Canaloplasty is the procedure by which catheterization of Schlemm’s canal is achieved via an ab externo approach in order to restore and enhance physiologic aqueous outflow through the conventional pathway thus avoiding a subconjunctival bleb. Expansion of Schlemm’s canal was first described by Stegmann as viscocanalostomy, a nonpenetrating procedure, wherein two cut ends of the canal were inflated with viscoelastic spanning a few clock hours [35]. Various implants were used in an attempt to enhance the success of nonpenetrating surgery in the early 2000s, but these procedures continued to rely on a bleb for IOP control [37–39]. Recently, a device has been developed to allow 360° cannulation of Schlemm’s canal to expand the entire circumference with viscoelastic and also to allow a suture to be delivered within the canal to exert centripetal force maintaining expansion of the canal. This procedure, termed canaloplasty, aims to restore physiologic outflow via the conventional pathway with suture-assisted canal distension, foregoing the need for a bleb or fistula.

Although in theory Schlemm’s canal can be accessed from any location, the usual chosen surgery site is the superior sclera, for eyelid coverage, in the possible event of bleb formation, as well as for patient comfort. This requires the patient to maintain a down-gaze position for the majority of the procedure. While, in the authors’ experience, topical anesthesia and patient cooperation usually is sufficient for successful performance of the surgery, a traction suture may be utilized to assist in globe positioning and retrobulbar or peribulbar block as well. In some cases, general anesthesia may be required. If a corneal traction suture is utilized, careful site selection is required to ensure the suture is placed several clock hours away from the intended surgical site.

A fornix-based conjunctival peritomy is created, leaving an anterior skirt of conjunctiva attached to the limbus. Blunt dissection is carried out posteriorly to ensure that the posterior edge is relaxed sufficiently to allow for creation of a 5-mm × 5-mm parabolic scleral flap. The posterior conjunctiva should also be easily brought forward to appose to the anterior lip to allow for easy closure at the conclusion of surgery. Light cautery is then applied to the sclera, being careful to avoid aqueous and ciliary veins. Observation of the location of such vessels should also affect the initial selection of where to perform the dissection. A superficial parabolic scleral flap of approximately 5-mm anterior–posterior length by 5-mm width is then outlined on the scleral surface (Fig. 27.8). Although this scleral flap may be any shape, it is the authors’ preference to create a parabolic flap to facilitate watertight closure at the end of surgery. A crescent knife is then used to fashion the superficial flap of approximately one-third scleral thickness (typically 200–300 μm) forward into clear cornea. A deep inner scleral flap is then outlined approximately 1 mm inside from the edge of the superficial scleral flap. Once again, the crescent knife is used to fashion the deep flap and carry it forward directly into Schlemm’s canal. An approximately 100-μm-thick layer of sclera should be left covering the choroid at the base of the deep dissection (Fig. 27.9). It is not uncommon to be left with a full-thickness dissection at some points of the dissection of the deep flap. Care must then be taken to reestablish a tissue plane leaving a thin layer of sclera in the bed of the dissection. It is of utmost importance that the surgeon maintains an adequate depth during the deep flap dissection in order to unroof Schlemm’s canal (Figs. 27.10 and 27.11). If the dissection is too deep, then penetration into the globe occurs, while if the dissection is too superficial, which is more common, it is possible to dissect and pass right over Schlemm’s canal into clear cornea without exposing the canal itself. This results in a difficult situation where a deeper plane of dissection needs to be established with only a thin layer of residual sclera in the bed. Identification of the proper anatomical landmarks is often challenging in these situations, and there is a resultant higher likelihood of penetration into the anterior chamber unintentionally.

