Focus on Diagnostics

Anterior-segment OCT in glaucoma management

Gaining a clearer picture of the angle and IOP than traditional methods.

By Syril K. Dorairaj, MD, and Michael W. Stewart, MD

Since anterior segment OCT (AS-OCT) was introduced in 2006, clinicians have frequently used it to gather data for a multitude of patient presentations. Increasingly, ophthalmologists have used AS-OCT to gather data for evaluating glaucoma and monitoring its progression.

Measuring central corneal thickness (CCT) is important for monitoring glaucoma progression in patients with ocular hypertension and primary open-angle glaucoma, and correcting IOP measurements made by Goldman applanation tonometry.¹

This second of two articles on AS-OCT reviews the role AS-OCT has in evaluating the angle, post-SLT blebs and secondary glaucomas.

EVALUATION OF THE ANGLE

Primary angle closure (PAC) is the leading cause of glaucoma-related blindness in East Asians.² To make this diagnosis, the examiner must evaluate the anterior chamber angle (ACA) and its inlet in darkness to determine the risk for closure.

Drawbacks of gonioscopy and UBM

Gonioscopy is the standard for evaluating the ACA, but its drawbacks include subjective and semi-quantitative assessment methods, need for illumination, inter-examiner variability, and its inability to determine angle configuration when the goniolens contacts the cornea directly.^3,4

Ultrasound biomicroscopy (UBM) provides valuable information about ACA configuration and allows detailed imaging of the ciliary body and the posterior chamber, but it requires close-contact immersion.

Benefits of AS-OCT

Non-contact AS-OCT visualizes spatial relationships within the anterior segment and objectively measures the ACA. In addition, the infrared laser combined with the real-time eye position monitor permits the precise capture of angle morphology in the dark. Darkroom conditions are imperative in angle imaging to increase the detection of angle closure (Figure 1).

Figure 1: Anterior chamber angle imaged with AS-OCT. Block arrows show an open angle in lighted conditions (A) and closure of the angle in darkness (B).

With higher scanning speed (8 frames/sec), SL-OCT provides quantitative, spatial information regarding dynamic changes of the angle configuration, which standard gonioscopy and UBM cannot visualize. Swept-source OCT has been used to successfully view the anterior chamber drainage angle (Schlemm’s canal, trabecular meshwork and iris configuration).⁵

In plateau iris, large and/or anteriorly situated ciliary processes obliterate the ciliary sulcus so that the ciliary body supports the iris against the trabecular meshwork. A recent study used indentation AS-OCT to evaluate plateau iris and other causes of angle closure.⁶

POST-TRABECULECTOMY

Imaging of filtering blebs

Clinicians have used UBM to image blebs after trabeculectomy,^7-9 but the need for conjunctival contact limits its use. Alternative techniques for imaging of blebs have included in vivo confocal microscopy¹⁰ and AS-OCT.^11,12

Non-contact OCT imaging is a much safer way to examine bleb morphology in the early postoperative period, offering an opportunity to study the longitudinal healing and remodeling processes inside the blebs.^11-16 Four-different AS-OCT patterns of intra-bleb morphology have been identified — diffuse filtering blebs, cystic blebs, encapsulated blebs and flattened blebs — and have been correlated with slit lamp appearance and bleb function (Figure 2).¹²

Figure 2: AS-OCT shows thickened, homogeneously spongy filtering bleb (arrow) with fluid-filled spaces separated by septa and closed intrascleral fistula.

Combining clinical and imaging information could provide new insight for understanding surgical outcomes after trabeculectomy, which may refine surgical techniques and better evaluate adjuvant surgical treatments for glaucoma.

A recent Singapore study that used SD-OCT (Cirrus SD-OCT, Carl Zeiss Meditec, Dublin, Calif.) to identify bleb morphology found SD-OCT identified bleb wall details, but could not image deeper structures such as internal ostium or flap position.¹⁶ Also, AS-OCT has been found to aid in determining the position, course and patency of drainage implants.^18,19

Secondary glaucoma, iris disorders

AS-OCT has been useful for evaluating post-penetrating keratoplasty eyes with secondary glaucoma,^20,21 after laser iridotomy²² and after cataract surgery.²³ AS-OCT has also been used to visualize the position and patency of aqueous shunt devices in the anterior chamber that corneal edema may partially obscure.²⁴

AS-OCT penetrates full thickness iris lesions and allows three-dimensional measurement of lesion size. Compared to slit-lamp photography and ultrasound biomicroscopy, AS-OCT is helpful in evaluating iris tumors, including focal iris nevus, diffuse iris nevus, amelanotic iris nevus, iris melanocytosis and iris melanoma.²⁵ OM

Acknowledgement

The authors acknowledge the contributions of Vishal Jhanji, MD, from Chinese University of Hong Kong for initial drafting of the article, and Alison Dowdell, Mayo Clinic, Florida, for editing the article.

REFERENCES

About the Author
	Syril K. Dorairaj, MD, is an assistant professor at Mayo Clinic, Jacksonville. His e-mail is Dorairaj.syril@mayo.edu.
	Michael W. Stewart, MD, is an associate professor and chairman of ophthalmology at Mayo Clinic, Jacksonville. His e-mail is stewart.michael@mayo.edu.
	Disclosures: Dr. Dorairaj has no conflicts to disclose. Dr. Stewart disclosed he is a consultant to Allergan, Boehringer-Ingelheim and Regeneron.