In the past 2 decades, treatment approaches across several ophthalmic subspecialties have been revolutionized. Sutureless microincisional phacoemulsification with implantation of premium IOLs for cataracts; sutureless microincisional transconjunctival pars plana vitrectomy for retinal diseases; intravitreal pharmacologic therapy for neovascular age-related macular degeneration; laser refractive surgery for refractive errors; and less-invasive techniques for corneal diseases including Descemet membrane endothelial keratoplasty, deep anterior lamellar keratoplasty, and corneal collagen crosslinking for corneal diseases have all become standard. Consequently, complication rates have been reduced and outcomes improved dramatically. Some patients now see better after surgery than before. Laser-assisted cataract surgery will change our techniques yet again.
But what about glaucoma? Has it become easier for surgeons to differentiate ocular hypertension from early glaucoma? Are we able to locate the structure causing elevated intraocular pressure (IOP)? Is medical treatment sufficient, cost-effective, and safe in the long term? Is trabeculectomy really the gold standard in the surgical treatment of glaucoma, and, if not, what is? And shouldn’t we perform surgery earlier?
This article details two approaches to the treatment of glaucoma—canaloplasty and minimally invasive glaucoma surgery (MIGS)—and explores the potential improvement that these procedures could offer to glaucoma patients, similar to the results provided in other ophthalmic subspecialties.
CANAL-BASED SURGICAL INTERVENTIONS
Viscocanalostomy. Nonpenetrating glaucoma surgery was introduced in the late 1990s. Viscocanalostomy, developed by Robert Stegmann, MD, was the first bleb-independent glaucoma surgery, but it was not very popular for several reasons. It had a long learning curve, and some surgeons reported poor outcomes, most of which were related to early difficulties in adoption of the procedure, useless variations of the technique, or incorrect patient indications.
Additionally, viscocanalostomy treated only a small portion of the trabecular meshwork and Schlemm canal. There is increasing evidence that Schlemm canal plays an important role in the development of open-angle glaucoma. The canal can be partially or totally collapsed, and herniation of the trabecular meshwork into the ostia of collector channels can block aqueous humor outflow distal to the trabecular meshwork.
Canaloplasty. Canaloplasty was the first surgical intervention to treat the entire Schlemm canal. As with deep sclerectomy and viscocanalostomy, a superficial flap and a smaller, deeper scleral flap are created. Then the entire Schlemm canal is catheterized with one of two available microcatheters (iTrack; iScience Interventional or Glaucolight; DORC; Figure 1). A polypropylene suture is fixed to the microcatheter, and when the microcatheter is retracted the suture is threaded through Schlemm canal. If this suture is tensioned, the inner wall of Schlemm canal is detached and the trabecular fibers are stretched. This increases outflow facility through the trabeculum and allows circumferential flow in Schlemm canal, naturally increasing the facility of aqueous humor outflow. Finally, the superficial scleral flap is closed watertight with five sutures to prevent subconjunctival filtration and bleb formation, and the conjunctiva is refixated.
We began performing canaloplasty in 2006, and this procedure has since become our gold standard for the surgical treatment of open-angle glaucoma. Intra- and postoperative complications are rare. Our own experience and multiple published studies suggest that, in most cases, postopera tive IOP reaches the low 10s mm Hg, with most patients off medication. We found that canaloplasty was useful for eyes with not only moderate but also advanced glaucoma and for single-eye patients, as the complication rate is very low. We have stopped performing circumferential injection of ophthalmic viscosurgical device (OVD) into Schlemm canal, as this could cause uncontrolled rupture of the canal wall or Descemet membrane detachment.
In a study comparing iTrack-assisted viscocanaloplasty and Glaucolight-assisted canaloplasty, we found equal postoperative IOP with no Descemet membrane detachment in the Glaucolight group, and we concluded that the OVD injection step is not needed for successful canaloplasty 1
Combination with phacoemulsification, through a separate or shared incision, is safe and seems to lower IOP even more effectively than viscocanalostomy alone. In contrast with trabeculectomy, a separately performed cataract surgery even years later does not cause late failure of canaloplasty. Another study found that canaloplasty is more economical then trabeculectomy. 2 In the past year, we have recognized an increasing interest in canaloplasty, and many surgeons have attended our courses, visited us for live surgery, or received hands-on training.
