Dear Editor

Regarding the editorial by Khaw et al.[1] we are surprised that after quite a few years now non- penetrating filtering surgery (NPFS) remains only partly understood by many ophthalmologists. There are at present two main NPFS: viscocanalostomy as described by Stegmann, in which outflow filtration is at least in theory not subconjunctival, and deep sclerectomy with or without an implant or even with viscoelastics, the success of which depends on several outflow routes - an important one being subconjunctival. Careful postoperative follow up becomes therefore mandatory and is at least as important as the procedure itself. If needed subconjunctival injection of antimetabolites, needling or use of lasers for goniopuncture, iris desincarceration and attempts of possible bleb reduction or closure of possible seidel may be required. It also becomes evident that antimetabolites play an important role in high risk cases for filtration failure undergoing NPFS (apart from viscocanolostomy as describd by Stegmann). Furthermore in our hospital we have been using antimetabolites also in cases requiring low postoperative IOP such as normal tension glaucoma since 1994. We do not understand the comment made stating that greater care should be taken with antimetabolites used during NPFS, since we intraoperatively use these agents before the deeper scleral flap is being excised or even created. Thus at this stage this does not differ with trabeculectomy. Later on the anterior chamber is not entered and furthermore the deep scleral flap which has been exposed to antimetabolites is being excised making the danger of intraoperative intraocular penetration considerably less than with trabeculectomy (even with unintended macroperforation during NPFS).

Additional sutures are added in cases of accidental macroperforaton so that the incidence of early significant hypotony in the hands of experienced surgeons with NPFS is not high. Moreover postoperative suturelysis may then be required in these cases if the sutures have been made too tight or too numerous.

References are being made to the article by Brart et al. However how reliable is it to compare the efficacy of two procedures without giving the same chances of success to both? Intraoperative antimetabolites were used for all trabeculectomies and yet never with NPFS. Yet strangely enough, postoperative antimetabolites as well as needling with an attempt to lift the scleral flap in some, were used in both groups. The author also writes in the discussion that patients with successful drainage at 6 and 12 months following viscocanalostomy had evidence of subconjunctival drainage of aqueous as opposed to eyes without successful drainage. Later on he further states that 'with our viscocanalostomy technique, the subconjunctival route is the main pathway' and 'observation of the disappearance of subconjunctival blebs in our patients with drainage failure after viscocanalostomy appears to suggest that subconjunctival fibrosis is responsible'. Clearly, if antimetabolites are being used in trabeculectomies, it should be used in NPFS before any reliable conclusions can be drawn. Goniopunture was also only done after 18 months which of course will not be of great help if the outflow route after the trabeculodescemet's membrane has scarred down. Thus to promote good filtration in addition to intraoperative and postoperative use of antimetabolites, goniopuncture can help in enhancing and thus in maintaining a flow under the scleral flap. Using lasers for suturelysis in trabeculectomies or for goniopuncture in NPFS is part of the armementarium we have in glaucoma surgery. The final aim for the patient is not to know whether the procedure is penetrating or not, but rather how effective it is so that the discussion on whether or not to use goniopunture is futile.

For ophthalmologists performing NPFS, the later is compared to trabeculectomy what phacoemulsification was to extracapsular cataract extraction. They will never go back to it unless obliged to do so. However it is clear that there is a learning curve to this surgery and that it is not as forgiveful as is trabeculectomy.

E. Ravinet, MD
A. Mermoud, MD

Reference

(1) Khaw PT, Wells AP, Lim KS. Surgery for glaucoma in the 21st century. Br J Ophthalmol 2002;86: 710-711.