Ab-interno Canaloplasty (ABiC)
Canaloplasty performed with an ab-interno approach to Schlemm’s canal – is a new MIGS procedure that can comprehensively restore the natural outflow pathways in glaucoma patients.
Performed via a self-sealing, clear corneal incision, ABiC conserves the clinicallyproven benefits of 360° viscodilation of Schlemm’s canal provided by traditional Canaloplasty, but with the speed and ease of implementation of a MIGS procedure.
Unlike other currently available MIGS procedures however, ABiC preserves tissue and does not require the permanent placement of an implant in the eye.
It has also been shown to be effective as both a standalone procedure and when combined with cataract surgery. It is also the only MIGS indicated for use outside cataract surgery.
How the glaucoma treatment paradigm has shifted from long-term use of topical medications to restoring natural outflow.
The primary treatment for glaucoma is to lower the intraocular pressure (IOP), whether through medications, laser procedures, or surgical procedures.
Ab interno canaloplasty is a surgical procedure intended to lower the IOP in patients with glaucoma. This surgery involves an internal approach to viscodilation of Schlemm’s canal through a clear corneal incision.
Ab interno canaloplasty is one of a variety of surgical procedures described as minimally invasive glaucoma surgery (MIGS). These surgeries differ from traditional glaucoma surgery in that the conjunctiva is not disturbed and no bleb is created.
In addition, efforts are made to improve or rehabilitate the function of the normal outflow pathway rather than creating a new outflow pathway.
Ab interno canaloplasty is unique amongst MIGS procedures in that no devices or hardware are implanted in the eye and no significant amount of tissue damage occurs.
Many of the MIGS procedures attempt to increase the function of the outflow pathway of the eye by implanting devices into the trabecular meshwork and/or Schlemm’s canal or by cutting or removing part of the trabecular meshwork.
Ab interno canaloplasty involves only a small perforation of the trabecular meshwork followed by cannulation and viscodilation of Schlemm’s canal.
The past decade has been very exciting for glaucoma surgeons. The advent of minimally invasive glaucoma surgery (MIGS) has filled the frustrating treatment gap between topical medications and invasive filtration surgery for patients who have uncontrolled intraocular pressure (IOP) despite topical therapy.
It is a long-awaited change, and one that supports growing awareness of the need to preserve patient qualify of life.
Although filtration surgery such as trabeculectomy and aqueous shunts is undeniably effective, it is invasive and often associated with infections and bleb-related problems that negatively impact patient quality of life.
In contrast, topical medication use may seem innocuous in the short term but, because glaucoma is a chronic disease, it is important to consider the consequences of long-term exposure that can lead to a higher risk of ocular surface disease.
For example, the damaging effect of benzalkonium chloride (BAK), the most commonly used preservative in topical glaucoma medications, on trabecular meshwork cells may be underappreciated.
Exposure to BAK is toxic to trabecular meshwork cells and was found responsible for inducing apoptosis in the endothelial cells and along the trabecular columns.
The deterioration of the trabecular cells might cause major changes, including fusion of the trabecular meshwork, with collapse of Schlemm’s canal and obstruction of the collector channels, compromising the natural outflow and the viability of other treatments.
Ultimately, glaucoma patients often fail to adhere to topical treatment as advised, leading to optic nerve damage and disease progression.
Bearing in mind that patients with mild to moderate glaucoma, who are controlled with multiple antiglaucoma therapies, may benefit from a reduction in the number of topical medications required to maintain their IOP, finding a safer but equally effective way to lower IOP has long been the goal in glaucoma treatment, especially because just a 1-mmHg reduction in IOP is associated with an 11% decrease in disease progression.
This is why I was an early adopter of canaloplasty via an ab-externo approach as an alternative to trabeculectomy. Indeed, its improved safety and efficacy profile as compared to trabeculectomy is the reason why we continue to use it in advanced glaucoma patients.
