Novel Approach for Anterior Chamber Angle Analysis

Abstract

Primary angle-closure glaucoma is a leading cause of blindness in East Asian populations, and for this reason, examining the anterior chamber angle is an indispensable component in the ophthalmic assessment of any individual who is diagnosed with or suspected of having glaucoma.¹ Although gonioscopy allows a direct visualization of the anterior chamber angle under slitlamp, the examination is essentially subjective and qualitative. The availability of high-frequency ultrasound biomicroscopy (UBM) makes possible high resolution, cross-sectional, in vivo anterior chamber angle imaging. Quantitative assessment of the angle width with UBM was originally proposed by Pavlin et al.² The angle of opening, called the trabecular-iris angle (TIA), was defined as an angle measured with the apex in the iris recess and the arms of the angle passing through a point on the trabecular meshwork 500 μm from the scleral spur and the point on the iris perpendicularly (Figure 1A). Because of the variability of iris curvature and the inner scleral-corneal surface, TIA measurements were considered imprecise and less reproducible.² Instead of measuring the anterior chamber angle in degree, Pavlin et al² measured the opening of the angle as the perpendicular distance between a fixed point anterior to the scleral spur and the opposing iris (Figure 1B). It was defined as the angle opening distance (AOD) and became the gold standard parameter in subsequent studies involving measurements of the anterior chamber angle.^3-5