Canal-Based MIGS

In recent years, canal-based MIGS procedures have evolved rapidly, reflecting a growing recognition that patient-specific goals can be achieved with greater success by addressing the entire aqueous outflow system. This article explores changes in canal-based MIGS and its implementation in cataract surgery.

GONIOSCOPY-ASSISTED TRANSLUMINAL TRABECULOTOMY 

The development of gonioscopy-assisted transluminal trabeculotomy (GATT) was an important milestone in glaucoma surgery because the procedure addresses outflow resistance from an ab interno viewpoint and reliably improves aqueous outflow.¹ In our clinical experience, GATT can reasonably decrease IOP and significantly reduce the number of topical medications in select glaucoma patients. For example, in pseudoexfoliation glaucoma, obstruction occurs predominantly with proximal outflow, so patients who have this form of the disease often respond dramatically to GATT.²

The procedure is highly effective at reducing resistance at the trabecular level, but GATT may not overcome pathology distal to Schlemm canal. In patients with preoperative IOPs in the high teens or low 20s mm Hg, GATT often achieves stable postoperative IOPs in the midteens on fewer medications. Individuals who need pressures near 10 to 12 mm Hg, however, frequently experience surgical failure over time. 

In other words, there are physiologic limitations of the conventional aqueous outflow pathway and thus a floor beyond which IOP reduction becomes difficult regardless of the extent of classic canal-based MIGS. Filtration surgery may be required to achieve the low target IOPs demanded by advanced glaucoma.³

INTERVENTIONAL VALVE-ENHANCING TRABECULOTOMY 

A Conceptual Shift

Just as cataract surgery has evolved from large-incision extracapsular extraction to highly refined microincisional phacoemulsification, canal-based MIGS appears to be shifting from generalized, passive-based procedures toward the more physiologic restoration of aqueous outflow, allowing ophthalmologists to individualize treatment based on the patient’s anatomy.  

In traditional models of aqueous outflow, the trabecular meshwork and Schlemm canal serve as passive hydrodynamic resistance to aqueous outflow. More recent models account for pulsatile dynamics in aqueous outflow and for intracanalicular valvelike structures that may facilitate drainage. This pulsatile motion is based on pressure-sensitive tissue that distends and recoils in response to physiologic IOP fluctuations generated by the ocular pulse, blink, and eye movement. This cyclic motion produces corresponding volume changes within Schlemm canal that work in tandem with the movement of these delicate intracanalicular structures that are linked to distal outflow.⁴ Tube-like inlet valves within Schlemm canal allow aqueous to enter, whereas outlet valves consisting of hinged collagen flap leaflets at the collector channel ostia regulate aqueous egress. Mechanical coupling between the trabecular meshwork and these structures enables coordinated control of aqueous outflow.^5,6 For simplicity, the authors combine the inlet and outlet valvelike structures described by Johnstone into one term: intracanalicular valvelike structures (IVaLS).⁷

During ab interno interventional valve-enhancing trabeculotomy (iVEnT), a specialized spatula is used to cleave the nonfiltering trabecular meshwork just below Schwalbe line, away from the IVaLS, which are typically located in the posterior part of the canal. Classic trabeculotomy procedures rely mainly on bulk flow through a wide surgical opening with minimal consideration of the valvelike structures. IVEnT was designed to protect IVaLS and enhance their function, potentially augmenting pulsatile outflow through surgical reduction in tissue stiffness. 

Initial Data

Our early data⁸ on 218 eyes with varying degrees of glaucoma damage (the majority were categorized with a visual field mean deviation of -12 to -3 dB) that underwent phaco-IVEnT showed a statistically significant and sustained reduction in mean IOP from 16.2 mm Hg preoperatively to 13.5 mm Hg at 24 months (a 12.4% decrease). The number of topical antiglaucoma medications decreased from a preoperative average of 2.18 drops to an average of 0.79 drops at 24 months (a 60.1% reduction). Regardless of the starting IOP, 55% of the eyes achieved a postoperative IOP in the low teens (13.1 mm Hg) with no antiglaucoma medication after surgery. Even in the low-pressure group (IOP ≤ 14 mm Hg), the baseline mean IOP was 12.3 mm Hg and 12.5 mm Hg at 18 months, but the average number of medications decreased from 2.29 to 0.6.  

Regarding safety, hyphema rates (4%) were relatively low with IVEnT compared to those reported for conventional trabeculotomy and goniotomy procedures.^1-3 

From a mechanism of action viewpoint, the authors noted enhanced visualization of IVaLS, hinged collagen flaps, collector channel openings, and distal outflow channels with clinic-based HD-OCT imaging in eyes that had undergone IVEnT.⁷ This suggests evidence for a motion-dependent component of aqueous outflow and paves the way for clinicians and researchers to use OCT for Schlemm canal imaging to better understand their clinical outcomes. 

CANAL-BASED MIGS COMBINED WITH CATARACT SURGERY 

A practical question when combining canal-based MIGS with phacoemulsification is when to intervene. The majority of patients needing combined surgery will have mild to moderate glaucoma and varying degrees of cataract, an excellent group to benefit from combined phaco-canal-based MIGS. Surgeons now have the opportunity to improve not only visual acuity but also aqueous outflow in a more physiologic fashion before more advanced distal outflow dysfunction develops. This enhances quality of life by improving visual acuity, reducing medication burden, improving ocular surface health, and delaying the need for more aggressive filtration surgery. Compared to many angle surgeries, the authors believe valve-sparing canal-based surgeries may enhance long term physiologic function by promoting factors related to the motion of aqueous outflow.

As procedures evolve, the planning of cataract surgery combined with MIGS, both canal-based and bleb-based, increasingly depends on surgeon judgment regarding patient-specific factors such as the target IOP, disc damage combined with medication burden and compliance, angle anatomy, recovery profile, and surgeon experience. Safer, less invasive options to increase physiologic outflow allow earlier intervention to reduce the global burden of blindness from glaucoma, while still allowing for more invasive options later, if needed. 

Authors’ note: Intellectual property rights relating to the IVEnT technology are jointly held by RAICO International, Eyeinnovative, and Innovative Glaucoma Consulting.