Schlemm Canal-Based Surgery

Richard A. Lewis

Jacob W. Brubaker

INTRODUCTION

The surgical treatment of elevated intraocular pressure (IOP) at the site of outflow obstruction has long been a goal of glaucoma management. Canal-based surgery is not new. In fact, trabeculectomy was first described as a way to enhance outflow through the excised trabecular meshwork (TM) into the canal. Although further studies revealed that trabeculectomy functioned by directed flow to the subconjunctival space, the search for a safe and effective canal-based surgery did not stop. The current interest in procedures in and around the canal is the result of a number of factors, including the complications that arise in the early and late postoperative period after trabeculectomy. It is also the result of the development of more sophisticated surgical instruments and devices that allowed easier access to the canal. These procedures can now be partitioned into three basic categories.

Canal dilating: ab externo canaloplasty, ab interno canaloplasty, and Visco360

TM ablating: Trabectome, Kahook Dual Blade (KDB), gonioscopy-assisted transluminal trabeculotomy (GATT), and Trab 360

Canal stenting: iStent

CANALOPLASTY

The concept of nonpenetrating glaucoma surgery has evolved as an alternative to full- or partial-thickness procedures that rely on
subconjunctival flow and a bleb. The complex and unique individual variability of these procedures because of wound healing led to the search for a more direct surgical treatment of glaucoma. The first nonpenetrating procedure to utilize a microcatheter (iTrack, Ellex Medical Lasers Ltd, Adelaide, Australia) to take advantage of the full extent of the canal was canaloplasty, first described in 2007.

Mechanism of Action

• Ultrasound studies of patients with primary open-angle glaucoma (POAG) demonstrate collapse or narrowing of the canal of Schlemm. During the procedure, the canal is dilated, the TM is tensioned, and after removal of the deep scleral flap, a Descemet window is created.

• Canaloplasty is thought to lower IOP primarily by enhancing conventional circumferential outflow through the canal and the collector system. Studies using dyes and viscoelastic have confirmed enhanced intraoperative outflow in this manner. The greater the canal suture tension, the greater the IOP-lowering effect. Whether this continues to function months to years postoperatively has not been validated. Structural dilation of the canal has been demonstrated for at least 2 years (Fig. 26-1).

• Other sites of drainage have been postulated. These include percolation or flow through Descemet window into a “scleral lake.” Some patients are noted to have formation of a bleb, suggesting a transscleral flow or a “mini” perforation through the window.

Technique

• After creating a superficial scleral flap with a 4-mm base, a deeper scleral flap is used to access the canal (Fig. 26-2).

• The iTrack microcatheter (Fig. 26-3) is placed in the canal and threaded for the full 360 degrees until it comes out the other end.

• A 10-0 prolene suture is attached to the distal end and the catheter is withdrawn while injecting viscoelastic. The remaining prolene suture is tied tightly, creating tension in the meshwork to enhance outflow into the canal.

• Before suturing the superficial flap and conjunctiva, the deeper scleral flap is excised, leaving a Descemet window to further enhance outflow (Fig. 26-4).

• Blebs are avoided.

Outcomes

• On the basis of the published data, canaloplasty efficacy results are comparable to the published reports of trabeculectomy. Mean IOP at 2 years decreased to 16.2 mm Hg in eyes having canaloplasty alone. In eyes undergoing combined canaloplasty and cataract surgery, IOP decreased to 13.7 mm Hg and medication use decreased to 0.6 and 0.2, respectively

• Studies demonstrate that canaloplasty is safer than trabeculectomy. Hypotony, choroidal detachment, and bleb infections were reported in less than 1% of all cases. The most common side effect is transient hyphema.

BIBLIOGRAPHY

Lewis RA, von Wolff K, Tetz M, et al. Canaloplasty: circumferential viscodilation and tensioning of Schlemm’s canal using a flexible microcatheter for the treatment of open-angle glaucoma in adults: interim clinical study analysis. J Cataract Refract Surg. 2007;33:1217-1226.

Lewis RA, von Wolff K, Tetz M, et al. Canaloplasty: circumferential viscodilation and tensioning of Schlemm’s canal using a flexible microcatheter for the treatment of open-angle glaucoma in adults: two-year interim clinical study results. J Cataract Refract Surg. 2009;35: 814-824.

Shingleton B, Tetz M, Korber N. Circumferential viscodilation and tensioning of Schlemm canal (canaloplasty) with temporal clear corneal phacoemulsification cataract surgery for open-angle glaucoma and visually significant cataract: one-year results. J Cataract Refract Surg. 2008;34:433-440.

FIGURE 26-1. Canaloplasty effect. Ultrasound image of canal preoperatively (A) and 2 years postoperatively (B). A. The collapsed canal in open-angle glaucoma suggests limited flow. B. Canaloplasty viscodilates the canal during surgery and facilitates drainage into the collectors. SC, Schlemm canal.

FIGURE 26-2. Scleral dissection. A guarded blade is used to create a 300-µm partial-thickness groove (A and B), followed by sharp dissection to create a sclerocorneal flap (C).

FIGURE 26-2. (continued) D. A deeper flap is dissected at approximately 99% depth. E. Once Schlemm canal is unroofed, a modified Drysdale spatula is used to bluntly dissect the inner corneoscleral flap from Descemet membrane with careful attention not to rupture and enter the anterior chamber. F. The appearance of both dissected flaps; there is some blood reflux from each end of the exposed Schlemm canal.

FIGURE 26-3. iTrack 250A canaloplasty microcatheter. The 200-µm-diameter catheter with a 250-µm tip is attached to a battery-powered light source with a second attachment to facilitate injecting viscoelastic to dilate the canal upon removal. (Courtesy of iScience Interventional.)

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