Surgical alternative to trabeculectomy

Introduction

During the last 20 years, many alternatives to trabeculectomy were proposed, but only ‘‘nonpenetrating glaucoma surgery’’ succeeded and was therefore included in guidelines as a surgical option for glaucoma (European Glaucoma Society, 2003; Carassa and Goldberg, 2005). Non-penetrating glaucoma surgery is represented by ‘‘deep sclerectomy’’ and by ‘‘viscocanalostomy’’ (which was introduced by R. Stegmann in the early 1990s), and is based on the original studies by Krasnov (1972) and by Zimmerman et al. (1984) on ‘‘non-penetrating trabeculectomy.’’ Similarly, both procedures are aimed at lowering intraocular

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pressure (IOP) by allowing drainage of the aqueous humor from the anterior chamber not through a patent scleral opening, but by slow percolation through the inner trabecular meshwork and/or Descemet’s membrane (‘‘sclerodescemetic membrane’’). This avoids sudden IOP drops, hypotonies, and flat anterior chambers. The absence of anterior chamber opening and iridectomy limits the risk of cataract and infection. Compared to deep sclerectomy, viscocanalostomy is a step forward. In fact, this procedure is aimed not only at taking the advantages of being nonpenetrating, as deep sclerectomy, but also, most important, in restoring the physiological outflow pathway, thus avoiding any external filtration. This would make the success of the procedure independent of conjunctival or episcleral scarring, the leading causes of failure in trabeculectomy, with fewer indications for wound healing modulation.

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Moreover, the reduced incidence of the filtering bleb avoids related ocular discomfort, and the procedure can be carried out in any quadrant.

Deep sclerectomy

Deep sclerectomy is aimed at reducing IOP by allowing external filtration of the aqueous humor. Differently from trabeculectomy, aqueous exits the eye not through a patent hole but by slow passage through the sclerodescemetic membrane formed by the internal portion of the posterior and anterior trabecular meshwork and by the adjacent Descemet’s membrane. The membrane is created by the removal of the inner wall of Schlemm’s canal and juxtacanalicular trabeculum (sites of the increased outflow resistance in glaucoma) and by the exposure of the anterior trabecular meshwork and Descemet’s membrane. After exiting the anterior chamber, aqueous fills an intrascleral space called ‘‘intrascleral lake’’ or ‘‘decompression chamber,’’ and it is finally drained into the subconjunctival space or is partially reabsorbed into the suprachoroidal space (Chiou et al., 1996, 1998b; Marchini et al., 2001; Sarodia et al., 2007).

The surgical technique varies among surgeons. Different methods of anesthesia are used as topical, local infiltration, peribulbar, or general anesthesia based on surgeon preference. Nevertheless, nonpenetrating surgery is a long and difficult procedure; thus, deep and long-lasting anesthesia and akinesia, as the peribulbar block, are to be preferred.

After performing a wide limbus-based conjunctival flap, a 5 5 mm rectangular superficial flap, approximately one-third of scleral thickness, is dissected. A limbus-based triangle of deep sclera is then dissected as deep as to leave a thin layer of sclera over the choroid and the ciliary body. The dissection is carried anteriorly until Schlemm’s canal is deroofed and 1–2 mm of Descemet’s membrane is exposed. At this stage of the procedure, aqueous humor should be seen percolating through the ‘‘trabeculodescemetic membrane.’’ In order to increase percolation, some surgeons suggest the removal of the inner wall of Schlemm’s canal (‘‘external trabeculectomy’’). The inner flap is then removed, and, in order to maintain the space

(the ‘‘intrascleral lake’’ or ‘‘decompression chamber’’) and avoid postoperative scarring, different implants are often used. Absorbable porcine collagen implant (Aquaflowt, Staar surgical AG, Nidau, Switzerland), reticulate hyaluronic acid implant (Skgelt, Corneal Laboratoires, Paris, France), non-absorbable implant (T Fluxt, Ioltech Laboratoires, La Rochelle, France), or PMMA implant (Homdec SA, Belmont, Switzerland) can be sutured or positioned in the intrascleral space. The superficial scleral flap is then repositioned and sutured with two 10-0 nylon sutures. Finally, the conjunctiva is tightly closed.

Some authors, before dissecting the internal flap or just after the opening of the Schlemm’s canal, in all or in selected cases, apply a sponge soaked with Mitomicin C (0.1–0.3 mg/ml) over the sclera for 1–3 min in order to avoid excessive scarring, thus increasing the success rate of the procedure. In the postoperative time, up to 60% of the eyes need to be treated with a Nd:YAG laser goniopuncture of the ‘‘trabeculodescemetic’’ membrane. With a gonioscopy contact lens, the aiming beam of the laser is focused on the semi-transparent trabecularDescemet’s membrane, which often has a concave appearance. In the free-running Q-switched mode with a power of 4–8 mJ, 4–15 shots are applied. This procedure, by creating openings in the membrane itself, increases the outflow of aqueous, thus reducing the IOP.

