MIGS: The ABC of ABiC

What are MIGS procedures, and why are they increasing in popularity?

MIGS (micro or minimally invasive glaucoma surgeries) are a group of procedures that reduce IOP and/or medication burden (and associated compliance issues) with a safety profile that exceeds that of older, more aggressive glaucoma filtration procedures. They work by enhancing aqueous filtration through natural outflow systems, whether it’s the conventional outflow system or via the suprachoroidal space. We’ve had experience with some MIGS procedures for close to a decade and found them to be highly effective, low-risk interventions, either when performed during or independently of cataract surgery.

Is it possible to define the ideal MIGS patient?

When performing procedures that manipulate the conventional outflow system, the distal system must remain intact, relatively patent, and amenable to rejuvenation – but we lack the tools necessary to clinically evaluate the functionality of the distal system pre-operatively and must therefore rely on visual cues in the proximal system, such as increased pigmentation of the trabecular meshwork (TM) in regions of increased outflow. However, when significant disease is present in TM, patients benefit from certain MIGS procedures, in particular, trabeculo-ablative interventions, such as a trabeculotomy or goniotomy. Similarly, patients with a sclerosed distal outflow system would benefit from bypassing it altogether with a suprachoroidal shunt.

The point is this: we can now match MIGS procedures to patients. My practice has changed considerably over the last five years following the adoption of MIGS. I once had to perform filters approximately 8–10 times a week, but since MIGS, I have seen a drastic drop: I now perform 1–2 filters a month, in patients where MIGS have failed to achieve target pressures or in those where pressures in the single digits are a must.

In what ways do MIGS procedures fall short of the ideal?

It’s perceived that MIGS is not as effective as filtration procedures at reducing IOP. While we may obtain pressures in the single digits with MIGS, most patients with glaucoma (especially those with mild-to-moderate disease) don’t require pressures that hover just above hypotony. We do typically obtain pressures in the mid and even low teens – even in patients with pre-operative pressures in the twenties who are already on 3 or 4 eyedrops. This is often sufficient. But if a surgeon opines that single digit pressures are needed to avoid progressive glaucomatous optic neuropathy and vision loss, as is typically the case with end-stage glaucoma, MIGS procedures likely won’t offer the desired result.

Adopting MIGS procedures does involve a little bit of a learning curve – not only with the individual procedures, but also with performing intra-operative gonioscopy. All angle-based surgeries rely on our direct visualization of the nasal drainage angle, which requires direct gonioscopy. Most of us “cataract surgeons” had little experience using direct gonioscopy lenses during our training, much less manipulating structures of the outflow systems via an ab-interno approach. These small hurdles can be overcome with a little time, practice and patience, which is important given the rapid acceptance and adoption of MIGS devices.

Each MIGS device, when used in the appropriate patient can yield amazing results. The conventional outflow system has pathologic changes along each layer of the system, from the uveal portion of the TM all the way through the scleral plexus. Minimizing treatment to one layer of disease, while leaving other areas of disease untreated may lead to sub-optimal results. However, one procedure can addresses the entire drainage system through transluminal viscodilation – ab interno canaloplasty or ABiC.

How is ABiC an improvement over traditional canaloplasty?

Traditional canaloplasty is an elegant but laborious procedure that can take up to an hour to perform. It requires large conjunctival and scleral dissections, creation of a Descematic window and scleral lake, and placement of a tensioning suture. The extensive dissections also violate conjunctiva. By contrast, ABiC preserves the conjunctiva and therefore does not preclude performance of any glaucoma procedures, should they be needed. The procedure is also rapid – perhaps less than five minutes – and requires virtually no recovery time.

Despite their differences, there is one very important similarity – efficacy. We once thought that the tensioning suture was paramount to the success of the procedure, but evaluation of three-year canaloplasty data proved otherwise – a subgroup of patients with successful viscodilation of the canal, who were unable to receive a tensioning suture experienced very similar reductions in IOP and medication burden to those who received a tensioning suture (2). With this data, we developed, adopted and evaluated ABiC. I compared the efficacy of ABiC and canaloplasty in a small case series of 12 patients – ABiC in one eye and canaloplasty in the other – and found that the results mirrored each other. We have since performed ABiC in several hundred patients and have anecdotally seen similar results.

One advantage of ABiC over canaloplasty is that ABiC can be performed more easily in patients in whom trabeculectomies and tube shunts have failed. No further conjunctival dissection is required, which can be challenging in patients that have had filters (or other procedures that damage conjunctiva), especially if a large 5 mm scleral flap is needed (as is with canaloplasty).

ABiC is less traumatic to the eye, preserves conjunctiva, decreases surgical and recovery times and recovery is similar to that of an uncomplicated, routine cataract extraction. In stand-alone procedures, patients are within one line of their best, corrected visual acuity on day one and return to their baseline pre-operative visual acuity within one week. It’s a relatively comfortable procedure with little to no post-operative discomfort. Furthermore, many patients are able to reduce their medication burden after ABiC, which has the benefit of reducing cost to the patient, and exposure to the caustic chemical, benzalkonium chloride (BAK). This benefits not just the ocular surface but the TM; BAK induces apoptosis to endothelial cells of the cornea and cells lining trabecular columns, which can lead to fusion of adjacent damaged columns and an eventual reduction of the effective filtration area.

Despite all of the positives of ABiC, I still favor canaloplasty in patients with corneal endothelial disease, such as Fuch’s endothelial dystrophy. I fear manipulation of devices within the anterior chamber in such patients could lead to further endothelial disease and possible corneal decompensation.

Compared with other MIGS procedures, what is ABiC’s niche?

ABiC does not have to be coupled with cataract extraction. It can be done in phakic and pseudophakic patients or as an adjunct to cataract extraction. It is also not limited to mild-to-moderate disease and could be performed in patients with severe glaucoma (although outcomes may be more variable in such cases). It also treats 360° of the outflow system – and every layer from the uveal TM to the scleral plexus. Circumnavigation of the canal with a the iTrack is the only method that has the ability to lyse all herniations obstructing the collector channels, comprehensively treating the entire conventional outflow system. It is less traumatic and disruptive compared to most MIGS procedures – without ablating the TM or inner wall of Schlemm’s canal, ABiC leaves the natural blood-aqueous barrier intact, avoiding the possibility of recurrent hyphema, as seen with some MIGS procedures. It is as effective as any other MIGS procedure; you don’t lose efficacy by avoiding stent implantation or tissue ablation, and because all structures are left intact, performing other procedures is not precluded. Also, having done a multitude of procedures, I can say that ABiC is very safe with minimal risk.

How important is the iTrack device to ABiC?

The iTrack catheter’s coating allows it to glide easily through the drainage canal. The fiber optic tip allows visualization of the device’s location at all times. It’s very easy to push a normal device into the suprachoroidal space; by contrast, you can follow iTrack’s illuminated tip and direct it accordingly. The catheter is relatively long, so if you encounter a blockage within Schlemm’s canal close to the intubation site, you simply circumnavigate in the opposite direction – you’ll still get 360° of treatment. Further, the iTrack permits control of viscoelastic infusion.  In a canal with many structures, you can infuse more viscoelastic to achieve better dilation; similarly, if you encounter an adhesion, you can bypass it by infusing additional viscoelastic to expand the canal sufficient for catheter advancement. No other device permits procedure parameters to be modulated in this fashion.