Once the white limbus-parallel fibers of the scleral spur are visible at the deep dissection, fibers of the outer wall of Schlemm’s canal should be visible by lifting of the deep flap with a toothed forceps. Aqueous humor may be observed to percolate through the Schlemm’s canal, and blood regurgitation may be encountered from the cut ends of the canal (Fig. 27.12). A paracentesis incision should be made in the clear cornea away from the surgical site to lower the IOP to single-digit levels to prevent outward bulging of Descemet’s membrane and the inner wall of Schlemm’s canal, lowering the likelihood of penetration into the anterior chamber during the ensuing delicate dissection. The deep flap is now advanced forward approximately another 1 mm to expose Descemet’s membrane. Aqueous humor may again be observed to percolate through Descemet’s membrane in the anterior bed of the deep dissection commonly known as the trabeculodescemet window (TDW). In some instances, to increase aqueous percolation through the TDW, a Mermoud forceps can be used to delicately strip the inner wall of Schlemm’s canal away. The corneal stroma should be separated from Descemet’s window with surgical sponges such as Merocel (Merocel Corp., North Mystic, Connecticut) and Weck-Cel (Medtronic, Jacksonville, Florida) sponges carefully and gently pushing down on Schwalbe’s line and Descemet’s membrane. Excessive downward pressure, sudden movements, or a dry sponge may easily perforate the TDW and enter the anterior chamber. For this reason, it is the authors’ recommendation that the very tip of the surgical sponges be moistened with a minute amount of balanced saline solution prior to use on the surgical field. Once the TDW has been satisfactorily fashioned, the underside of the deep flap is scored with a sharp-tip blade at the very anterior aspect and cut off with Vannas scissors. Each cut end of Schlemm’s canal is then intubated with a 150-mm outer bore viscocanalostomy cannula, and a very small amount of high viscosity sodium hyaluronate, such as Healon GV (Advanced Medical Optics Inc., Santa Ana, California), is injected into each end to dilate the ostia and facilitate entrance of the iScience device into the canal.

In the normal human eye, Schlemm’s canal is known to be of 300 μm in diameter. Studies have shown evidence to suggest that the canal, however, collapses in glaucoma as a result of increased trabecular meshwork and inner wall resistance [76]. A vicious cycle may then be set up, wherein the elevated IOP further compresses Schlemm’s canal because of the reduced circumferential flow of aqueous from the canal into its collector channels. The iScience device aims to restore the patency and re-expand the canal using a 45-mm working length flexible polymer microcatheter of 200-μm shaft diameter with a rounded 250-mm tip diameter designed to be atraumatic to, and to guide the 360° passage and catheterization of, Schlemm’s canal (iScience Interventional Inc., Menlo Park, California). The catheter consists of a central support wire designed to provide a backbone for guidance during advancement and to add resistance to potential kinking of the microcatheter. The optical fibers in the microcatheter allow for transmission of a red blinking light from a laser-based micro-illumination system to the tip to assist in visualization and localization of the tip during passage. Thirdly, the microcatheter possesses a true lumen for the delivery of substances such as viscoelastic to expand the canal during passage or retraction (Fig. 27.13). The proximal end of the device connects to the nonsterile laser-based micro-illumination light source on a mayo stand from one arm, with another arm connected to a sterile screw-mechanism syringe designed to assist in controlled injection of viscoelastic into Schlemm’s canal. The microcatheter is then secured to the surgical drape with surgical tape such as Steri-strips (3 M, St. Paul, Minnesota). Two nontoothed forceps are then used to introduce the microcatheter into one of the cut ends of Schlemm’s canal and advanced 360° until the tip emerges from the other cut end of the canal. Although in the vast majority of patients, this passage is possible, a minority of patients will not allow successful catheter passage through the entirety of the canal. In addition, the authors have observed the microcatheter pass into the suprachoroidal space posterior to Schlemm’s canal. In these cases, early recognition of unusual location of the blinking red light is of utmost importance and the catheter retracted to attempt passage again with scleral depression or removal of the entire microcatheter and passage attempted in the opposite direction.

Once the microcatheter has been passed 360° and the tip has emerged, a 10-0 Prolene suture with the needles cut off is tied around the shaft of the device near the tip with the two loose ends tied to the loop (Fig. 27.14). The device is then withdrawn carefully in the reverse direction to which it was passed, with controlled injection of viscoelastic into the canal by a surgical assistant. Care must be taken not to inject an excessive amount of viscoelastic into Schlemm’s canal as a Descemet’s detachment can occur. Regurgitation of heme into the anterior chamber is also commonly seen during passage or withdrawal of the microcatheter. Once the catheter has been removed, the 10-0 Prolene is cut from the tip, essentially leaving two single 10-0 Prolene sutures in the canal with two loose ends emerging from each cut end of Schlemm’s canal. The surgeon must then identify the corresponding ends, and each suture is tied to itself in a slipknot fashion with some back and forth movement in the canal, known as “flossing,” to ensure that the suture sits anteriorly in Schlemm’s canal. Suture tension is then assessed by observing the amount of indentation of the TDW, as well as by pulling the suture knot posteriorly, until it is only barely able to reach the scleral spur (Figs. 27.15 and 27.16). The suture is postulated to produce a surgical pilocarpine-like effect by putting the trabecular meshwork on tension and thus enhancing flow through Schlemm’s canal and its collector channels. Suture tension is felt to play an important role in canaloplasty, where a greater suture tension results in more distension of Schlemm’s canal with resultant greater IOP reduction and increased flow [42].

