Assessment of angle width using anterior segment optical coherence tomography

Primary angle closure glaucoma (PACG) accounts for nearly half of glaucoma related blindness in the world with majority occurring in Asian populations (1,2). A range of phenotypes are recognized within the spectrum of primary angle closure disease (PACD), namely primary angle closure suspects (PACS, occludable angles with normal intraocular pressure and optic disc), primary angle closure (PAC, occludable angles with features indicative of trabecular outflow obstruction or raised intraocular pressure but absence of glaucomatous optic neuropathy) and finally PACG (presence of glaucomatous optic neuropathy) (3). Angle assessment plays an important role in the diagnosis and management of PACD. Traditionally the diagnosis is established using gonioscopy. Though to date gonioscopy remains the gold standard technique for angle assessment, it is not without limitations. Gonioscopy is a highly subjective procedure and interpretation can vary based on the skill and experience of the examiner performing this procedure leading to intra and inter-observer discrepancies. Additionally, the results and accuracy of gonioscopic examination can vary drastically as a result of changes in illumination which can falsely open the anterior chamber angle (ACA) as well as patient cooperation and direction of gaze. With the advances in technology it is now possible to perform objective, reproducible, high resolution imaging of the ACA using devices such as high frequency ultrasound biomicroscopy (UBM), Scheimpflug imaging system, and anterior segment optical coherence tomography (ASOCT). These imaging techniques help us to understand the pathophysiology of angle closure in a better way. Compared to UBM, AS-OCT achieves higher resolution and does not require contact with the ocular surface. AS-OCT is a noncontact, rapid imaging device that uses low-coherence interferometry to obtain cross-sectional images of the anterior segment (4). The measurements obtained from ASOCT are semi-automated, reproducible (5,6) and unlike gonioscopy these measurements are operator independent. The swept source OCTs (SS-OCT), such as the Tomey CASIA SS-1000 (Tomey corporation, Nagoya, Japan), utilize a laser wavelength of 1,310 nm and employ scan speed of 30,000 A-scans/s with an axial resolution of 10 μm. SS-OCTs have recently become commercially available and are able to capture extremely high-resolution images. All automated biometric analyses of the AC angle using AS-OCT require a reference landmark from which the angle measurements are derived. Typically, the scleral spur is used as a reference point for parameters such as the iris area and volume, angle opening distance (AOD), angle recess area, scleral thickness, trabecular meshwork-ciliary process distance, trabecular iris angle (TIA), and trabecular iris space area (TISA). The other possible biometric measurements include iris thickness (IT), iris curvature (IC), anterior chamber depth (ACD), anterior chamber width (ACW), and lens vault (LV). As the location of the SS must be manually determined, this introduces some level of intra and inter-observer variability to these imaging techniques. Additionally, the location of the SS is not always apparent, especially when image quality is poor, resulting in Editorial Commentary