Optical coherence tomography (OCT) is a noninvasive imaging technique widely utilized in ophthalmology. In glaucoma, OCT has primarily been used for the diagnosis, monitoring, and management of disease. Posterior-segment OCT aids in evaluation of the retinal nerve fiber layer and ganglion cell layer thickness, whereas anterior-segment OCT (AS-OCT) uses a higher wavelength to evaluate the anterior segment, including the lens and angle structures. 

Recently, AS-OCT has also been applied intraoperatively for glaucoma surgeries.¹ There are currently 3 microscope-integrated commercial intraoperative OCT (iOCT) platforms approved by the US Food and Drug Administration (FDA) — the Haag-Streit iOCT system,  the Zeiss Rescan 700, and the Leica EnFocus system.² This review will discuss various applications of iOCT in glaucoma surgery.

Trabeculectomy

Trabeculectomy is a filtering procedure that lowers intraocular pressure (IOP) by creating a scleral flap to facilitate aqueous humor drainage to the subconjunctival space. The iOCT has been used to enable real-time visualization of the scleral incision depth and better appreciation of flap regularity and thickness during trabeculectomy surgery.³ The sclerotomy and iridectomy may also be visualized with iOCT imaging. However, posterior shadowing from metallic surgical instruments may at times obscure the view of structures during the procedure.⁴ 

Glaucoma Drainage Devices

Glaucoma drainage device (GDD) implantation enables aqueous humor to be shunted from the anterior chamber (AC) to the subconjunctival space. Introduction of the GDD into the AC may be considered a “blind” procedure, and accurate entry and placement can be challenging, especially in eyes with synechial angle closure or opaque corneas. During implantation of the Ahmed glaucoma valve, Kumar et al found iOCT helpful in ensuring avoidance of peripheral anterior synechiae (PAS) during the GDD’s entry into the AC.³ Swaminathan et al demonstrated the utility of the iOCT in guiding sulcus placement of the GDD.⁵ Precise entry into the AC will minimize complications such as hyphema, endophthalmitis, and tube malposition.

Bleb Management

Bleb fibrosis following subconjunctival filtering procedures may require needling with antimetabolite administration to restore aqueous flow. Kumar et al used iOCT to facilitate bleb needling with mitomycin C and were able to visualize adhesions and loculations prior to needling.³ After needling, iOCT showed a single hyporeflective cavity that represented procedural success. Dada et al also reported clear visualization of the bleb wall and adhesions with iOCT and proposed that visualization of the needle within the bleb during needling may reduce complications such as blebitis, hyphema, or bleb leak after surgery.⁶

Hypotony is a complication that may follow bleb-forming procedures, with accompanying hypotonous maculopathy. In this scenario, overfiltering blebs may be sutured to prevent choroidal detachment and further vision deterioration. Angmo et al used iOCT to perform targeted transconjunctival bleb suturing in 2 eyes with persistent hypotony and hypotonous maculopathy following trabeculectomy.⁷ The iOCT allowed for visualization of areas of excessive filtration within the bleb and guided suture placement at specific sites.

Goniosynechiolysis

Goniosynechiolysis involves mechanically releasing PAS to widen the iridocorneal angle. This is typically performed under intraoperative gonioscopy. However, iOCT may allow this to be performed without gonioscopy, with the potential of demonstrating posterior displacement of the iris after a successful procedure.³

MIGS

Minimally invasive glaucoma surgeries (MIGS) employ specialized surgical devices and microinvasive maneuvers to target specific components of the aqueous outflow pathway. Compared to traditional glaucoma surgical techniques, MIGS offer less-invasive alternatives and have demonstrated a lower risk of complications. FDA-approved MIGS may be broadly classified as angle-based or subconjunctival. Angle-based MIGS include devices that enhance aqueous outflow in 1 of 3 ways. Some devices, such as the iStent series (Glaukos) or Hydrus Microstent (Alcon), bypass the trabecular meshwork (TM). Others are used to dilate Schlemm’s canal, such as the iTrack Advance (Nova Eye Medical) or Omni Surgical System (Sight Sciences). A third angle-based approach is to excise TM, using devices like the Trabectome (MST) or Kahook Dual Blade (New World Medical). Subconjunctival MIGS, such as the Xen45 gel stent (AbbVie), divert aqueous humor to the subconjunctival space. The iOCT may be used in conjunction with various MIGS to improve anatomical visualization and ensure optimal device placement during surgery.⁸ 

