Canaloplasty offers an alternative to trabeculectomy

The nonpenetrating procedure restores the eye’s natural outflow system while eliminating problems associated with blebs.

Thomas John

Glaucoma is a commonly encountered clinical disease entity that may not be fully controlled by medical treatment. When surgical intervention becomes necessary, there are two main avenues to choose from, penetrating procedures such as trabeculectomy and nonpenetrating procedures. While trabeculectomy has been time-tested, it can be associated with various clinical challenges, especially in the immediate and late postoperative periods, including infection, irritation and aqueous humor leakage. Hence, nonpenetrating procedures such as canaloplasty may be an alternative in many cases of medically uncontrolled open-angle glaucoma.

The long-term efficacy and safety of canaloplasty has been demonstrated in published studies. Canaloplasty has many advantages over trabeculectomy including that it restores the eye’s natural outflow system, is a non-bleb procedure and requires no antimetabolites. Thus, canaloplasty eliminates the problems associated with a bleb in addition to hypotony-related issues such as maculopathy. Also, contact lens wearers may be good patients for canaloplasty. However, the procedure can be surgically challenging in the initial stages and requires a dedicated learning period to successfully perform.

In this column, Dr. Grieshaber describes a step-by-step approach to performing a canaloplasty procedure and also provides useful surgical pearls.

Thomas John, MD
OSN Surgical Maneuvers Editor

by Matthias C. Grieshaber, MD, FEBO

Matthias C. Grieshaber

In the majority of patients, I use a combination of topical anesthesia (tetracaine 1%) and subconjunctival anesthesia (mepivacaine 1.5%), which also acts as viscodissector and alleviates scleral exposure (Figure 1). For the learning surgeon, I suggest using general anesthesia because the operating time is longer and one should never rush doing surgery, especially while learning a new technique.

After two corneal anchorage sutures are placed, the conjunctiva and Tenon’s capsule are opened at the limbus and the sclera is exposed. No wet-field hemostasis cautery should be performed to preserve the episcleral collector channels, but vasoconstrictive agents (eg, ornipressin) may be used to reduce excessive bleeding. The application of mitomycin C is also avoided, not only because of side effects but simply because a filtering bleb is unwanted. A superficial flap and deep scleral flap are dissected. The superficial flap is about one-third scleral thickness and has a parabolic shape (about 5 mm × 5 mm) to ensure watertight closure at the end of surgery. A parabolic impression, made with a Kearney marker, helps to outline the superficial flap. The margins of the superficial flap should extend 1 mm into clear cornea to facilitate the preparation of the trabeculo-Descemet’s membrane window later (Figure 2). The deep flap (about 4 mm × 4 mm) is dissected just above the choroid. It is important to progress in this plane anteriorly to find Schlemm’s canal, crossing the scleral spur, which is identified at the base of the canal by a change in the pattern of collagen fibers from crisscross to ligament-like (Figure 3).

At this stage of the procedure, release the anchorage sutures and perform a paracentesis (Figure 4) to lower IOP in order to reduce the risk of rupture of the trabeculo-Descemet’s membrane window and to create blood reflux into Schlemm’s canal to aid in identifying the ostiae. The trabeculo-Descemet’s membrane window should be large enough, about the size of the mini blade, to provide good aqueous percolation and have a backup for Nd:YAG laser goniopuncture after surgery, if necessary.

Figure 1. Subconjunctival injection of anesthetics.

Figure 2. The superficial flap is dissected with the margins extending about 1 mm into clear cornea to facilitate preparation of the trabeculo-Descemet’s membrane window during dissection of the deep scleral flap. Images: Grieshaber MC

Figure 3. The deep scleral flap is dissected just above the choroidal tissue, and Schlemm’s canal is partially de-roofed.

Figure 4. A side-port incision is made to lower IOP to reduce the risk of the rupture of trabeculo-Descemet’s membrane window.

Figure 5. Dilation of the ostiae with a microcannula and OVD facilitates the insertion of the microcatheter.

