The changing geometry of angle closure

Abstract

Although significant advances have been made in recent years in understanding the epidemiology of primary angle closure and primary angle-closure glaucoma, there is continued debate on the pathogenesis and optimal medical management of this condition. The way forward will be to embrace advances in anterior segment imaging, as this technology appears to hold promise to change our current management algorithms. Our present classification of this disease depends on clinical gonioscopic evaluation of the drainage angle, a skill that can be highly subjective and show significant interobserver variation.1 These limitations have fuelled the development of imaging devices that can achieve both objective and reproducible quantification of anterior segment dimensions resulting in an ability to accurately grade angle closure. In this issue, Friedman and colleagues provide a refreshing overview on both the current limitations we have in the diagnosis and management of primary angle-closure glaucoma and discuss new technologies and clinical trials that are on the horizon.2 Although it is not disputed that ocular anterior segment anatomy predisposes to acute angle closure, the factors that predispose to chronic angle-closure glaucoma are less well defined.3 The authors highlight the shortfall in understanding of the circumstances that determine which individuals with angle closure will ultimately develop glaucoma. Current practice is to offer prophylactic laser iridotomy to most angle-closure suspects in the hope that a change in anterior segment configuration will reduce the risk of subsequent acute angle closure and chronic angle-closure glaucoma. This approach is neither cost-effective nor evidence-based, when we know that 85–90% of these subjects, if untreated, will not develop acute angle closure in their lifetime.4 Similarly, most eyes with an ‘occludable’ angle will not develop chronic angle-closure glaucoma. Randomized, controlled trials are already guiding the surgical management of angle-closure glaucoma, and there is a strong focus to make similar headway with targeted preventive management.5,6 Two large, randomized, controlled trials that aim to further define best practice for primary angle closure are the Zhongshan Angle-Closure Prevention (ZAP) and the Effectiveness in Angle-Closure Glaucoma of Lens Extraction (EAGLE) studies.7,8 The ZAP trial aims to determine whether prophylactic laser iridotomy in high-risk angle-closure suspects influences the subsequent development of angle-closure glaucoma. The EAGLE study is an international multicentre, randomized study comparing lens extraction versus laser iridotomy in patients presenting with angle closure and angle-closure glaucoma. The fundamental anatomical contribution of the morphology and intraocular position of the natural lens to the aetiology of angle closure certainly supports a role for lens extraction in disease management, with the premise that phacoemulsification surgery would effectively eliminate the lenticular basis of angle closure and angle-closure glaucoma. The EAGLE study is therefore intended to provide clear guidance on the clinical value of each intervention in primary management as well as a measure of patient-reported outcomes. Interestingly, a trend of falling incidence of primary angle-closure glaucoma has been postulated to relate to the increasing rate of cataract extraction surgery.9 It should be borne in mind that many of the large angle-closure trials are being conducted in populations that are racially different to Western white populations, and therefore, the results of these studies may not be universally applicable. In the future, accurate diagnosis is likely to become more reliant on technology. An exciting anterior segment imaging modality is swept source ocular coherence tomography (SSOCT). A form of Fourier domain ocular coherence tomography (OCT), SSOCT allows simultaneous three-dimensional evaluation of the entire cross-section of the anterior chamber for the entire circumference of the angle.10 Along with ultrasound biomicroscopy, anterior segment OCT and spectral domain OCT, modern imaging will help to uncover the underlying anatomical or physiological factors that relate to the variation in angle-closure risk in different ethnic groups and also between individuals within a population. These factors may also help to explain why the reported incidence of malignant glaucoma among angle-closure patients is lower in Southeast Asia compared with Western populations.11 Precise imaging will enable more detailed evaluation of iris dynamics to determine if volumetric changes with pupil dilation are an important predictor of risk for angle closure.12 Eventually, novel classification schemes may be derived from objectively measured parameters that form part of risk-prediction models to identify the individuals most likely to benefit from interventional therapy, continued monitoring by glaucoma specialists, and those who require neither monitoring nor treatment. Chronic angle-closure glaucoma remains a major cause of global blindness and is more likely to cause visual loss than its open-angle counterpart. It is apparent that current diagnostic criteria of angle closure based on static gonioscopy are not adequate and are likely to be augmented in the near future with the use of modern imaging modalities. Akin to cardiac echocardiography now being mandatory to supplement the art of cardiac auscultation, we should embrace the use of imaging devices in glaucoma practice and encourage their introduction to mainstream clinical practice. Objective and repeatable assessments of angle anatomy can be used to elucidate those factors that are important in the subsequent development of angle-closure glaucoma. Pinpointing and targeting therapy towards the one individual out of 10 at real risk of this blinding disease should be cost-effective, accurate and avoid unnecessary intervention along with its potential morbidity. With burgeoning pressure on limited health-care resources, the ability to personalize the management of angle closure based on an objective evaluation of the anatomical structure of the anterior segment within a risk-prediction model is a highly desirable goal.