Since the evolution of minimally invasive glaucoma surgery (MIGS), ophthalmologists have needed to understand the pathophysiologic changes that occur in the natural aqueous outflow pathways to better understand results following MIGS intervention as well as to determine which MIGS procedure or device to use for each patient. Contrary to established teaching, primary open-angle glaucoma is not glaucoma in an otherwise normal eye; rather, it is glaucoma in an eye with substantial disease extending from the innermost portions of the trabecular meshwork through the episceleral venous plexus. There is a dropout of the endothelial cells that line trabecular columns, which leads to adhesions of adjacent lamellae, reducing the effective filtration area.^1,2

There are other pathologic changes within the trabecular meshwork that increase the resistance to outflow, including an increased rigidity of the uveal and corneoscleral meshwork due to an increase in the contraction of actin-myosin complexes within trabecular columns and abnormal turnover of extracellular matrix proteins in the juxtacanalicular space.^3-5 There is also strong evidence supporting the theory that aqueous is able to transcend the otherwise impermeable right junctions of the inner wall of Schlemm’s canal via intercellular and intracellular micropores, but research has shown a reduction of the micropore population in glaucoma patients, which adds to the overall increased resistance to outflow.^6,7 Downstream, we continue to see deleterious changes, such as a decreased diameter and latency of Schlemm’s canal as well as obstructions to a significant percentage of collector channels from herniations of the trabecular meshwork and inner wall of Schlemm’s canal.^8,9

Modification of these abnormalities has been a target of therapy for decades, both surgically and medically. In the 1980s, pioneers like Robert Stegmann, MD, found that by viscodilating Schlemm’s canal via viscocanalostomy, patency can be regained and pressure effectively lowered.¹⁰ Viscocanalostomy has been shown to be quite effective, but canal dilation is limited to a few clock hours distal and lateral to the site of treatment. In the early 2000s, iScience developed the iTrack catheter, which is the world’s smallest fiberoptic microcatheter, and this allowed for the entire canal to be circumnavigated and viscodilated, a procedure now known as canaloplasty. Work by these pioneers showed that the forceful injection of viscoelastic into the canal using the iTrack catheter (now owned by Ellex) helped modify some of the above-mentioned pathologic changes by evaluating cadaveric eyes with electron microscopy and in vivo using high definition ultrasonography. Trabecular lamellae separated, microperforations in the inner wall were created, the canal diameter and collector channels were greatly dilated (Figure 1), and previously stenotic canals were transformed into conduits of flow.

The procedure was quite elegant and is still commonly used today. During the critical evaluation phase or pivotal trial of the device, there was a subset of patients who were unable to receive the tensioning suture, primarily because abnormalities within the canal prohibited 360 degrees of canal intubation with the catheter.¹¹ When the 3-year data were evaluated, there was not much difference between those who received and did not receive the tensioning suture. This led glaucoma surgeons to question whether such a large conjunctival dissection was necessary, or if a scleral lake or descematic window was needed. Could surgeons merely introduce the catheter into the canal via a less invasive approach and limit the intervention to comprehensively viscodilating the canal?

Development of Ab-Interno Canaloplasty

We developed a technique in 2016 called ab-interno canaloplasty (ABiC) and evaluated the safety and efficacy outcomes in 12 patients in whom 1 eye had “traditional” canaloplasty and the other ABiC. Short-term analysis (12 months) showed similar outcomes in IOP and medication reduction between the 2 eyes.¹² Unlike canaloplasty, ABiC (viscodilation using the iTrack catheter) comprises all facets of MIGS. It is minimally invasive, conjunctival sparing, performed via an ab-interno approach through clear corneal incisions, blebless, and safe (Figure 2). The microcatheter is introduced into the canal through a very small otomy created in the trabecular meshwork, and microsurgical instruments are used to feed the catheter through the canal. Viscoelastic solution such as Healon GV (Abbott Laboratories) is typically injected into the canal upon withdrawal of the catheter. As the catheter is withdrawn, the surgical technician is instructed to turn or “click” the injector, which forces the viscoelastic through the catheter into the canal and distal collector system, which can be visualized externally by blanching of the episceleral venous system (Figure 3). I typically remove the catheter at a rate of 1 clock hour per second and instruct my technician to click the injector 2 times per clock hour.

To further our analysis, others and myself have greatly expanded our utilization of the iTrack catheter via ABiC and have shared our data at national meetings through podium and poster presentations.¹³ Our most recent paper showed favorable results in both IOP and mediation burden when performed as a stand-alone or combined with cataract surgery.¹⁴ In the combined subgroup (n=34 eyes), the mean IOP decreased from 19.4±3.7 mmHg preoperatively to 13.0±1.8 mmHg (P=.001) and mean medications used decreased from 2.6±1.0 preoperatively to 0.8±0.2 at 12 months after surgery (P=.001). In the standalone ABiC subgroup (n=41), the mean IOP decreased from 21.2±5.3 mmHg to 13.7±1.9 mmHg (P=.001) and the mean medications used decreased from 3.0±0.7 medications preoperatively to 1.3±1.1 medications at 12 months postoperatively (P=.001).

A device that was previously used to introduce viscoelastic into Schlemm’s canal was the Visco360 system manufactured by Sight Sciences. This device was a single-piece unit that allowed for the introduction of a viscoelastic-primed catheter into the canal through an injector. The device would safely introduce the catheter into the canal through a small goniotomy in the trabecular meshwork and intubate at 180-degree increments. One half of the canal would be intubated and upon withdrawal of the catheter back into the delivery device, Healon GV would be injected into the canal. The injector would then be moved in the opposite direction and the other 180 degrees would be treated. This product is currently not being manufactured. A new device, however, has come to market manufactured by Sight Sciences called the Omni device. The Omni device is a viscoelastic drug delivery system that at times is also used for goniotomies. The device, like the Visco360 system, is primed with a viscoelastic and the canal can be viscodilated in a similar fashion to the Visco360 device. The one difference between the 2 devices is that following complete 360 degrees of viscodilation, the surgeon has the option of reintroducing the catheter into the canal and lysing a preferred number of clock hours of trabecular meshwork, creating a focal goniotomy. As this is a newer device, few data are available. The device is currently FDA approved. The company is also performing a prospective nonrandomized study evaluating the efficacy and safety to perform 360 degrees of viscodilation with 180 degrees of goniotomy in conjunction with cataract surgery.

Conclusion

As with all things in medicine, glaucoma surgery continues to advance. Tools become more elegant, procedures become less invasive, and glaucoma surgeons strive to maintain efficacy. It’s a great time to be a glaucoma surgeon. GP

References

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Mark J. Gallardo, MD, is a glaucoma specialist at El Paso Eye Surgeons in El Paso, Texas. Dr. Gallardo reports consultancy, speaker’s fees, and clinical trials with Glaukos, Alcon, and Ellex; clinical trials with Sight Sciences, New World Medical, and Ivantis; and speaker’s fees from Aerie Pharmaceuticals and Bausch + Lomb. Reach him at gallardomark@hotmail.com.