The Role of Canaloplasty in the Surgical Treatment of Open-Angle Glaucoma

Canaloplasty is a new surgical technique used to treat various types of open angle glaucoma, which is similar to Stegmann’s viscocanalostomy. It is a non-perforating bleb-less technique, which involves positioning a 10-0 prolene suture that is left tensioned within Schlemm’s canal that is dilated with the use of viscoelastic. This surgery aims at facilitating aqueous outflow through natural pathways (collector channels and aqueous veins) [1-9].

In brief, surgery involves first forming a fornix-based conjunctival flap, followed by a superficial scleral flap, similar to deep sclerectomy, which is dissected about 1.5 mm forward. A deep scleral flap is then sculpted, followed by the opening and deroofing of Schlemm's canal. This surgical step is performed after paracentesis to avoid Intra-Operatory Intraocular Pressure (IOP) spikes, thus reducing the risk of perforation of the trabeculo-descemet membrane. After deep scleral flap removal, the two ostia of the canal are dilated using high molecular weight hyaluronic acid (Healon GV). A 200 micron microcatheter (iTrack by iScience Interventional, Menlo Park, CA, USA) connected to a laser flickering red light source is used to facilitate the identification of the distal tip through the sclera ( Figure 1) while it is inserted and pushed though the entire 360° of Schlemm's canal until it comes out of the other end of the of the canal opening. Once the microcatheter exits the ostia, a single or double 10-0 prolene suture is tied to the distal tip and then withdrawn and pulled back in the opposite direction through the canal while a small amount of viscoelastic agent is delivered in at every two or three clock hours. The suture is then knotted under tension in order to inwardly distend the trabecular meshwork. The superficial scleral flap is then tightly sutured to ensure a watertight closure, thus preventing any bleb formation. The conjunctival flap is then sutured with 10-0 vicryl sutures.
IOP tends to be in the mid teens after surgery, often without any medical therapy. Canaloplasty is an interesting surgical alternative mostly because, unlike traditional trabeculectomy, a filtering bleb is not needed to enhance aqueous outflow. Postoperative results tend to be quite good; after just a few weeks the eye shows little signs of surgery, inflammation or discomfort. Gonioscopy can be used to show that even after several years, the prolene suture remains intact within the canal.
Canaloplasty can be a viable alternative in patients with early to moderate glaucoma, which does not need a low target IOP. The procedure may prove to be ineffective in a small percent of patients, which may be due to a non-reversible collapse of collector channels or other outflow pathways that cannot be enlarged due to anatomical factors.
Canaloplasty is indicated for the following types of patients: 1) primary open-angle glaucoma; 2) pseudoexfoliation glaucoma; and 3) pigmentary glaucoma. Canaloplasty can also be considered in patients with failed trabeculectomy or previous filtrating surgeries, as long as Schlemm's canal has been left undamaged and intact [10].

Abstract
Canaloplasty is a relatively new surgical option in the treatment of open-angle glaucoma. This non-perforating technique uses a microcatheter connected to a flickering red light laser source, which is inserted within Schlemm's canal. A 10-0 prolene suture is tied to the distal tip of the microcatheter, which is gently guided out of the canal in the opposite direction and then left tensioned while a small amount of viscoelastic agent is injected in Schlemm's canal. The aim of this surgery is to facilitate aqueous outflow through natural pathways, with less risks of post-operative complications. Canaloplasty offers several important advantages when compared to traditional trabeculectomy [11,12], which include: 1) no subconjunctival bleb formation; 2) no need for antimetabolites; 3) better and faster post-operative healing; 4) simplified and less frequent post-operative follow-ups; 5) limited post-operative complications that are seldom sight threatening; and, 6) postoperative results and IOP tend to be stable over time.
There are some disadvantages to canaloplasty, which include: 1) long and difficult learning curve; 2) necessity of specifically designed (and expensive) instruments; 3) average post-operative IOP levels tend not to be very low; and, 4) impossibility to cannulate Schlemm's canal in about 10% of eyes. Canaloplasty can easily be converted in these cases into either a deep sclerectomy or a viscocanalostomy.
Although canaloplasty is a demanding and rather difficult technique, this surgical procedure tends to provide promising surgical outcomes in patients with various types of open-angle glaucoma. Further multicenter prospective studies that involve a larger group of different types of glaucoma patients randomly selected to undergo canaloplasty versus other traditional glaucoma surgeries are needed to confirm the advantages, efficiency in IOP lowering over time, and limiting complications of canaloplasty. Moreover, the correct indications for canaloplasty need to be further assessed in prospective studies.