MIGS. Less-invasive glaucoma surgeries fall under the category of MIGS. In the past decade, efforts have focused on using small tubes to bypass the trabecular meshwork, creating a direct route from the anterior chamber into Schlemm canal. The first models were made of flexible silicone and were several millimeters in length, and they were implanted ab externo during a deep sclerectomy procedure. Initial reports indicated that IOP was lowered to 16.5 mm Hg with these implants and the addition of medications. These microtubes had an inner lumen of 50 μm, but they could cause obstruction of the route from the surgical site ostia to the collector channels adjacent to the implant, which could lead to failure.
The first US Food and Drug Administration (FDA)- approved device for this kind of intervention was the Ex-Press Mini Glaucoma Shunt (Alcon). Although it was initially developed for subconjuntival implantation, in recent years, the Ex-Press shunt has been used under a scleral flap. This stainless-steel implant is designed to self-fixate after it is implanted through a 27-gauge incision under the flap. The inner lumen allows more controlled flow, and an advantage over traditional trabeculectomy is that no iridectomy is required. The success rate is comparable to trabeculectomy but with fewer postoperative complications.
iStent. Another device designed to bypass the trabecular meshwork is the iStent (Glaukos Corp.; Figure 2). This heparin-coated bypass stent is made of nonferromagnetic titanium and is L-shaped with a curved open lumen. The tip is pointed to allow insertion by penetrating through the trabecular meshwork into Schlemm canal. Just behind the tip is the canal portion, which is about 1 mm in length and has an outside diameter of 180 μm. The canal portion is half-pipe–shaped and designed to fit within the lumen of Schlemm canal, which averages about 250 μm. Finally, there is an anterior chamber portion.
The half-pipe portion of the microstent is placed with the convex side against the outer wall of the canal to avoid blockage of the ostia of collector channels. Three barblike ridges on the side opposite the half-pipe are designed to prevent dislocation. A smaller-sized, snorkel-like tube at the other end of the device is designed to traverse the meshwork and connect with the anterior chamber. The microstent weighs approximately 0.1 mg.
The surgical technique for iStent implantation has its own learning curve. The surgeon must especially adapt to visualization, as this is important in performing controlled surgery. The operating microscope is usually tilted about 30° to 40°, or the patient’s head is turned approximately 30° and the surgeon works from the temporal site. Between the patient position and the scope tilt, it is possible to view the angle.
A clear corneal incision of about 2 mm is created for insertion of the implant into the anterior chamber. Then the anterior chamber is filled and the angle is deepened with an OVD. The surgeon should check that the orientation of the stent on the applicator is appropriate for the desired nasal implantation. A gonioprism (Swan-Jacobs gonioscope; Ocular Instruments) is used to view the nasal angle.
The iStent is preloaded on a disposable applicator and inserted through the temporal clear corneal incision. Under the view through the gonioprism, the iStent on the applicator tip is advanced to the opposite nasal anterior chamber angle. The stent is positioned parallel to the trabecular meshwork, and the leading edge of the device is slid gently through the trabecular meshwork and placed into Schlemm canal with the tip of the microstent directed inferiorly. Blood reflux from Schlemm canal through the stent provides an indication of suitable placement of the device. Next, the device is released by pushing the button on the applicator. The snorkel can be gently nudged to confirm that the stent is seated in the canal. It should be parallel with the iris plane, the base should be well seated and fully through the trabecular meshwork, and the snorkel should be opening perpendicular to the meshwork.
This implant works best if placed directly over an ostium of a collector channel. Unfortunately, it is difficult to locate a collector channel ostium intraoperatively. Some surgeons advocate for implantation of multiple iStents. Although this may increase effectiveness, it also dramatically raises the cost of surgery. We have studied intraoperative flow with injection of dye (trypan blue or indocyanine green) into the iStent ostium. If dye appeared in the episcleral veins, there was a better chance for success. If it did not, the iStent was explanted from this site and reimplanted.