Figure 6. The shaft of the microcatheter must be inserted in line and in plane with the ostia. Note the red beacon tip.

Figure 7. After 360° cannulation, a 10-0 polypropylene suture is tied to the distal tip of the microcatheter.

Figure 8. The suture is locked under tension to distend the inner wall of Schlemm’s canal.

Figure 9. Ultrasound biomicroscopy (80 MHz) of the chamber angle. Note the dilated Schlemm’s canal and the sonographic shadow of knot of the suture below the area of Schwalbe’s line.

Figure 10. Watertight closure of the superficial flap with seven interrupted 10-0 nylon sutures.

Figure 11. Closed conjunctival flap. Completed view of the procedure.

Figure 12. No bleb formation 2 months after canaloplasty.

The ostiae are dilated with the ophthalmic viscosurgical device (OVD) Healon GV (sodium hyaluronate 1.4%, Abbott Medical Optics) with a microcannula (Figure 5) to facilitate the insertion of the flexible microcatheter. Before starting with the cannulation, the microcatheter should be primed with OVD to ensure it is patent, and the microscope lights should be dimmed so the path of the illuminated beacon tip, which must be visible at all times, can be tracked easily through the sclera. Also, it is important to keep the shaft in line and in plane with the ostia during insertion of the microcatheter to prevent perforation into the anterior chamber or suprachoroidal space (Figure 6). Further, one should not inject more than 0.5 µL of OVD (equaling one-eighth of a turn on the injector knob) every two clock hours during advancement of the tip to avoid cyclodialysis or detachment of trabeculo-Descemet’s membrane.

Usually, cannulation of Schlemm’s canal is a smooth and uneventful step of this procedure. However, if the tip gets stuck during cannulation, the catheter should be retracted a bit and passed again while applying a slight counter-pressure over the distal tip on the limbus. Alternatively, the tip can be bent slightly according to the curvature of Schlemm’s canal. This may prevent passage into a large collector channel leaving from the outer wall of Schlemm’s canal. If there is still no passage, the catheter must be retracted completely and inserted from the opposite side for 360° cannulation. In such cases, further injections of OVD must be avoided absolutely because these could cause further dilation of the collector channels, making cannulation increasingly difficult, and could also lead to detachment of trabeculo-Descemet’s membrane or cyclodialysis.

After complete circumferential dilation, a 10-0 polypropylene suture is tied to the distal tip tightly enough that it will not slip off. In case viscodilation is performed during withdrawal of the catheter, the suture should not be tied too tightly to ensure that OVD flow is not impeded by the tip (Figure 7). After looping the suture through Schlemm’s canal, both thread ends are pulled gently toward each other simultaneously. This prevents “cheese-wiring,” a commonly voiced concern but rarely encountered complication. The suture is placed against Schwalbe’s line, which ensures correct suture position for consistent tensioning of the inner wall. Slip-knot techniques are used to lock the suture under tension to stretch the Schlemm’s canal and trabecular meshwork circumferentially (Figure 8). Suture tension is judged clinically by observing the dimple of the suture in the trabeculo-Descemet’s window or by high-resolution ultrasound biomicroscopy, if available (Figure 9).

The superficial flap is closed watertight with five to seven interrupted 10-0 nylon sutures starting at the apex. Good tissue apposition prevents the formation of blebs and promotes backflow of aqueous into Schlemm’s canal (Figure 10). The anterior chamber is reformed with balanced salt solution, and the wound is tested for leakage. If there is any leakage, additional sutures are placed until no leakage is detected. I prefer to close the conjunctiva and Tenon’s capsule with 8-0 absorbable sutures (Figure 11). At the end of surgery, the eye is left with a pressure of about 20 mm Hg to reduce the risk of refluxed hyphema or iris incarceration and to activate aqueous outflow through re-established pathways.

Surgical pearls

1. Use vasoconstrictive agents instead of wet-field hemostasis cautery to reduce excessive bleeding and to preserve the episcleral collector channels for aqueous outflow.

