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Mary Ben L Apatoff, Joel S Schuman, Mengling Liu, Gadi Wollstein, Hiroshi Ishikawa, Mengfei Wu, Ronald Zambrano, Ezekiel Ede, Priya Achanta, Larry Kagemann; Variability in Schlemm Canal Anatomical Measurements. Invest. Ophthalmol. Vis. Sci. 2021;62(8):472.
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© ARVO (1962-2015); The Authors (2016-present)
Schlemm canal (SC) is characterized by high local variations in morphology. Previously, we reported characteristics of SC using SC area measurements by optical coherence tomography (OCT) in healthy eyes. Herein, we examine the interobserver variability of SC height, width, and area in glaucomatous and healthy eyes.
The anterior segment of six eyes from three subjects (1 female, 2 male) were imaged using OCT (Cirrus HD-OCT, Zeiss, Dublin, California, USA). A 4x4mm volumetric image of the limbus (depth of 2mm) was acquired with the Anterior Segment Cube scan protocol, comprised of 128 horizontal B-scans composed of 512 A-scans. SC was positioned to the side of the image to maximize visualization of aqueous humor vessel crossings. Scans were processed to maximize visualization of SC; image volumes were averaged (3x3x3 kernel) and contrast was enhanced with the local histogram algorithm using Fiji (version 2.10/1.53c). A cross-sectional B-scan and the two B-scans +/- 5 frames were identified as three reference frames, based on best visualized SC location (Fig. 1). Three independent observers performed manual segmentation to measure SC width, height, and cross-sectional area for these three reference frames per volume. Width was defined as the longest measure of SC and height as perpendicular to the line used for width measurement. The observers performed these measurements on 15 volumes for a total of 45 analyzed frames each. The coefficient of variation was calculated based on standard deviations estimated using hierarchical multi-level random-effects models. Interobserver variability was quantified with a two-way ANOVA to calculate the intraclass correlation coefficient (ICC).
Participants had a mean age of 72.0 ± 7.47 years (range: 66 to 82) and consisted of one healthy subject and two with primary open angle glaucoma. Measurement means and variation are presented in Table 1. The ICCs for interobserver variability are excellent for width measurements and low to moderate for height and area (Table 2).
Excellent ICC for interobserver variability of SC width suggests it is suitable for use in clinical trials.
This is a 2021 ARVO Annual Meeting abstract.
Figure 1. Schlemm canal measurements in B-scans post image processing
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