Reproducibility of scleral spur identification and angle measurements using fourier domain anterior segment optical coherence tomography

Abstract

Purpose. To evaluate intraobserver and interobserver agreement in locating the scleral spur landmark (SSL) and anterior chamber angle measurements obtained using Fourier Domain Anterior Segment Optical Coherence Tomography (ASOCT) images. Methods. Two independent, masked observers (SR and AZC) identified SSLs on ASOCT images from 31 eyes with open and nonopen angles. A third independent reader, NPB, adjudicated SSL placement if identifications differed by more than 80 μm. Nine months later, SR reidentified SSLs. Intraobserver and interobserver agreement in SSL placement, trabecular-iris space area (TISA750), and angle opening distance (AOD750) were calculated. Results. In 84% of quadrants, SR's SSL placements during 2 sessions were within 80 μm in both the X- and Y-axes, and in 77% of quadrants, SR and AZC were within 80 μm in both axes. In adjudicated images, 90% of all quadrants were within 80 μm, 88% in nonopen-angle eyes, and 92% in open-angle eyes. The intraobserver and interobserver correlation coefficients (with and without adjudication) were above 0.9 for TISA750 and AOD750 for all quadrants. Conclusions. Reproducible identification of the SSL from images obtained with FD-ASOCT is possible. The ability to identify the SSL allows reproducible measurement of the anterior chamber angle using TISA750 and AOD750.

Figure 1

Anterior chamber angle measurements and scleral spur landmark (SSL). Illustrations are angle opening distance (AOD) and trabecular-iris space area (TISA) for an (a) open angle and (b) nonopen angle. AOD is defined as the length of a line drawn perpendicular to the cornea anterior to the SSL and ending on the anterior surface of the iris. TISA is the area bordered centrally by the AOD line, anteriorly by the posterior corneoscleral-aqueous interface, and posteriorly by the anterior surface of the iris. The peripheral border is a line segment starting at the SSL perpendicular to the corneoscleral-aqueous interface and ending on the anterior surface of the iris.

Figure 2

Effect of scleral spur landmark (SSL) location on TISA750 and AOD750. Illustrations show the impact of SSL location on trabecular-iris space area at 750 μm (TISA750) and angle opening distance at 750 μm (AOD750) calculations for nonopen-angle eyes (a) and open-angle eyes (b). If the SSL location is moved by h to SSL′ location, TISA750, now based on SSL′, gains the grey area and red area and loses the green area. Both the green area and red area are small relative to the grey area, especially in nonopen-angle eyes (a). In addition, the difference between the green and red areas are negligible (gain red and lose green). Thus, when SSL is moved to SSL′, TISA750 is changed by h × AOD750 based on SSL (longer blue line). The SSL location's impact on AOD750 is dependent on angle width and iris shape (the length of the anterior border of red area). The AOD750 changes are smaller if the angle is nonopen and the iris is straightener.

Figure 3

Bland-Altman agreement plots for adjudicated pairs for (a) SSL location in X-axis, (b) SSL location in Y-axis, (c) TISA750, and (d) AOD750.