The superficial scleral flap is then placed back into position and sutured in a watertight fashion with five interrupted 10-0 nylon sutures. High viscosity sodium hyaluronate is then injected under the superficial scleral flap using the viscocanalostomy cannula in order to maintain the scleral lake – the space where aqueous humor that has percolated through the TDW accumulates and is then absorbed into episcleral, scleral, and choroidal circulation. The conjunctiva is then closed over the surgical site in a watertight fashion with a 10-0 Vicryl suture in a running horizontal mattress fashion.

Canaloplasty seeks to restore aqueous outflow through the conventional outflow pathway into Schlemm’s canal and its collector channels to control IOP. However, the potential space under the superficial scleral flap, or the scleral lake, also allows for aqueous humor to drain into multiple pathways such as the cut ends of Schlemm’s canal, the surrounding scleral and episcleral vasculature, the suprachoroidal space, and even subconjunctivally in some patients, resulting in a bleb despite a watertight closure. It has been the authors’ experience that fibrosis of the TDW can occur postoperatively with a resultant elevated IOP that requires YAG (yttrium–aluminum–garnet) laser to puncture the TDW, restoring aqueous flow to the scleral lake and resulting in IOP control (Figs. 27.17 and 27.18). A recent study demonstrated that 94 patients who underwent canaloplasty had an IOP reduction from 24.6 to 14.9 mmHg with a decrease in medication usage from 1.9 to 0.6 medications [42]. A similar efficacy was observed in patients undergoing combined phacoemulsification and intraocular lens implantation surgery with canaloplasty [77]. Complications reported were elevated IOP and hyphema, most commonly with Descemet’s detachment, hypotony, and choroidal effusion also being reported.

Careful patient selection must occur for successful canaloplasty. As the suture in Schlemm’s canal places centripetal tension on the inner wall, it draws the trabecular meshwork in toward the pupil (Fig. 27.19). Although this is a minute distance, in narrow-angle patients or those with crowded anterior segments, this may result in constant or intermittent iridotrabecular touch, peripheral anterior synechiae, and angle closure. As a result, it is advisable to exclude patients with narrow angles or crowded anterior segments from being potential candidates for canaloplasty. Preoperative gonioscopy, and in some cases adjunctive anterior segment imaging, is of great importance in assessing the angle and iris profile when a patient is being considered for this procedure. It has been the authors’ experience that postoperatively even the patient who had an unequivocally open angle may develop peripheral anterior synechiae and iris incarceration into microperforations in the TDW. An intact Schlemm’s canal is also a prerequisite to successful canaloplasty; thus, patients with prior surgery such as a trabeculectomy or patients with obvious scarring in Schlemm’s canal due to prior medication use, laser, surgery, and corneoscleral trauma at the limbus may not be good candidates for canaloplasty.

In summary, canaloplasty is the procedure of catheterizing and distending Schlemm’s canal with an ab externo nonpenetrating dissection and using a microcatheter to deliver viscoelastic and sutures to restore aqueous outflow through the conventional pathway and into a scleral lake created at the surgical site. The aim is to lower IOP in glaucomatous eyes without the reliance on a subconjunctival bleb, thus avoiding both short- and long-term complications of subconjunctival filtration surgery. Studies have shown efficacy at 1 year in terms of IOP reduction and dependence on glaucoma medications with a low complication profile and minimal postoperative management. However, canaloplasty remains technically challenging, and other issues remain to be studied such as the optimal tension of the suture in Schlemm’s canal, the yet-undetermined long-term implications of a suture in the canal, fibrosis of the TDW resulting in the need for postoperative YAG laser, and closure and contraction of the intrascleral lake, and its implications are yet poorly understood. Finally, because of episcleral venous pressure, it would seem that in theory, a ceiling to the maximal amount of physiologic outflow should exist and thus a limit in the lowest IOP attainable by this procedure. This too is yet to be well understood and requires further investigation.