More recently, inspired by MIGS, a less invasive yet equally effective approach to canaloplasty was introduced for mild to moderate glaucoma patients: ab-interno canaloplasty. Ab-interno canaloplasty ticks all the boxes of MIGS, but then it excels.
It uses an ab-interno approach, has a high safety profile, does not damage eye tissue, is effective at lowering IOP, and has a rapid recovery time. Importantly, it addresses and treats both the proximal and distal portions of the conventional outflow pathway.
Most types of MIGS are focal in their approach and aim to improve outflow facility using an implant to bypass the trabecular meshwork or to increase drainage via the subconjunctival or suprachoroidal space, but this can lead to complications.
More importantly, however, they fail to address that glaucoma is associated with a pathophysiology of the entire outflow system, both proximal and distal.
In contrast, ab-interno canaloplasty employs a comprehensive approach designed to restore the natural outflow of the eye by targeting all sites of resistance in both the proximal and distal outflow pathways, such as Schlemm’s canal, the trabecular meshwork, and occluded collector channels.
And it does it atraumatically, sparing the conjunctiva from manipulation and allowing future procedures to be performed if necessary.
The Surgical Procedure
In my practice, we have been using the iTrack canaloplasty microcatheter (Nova Eye Medical) for more than a decade with excellent results and no significant complications. During the ab-interno procedure, the iTrack canaloplasty microcatheter is inserted via a clear corneal incision into Schlemm’s canal.
The microcatheter has an LED light at its tip, which allows the surgeon to know exactly where it is going and feel confident that it is in the right position while passing through the full length of Schlemm’s canal, breaking adhesions and separating herniations of the inner wall of the trabecular meshwork.
Once the device has circumnavigated all 360° of Schlemm’s canal, the surgeon withdraws it while injecting high-molecular-weight, hyaluronic-acid-based ophthalmic viscosurgical device (OVD) through the microcatheter.
The OVD delivery is not automatic, but performed by clicking the viscoinjector device and counting, allowing the surgeon to precisely control the amount of OVD used to dilate the canal. The speed of withdrawal of the iTrack microcatheter can also be adjusted to impact the volume of OVD delivered.
Pressurized OVD delivery with the iTrack canaloplasty microcatheter has been shown to dilate the full circumference of Schlemm’s canal, leading to enhanced outflow facility.
In our experience, 40 clicks, on average (using Healon Pro, Johnson & Johnson Vision), successfully viscodilates the canal over 360°, but this is dependent on the patency of the canal. If I consider the canal to be particularly stenotic or occluded, I will deliver 50 or more clicks.
One of the benefits of the iTrack canaloplasty microcatheter is that the surgeon can adjust the amount of OVD delivered on an individual patient basis.
It is a procedure designed to leave no implant or tensioning suture in place, therefore allowing the eye to re-establish its natural functions.
The noninvasive approach also avoids damage to delicate ocular tissue, which is one of the reasons why the intraoperative and postsurgical complications rate in my practice and in the published literature is so low.
To date, no serious adverse events have been recorded. Intraoperative bleeding at the goniotomy site, postoperative IOP elevation, and postoperative microhyphema development are the only notable side effects associated with ab-interno canaloplasty, but these usually resolve within 1 week postoperatively and without sequelae.
Reducing Patient Reliance on Glaucoma Medications
As noted earlier, reducing the burden of glaucoma medications can contribute significantly to improved quality of life for glaucoma patients.
In a consecutive case series at our eye center (Dean McGee Eye Institute, University of Oklahoma; M.A. Khaimi, MD, unpublished data, 2020), we assessed the ability of ab-interno canaloplasty to reduce the mean number of medications required to maintain IOP below 18 mmHg in a cohort of 45 eyes of 35 patients (mean age of 73 years) with controlled glaucoma.
The baseline mean number of medications was 1.89±0.93. At 36 months postoperatively, the mean number of medications fell to 0.60±0.82 — a 61% reduction from baseline.
