Simultaneous Implantation of an Ahmed and Baerveldt Glaucoma Drainage Device for Uncontrolled Intraocular Pressure in Advanced Glaucoma: Comment

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To the Editor:
The management of refractory glaucoma is challenging and often not rewarding. In this context, the article by Rao et al1 was an interesting and thought provoking read and we congratulate the authors for providing new insight into management of refractory glaucomas. However, few observations need a deeper scrutiny.
Most of prior studies on single glaucoma drainage device (GDD) insertion have included eyes with higher mean preoperative intraocular pressures (IOP) [38.8±6.2, 38.5 (32-43), 34.6 (26-43) mm Hg]2,3 with a reported risk of hypotony ranging from 7%3 to 32%.4 Although the authors have failed to mention the exact number of eyes with IOP<5 mm Hg, many of the complications mentioned in Table 3 [namely shallow or flat anterior chamber (AC), hyphema, fibrin in AC, choroidal effusion, supra choroidal hemorrhage, vitreous hemorrhage, cystoid macular edema, corneal edema, and need for tube removal] can be attributed to hypotony. It is alarming, but not surprising considering placement of 2 devices in 1 eye with lower mean IOP of 25.5±9.8 mm Hg.1
We agree with the authors that being of retrospective nature, the study included it’s inherent faults of selection bias (which was subjected to surgeon’s discretion) and considerable loss of follow-up data (7 patients at 5 years).
The surgery was performed by 1 of 6 glaucoma surgeons. As implantation of GDD is complex, with a steep learning curve, the surgical outcomes may have been affected by the expertise of the respective surgeons.
Patients included in the study had undergone previous glaucoma surgeries, including prior GDD implantation. It was not clear from the text whether the preexisting implant was retained or explanted. Moreover, for hypotony, were both the tubes ligated/removed?
In addition, pars plana insertion of the tube may have potential advantages over insertion in AC.5 Though the text mentions use of viscoelastic in AC for tube insertion,1 the site of insertion has not been mentioned.
Final IOP achieved is determined by the thickness of fibrous capsule around the plate, which in turn is affected by its surface area6 and biomaterial.7 Could the different models used play confounding variables in the final outcome?
There is also a concern regarding the follow-up schedule. Patients were not followed-up between 6 and 8 weeks, which is the critical period to assess the hypertensive phase with Ahmed valves8 and hypotony due to lysis of absorbable suture with Baerveldt valves.9 The text also mentions a referral by the local ophthalmologists back to the surgical centre if and when any complication was noted. Here, a type II error cannot be ruled out as the complication rate would be guided by the clinical acumen of the local ophthalmologist.
It would be interesting if the authors could access the complication data (tube erosion, leak, blebitis) comparing the 2 implants.
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