response in the conjunctiva as a consequence of previous glaucoma surgery.[19–21] Prior cataract surgery and vitrectomy were not identified as risk factors for failure in this study. All cataract surgeries were performed using clear corneal incisions, leaving the conjunctiva untouched, thereby reducing damage that could complicate any subsequent glaucoma surgery.[22] In addition, all vitrectomies were performed using a 23 G transconjunctival sutureless technique, which causes less conjunctival scarring, and so may be beneficial for preserving the conjunctiva.[23]
This study has some limitations. The power of the Cox proportional hazard model analysis may have been affected by the small sample. In addition, the retrospective nature of the experimental design, lack of randomization, and absence of a control group were also limitations. A long-term, prospective, randomized study is needed to confirm the results of this study.
In conclusion, the implantation of a second AGV following a failed previous AGV implantation was found to be effective in controlling IOP in refractory glaucoma. Although corneal decompensation was found in some cases, there were no other serious complications. A second AGV implantation may be a good option for the surgical treatment of glaucoma when an initial AGV implantation has failed to control IOP.
J Glaucoma. 2016;25(6):e620-e624. © 2016 Lippincott Williams & Wilkins