Even more encouraging was that 56% of eyes were medication free at 36 months. In this cohort, 11 eyes underwent ab-interno canaloplasty outside of cataract surgery (iTrack-alone) while 34 eyes underwent ab-interno canaloplasty in combination with cataract surgery (iTrack+phaco).
At 12 months and 36 months there was no significant difference in the percentage reduction in medication use between the groups. Mean IOP was within the target at baseline (14.42±2.2 mmHg) and remained under control at 12, 24, and 36 months (14.19±2.91 mmHg).
Reducing IOP and Medications
In addition to those 36-month results demonstrating the ability of ab-interno canaloplasty to effectively reduce the number of glaucoma medications required to control IOP, results presented at the 2020 annual meeting of the American Society of Cataract and Refractive Surgery arise from 859 eyes treated with ab-interno canaloplasty at our eye center (594 eyes in the iTrack+phaco and 275 eyes in the iTrack-alone group) and they show a meaningful reduction in IOP and medications in both groups.
At the 24-month follow-up, the percentage reduction in IOP among the iTrack+phaco group remained in line with that of the iTrack-alone group, particularly in patients with a high baseline IOP.
Specifically, at 24 months, patients with a baseline IOP of 22 mmHg or higher experienced a fall in IOP of 34% and 39% in the iTrack+phaco and iTrack-alone groups, respectively.
And among patients with a baseline IOP of 25 mmHg or higher, IOP fell by 38% and 39% in the iTrack+phaco and iTrack-alone groups, respectively.
At 24 months, the number of medications used had fallen from 1.7 and 2.1 in the iTrack+phaco and iTrack-alone groups to 0.7 and 1.2, respectively, reflecting reductions of approximately 59% and 43%.
When stratifying the data to account for baseline IOP, there was a reduction of 47% and 38% at 24 months in the iTrack+phaco and iTrack-alone groups, respectively, in patients with a baseline IOP of 22 mmHg or higher. In patients with a baseline IOP of 25 mmHg or higher, the number of medications fell by 44% and 41% in the iTrack+phaco and iTrack-alone groups at 24 months.
There were few adverse events reported across both treatment groups: postoperative IOP elevation (≥30 mmHg, predominantly at 1 week), which resolved spontaneously, occurred in 8% and 10% of eyes in the iTrack+phaco and iTrack-alone groups, respectively.
Two eyes (0.3%) in the iTrack+phaco group required additional surgery to address IOP spikes. In total, 6% of iTrack+phaco eyes and 14% of iTrack-alone eyes underwent additional surgery to keep IOP under control during follow-up.
Other minor complications included intraoperative bleeding at the goniotomy site and microhyphema development, but these resolved spontaneously.
iTrack and the Evolving Glaucoma Treatment Algorithm
Ab-interno canaloplasty represents a highly effective, minimally invasive glaucoma treatment that enables glaucoma specialists to reduce both medication dependence and IOP — and so to enhance patient quality of life.
Importantly, it also provides surgeons with an opportunity to rethink their use of medications in the management of glaucoma patients.
There is a broad consensus that long-term use of topical glaucoma medications can be linked to ocular surface diseases in both the cornea and conjunctiva and, based on the excellent clinical results
We have achieved with ab-interno canaloplasty, I am of the opinion that, in the treatment of mild-to-moderate glaucoma, topical medications should be avoided or used occasionally as an adjunct between different procedures like selective laser trabeculoplasty or ab-interno canaloplasty.
Based on the results achieved to date, the safety profile of ab-interno canaloplasty with the iTrack microcatheter and its long-term efficacy make it an excellent option for patients with mild-to-moderate glaucoma undergoing cataract surgery, as well as for those who need a standalone procedure.
Although our 24-month and 36-month results are promising, I am very much looking forward to confirming these data with the prospective, randomized, multicenter MAGIC study (NCT0469453) currently involving 8 surgery centers in the United States.
In the meantime, current evidence suggests that ab-interno canaloplasty provides a tissue-preserving, implant-free glaucoma procedure that will greatly benefit patients and surgeons alike.