2. The parabolic shape of the scleral flap and proper perpendicular incisions allow for tight closure and prevention of leakage at the end of surgery.

3. Verify the depth of the deep scleral dissection. Stay in the deepest layer just above the choroid, and identify the scleral spur at the base of Schlemm’s canal by the change of the collagen fiber composition from crisscross to regular. By following these rules, Schlemm’s canal is easily found.

4. Release the anchorage suture and perform a paracentesis to lower IOP and reduce the risk of perforation of the trabeculo-Descemet’s membrane.

5. Creating a large trabeculo-Descemet’s membrane window not only helps in inserting the catheter during surgery but also serves as a backup for Nd:YAG laser goniopuncture after surgery.

6. Keep the shaft of the catheter in line and in plane with the canal ostia during the cannulation to prevent penetration into the anterior chamber or suprachoroidal space.

7. Never dilate Schlemm’s canal with OVD more than one-eighth of a turn on the injector knob every two clock hours. Overdilation increases the risk for trabeculo-Descemet’s membrane detachment and cyclodialysis.

8. Tie the suture to the catheter tip tightly enough that it does not slip off during looping through Schlemm’s canal, but not so tightly as to impede OVD flow by the suture.

9. Gently pull both thread ends synchronously and equally toward each other as if closing a sling to prevent rupture of the inner wall. Note that the inner wall region is more resistant to tension but not friction than one might expect.

10. Judgment of the tensioning suture is performed by observing the inward distension of the trabeculo-Descemet’s membrane on the hypotonous eye.

11. Close the superficial flap watertight to avoid bleb formation and to promote outflow through re-established pathways.

The postoperative treatment is topical steroids and antibiotic drops and/or ointments up to 8 weeks.

References:

• Bull H, von Wolff K, Körber N, Tetz M. Three-year canaloplasty outcomes for the treatment of open-angle glaucoma: European study results [published online ahead of print July 6, 2011]. Graefes Arch Clin Exp Ophthalmol. 2011;doi:10.1007/s00417-011-1728-3.
• Grieshaber MC, Fraenkl S, Schoetzau A, Flammer J, Orgül S. Circumferential viscocanalostomy and suture canal distension (canaloplasty) for whites with open-angle glaucoma. J Glaucoma. 2011;20(5):298-302.
• Grieshaber MC, Pienaar A, Olivier J, Stegmann R. Canaloplasty for primary open-angle glaucoma: long-term outcome. Br J Ophthalmol. 2010;94(11):1478-1482.
• Grieshaber MC, Pienaar A, Olivier J, Stegmann R. Comparing two tensioning suture sizes for 360 degrees viscocanalostomy (canaloplasty): a randomised controlled trial. Eye (Lond). 2010;24(7):1220-1226.
• Lewis RA, von Wolff K, Tetz M, et al. Canaloplasty: Three-year results of circumferential viscodilation and tensioning of Schlemm canal using a microcatheter to treat open-angle glaucoma. J Cataract Refract Surg. 2010;37(4):682-690.
• Peckar C, Koerber N. Canaloplasty for open angle glaucoma: a three years critical evaluation and comparison with viscocanalostomy. Spektrum Augenheilkd. 2008;22(4):240–246.
• Stegmann RC. Visco-canalostomy: a new surgical technique for open angle glaucoma. An Inst Barraquer. 1995;25:229-232.

• Thomas John, MD, is a clinical associate professor at Loyola University at Chicago and is in private practice in Oak Brook, Tinley Park and Oak Lawn, Ill. He can be reached at 708-429-2223; fax: 708-429-2226; email: tjcornea@gmail.com.
• Matthias C. Grieshaber, MD, FEBO, is a consultant at University of Basel, Department of Ophthalmology, Basel, Switzerland; and is in private practice in Schaffhausen, Switzerland. He can be reached at 41-61-265-87-81; fax: 41-61-265-86-52; email: mgrieshaber@uhbs.ch.
• Disclosure: Drs. John and Grieshaber have no relevant financial disclosures.