Viscocanalostomy - Clinical Tree

Summary

Viscocanalostomy is an effective surgery for lowering IOP in glaucomatous eyes. It has several advantages over trabeculectomy, such as lower complication rate, lower incidence of cataract, less risk of infection-related side effects, fewer refractive changes, significantly less eye discomfort, and an easy postoperative management. When considering final IOPs between 16 and 21 mmHg, the rate of failure over time is similar between the two procedures, while with trabeculectomy there is a greater chance to reach lower IOPs. Viscocanalostomy is, nevertheless, technically demanding, it requires a long learning curve, and it is a longer procedure than trabeculectomy.

Introduction

In recent years, nonpenetrating glaucoma surgery received great interest as a possible alternative to trabeculectomy. This class of procedures is mainly represented by ‘ deep sclerectomy ’, and by ‘ viscocanalostomy ’ (and by its more recent variation ‘ canaloplasty ’), and they are based on the original studies by Krasnov and by Zimmerman et al. on ‘nonpenetrating trabeculectomy.’ All procedures are aimed at 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 intraocular pressure (IOP) drops, hypotonies, and flat chambers. The absence of an anterior chamber opening and iridectomy limits the risk of cataract and infection. Compared to deep sclerectomy, viscocanalostomy is a step forward. This procedure is aimed not only at taking advantage of being nonpenetrating, as deep sclerectomy, but, most important, in restoring the physiological outflow pathway, thus avoiding any external filtration. This would make the success independent of conjunctival or episcleral scarring, the leading cause of failure in trabeculectomy, with fewer indications for wound healing modulation. Moreover, the absence of the filtering bleb avoids related ocular discomfort, and the procedure can be carried out in any quadrant.

Mechanism of Action

Viscocanalostomy increases the aqueous outflow through different mechanisms of action. 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 the trabecular outflow facility and making the procedure act as a trabeculotomy ( Fig. 98-1 ). Aqueous outflow facility is also increased by damage to the inner wall of Schlemm's canal and adjacent trabecula 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, or by external filtration into the subconjunctival space, or by reabsorption into the subchoroidal space. External filtration and filtering blebs are uncommon in viscocanalostomy and can be detected only in up to one-third of eyes, whereas a supraciliary hypoechoic area suggesting aqueous drainage into the subchoroidal space has been shown by the use of ultrasound biomicroscopy.

Figure 98-1, Trabecular meshwork of a viscocanalostomy-treated monkey eye about 4 mm from the 12-o'clock limbus where surgery was performed 2 months earlier. Larger defects (arrows), about 1–5 µm long, are present in the inner and outer wall of Schlemm's canal (SC).

Indications

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 in previously operated eyes where an extensive cautery of the perilimbal area has been made. Due to its final results, the procedure is indicated in primary open-angle glaucoma when target IOP is not very low. The advantage of the absence of (or very reduced) external filtration makes the technique safe and is particularly indicated in eyes with chronic blepharitis, in contact lens wearers, or when the surgery has to be performed in the lateral or inferior quadrants. Viscocanalostomy was also shown to be effective in uveitic glaucomas with well-controlled inflammation, in juvenile glaucomas, and in congenital glaucomas.

Preoperative Considerations

Preoperative assessment includes considering the risk factors for surgical failure, such as previous conjunctival or limbal surgery, angle synechiae in the surgical quadrant, neovascularization of the angle, iridocorneal endothelial syndrome or scleral thinning. Anticoagulants should be discontinued prior to surgery to minimize the risk of bleeding with subsequent need for excessive cautery, and to reduce the risk of intraocular hemorrhage. In inflamed eyes, pretreatment with topical steroids can be considered, such as prednisolone acetate 1%, beginning 1 week preoperatively.

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