Effect of Light Intensity on the Geometric Iridocorneal Angle and Ciliary Cleft Parameters Using Ultrasound Biomicroscopy in Normal Purpose-Bred Beagle Dogs.

OBJECTIVE: To evaluate the effect of varying light intensity on geometric iridocorneal angle (ICA) and ciliary cleft (CC) parameters using ultrasound biomicroscopy (UBM) in normal canines. ANIMALS STUDIED: Twenty clinically normal adult purpose-bred Beagles (40 eyes). PROCEDURES: All dogs underwent UBM of the CC at two locations (dorsal and temporal) in each eye under dark ( 1500 lx) light conditions. The following parameters were measured: geometric ICA, width of CC entrance, width of mid-CC, and length of CC. Digital color and infrared photographs were obtained to allow for pupil diameter measurement under each light condition. RESULTS: Increasing light intensity resulted in significant decrease in the geometric ICA (dark: 47.27° ± 11.09°; dim: 40.48° ± 9.71°; bright: 26.72° ± 7.63°; p = 0.0001) with marginal effect on the width of CC entrance (dark: 0.25 ± 0.04 mm; dim: 0.27 ± 0.06 mm; bright: 0.28 ± 0.08 mm; p = 0.0548), and no effect on width of mid-CC, or length of CC. Regardless of light intensity or laterality of eye, geometric ICA and width of CC entrance were significantly greater dorsally (ICA: 39.82° ± 12.67° dorsal vs. 36.5° ± 12.8° temporal; CC entrance: 0.28 ± 0.07 mm dorsal vs. 0.26 ± 0.06 mm temporal; p = 0.0003 respectively) and CC length was significantly greater temporally (2.36 ± 0.28 mm temporal vs. 2.2 ± 0.27 mm dorsal; p = 0.0028). CONCLUSIONS: Aside from the geometric ICA, varying light intensity does not significantly impact CC measurements. ICA and CC measurements varied by quadrant. The authors recommend performing UBM of the CC in dim light conditions as this is most applicable to clinical settings and subjectively provided the greatest image quality.