Trabeculotomy

The Trabectome is an angle-based MIGS device used in ab interno trabeculotomy, where the TM is ruptured to increase aqueous outflow. Two studies have detailed the use of iOCT in this context. Junker et al demonstrated views of the iridocorneal angle and Schlemm’s canal during the procedure, describing overall good quality images with image acquisition only adding a couple of minutes to total surgical time, after initial adaptation.⁹ Heindl et al, with another iOCT platform, visualized the Trabectome tip and aspiration canula in the AC, and were able to obtain clear views of the TM in porcine eyes.¹⁰ Both studies described limitations of iOCT used in this context, including limited visualization of key angle structures due to the high reflectivity of the conjunctiva and sclera that caused shadowing artifacts.

Tanito et al used iOCT during ab interno trabeculotomy with the Tanito microhook (Inami and Company).¹¹ While difficulty was encountered in visualization of Schlemm’s canal due to the poor penetration of iOCT, a combination of iOCT and gonioprism use enabled successful observation of a trabeculotomy cleft and the Schlemm’s canal lumen.

Canaloplasty

Canaloplasty is an angle-based MIGS procedure that involves dilation of Schlemm’s canal to improve aqueous outflow. The use of iOCT during ab externo canaloplasty performed with the iTrack 250A microcatheter was reported by Siebelmann et al,¹² who demonstrated visualization of all anterior angle structures except for the Schwalbe’s line. Authors successfully demonstrated visualization of the suture positioned in Schlemm’s canal, as well as structural changes of the AC angle after tensioning of the suture. 

Trabecular Bypass Devices

The iStent series includes microimplants inserted into Schlemm’s canal to bypass the TM and enhance aqueous outflow. Although device implantation involves minimal risk, both under- and over-implantation of stents have been reported, and studies utilizing postoperative OCT have demonstrated reduced IOP-lowering effect in these circumstances.^13,14 Ang et al recently demonstrated high-resolution imaging of the iStent Inject W device using iOCT,¹⁵ successfully visualizing both the device flange and lumen following implantation, confirming optimal placement of the stent (Figure 1). 

Ang et al also used iOCT during Hydrus Microstent implantation.¹⁵ The Hydrus Microstent is another TM bypass device inserted directly into Schlemm’s canal; it also provides structural support to maintain canal patency, thereby enhancing aqueous outflow. Malpositioning of the device can result in suboptimal outcomes. Following successful implantation, the 3 windows of the Hydrus Microstent should be visible in Schlemm’s canal. However, visualization can be challenging in patients with a heavily pigmented TM or dense iris processes. Ang et al described the expected appearance of an accurately placed Hydrus Microstent on iOCT images, across various segments of the device (Figures 2 and 3). 

Future Directions

There is growing use of iOCT in glaucoma surgery, with potential benefits in both traditional filtering surgeries and MIGS, in increasing surgical precision and confirming accurate device placement. Currently, iOCT already complements various surgeries and may have the potential to eventually replace intraoperative gonioscopy in selected glaucoma procedures. However, several challenges remain, including the presence of artifacts, shadowing from metallic instruments, insufficient penetration into deeper tissue, and the presence of a small but discernable time lag during real-time use. 

iOCT technology continues to evolve, and some of its limitations may, in future, be overcome by newer swept-source OCT technology, with its enhanced speed and visualization capabilities, as well as by the development of nonmetallic surgical instruments, which will mitigate the shadowing effect of metallic instruments observed in iOCT imaging today.^16,17 Additionally, the benefits of the iOCT in glaucoma surgery would be better defined by further studies assessing the impact of the iOCT on actual postoperative clinical outcomes. GP 

Syril K. Dorairaj, MD, and, is a professor of ophthalmology at the Mayo College of Medicine and Science, and is the director of the glaucoma fellowship program at the Mayo Clinic in Jacksonville, Florida. He reports consultancy to New World Medical. Reach Dr. Dorairaj at dorairaj.syril@mayo.edu.

Bryan C. H. Ang, MBBS, FAMS, FRCOphth, is a consultant with the glaucoma service at the National Healthcare Group Eye Institute at Tan Tock Seng Hospital in Singapore and a research collaborator with the Mayo Clinic in Florida. He reports consultancy to and speakers’ fees from Carl Zeiss Meditec, Glaukos, and Alcon. Reach Dr. Ang at drbryanang@gmail.com.

P. Connor Lentz, BS, is a medical student at the Mayo Clinic Alix School of Medicine in Jacksonville, Florida. He reports no relevant disclosures.

References

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