Glaucoma surgery has undergone a paradigm shift with the rise of microinvasive glaucoma surgeries (MIGS). These techniques are minimally invasive, have a high safety profile relative to traditional procedures, and are effective IOP lowering therapies.¹ The popularity of these surgeries is not only growing due to novelty, but out of necessity as well. Cataract surgery is the most common surgery in the United States, with 3.8 million performed in 2015,² and 20% of patients having comorbid glaucoma.³ It has become increasingly popular for cataract surgery to be performed with MIGS.⁴ These combination surgeries are a testament to the ingenuity of modern glaucoma therapies.

A longitudinal study done by the Mayo Clinic found that the 20-year probability and population prevalence of blindness from open-angle glaucoma in at least 1 eye decreased from 25.8% to 13.5% between 1965 and 2009 because of more aggressive management of glaucoma.⁵ MIGs began in the United States with the FDA approval of the Trabectome (NeoMedix) in 2004, and it has expanded to include a multitude of surgeries and devices that improve aqueous humor outflow (AHO) via 1 of 3 main mechanisms: enhancement of Schlemm canal (SC) flow, shunt to the suprachoroidal space, and shunt to the subconjunctival space (Figure 1).¹

However, these surgeries have a learning curve and need to be precise for the best outcomes. High-definition anterior segment imaging has revolutionized our understanding of the pathophysiology of glaucoma. But, we currently have no imaging protocol to help us decide among the many options in our surgical armamentarium. This limits the surgeon’s ability to individualize device selection and optimize patient outcomes.

Optical coherence tomography (OCT) and ultrasound biomicroscopy (UBM) are currently the mainstays of structural imaging of the anterior segment (AS) because they are noninvasive, objective, and easy to use (Figure 2).^6-9 Each modality has its benefits and drawbacks that should be used to guide clinical applications in MIGS.

There is one dedicated anterior-segment OCT (AS-OCT) platform available in the United States, the Visante (Carl Zeiss Meditec). It is best used when the cornea is clear and the patient is sitting upright. Its ease of use and high resolution makes it a good option for longitudinal study.^6,7 Its main drawback is the inability to view structures posterior to the iris pigment epithelium and in corneal edema.⁷

Unlike AS-OCT, UBM has been shown to have better visibility of structures posterior to the iris pigment epithelium.^9,10 A significant disadvantage to UBM is it is a contact procedure and could be uncomfortable in the immediate postoperative period. Ultrasound biomicroscopy platforms are available in the United States from several manufacturers (UBM Plus by Accutome, VuMAX by SonomedEscalon, Aviso UBM by Quantel Medical, Reflex UBM by Reichert Technologies, and Scanmate Flex by DGH Technology).

Our discussion will focus on the most popular FDA-approved MIGS in the United States and how AS-OCT and UBM can augment perioperative care. The success of angle surgery is dependent on precision with a margin of error no larger than a couple hundred nanometers (Figure 3).¹¹ Preoperative imaging can be an effective way to determine surgical candidates and familiarize the surgeon with angle anatomy prior to intervention to avoid complications. For preoperative assessment and selection of procedures and device, research has shown that AHO is not circumferential but segmental, and not static but dynamic.⁶ This explains in part why the MIGS outcomes show some inconsistency.⁶

Aqueous angiography on postmortem eyes showed that stent placement in low-flow areas improved AHO.¹² However, studies done on living primates and humans showed the AHO increase in regions of initially low flow, and decrease in regions of initially higher flow.^13,14 This suggests that physiologic changes in outflow pathways alter the pressure gradients and shunt fluid to other segments.⁶ The decision to select appropriate stent and number of stent placement poses quite a challenge. As we continue to map the structural and functional pathways in the AS, our strategies for effective intervention will continue to improve. Ultrasound biomicroscopy and AS-OCT can be effectively used to assess stent placement,⁷ the latter with the potential for intraoperative assessment using AS-OCT mounted microscopes.

