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Schlemm’s Canal Surgery for Glaucoma Management

Steven R. Sarkisian Jr, MD and Marcos Reyes, MD

In the era of minimally invasive glaucoma surgery (MIGS), operating in and around Schlemm’s canal has received the greatest attention from surgeons and is the prototypical space in modulating the natural outflow pathway. In order for an approach to be considered “MIGS” in the minds of most surgeons, the intervention must not involve any conjunctival incision; however, for historical purposes it is prudent to review not only the recent development of ab interno Schlemm’s canal surgery, but also its predecessor ab externo treatments such as canaloplasty and nonpenetrating deep sclerectomy (NPDS). Of course it should be mentioned that the development of these surgeries has been due to the profound and sight-threatening complications that can occur with conventional glaucoma surgery such as trabeculectomy and glaucoma drainage implants. These complications include blebitis, endophthalmitis, hypotony, astigmatism, bleb dysesthesia, bleb leak, strabismus, penetration of the eye, and wipeout syndrome.

The goal of this chapter will be to review surgical techniques for both ab externo and ab interno surgery on Schlemm’s canal. The 2 ab externo techniques that we will review are NPDS and ab externo canaloplasty. The ab interno surgeries will include both stenting or trabecular microbypass and a variety of methods used to complete goniotomy, ab interno trabeculotomy and ab interno viscodilation/canaloplasty. Other chapters in this textbook may include information on these procedures that could differ slightly from our own approach and understandings. It is our goal to show our own personal practice patterns so that the reader can appreciate the subtle differences between surgeons.

AB EXTERNO SURGICAL TECHNIQUES

Nonpenetrating Deep Sclerectomy

Though not a completely new idea, NPDS has changed dramatically over the past 50 years. The first documented success was in the late 1950s by Edward Epstein, in South Africa, after he found oozing in the area over Schlemm’s canal after deep excision of a pterygia.¹

The surgery Epstein devised consisted of 180 degrees of deep sclerectomy in the area over Schlemm’s canal that he then covered with conjunctiva.¹ This approach resulted in a functioning bleb, which lasted for a few months but then scarred over. In the late 1960s, Krasnov² performed a fistulizing procedure of Schlemm’s canal by removing the outer portion of the canal (sinusotomy). This was performed only in patients that, they felt, had obstruction of intrascleral collector channels.³ In the late 1980s, another group documented success with NPDS⁴ and also pioneered the use of a lyophilized porcine collagen implant under the superficial flap as a space maintainer to obtain lower intraocular pressure (IOP).

The traditional NPDS consists of dissection of a superficial sclera flap the size of which is surgeon dependent. Underneath this superficial flap, a deeper flap is dissected (the deep sclerectomy) until there is only a thin layer of sclera above the uvea, usually seen as a bluish hue. This flap is extended anteriorly into corneal stroma until aqueous humor is seen percolating through a window of thin trabeculo-Descemet’s membrane. This deep sclerectomy, which includes the outer wall of Schlemm’s canal, is removed en bloc fashion. At this point, one can use a collagen implant sutured to the sclera floor, popularly known as the Aquaflow implant (Staar Surgical Co), or leave the area empty as studies have shown equal success with and without the implant. The superficial flap is closed with several sutures, and the conjunctiva is reapproximated in the normal fashion based on surgeon preference.4,5

This procedure effectively creates a scleral lake into which aqueous can percolate, thereby lowering IOP without a bleb. The absence of a bleb likely results in a decrease of complications, such as infection (blebitis or endophthalmitis), bleb dysesthesia, and other issues associated with traditional penetrating procedures. In one study with 6-year postoperative follow-up, the data showed reasonable success. The mean preoperative IOP was 24.47 mm Hg ± 5.92 (SD), and after 6 years, there was an average lowering of 33.73% ± 20.9% with the mean IOP of 15.81 mm Hg ± 3.79 (SD).⁶ There are a few studies prospectively comparing the NPDS approach to that of traditional trabeculectomy, and this procedure remains accessible (due to surgical training) to only a handful of surgeons. Typically, a very low IOP is not achieved with this technique; however, this may be a beneficial procedure for less advanced glaucoma when a pressure of 15 to 16 mm Hg is targeted.

Canaloplasty

Canaloplasty was first described by Dr. Roger Stegmann and his team out of the Medical University of South Africa (Johannesburg). From astute observations made during NPDS surgery, Stegmann began methodically altering his surgical technique until he developed a procedure he named viscocanalostomy, which became a precursor to canaloplasty. Viscocanalostomy was developed in various stages during the 1990s.⁷ The final leap in innovation was unveiled at the 2005 annual American Society of Cataract and Refractive Surgery meeting where Stegmann⁸ shared the results of his newest stage of development, a 360-degree microcanaloplasty, now popularly named canaloplasty.

