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Tan and Hitchings should be commended on attempting to tackle the issue of sclerectomy with collagen implant.1 Nevertheless we do feel that their fundamental points and principal arguments merit reconsideration.
The authors state categorically that “long term outcomes do not exit for the newer non-penetrating surgery technique” when, in fact, long term (43.2 months (SD 14.3)) results for deep sclerectomy with collagen implant have been published some time ago.2 The study provided a qualified success rate of 94.8% and the complete success rate of 61.9% after 60 months (survival analysis), with a mean IOP at end of follow up of 11.8 (SD 3) mm Hg. The study reported no surgically induced cataract in the whole series of 105 patients.
The authors, unfortunately, failed to cite a landmark study3 comparing deep sclerectomy without an implant with trabeculectomy in the two eyes of the same patient in a prospective randomised fashion. At 12 months, mean IOP reduction was 12.3 (4.2) mm Hg (sclerectomy) versus 14.1 (6.4) mm Hg (trabeculectomy) (p = 0.15), and an IOP ≤21 mm Hg was achieved in 36 (92.3%) and 37 eyes (94.9%) (p = 0.9), respectively. The authors of this study concluded that deep sclerectomy might provide comparable IOP reduction with fewer complications in the management of primary open angle glaucoma.
Furthermore, the authors cite a study by Gandolfi (personal communication) supposedly providing evidence that “trabeculectomy produces lower and better sustained IOP control than either viscocanalostomy or deep sclerectomy.” The authors fail to mention however, that in this particular study postoperative YAG goniopunctures were considered as a failure criterion. Excluding goniopuncture from the success criteria would easily be compared to considering laser suture lysis or even YAG capsulotomies to be failure criteria of glaucoma or cataract surgeries. Furthermore, Gandolfi concluded that deep sclerectomy was associated with lower perturbation of lens nuclear transparency (personal communication, January 2002).
In another point worthy of reconsideration the authors cite a study4 that allegedly draws attention to high rates of hypotony and hyphaema after intraoperative conversion of deep sclerectomy to trabeculectomy following accidental intraoperative perforation of the trabeculo-Descemet’s membrane (TDM). However, the authors again fail to mention, when quoting this specific study, that “when deep sclerectomy is complicated with a perforation of the TDM, the long term success rate of the surgery is similar to that of trabeculectomy.” This conclusion would encourage the surgeons to start their surgery as a deep sclerectomy, knowing that in case of a perforation and a subsequent transformation to trabeculectomy, the chances of success would be similar to initial trabeculectomy.
The authors of the paper at hand compare in their figures, to each other, for viscocanalostomy, deep sclerectomy without an implant, deep sclerectomy with collagen implant, and deep sclerectomy without suturing the superficial flap, thus drawing certain conclusions. The different techniques have one thing in common, the element of non-perforation. It is not useful to compare apples and pears.
A major factor in the conflicting, often contradictory, results available is the element of long learning curves.
The same group also analysed the depth of their dissection of the deep sclera7 to find that they dissected too superficially in 48% of their cases and too deeply in 17%; meaning that the proper depth of dissection, which should bisect transversally the Schlemm’s canal deroofing it, was not achieved in the majority of their cases.
To achieve successful non-penetrating surgery, the dissection of the deep sclerectomy needs to be correct. This entails a total excision of corneal stroma behind Descemet’s membrane and the excision of the inner wall of Schlemm’s canal and the juxtacanalicular trabeculum. An implant has to be used to maintain the scleral space patent.8 Laser goniopuncture should be performed at any postoperative stage when IOP mounts beyond the target pressure.
We do, however, wholeheartedly agree with the authors on the importance of conducting a large scale multinational randomised prospective trial as the only possible method to compare non-penetrating glaucoma surgery, or any other new surgical practice, to trabeculectomy.