Results vary between studies due to different follow-up and technique used. The mean final IOP without adjunctive therapy is in the midto highteens ranging from 11 to 20.9 mmHg, while the achievement of an IOP below 21 mmHg is obtained in 57–92.6% at 12 months, in 40–69% at 24 months, in 44–77% at 36 months, and in 34–63% at 48 months (Demailly et al., 1997; Sanchez et al., 1997; Bas and Goethals, 1999; Bechetoille, 1999; Hamard et al., 1999; Karlen et al., 1999; Massy et al., 1999; Sourdille et al., 1999; Dahan and Drusedau, 2000; Mermoud, 2000;

et al., 2002; Auer et al., 2004; Lachkar et al., 2004; Neudorfer et al., 2004; Shaarawy et al., 2004; Anand and Atherley, 2005; Shaarawy and Mermoud, 2005; Khairy et al., 2006; Mansouri

et al., 2006; Mielke et al., 2006). In a meta-analysis of 29 articles conducted in 2004 (and thus not including the most updated articles), the success rate of deep sclerectomy (as an IOPo21 mmHg without medications) was 69.7% without implant, 59.4% with collagen implant, and 71.1% with reticulated hyaluronic acid implant. No significant difference was found among the three techniques (Cheng and Wei, 2004). When compared with trabeculectomy, the success in achieving an IOP below 21 mmHg is comparable even though final IOPs were found lower with trabeculectomy, in some studies (Chiou et al., 1998a; Mermoud et al., 1999; El-Sayyad et al., 2000; Chiselita, 2001; Ambresin et al., 2002; Cillino et al., 2004; Schwenn et al., 2004). The adjunctive use of implants, of antimetabolites, and of goniopuncture allows a greater success rate and lower IOPs, comparable to those obtained with MMC trabeculectomy (Neudorfer et al., 2004; Schwenn et al., 2004; Anand and Atherley, 2005).

Complications are minor and fewer than those reported after trabeculectomy. A reduced induced corneal refractive change was also showed in one study (Egrilmez et al., 2004).

Deep sclerectomy has specific indications and contraindications, based on its intrinsic characteristics and on clinical results. The procedure is not indicated in angle-closure glaucomas, in neovascular glaucoma, and in eyes with wide-angle synechia or diffuse scarring of the conjunctiva in the surgical quadrant. As suggested by the

Terminology and Guidelines for Glaucoma of the European Glaucoma Society (2003), deep sclerectomy is indicated in primary open-angle glaucoma when target IOP is not very low. The advantages of being non-penetrating make the procedure particularly useful in aphakic eyes with vitreous in the anterior chamber, or in cases where a sudden drop in IOP should be avoided, such as eyes with uncontrolled high pressure or high myopia. The procedure was found effective in uveitic glaucoma (Auer et al., 2004).

Viscocanalostomy

Viscocanalostomy is aimed at reducing IOP by attempting to restore the physiological outflow

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pathway and not by allowing external filtration of the aqueous humor. The technique is similar to the one used for deep sclerectomy, but for the injection of high molecular weight sodium hyaluronate in Schlemm’s canal and the tight suture of the external scleral flap (aimed at making the ‘‘intrascleral lake’’ or ‘‘decompression chamber’’ watertight). The rationale of the technique is the evidence that the site of greater aqueous outflow resistance in openangle glaucoma is the trabecular meshwork. Viscocanalostomy was aimed at creating a bypass by which aqueous humor could reach Schlemm’s canal and leave the eye through the physiological pathway, without passing through the trabecular meshwork. This is made by producing a space inside the sclera (the ‘‘intrascleral lake’’ or ‘‘decompression chamber’’) directly communicating both with Schlemm’s canal and with the anterior chamber through the ‘‘sclerodescemetic membrane.’’ The aqueous will enter the ‘‘intrascleral lake’’ by percolating through the membrane and will then leave by entering the Schlemm’s canal. In reality, viscocanalostomy lowers IOP by increasing the aqueous outflow through different pathways. Injection of viscoelastic into the canal not only dilates the canal and associated collectors, but also disrupts the internal and external walls of Schlemm’s canal and adjacent trabecular layers, thus increasing trabecular outflow facility and making the procedure acting as a trabeculotomy (Tamm et al., 2004). Aqueous outflow facility is also increased by damage to the inner wall of Schlemm’s canal and adjacent trabeculum at the site of surgery, thus enhancing aqueous outflow into the scleral lake. From here aqueous can leave the eye via three different paths: through the cut ends and previously nonfunctional sectors of Schlemm’s canal to collector channels, by external filtration into the subconjunctival space, or by reabsorption into the subchoroidal space. External filtration and filtering blebs are uncommon in viscocanalostomy, while a supraciliary hypoechoic area suggesting aqueous drainage into the subchoroidal space has been shown by the use of ultrasound biomicroscopy (Carassa et al., 1998; Negri-Aranguren et al., 2002).