The initial steps of performing canaloplasty are similar to NPDS. A superficial sclera flap is made first, followed by the creation of a second deeper scleral flap (aka, deep sclerectomy), which is then extended over Descemet’s membrane to create a window. Once the canal is correctly identified and exposed, the iTrack microcatheter (Nova Eye Medical) is inserted into a surgical opening in the canal (Figure 66-1). The catheter has a blinking red tip so its location is known at all times. It also allows you to inject viscoelastic material through the catheter, aiding in the advancement of the tip through the canal. By slowly advancing the catheter through the entire canal with occasional injections of viscoelastic material, the 360-degree journey can be accomplished with full assurance that the catheter is in the canal at all times using the blinking red light as a guide. Once the tip has reached the opening in Schlemm’s canal, a 9-0 or 10-0 Prolene (Ethicon, Inc) suture is tied to it, and the catheter is slowly withdrawn, pulling the suture all the way around to encircle the entire inner wall of the canal. The suture is then tied to itself with a slip knot, which can be locked after the suture is tightened with enough tension to pull the trabecular meshwork (TM) inward (Figures 66-2 and 66-3). The superficial flap and then conjunctiva are sutured closed, tight enough to prevent leakage and bleb formation.

The initial pilot study of 33 patients presented at the American Society of Cataract and Refractive Surgery 2005 annual meeting reported a mean IOP of 15.2 mm Hg ± 4.7 (SD) at 6 months.⁷ Further studies by Lewis and colleagues⁹ have shown additional long-term promise. In their case series of 127 eyes of 127 patients, the mean IOP at 24 months was 16.0 mm Hg ± 4.2 (SD) down from a baseline of 23.6 mm Hg ± 4.8 (SD). Glaucoma medication usage also dropped from a baseline of 1.9 ± 0.8 to 0.5 ± 0.8. The results were even more impressive with combined canaloplasty and cataract surgery, which is surprising given that traditional trabeculectomy and cataract surgery, in our hands, usually results in a higher pressure than trabeculectomy alone. With combined canaloplasty and cataract surgery, Lewis and colleagues9 reported a mean IOP of 13.4 ± 4.0 mm Hg and 0.2 ± 0.4 medications down from a baseline IOP of 23.1 ± 5.5 mm Hg and 1.7 ± 1.0 medications. This appears to be an excellent choice for patients with mild to moderate glaucoma. Further studies are ongoing to evaluate long-term maintenance of IOP lowering.

AB INTERNO SURGICAL TECHNIQUES

Ab Interno Viscodilation/Canaloplasty

Since the advent of MIGS, ab externo canaloplasty and NPDS have become essentially obsolete. The ab interno techniques to viscodilate Schlemm’s canal have largely replaced ab externo canaloplasty. Almost simultaneously, 2 techniques to perform this procedure have arisen. The first technique involves the same microcatheter that was used for ab externo canaloplasty; however, instead of making a scleral flap, followed by a deep sclerectomy and finding Schlemm’s canal externally, a small goniotomy incision is created in an ab interno fashion by directly visualizing the angle with a surgical gonioprism, turning the head 30 degrees away from the surgeon, and tilting the microscope 30 degrees. Following the goniotomy, the iTrack microcatheter is fed into the canal using a temporal incision, a paracentesis incision, and microforceps. The microcatheter was not designed for ab interno use, so great care must be taken not to kink the shaft, which would then lead to difficulty in advancing the tip. Should this occur, a second microcatheter will be needed to complete the procedure. However, there is a distinct advantage of the microcatheter in that there is a red LED light indicating exactly where you are in the canal as you advance for 360 degrees. The other major distinction between ab interno and ab externo canaloplasty is that a retention suture is neither possible nor practicable, and therefore the average pressure after ab interno canaloplasty may be a few points higher; however, the first comparative study published in 2018 showed no significant difference in IOP or medication use 12 months postoperatively.¹⁰

Another publication of ab interno canaloplasty alone demonstrated the efficacy of this technique showing a 33% IOP reduction with combined phacoemulsification and canaloplasty and a 35% reduction with canaloplasty alone. In both groups the number of medications used dropped.¹¹

The other technique for doing ab interno viscodilation uses a device called the Omni (Sight Sciences, Inc). With this device, one temporal clear corneal incision is created, approximately 0.5 to 2 mm in diameter, and a gonioprism is used in similar fashion as when the iTrack is used. However, rather than using a microvitreoretinal blade or goniotomy knife to make the incision for the catheter, the tip of the Sight Sciences device is sharp and adequate to strip off a small portion of the TM in order to engage Schlemm’s canal.

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