Article

Targeting Schlemm’s Canal

A review of available options

The trabecular meshwork (TM) is thought to be the main site of resistance to aqueous outflow. In theory, bypassing, incising, or removing TM should lower this resistance and lead to improved IOP control. It comes as no surprise that there has been an explosion of innovation targeting this area. Here, we take a closer look at what devices and procedures are available that target the TM and Schlemm’s canal. All of the techniques listed below can be performed as a solo procedure or combined with cataract surgery.

Schematic drawing of the Kahook Dual Blade showing the cutting and stripping of the trabecular meshwork.

iSTENT

The iStent Trabecular Micro-Bypass Stent (Glaukos) was the first micro-invasive glaucoma surgery (MIGS) device to receive FDA approval. It consists of a heparin-coated, non-ferromagnetic titanium stent with a snorkel shape that facilitates implantation. The device allows aqueous humor to directly drain from the anterior chamber into Schlemm’s canal by bypassing the trabecular meshwork.

A study by Ferguson and colleagues1 evaluated 350 eyes that had undergone cataract surgery with placement of an iStent. The study found that mean preoperative IOP improved from 19.13 ± 6.34 mmHg to 15.17 ± 3.53 mmHg at 2 years, and the number of glaucoma medications was reduced from 1.19 ± 1.00 preoperatively to 0.61 ± 0.96 (P<0.0001) at 2 years postoperatively. This same group2 evaluated the placement of a single iStent in 42 pseudophakic patients. They found the mean preoperative IOP of 20.26 ± 6.00 mmHg was reduced to 16.34 ± 3.78 mmHg (P<0.01), and the mean number of glaucoma medications decreased from 1.95 ± 1.01 preoperative to 1.69 ± 1.28 (P>0.05) at 1 year postoperatively. At 2 years postoperatively, the mean IOP was 13.62 ± 4.55 (P<0.01).

Katz and colleagues3 evaluated the efficacy and safety of the implantation of one, two, or three iStents as a solo procedure. Subjects were submitted to washout and divided into groups to receive one (n = 38), two (n = 41), or three (n = 40) stents. Preoperative IOP after washout was 25.0 ± 1.1 mmHg, 25.0 ± 1.7 mmHg, and 25.1 ± 1.9 mmHg in the one-, two-, and three-stent groups, respectively. At 18 months, mean IOP was 15.6 ± 1.5 mmHg in the one-stent group, 13.8 ± 1.3 mmHg in the two-stent group, and 12.1 ± 1.2 mmHg in the three-stent group.

The iStent has a very favorable safety profile, with clinical trials and case series consistently reporting few to no adverse events following implantation.

KAHOOK DUAL BLADE

The Kahook Dual Blade (KDB) from New World Medical is a specially designed device that provides for more complete removal of the TM with minimal damage to adjacent structures. The KDB procedure can be performed through a single clear corneal incision and requires no additional materials or equipment. The sharp tip of the KDB allows it to pierce the TM and be seated in Schlemm’s canal, where it can be easily guided along the canal. The ramp of the KDB elevates the TM and guides it toward the two blades, which cut the TM and leave an intact strip of TM. Rotating the device 180 degrees and moving in the opposite direction to connect the previously incised TM allows for a free-floating TM strip, which can be removed via the irrigation and aspiration handpiece. The surgically created cleft is more resistant to closure, and leads to more sustained IOP control.

A recent multicenter, multisurgeon, prospective cohort study included 122 eyes that were treated with KDB.4 Glaucoma severity ranged from mild to end-stage, and patients underwent a variety of procedures that included KDB as a solo procedure and KDB plus cataract extraction. For all eyes in the study, the mean IOP at baseline was 18.7 ± 6.6 mmHg, and 12.9 ± 4.2 mmHg (P<0.001) 9 months postoperatively, a reduction of 5.8 mmHg, or 31%. The mean number of hypotensive medications was reduced from 1.8 ± 1.3 to 0.7 ± 0.8 medications at 9 months (P<0.001).

Seventy-three eyes in the same study underwent cataract surgery and KDB. The baseline IOP in this group was 17.5 ± 5.3 mmHg, and the mean number of hypotensive medications was 1.6 ± 1.3. At 9 months postoperatively, the mean IOP was 13.1 ± 3.0 mmHg, a decrease of 4.4 mmHg (23.4%) (P<0.001), and the number of medications decreased to 0.6±0.8 (P=0.005).

The safety profile of the KDB is favorable. The most common observation is intraoperative blood reflux, which is commonly seen in other MIGS procedures. This may result in retained blood in the anterior chamber at post-op day 1, and usually clears by the 1-week visit.

Top: Intraoperative photo of an iStent prior to insertion. The visible portion of the device will be placed within Schlemm’s canal and will allow the aqueous to bypass the trabecular meshwork; Bottom: Intraoperative photo of an iStent after insertion. Note the “snorkel” that is pointing back toward the inserter, which is the opening through which the aqueous will flow. The remainder of the device is seated within Schlemm’s canal.

TRABECTOME

The Trabectome (NeoMedix) was introduced in 2004 and allows a trabeculotomy to be performed via an internal approach. It uses electrocautery to remove a strip of the TM and unroofs Schlemm’s canal to allow aqueous to flow freely out of the eye. Surgeons are able to treat approximately 120 degrees through a single incision, although larger treatment areas can be achieved with additional incisions. The Trabectome does require additional equipment in the OR (i.e., a procedure pack and the irrigation/aspiration/cautery unit).

