The Use of Anterior-Segment Optical-Coherence Tomography for the Assessment of the Iridocorneal Angle and Its Alterations: Update and Current Evidence

Abstract

The introduction of anterior-segment optical-coherence tomography (AS-OCT) has led to improved assessments of the anatomy of the iridocorneal-angle and diagnoses of several mechanisms of angle closure which often result in raised intraocular pressure (IOP). Continuous advancements in AS-OCT technology and software, along with an extensive research in the field, have resulted in a wide range of possible parameters that may be used to diagnose and follow up on patients with this spectrum of diseases. However, the clinical relevance of such variables needs to be explored thoroughly. The aim of the present review is to summarize the current evidence supporting the use of AS-OCT for the diagnosis and follow-up of several iridocorneal-angle and anterior-chamber alterations, focusing on the advantages and downsides of this technology.

Keywords: AS-OCT; glaucoma; iridocorneal angle; plateau-iris configuration; primary-angle closure.

Figure 1

Tomey Casia 2 AS-OCT scan (Nagoya, Japan) of a patient with a narrow angle. (A) The white, bold vertical line from the corneal endothelium to the anterior surface of the lens corresponds to the anterior chamber depth (ACD), measuring 2.218 mm in the case reported, as shown in the bottom left table (green highlighted cell). (B) The white, bold vertical line from the anterior surface of the lens to the horizontal scleral-spur-to-scleral-spur line corresponds to the lens vault (LV), measuring 0.567 mm in the same patient.

Figure 2

Tomey Casia 2 AS-OCT scans (Nagoya, Japan). (A) The white, bold horizontal line corresponding to the scleral-spur-to-scleral-spur line shows the anterior chamber width (ACW) and measures 11.322 mm in this patient with narrow angles. (B) The green area, delimited by the corneal endothelium anteriorly, and the anterior surface of the iris and lens posteriorly, corresponds to the anterior chamber area (ACA) and measures 15.35 mm² in the same patient with narrow angles. (C) The ACA from a patient with open angles.

Figure 3

(A) Top panel. Schematic diagram of the angle-recess area at 500 microns from the scleral spur (ARA500, black area). Bottom panel. Tomey Casia 2 AS-OCT scan (Nagoya, Japan) of the angle showing in vivo ARA500 (red area). ARA is defined as the triangular area bordered by the anterior iris surface, corneal endothelium, and a line perpendicular to the endothelium drawn from a point anterior to the scleral spur to the iris surface. (B) Top panel. Schematic diagram of the trabecular iris-space area at 500 microns from the scleral spur (TISA500, black area, top left panel) and the angle-opening distance at 500 and 750 microns from the scleral spur (AOD500 and AOD750, respectively, top right panel). Bottom panel. MS-39 AS-OCT scan (CSO, Florence, Italy) of the iridocorneal angle showing in vivo TISA and AOD. AOD is the perpendicular distance from the iris to the trabecular meshwork, which can be measured at different distances anterior to the scleral spur, such as 250, 500, 750 microns (AOD250, AOD500, AOD750, respectively). TISA, which again can be evaluated at several distances from the scleral spur (SS), is defined as the trapezoidal area with the following boundaries: anteriorly, the AOD; posteriorly, a line perpendicular to the plane of the inner corneoscleral wall drawn from the scleral spur to the opposing iris; superiorly, the inner corneoscleral wall; and inferiorly, the anterior iris surface.

Figure 4

Tomey Casia 2 AS-OCT scans (Nagoya, Japan) showing iris thickness (A), area (B), and curvature (C) from a patient with narrow angles. (A) The iris thickness (IT) is the thickness of the iris at different distances from the SS. In the example, IT measured at 750 microns (IT750) from the scleral spur (white, bold vertical line). (B) The iris area is defined as the entire cross-sectional area of the iris, from the SS to the pupil. (C) The iris curvature is the perpendicular distance from the iris-pigmented epithelium at the point of greatest convexity to a line between the most peripheral and the most central points of the iris-pigmented epithelium (white, bold vertical line).

Figure 5

Left panel. MS-39 AS-OCT (CSO, Florence, Italy) from a patient with plateau-iris configuration (PIC). Right panel. Ultrasound biomicroscopy (UBM) of the same eye. While the AS-OCT scan provides details of the iris surface, iris root, and angle structures, its penetration depth does not make it possible to visualize the ciliary body. Conversely, UBM is characterized by poorer definition of the iris and angle structures, but it also allows the evaluation of the ciliary-body position. This is particularly important in PIC, as it is a condition associated with anterior ciliary-body displacement, which displaces the peripheral iris forward, eventually causing appositional angle closure.