Intervention with the Kahook Dual Blade (KDB; New World Medical), FDA approved in 2015, and Trabectome are particularly effective procedures that enhance SC flow by removing trabecular tissue obstruction.¹⁵ A KDB or Trabectome incision too anterior can cause corneal Descemet folds and edema, while an incision too posterior can cause a cyclodialysis cleft, iris injury, and hyphema.¹⁶ A cyclodialysis cleft may not be visible on gonioscopy, particularly in the presence of significant hypotony, opaque media, or abnormal structural anatomy (Figure 4).^17,18 Ultrasound biomicroscopy is an effective method of detecting cyclodialysis, ciliary body detachment, iridodialysis, lens displacement, zonular defects in the presence of hyphema, and opaque media.¹⁹ Postoperatively, AS-OCT and UBM can be used to evaluate the incisions to check for proper outflow alteration (Figure 5C and 5E).

The Xen gel stent (Allergan) has become an effective option for subconjunctival shunting of AHO since its FDA approval in 2016. Following placement, aqueous fluid drains into the potential space and is absorbed by conjunctival and episcleral veins.¹ The presence of conjunctival microcysts has shown to be a reliable indicator of a functioning bleb. Anterior-segment OCT imaging is an effective way to not only assess stent placement, but to monitor bleb function over time as well.^7,20,21 Current research suggests that microcysts, low reflectivity of the bleb wall, internal fluid cavities, internal ostia, and multiple internal layers are correlated with good AHO.^20,22,23 Tenon cysts overlying episclera and internal ostium can all be evaluated to assess scarring and outflow.⁷ A study by Jung et al showed a positive correlation between thin-walled blebs measured by AS-OCT and successful surgical outcomes.²¹ A study by Ahmed et al has shown the utility of using OCT for clinical management of postoperative Xen microstent placement. Their case study identified a fibrosed bleb 1 year postoperatively, and the structural changes that occurred after needling and flow restoration (Figure 5).²⁴ In the past, surgeons would have to watch and wait while the aqueous flow deteriorated, or blindly intervene based on clinical acumen. Now, imaging provides the opportunity to objectively access stent placement as it relates to anatomy and outflow in real time.

The Cypass Micro-Stent (Alcon), FDA approved in 2016, enhances outflow by providing a permanent conduit into the supraciliary space analogous to a controlled cyclodialysis cleft.¹ Postoperative AS-OCT and UBM can be performed to evaluate SC space after implantation while also providing the surgeon with accurate assessment of AHO.²⁵ Hypodensities of aqueous fluid surrounding the implant, particularly the anterior fluid accumulation (known as tenting) and the posterior fluid accumulation, are predictive of IOP reduction (Figure 5).²⁶

AS-OCT and UBM also allow for longitudinal observation of SC fluid over time giving the surgeon the opportunity to make clinical judgements earlier in the disease course as well as adjustments in surgical technique to optimize therapy. Choroidal effusion and hemorrhage are also able to be assessed using UBM.⁷ Unpublished imaging studies done at Mayo Clinic Florida have shown reduction of anterior-chamber volume and depth with an increase in supraciliary fluid. This in turn affects the lens position and refractive outcome. This fluid drawn into the supraciliary space is also influenced by the size of the localized cyclodialysis cleft around the Cypass. Prospective studies are needed to more clearly determine postoperative clinical outcomes.

Summary

Anterior-segment OCT and UBM provide surgeons with objective, measureable visualization of the structural components of the anterior chamber during the perioperative period. Such visualization is beginning to become an essential part of the glaucoma specialist’s clinical and operative toolkit. As we continue to collect data both clinically and in the laboratory, these various imaging modalities will shepherd surgeons into a more precise and predicable era of glaucoma surgeries. GP

References

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Kyle A. Batton, BS, is a third-year medical student at the Florida State University College of Medicine in Tallahassee, Florida.

Syril Dorairaj, MD, MBBS, is an associate professor of ophthalmology in the Department of Ophthalmology at the Mayo Clinic in Jacksonville, Florida.
The authors report no related disclosures. Reach Kyle Batton at kb10k@med.fsu.edu.