Effects of light variation on Schwalbe's line-based anterior chamber angle metrics measured with cirrus spectral domain optical coherence tomography

Evaluate the impact of variations in ambient lighting conditions on the reproducibility/reliability of Schwalbe's Line (SL)-based anterior chamber angle (ACA) metrics using spectral domain optical coherence tomography (SD-OCT).

Images were taken at Doheny Eye Centers-UCLA clinic, which were randomized, masked and graded twice by dual reading centre graders.

Twenty-five normal/healthy participants with open angles

Inferior angles were imaged using Cirrus SD-OCT under five light levels (foot-candles (fc) measured at camera-eye interface with Sper light-meter) with the instrument's corneal illumination function set to default level of 50 (CIdef) and low 5 (CIlow). Each eye was imaged 20 times, totaling 500 assessments.

SL-angle-opening-distance (SL-AOD) and SL-trabecular-iris-space-area (SL-TISA) measured using custom ImageJ software. Intra-/inter-grader variability analyses were completed using Statistical-Package-for-Social-Science and Bland-Altman plots demonstrated limits of agreement for comparisons.

Light level demonstrated a linear relationship with angle size, thus differences from highest to lowest light levels were compared. Decreasing light from 1.0 → 0.0fc at CIdef decreased SL-AOD from 394 µm +/- 137 µm → 356 µm +/- 137 µm (mean percent difference (MPD) = 10.71%,P < 0.001) and SL-TISA from 297 µm(2)  +/- 114 µm(2)  → 261 µm(2)  +/- 109 µm(2) (MPD = 13.7%, P < 0.001). Decreasing from 1.0 → 0.0fc at CIlow decreased SL-AOD from 366 µm +/- 136 µm → 329 µm +/- 122 µm (MPD = 10.9%, P < 0.001) and SL-TISA from 271 µm(2)  +/- 113 µm(2)  → 234 µm(2)  +/- 98 µm(2) (MPD = 15.8%, P < 0.001). There was 7.9/11.4% (both P < 0.001) difference for SL-AOD/SL-TISA between CIdef → CIlow at 1.0fc, and 7.7/9.4% (both P < 0.001) difference at 0.0fc. Intra-/inter-grader results showed high reproducibility for all metrics (MPD = 0.33-4.4%; CV = 0.96-1.36; PCC = 0.93-0.95(P < 0.001); R2 = 0.94-0.98). Bland-Altman plots did not demonstrate bias, with repeat-ability and agreement among measurements.

Using Cirrus SD-OCT, we found that SL-based ACA morphometrics are exquisitely sensitive to changes in ambient illumination and also corneal illumination by the OCT instrument. Consistently imaging in the darkest room possible is recommended (≤0.2 fce).