The surgical technique of viscocanalostomy is similar to the one described for deep sclerectomy.

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Nevertheless, some critical differences need to be emphasized.

During the conjunctival fornix-based flap dissection, cautery should be minimized in order to prevent damage to Schlemm’s canal and collector channels. In dissecting the internal scleral flap, Schlemm’s canal needs to be fully opened and deroofed, leaving two patent and clean openings on the lateral edges of the cut. A paracentesis should always be made in order to decrease IOP, to make incannulation of Schlemm’s canal easier, and to reduce bulging of Descemet’s membrane during its cleavage from the corneal stroma, which is at high risk of tear formation. To avoid external pressure on the eye, the traction on the bridle suture should also be removed. Using the specific 165-mm cannula, high molecular weight sodium hyaluronate is slowly injected into Schlemm’s canal by cannulating the two ostia at the lateral edges of the inner flap. To avoid damage to the canal endothelium, the insertion of the cannula should not exceed 1–1.5 mm from the ostia. The slow injection should be repeated six to seven times on each side to avoid tears and ruptures of the canal. The injection of viscoelastic substance allows progressive dilation of Schlemm’s canal over its circumference, disrupting its internal and external walls and adjacent trabecular layers. Moreover, sodium hyaluronate hemostatic properties avoid bleeding and fibrin clot formation, thus limiting healing processes and scarring of Schlemm’s canal openings.

Differently from deep sclerectomy, there is little evidence that the use of an implant as the absorbable porcine collagen implant (Aquaflowt, Staar surgical AG, Nidau, Switzerland), the reticulate hyaluronic acid implant (Skgelt, Corneal Laboratoires, Paris, France), and the non-absorbable implant (T Fluxt, Ioltech Laboratoires, La Rochelle, France) could be beneficial for the outcome of viscocanalostomy. This is probably related to the differences in the mechanism of action between the two procedures. Nevertheless, as described in the next paragraph, in order to maintain patency of the ‘‘intrascleral lake’’ during the days after surgery, high-weight sodium hyaluronate is used.

In order to seal the ‘‘intrascleral lake,’’ the outer scleral flap should be tightly sutured by placing six

or seven 10-0 nylon stitches. The step created by the different size of the two flaps allows a better and tight apposition of the external flap. Finally, in order to minimize bleeding and prevent collapsing and scarring of the intrascleral chamber, high molecular weight sodium hyaluronate is injected underneath the flap.

Viscocanalostomy has specific indications and contraindications. It cannot be effective when the angle is closed or neovascularized, or when Schlemm’s canal is likely to be damaged. This is the case of previously operated eyes where an extensive cautery of the perilimbar area was made. Due to its final results, the procedure is indicated in primary open-angle glaucoma when target IOP is not very low (as indicated by the Terminology and Guidelines for Glaucoma of the European Glaucoma Society (2003) and by the Consensus Series book by the Association of International Glaucoma Societies (Carassa and Goldberg, 2005)). The advantage of the absence (or very reduced) external filtration makes the technique safe and particularly indicated in eyes with chronic blepharitis, in lens contact wearer, or when the surgery has to be performed in the lateral or inferior quadrants. Viscocanalostomy was shown effective also in uveitic glaucomas with wellcontrolled inflammation (Miserocchi et al., 2004), in juvenile glaucomas (Stangos et al., 2005), and in congenital glaucomas (Noureddin et al., 2006).

Viscocanalostomy is an effective procedure in lowering IOP (Carassa et al., 1998; Stegmann et al., 1999; Sunaric-Me´gevand and Leuenberger, 2001; Luke et al., 2003; Shaarawy et al., 2003; Yarangumeli et al., 2005). The mean final IOP without adjunctive therapy is in the midto high teens ranging from 11.9 to 18.3 mmHg, while the achievement of an IOP below 21 mmHg is obtained in 30–86% at 12 months, in 21–85% at 24 months, and in 35.3–92% at 36 months. In a meta-analysis of eight articles conducted in 2004, the success rate of viscocanalostomy (as an IOPo21 mmHg without medications) was 72.0% with no significant difference when compared to deep sclerectomy (Cheng and Wei, 2004).

When compared to trabeculectomy, viscocanalostomy may provide higher final mean IOPs and a lower success rate in achieving an IOP below