title: Effects of light variation on Schwalbe's line-based anterior chamber angle metrics measured with cirrus spectral domain optical coherence tomography.
abstract: BACKGROUND
  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).
  
  
  METHODS
  Images were taken at Doheny Eye Centers-UCLA clinic, which were randomized, masked and graded twice by dual reading centre graders.
  
  
  METHODS
  Twenty-five normal/healthy participants with open angles
  
  
  METHODS
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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).
authors.affiliation: Doheny Image Reading Center, Doheny Eye Institute, Los Angeles, California, USA.
authors.forename: Kenneth M
authors.initials: KM
authors.lastname: Marion
authors.affiliation: Doheny Image Reading Center, Doheny Eye Institute, Los Angeles, California, USA.
authors.forename: Moritz
authors.initials: M
authors.lastname: Niemeyer
authors.affiliation: Doheny Image Reading Center, Doheny Eye Institute, Los Angeles, California, USA.
authors.forename: Brian
authors.initials: B
authors.lastname: Francis
authors.affiliation: Doheny Image Reading Center, Doheny Eye Institute, Los Angeles, California, USA.
authors.forename: Srinivas R
authors.initials: SR
authors.lastname: Sadda
authors.affiliation: Doheny Image Reading Center, Doheny Eye Institute, Los Angeles, California, USA.
authors.forename: Vikas
authors.initials: V
authors.lastname: Chopra
mesh_terms: D059925:Anatomic Landmarks; D000867:Anterior Chamber / Q000000981:diagnostic imaging*; D003315:Cornea / Q000033:anatomy & histology*; D004311:Double-Blind Method; D064368:Healthy Volunteers; D006801:Humans; D007498:Iris / Q000033:anatomy & histology*; D008029:Lighting; D015588:Observer Variation; D015203:Reproducibility of Results; D041623:Tomography, Optical Coherence
keywords: Schwalbe's line; angle closure; anterior chamber angle; anterior-segment OCT