Combination of the iStent with phacoemulsification was found to be useful and resulted in lower postoperative IOP than the iStent alone. Overall, most studies have found that, in moderate glaucoma, the iStent has the potential to lower IOP to the higher 10s mm Hg. The complication rate has been very low, and in combined surgeries there were no more complications than with cataract surgery alone.
Hydrus. The Hydrus stent (Ivantis; Figure 3) was designed to scaffold a much larger portion of Schlemm canal. The device is made of nitinol and is about 8 mm long. The anterior chamber angle is visualized with the same technique used for iStent implantation. After the anterior chamber is filled with OVD, the tip of the applicator is used to cut the trabecular meshwork. The applicator is then pushed toward the trabecular meshwork, and the Hydrus stent is injected into Schlemm canal. This expands about one quadrant of Schlemm canal on the nasal side and stretches the trabecular meshwork.
Although the Hydrus is longer than the iStent, implantation is more controlled, and we have found that it can be completed only if the Hydrus is well placed within Schlemm canal. This stent has a low complication rate and can be combined with phacoemulsification. Ongoing studies are being conducted to analyze whether the Hydrus stent is more effective than the iStent. Compared with trabeculectomy or canaloplasty, we have found that the Hydrus stent is less effective, but we do see an indication for use in less advanced glaucoma.
Suprachoroidal space. The suprachoroidal space is another area of interest. The first implant developed for this approach was the Gold Shunt (Solx). Most surgeons using this device were not satisfied, as it has a high failure rate due to fibrous encapsulation. Recently, the CyPass (Transcend Medical) was introduced. This polyimide microtube is about 6 mm long. It is implanted in the anterior chamber angle through a supracilliary microcleft toward the suprachoroidal space. Visualization requires the same technique as described above. Early results suggest results comparable with those of other MIGS procedures, but longer-term results are still needed. The CyPass could have a niche if aqueous humor outflow downstream from Schlemm canal is affected.
CONCLUSION
Additional MIGS devices are in the pipeline, and the coming years will be exciting for glaucoma specialists and cataract surgeons who are interested in this field.
A better understanding of the pathophysiology of aqueous humor outflow in open-angle glaucoma is needed. New techniques for visualization of the microstructures of the anterior segment, such as the trabecular meshwork, Schlemm canal, and collector channels, will help to localize affected areas (Figure 4). This may lead to more customized approaches to the treatment of glaucoma patients. Improved intraoperative visualization will reduce the learning curve of MIGS procedures. Bimanual techniques could increase the safety of current techniques or help to develop more delicate techniques for manipulation of the microstructures in the anterior chamber angle.
Currently, canaloplasty is the only procedure with the potential to become the gold standard treatment for moderate to advanced glaucoma. This approach recreates the natural outflow with a low rate of complications. In case of its failure, any other technique can be used.
With the increased interest in glaucoma surgery, there is hope that further improvement will lead to a technique that provides glaucoma patients with results comparable with those provided by phacoemulsification in our cataract cases. This will lead to earlier surgical intervention in glaucoma, which many studies have shown to yield better outcomes. We have seen patients more than 10 years after viscocanalostomy with no need for medical therapy and no sign of progression.
Gabor B. Scharioth, MD, PhD, is in private practice at the Aurelios Augenzentrum in Recklinghausen, Germany. Dr. Scharioth states that he is a consultant to Alcon, DORC International, and Ivantis Inc., and receives royalties for the Glaucolight. He may be reached at tel: +49 2361306970; e-mail: gabor.scharioth@augenzetrum.org.
- Scharioth GB. Glaucolight assisted canaloplasty vs. iTrack assisted viscocanaloplasty. Poster presented at: the 6th International Congress of Glaucoma Surgeons; 2012; Glasgow, United Kingdom.
- Brüggemann A, Müller M. Trabeculectomy versus canaloplasty-utility and cost-effectiveness analysis. Klin Monbl Augenheilkd. 2012;229(11):1118-1123.
