Visualizing the Angle

Before performing gonioscopy or swept-source anterior segment OCT (AS-OCT) imaging of the angle, it is essential to understand the anatomic landmarks of the anterior chamber angle. The key structures, from anterior to posterior, include Schwalbe line, the nonpigmented trabecular meshwork, the pigmented trabecular meshwork, the scleral spur, and the iris root.

Swept-source AS-OCT is a complement to gonioscopy rather than a replacement. Several platforms, including the CASIA2 (Tomey), the Anterion (Heidelberg Engineering), and the Triton DRI (Topcon), support quantitative angle imaging. This article is based on my experience with the Anterion. Click here to watch an accompanying video demonstration of angle anatomy, swept-source AS-OCT imaging, gonioscopy, and postoperative MIGS assessment.

Swept-source AS-OCT provides high-resolution, reproducible cross-sectional imaging. Gonioscopy provides dynamic, real-time evaluation. Together, they support a more complete angle assessment and long-term postoperative monitoring.

VIEWING THE ANGLE 

Swept-source AS-OCT provides high-resolution, reproducible cross-sectional imaging. Gonioscopy provides dynamic, real-time evaluation. Together, they support a more complete angle assessment and long-term postoperative monitoring.

Swept-Source AS-OCT

Swept-source OCT images can help confirm angle anatomy and support diagnosis and follow-up.

To visualize the angle, the patient is instructed to look in an extreme gaze direction, allowing the clinician to focus on the scleral spur and underlying angle structures (Figure 1). The Anterion imaging app uses multiple cross-sectional scans to capture and assess angle anatomy, particularly in eyes with glaucoma and for surveillance after MIGS.

Figure 1. Schematic (A) and gonioscopic (B) views of anterior chamber angle landmarks.

Gonioscopy

Gonioscopic photographs provide a permanent record of the angle anatomy, aiding diagnosis, treatment planning, and medicolegal documentation. Showing patients angle images can also help them understand their condition and the importance of treatment adherence.

Serial gonioscopic photographs may document progressive changes in the angle, including synechiae (Figure 2), pigment accumulation, and other signs of disease progression. Postoperative gonioscopic images can help monitor healing, detect complications, and guide further management.

Figure 2. Gonioscopic view of the anterior chamber angle demonstrating peripheral anterior synechiae.

APPLICATIONS IN MIGS 

MIGS procedures such as bent-needle ab interno goniectomy, goniotomy, iStent (Glaukos) implantation, and gonioscopy-assisted transluminal trabeculotomy benefit from precise angle images. Direct gonioscopic visualization and clear identification of the angle structures are prerequisites for MIGS and remain essential for postoperative assessment. 

Excisional Goniotomy

Excisional goniotomy involves precise removal of the trabecular meshwork to expose Schlemm canal, typically resulting in a clean, uniform cleft that can be evaluated with gonioscopy and swept-source AS-OCT. 

In my experience, a goniotomy performed with a Kahook Dual Blade (New World Medical) produces a characteristic postoperative appearance. The dual-edged blade excises a strip of trabecular meshwork to create a clean, uniform incision. On swept-source AS-OCT, this appears as a well-demarcated, W-shaped cleft with clear exposure of Schlemm canal (Figure 3). Gonioscopic view after Kahook Dual Blade goniotomy shows a smooth, uniform trabecular meshwork cleft (Figure 4).

Figure 3. Swept-source AS-OCT image after Kahook Dual Blade goniotomy demonstrating a well-demarcated, W-shaped cleft and exposure of Schlemm canal.

Figure 4. Gonioscopic view after Kahook Dual Blade goniotomy showing a smooth, uniform trabecular meshwork cleft.

Bent-Needle Ab Interno Goniectomy

A bent-needle ab interno goniectomy involves manual disruption of the trabecular meshwork. Swept-source AS-OCT can show a rugged cleft within the trabecular meshwork, often with visible tissue tags that reflect the mechanical nature of the disruption (Figure 5). Gonioscopy further supports this observation, revealing a jagged, nonuniform cleft at the site of intervention, consistent with the manual tearing technique (Figure 6).

Figure 5. Swept-source AS-OCT image after bent-needle ab interno goniectomy showing an irregular cleft within the trabecular meshwork with residual tissue tags.

Figure 6. Gonioscopic appearance after bent-needle ab interno goniectomy demonstrating an irregular cleft consistent with manual disruption of the trabecular meshwork.

iStent Technology

The iStent is a single snorkel-shaped microbypass device implanted in Schlemm canal to create a direct pathway for aqueous humor to exit the anterior chamber, helping to lower IOP in eyes with open-angle glaucoma. On swept-source AS-OCT, a distinct posterior shadow is visible at the implant site, which can help confirm positioning (Figure 7). On gonioscopy, a single stent is visible in the angle (Figure 8).

Figure 7. Swept-source AS-OCT image of the posterior shadow cast by an iStent at the site of implantation.

Figure 8. Gonioscopic view of a single iStent implant in the anterior chamber angle.

Second-generation iStent technology consists of two microbypass implants designed to improve outflow through multiple collector channels. On swept-source AS-OCT, dual posterior shadows confirm the location and depth of both devices (Figure 9). Gonioscopy shows two stents in the angle (Figure 10).

Figure 9. Swept-source AS-OCT image of second-generation iStent inject implants showing dual posterior shadowing that help verify implant location and depth.

Figure 10. Gonioscopic view showing second-generation iStent inject implants positioned in the angle.

AN INTEGRATED APPROACH 

Combining gonioscopy and anterior segment imaging provides more information on MIGS outcomes. Gonioscopy offers real-time, direct visualization of angle structures, aiding intraoperative decision-making and postoperative assessment. Swept-source AS-OCT provides high-resolution, cross-sectional images of the anterior segment. These complementary tools can increase surgeon confidence; support patient selection, surgical planning, and personalized care; and strengthen postoperative follow-up.