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In a small randomised, prospective study, O‘Brart and co-workers1 attempted to prove that trabeculectomy (n=25) provided better intraocular pressure (IOP) control than viscocanalostomy (n=23). However, we have serious concerns about the methodology of this study that need to be addressed.
Viscocanalostomy is characterised by the injection of viscoelastic into Schlemm‘s canal.2 However, this was not performed in half of the patients randomised to this group. This would be as inappropriate as using an antimetabolite in only half the patients in the trabeculectomy group, and then comparing this entire group to the viscocanalostomy group. Although the authors do state that the subgroup of patients with intracanalicular viscoelastic injection was inferior to trabeculectomy, this cohort consisted only of 12 patients and it is not known how many had completed follow up after 18 months. Surely, these small numbers are insufficient to draw such conclusions.
We are also concerned that the use of intraoperative antimetabolites in the trabeculectomy group added a major confounding variable in this study. This is particularly perplexing as the authors‘ viscocanalostomy technique primarily relied on subconjunctival filtration, as evidenced by their triangular scleral flap design and looser suturing technique in which only three 10/0 nylon sutures were used. Furthermore, their excellent early success rate of viscocanalostomy (95% at 6 months), the presence of filtering blebs in their successful viscocanalostomy procedures, the lack of one in their failures, and the need for postoperative bleb needling and 5-fluorouracil injections all indicate this. The use of mitomycin-C and 5-fluorouracil improves the success rate of glaucoma filtering surgery by reducing episcleral fibrosis,3 and probably explains the difference in success rates in this study. We wonder what the results would have been had the use of intraoperative antimetabolites been used in all groups, or if none was used at all.
In contrast with the authors‘ technique, we employ Stegmann‘s approach to viscocanalostomy in using a parabolic superficial flap secured tightly with five sutures in a relatively water tight fashion.2 Although low lying blebs may develop, most patients achieve IOP lowering through multiple alternate pathways including uveoscleral, through Schlemm‘s canal, and subconjunctival.4 Certainly in higher risk cases, we feel that there is a role for antimetabolites in non-penetrating surgery, and have found it to be safer than when used with trabeculectomy.
It should be mentioned that quite often we do rely on subconjunctival filtration in non-penetrating procedures (that is, deep sclerectomy) but advocate the use of a collagen wick5 or hyaluronic acid6 implant with an intraoperative antimetabolite in higher risk cases to obtain optimal IOP control.
Fibrosis and loss of permeability of the trabeculo-Descemet‘s window (TDW) is a well described cause of postoperative elevation in IOP after non-penetrating glaucoma surgery. Postoperative Nd:YAG goniopuncture of the TDW in these cases is a relatively easy adjunctive procedure and may be needed in up to 41% of non-penetrating procedures.7 It has been reported to successfully lower postoperative IOP in over 80% of cases.7 Yet, we are dismayed that the authors decided not to attempt laser goniopuncture in those viscocanalostomy cases with postoperative IOP elevations because “such interventions clearly convert a ‘non-penetrating‘ technique into a penetrating, full thickness procedure.” We vehemently disagree with this line of reasoning as we feel goniopuncture is an extremely useful adjunctive procedure and converting to a penetrating (not “full thickness”) procedure in the safety of the controlled postoperative period is completely reasonable. This is akin to suture lysis in trabeculectomy. Would the authors feel that performing suture lysis constitutes conversion of a guarded trabeculectomy into a full thickness unguarded trabeculectomy and thus cannot be fairly compared? Although goniopuncture was performed in only three eyes at 18 months, we wonder what results would have be obtained if this was done in all cases with uncontrolled IOPs at any point in the postoperative period.
Although we are critical of this study, we applaud O‘Brart and colleagues for attempting to investigate this evolving area of glaucoma surgery. Although they may have shown that trabeculectomy with the use of an antimetabolite is superior to a modified form of viscocanalostomy dependent on subconjunctival filtration without the use of an antimetabolite, this study unfortunately does not fairly compare the efficacy of Stegmann‘s viscocanalostomy technique versus trabeculectomy. Non-penetrating glaucoma surgery certainly has an improved safety profile and surely as future well designed controlled studies become available, the efficacy of these procedures compared to trabeculectomy will become clearer.