A study by Jordan and colleagues5 evaluated the effect of Trabectome in both primary open angle glaucoma (POAG) and pseudoexfoliative glaucoma (PXG). POAG eyes (n=261) had a preoperative IOP of 24 ± 5.5 mmHg and were on 2.1 ± 1.3 medications. At 200 days postoperatively, IOP was reduced to 18 ± 6.1 mmHg, and medication was reduced to 1.2 ± 1.1. For the PXG eyes (n=173), the IOP was reduced from 25 ± 5.9 mmHg to 18 ± 8.2 mmHg, and medication was reduced from 2.0 ± 1.2 to 1.1 ± 1.1 over the same time period. The safety profile for the Trabectome is favorable and similar to the KDB.

Intraoperative photo of the Kahook Dual Blade. Note the bunching of the trabecular meshwork to the left of the device. To the right of the device, the trabecular meshwork has already been removed, and the back wall of Schlemm’s canal is visible.

Intraoperative photo of the Kahook Dual Blade. Note the pointed tip, which will allow penetration and stretching of the trabecular meshwork. The blades are located on either side of the tip, allowing it to cut the meshwork.

AB-INTERNO CANALOPLASTY

The ab-interno canaloplasty procedure is a MIGS procedure that flushes out the natural outflow channels without damaging tissue. The ab-interno canaloplasty’s aim is to reestablish the eye’s natural outflow drainage system by accessing, catheterizing, and viscodilating the trabecular meshwork, Schlemm’s canal, and also the distal outflow system. The procedure involves making a small incision in the TM and then threading the iTrack catheter (Ellex) into Schlemm’s canal for a full 360 degrees. The surgeon then slowly withdraws the catheter while simultaneously injecting viscoelastic to viscodilate the TM, Schlemm’s canal, and the collector channels. The procedure involves added equipment as it requires the iTrack catheter.

In a small case series (n=14) of patients who underwent ab-interno canaloplasty as a stand-alone procedure, mean pre-op IOP was reduced from 21.5 ± 7.4 mmHg to 13.6 ± 1.2 mmHg, and medications were reduced from 3.0 ± 1.0 to 1.0 ± 1.0.6 When the procedure was combined with cataract surgery (n=34), mean pre-op IOP was reduced from 17.1 ± 5.0 mmHg to 13.1 ± 2.1 mmHg, and medications were reduced from 2.0 ± 1.0 to 1.0 ± 1.0.

Because of the minimal tissue disruption, the safety profile for this procedure is favorable.

GONIOSCOPY-ASSISTED TRANSLUMINAL TRABECULOTOMY

The gonioscopy-assisted transluminal trabeculotomy, or GATT procedure, is a minimally invasive, ab-interno approach to a circumferential 360-degree trabeculotomy. This procedure is performed in a similar manner as the ab-interno canaloplasty, but instead of slowly withdrawing the catheter, the leading edge is grasped and externalized, which creates the first half of the trabeculotomy. Traction is then placed on the trailing end of the catheter to complete the 360-degree trabeculotomy.

Grover and colleagues7 published a retrospective chart review of 85 patients who underwent the GATT procedure. After 12 months, 57 patients with primary open-angle glaucoma experienced an 11.1 mmHg (39.8%) decrease in IOP and needed 1.1 fewer glaucoma medications. In 28 patients with secondary glaucoma, IOP decreased by 19.9 mmHg (56.8%) with an average of 1.9 fewer glaucoma medications needed at 12 months. Because of the amount of tissue disruption, the most common complication of the procedure was transient hyphema and was seen in 30% of patients at the 1-week visit.

STAY INFORMED

There are a variety of devices and techniques available that act to bypass the trabecular meshwork. The current data are favorable and allow for a safe and effective approach to decrease IOP.

Factors, such as insurance coverage, reimbursement, whether or not it will be combined with cataract surgery, safety, and efficacy may ultimately influence your decision-making process. For instance, a patient with more advanced glaucoma may need a procedure that has a lower safety profile or greater risk, but can provide greater IOP lowering.

A patient with mild glaucoma may have limited insurance coverage for one procedure, and you may need to choose an alternative to avoid a large expense to the patient. Because more than one option may be appropriate for a given case, the surgeon will want to familiarize himself with each approach and make a treatment plan based on the information that is available. GP

References

  1. Ferguson TJ, Berdahl JP, Schweitzer JA, Sudhagoni R. Clinical evaluation of a trabecular microbypass stent with phacoemulsification in patients with open-angle glaucoma and cataract. Clin Ophthalmol. 2016;10:1767-1773.
  2. Ferguson TJ, Berdahl JP, Schweitzer JA, Sudhagoni R. Evaluation of a trabecular micro-bypass stent in pseudophakic patients with open-angle glaucoma. J Glaucoma. 2016;25(11):896-900.
  3. Katz LJ, Erb C, Carceller GA, et al. Prospective, randomized study of one, two, or three trabecular bypass stents in open-angle glaucoma subjects on topical hypotensive medication. Clin Ophthalmol. 2015;9:2313-2320.
  4. Greenwood MD, Abdullah S, Radcliffe NM. A novel dual blade device for goniotomy: 9-month follow up. ASCRS 2017.
  5. Jordan JF, Wecker T, van Oterendorp C, et al. Trabectome surgery for primary and secondary open angle glaucomas. Graefes Arch Clin Exp Ophthalmol. 2013;251(12):2753-2760.
  6. Gallardo MJ, Khaimi MA. Ab-interno Canaloplasty: 12 month preliminary results. Available at http://www.ellex.com/us/physicians/treatment-portfolio/abic/ ; accessed Feb. 8, 2017.
  7. Grover DS, Godfrey DG, Smith O, Feuer WJ, Montes de Oca, Fellman RL. Gonioscopy-assisted transluminal trabeculotomy, ab interno trabeculotomy: technique report and preliminary results. Ophthalmology. 2014;121(4):855-861.