The main abnormality leading to outflow impairment in angle-closure glaucoma is mechanical obstruction of the trabecular meshwork by the peripheral iris. In most cases, this angle-closure process precedes a rise in intraocular pressure (IOP) and glaucomatous optic neuropathy. Previous studies suggest that if effective prophylactic treatment such as laser peripheral iridotomy is performed at an early and appropriate time for eyes with anatomically narrow angles, glaucomatous optic neuropathy can be prevented with no further treatment. Thus, screening for angle closure deserves consideration, ideally for the early phases of the disease such as eyes with narrow angles (a stage termed primary angle-closure suspects), before peripheral anterior synechiae (PAS) formation and/or elevation of IOP occurs.

In 2005, Johnson and Foster reviewed previous studies that evaluated the burden of blindness attributable to primary angle-closure glaucoma (PACG) in the Inuit population in Greenland. In the 1960s and early 1970s, approximately two-thirds (64%) of all blindness in this population was attributable to glaucoma, 88% attributable to PACG. In the face of this problem, screening for subjects at risk for angle closure was performed by evaluating the central and peripheral anterior chamber depth (ACD). Those considered to be at risk then underwent gonioscopy, and those found to have occludable angles received surgical or laser iridectomies. The rates of blindness attributable to glaucoma subsequently dropped from 64% in 1962 to 9% in 2003. This reduction in glaucoma blindness may have occurred because of a combination of different factors during this period, and there are limited data on PACG incidence in subjects with narrow angles. Nevertheless, this inspiring action showed us that if resources are available, screening for angle closure is a feasible task that can reduce the burden of blindness caused by this disease, at least in certain populations.

Theoretically, the ideal population-based screening test should be clinician-independent, quick, and noninvasive, and have a high specificity. Gonioscopy is the current reference-standard examination to evaluate the anterior chamber angle, and it is the only technique that can differentiate appositional from synechial angle closure. This represents an important advantage, as within the angle-closure process continuum, PAS formation represents structural sequelae of an ongoing pathologic process, and its presence would indicate the need for treatment. However, gonioscopy has some drawbacks: it involves contact with the eye, it requires highly trained examiners and the use of a slit lamp, and its results may be affected by light and/or inadvertent indentation. Thus, gonioscopy may be considered unsuitable for mass screening for angle closure.

A variety of techniques and devices with varying levels of sophistication have been developed in order to evaluate risk for angle closure. In population-based studies performed in Mongolia and Singapore, optical central ACD measurements yielded an area under the ROC curve (AUC) of 0.93 and 0.86 respectively to detect eyes with narrow angles identified by gonioscopy. These results were similar to the ones obtained with the peripheral ACD measurements with the van Herick technique in these populations (AUC of 0.93 and 0.86, respectively). Recently, a community-based study in Singapore evaluated the performance of automated noncontact devices in screening for angle closure in more than 2000 subjects over 50 years of age. Qualitative evaluation of angle images by anterior-segment optical coherence tomography (AS-OCT), quantitative evaluation of the central ACD measured by IOLMaster, and peripheral ACD automatically assessed by the Scanning Peripheral Anterior Chamber Depth Analyzer (SPAC) performed reasonably well (AUC <0.83) in detecting eyes with narrow angles; however, all 3 devices showed low specificity values (less than 77.7%), which would limit the usefulness of these devices in screening for angle closure in the general population. Within the same study population, quantitative measurements of anterior segment parameters obtained with anterior-segment OCT showed slightly better performance. Parameters such as anterior chamber area and volume yielded an AUC of 0.88, while angle parameters like angle opening distance and trabecular-iris space area had an AUC of 0.91 and 0.88, respectively. However, one important limitation of quantitative analysis of angle parameters at this time is the poor definition of the scleral spur, which limits the analysis in approximately 25% of the time-domain AS-OCT images. Another limitation of all the techniques mentioned above is the inability to detect PAS. As technologies evolve, the diagnostic performance of different techniques/instruments may soon reach acceptable specificity and sensitivity levels for mass screening for angle closure.

While there may be appropriate technologies to screen for narrow angles in the future, the main limitation at this time is our inability to identify those who actually will develop PACG. Information about the natural history of PACG is limited at the current time. The previous studies mentioned before were cross-sectional, and adopted gonioscopy as the reference standard to determine the presence of narrow angles (defined as eyes in which the posterior trabecular meshwork is not seen in more than 2 or 3 quadrants). However, longitudinal studies have shown us that only 20% of the eyes considered to have narrow angles on gonioscopy actually develop PAS and/or elevated IOP over the relatively short term of 3 to 5 years. Thus, while we can identify subjects with narrow angles who are at risk for PACG, we are still unable to identify those who will indeed develop the disease. Prospective randomized clinical trials evaluating eyes with narrow angles are ongoing in China and Singapore. In these studies, primary angle-closure suspect (PACS) subjects will have 1 eye submitted to laser peripheral iridotomy while the fellow eye will be left untreated. These clinical trials will reveal the efficacy of laser iridotomy in preventing the development of angle-closure glaucoma, demonstrate the rates of conversion from narrow angles to primary angle closure (PAC) and PACG in untreated eyes, and more importantly, identify risk factors for the development of PACG.

In conclusion, screening for angle-closure disease represents a challenging task. There is limited information for us to identify who is at risk for developing primary angle closure and subsequent glaucomatous optic neuropathy. However, as demonstrated in the Inuit population, screening for narrow angles may be warranted in some high-risk populations to reduce the burden of blindness from PACG. Targeted screening in communities with high prevalence of angle closure may be particularly cost effective. Studies are ongoing to better identify risk factors related to the angle-closure process, and it is likely that ideal screening methods will have to be adapted according to the different regions of the world, the mechanism(s) of angle closure, and the availability of resources.