Ab interno canaloplasty (ABiC) with the Nova Eye Medical iTrack

Ab interno canaloplasty: what is it?

Ab interno canaloplasty, otherwise abbreviated to ABiC, is an elegant micro-invasive glaucoma surgery (MIGS) that I perform regularly. It is a procedure that treats both upstream and downstream resistance of the trabecular outflow system and is thereby the most comprehensive MIGS procedure available. Importantly, this allows for excellent pressure lowering which is a standard glaucoma treatment. In particular, many patients like the fact that ABiC doesn’t involve placement of a permanent stent inside the eye. As with all MIGS procedures, proper patient selection and atraumatic surgical technique are critical.

Here I discuss and explain the ABiC procedure and its mechanism of action.

The procedure for Ab interno canaloplasty (ABiC)

I have explained the rationale for ABiC in my publication, “A review of aqueous outflow resistance and its relevance to micro-invasive glaucoma surgery“.1 This is the most comprehensive review article on MIGS to date and was published in the blue-ribbon review journal for ophthalmology in 2016.

Nova Eye Medical iTrack 250A microcatheter

I perform ABiC using the Nova Eye Medical iTrack 250A microcatheter.  The iTrack is a flexible, hollow tube that is 12.5cm long and just 220um in diameter. A fibre-optic light source that flashes red illuminates its tip.

How it is used

The opthalmologist will introduce the microcatheter into the eye and feed it into Schlemm’s canal, which is the natural drainage channel of the eye. The surgeon advances the iTrack  360 degrees using a microforcep, and the flashing light keeps the surgeon informed of exactly how far the tip has travelled. As the microcatheter is withdrawn, a sterile surgical gel called cohesive Ophthalmic Viscosurgical Device (OVD) is injected into Schlemm’s canal. This flushes and dilates open Schlemm’s canal and the downstream collector channels.

OMNI 720

Recently, a company called Sightsciences released a device called the ‘OMNI 720’. This is a modified version of the ‘VISCO 360’ device, but it is not yet available in Australia.  However,  I was fortunate enough to get significant experience with it during my training with Ike Ahmed in Toronto. The  iTrack and the OMNI 720 essentially achieve the same thing. There are advantages and disadvantages of each, but a discussion of these differences is beyond the scope of this article.  You can watch a video of this procedure by clicking on this link:

Mechanism of action

The trabecular outflow pathway is the main drainage system of the eye. It can be thought of like an electronics circuit with two resistors in series (Figure 1). The pathway has upstream resistance and downstream resistance. Aqueous humor first drains through the trabecular meshwork into Schlemm’s canal (the upstream resistance).

This is where MIGS stents work, such as iStent and Hydrus. From Shclemm’s canal, aqueous drains into downstream collector channels (the downstream resistance). ABiC lowers both upstream and downstream resistance, and is the only procedure available that treats downstream resistance.

Upstream downstream resistance

Figure 1. Schematic representation of

trabecular outflow resistance

In glaucoma, Schlemm’s canal collapses and the trabecular meshwork herniates into the entrances of the collector channels, blocking them off. By injecting cohesive OVD into Shclemm’s canal, ABiC releases all of these herniations and re-expands both Schlemm’s canal and the downstream collector channels. The result is that fluid can drain from the eye more easily.

Theoretical limitations of ABiC

The theoretical limitations of ABiC are that it does not significantly increase the permeability of the trabecular meshwork. This is a key site of resistance in glaucoma. Although ABiC is marketed as causing “microperforations” in the trabecular meshwork, in my opinion these are likely to heal within days. I say this because this is what occurs when a cutting laser is used to make microholes in the trabecular meshwork.2 Because ABiC does not definitively treat trabecular meshwork resistance, I always combine it with an additional procedure such as a trabecular stent, an excisional goniotomy, or a Gonioscopy Assisted Transluminal Trabeculotomy (GATT).

The other theoretical limitation of ABiC relates to its duration of action. ABiC dilates open the drainage pathways by injecting a single pulse of OVD. For how long would these channels be expected to remain open for? Hours, days, months? It would make sense that the channels collapse again soon after the OVD clears.

Clinical Results of Ab interno canaloplasty (ABiC)

Interestingly, the clinical results speak against this. Following Ab interno canaloplasty (ABiC), patients tend to get a sustained pressure reduction. The caveat here is that our best long-term data with ABiC (three years) comes from studies that placed a ‘tensioning suture’ in Schlemm’s canal. This means that after the ABiC was completed, a suture was threaded all the way around Schlemm’s canal and tied under light tension and this suture held Schlemm’s canal open.3 A subanalysis of cases done without a tensioning suture also had sustained pressure reduction but the number of these cases was small.

nick andrew review microinvasive glaucoma surgery

References

  1. Andrew NH et al. A review of aqueous outflow resistance and its relevance to microinvasive glaucoma surgery. Surv Ophthalmol. 2019 Aug 16.
  2. Melamed S et al. Q-switched neodymium-YAG laser trabeculopuncture in monkeys. Arch Ophthalmol. 1985 Jan;103(1):129-33.
  3. Lewis et al. Canaloplasty: Three-year results of circumferential viscodilation and tensioning of Schlemm canal using a microcatheter to treat open-angle glaucoma. J Cataract Refract Surg. 2011 Apr;37(4):682-90.

Keywords

ABiC; Ab interno canaloplasty; iTrack ; Canaloplasty; Viscocanaloplasty; MIGS; Micro invasive glaucoma surgery; Minimally invasive glaucoma surgery; Glaucoma surgery