Diagnostic tools for the detection of primary angle closure include old stalwarts such as gonioscopy and slit-lamp estimation of limbal anterior chamber depth1,2 as well as new technologies that yield high-resolution, cross-sectional images of the angle. In addition to their diagnostic potential, anterior segment optical coherence tomography (AS-OCT) and ultrasound biomicroscopy (UBM) have provided insight into newly identified mechanisms of primary angle closure that go beyond the effect of relative pupillary block. These etiologic insights may enhance diagnostic accuracy, prognostication, and treatment selection. Other imaging modalities, including Scheimpflug photography1 and the scanning peripheral anterior chamber depth analyzer,3 are currently under investigation but do not provide direct images of the angle recess.
GONIOSCOPY, AS-OCT, AND UBM
Gonioscopy is the most comprehensive diagnostic tool for angle closure. It can assess the angle/iris relationship, distinguish between appositional and synechial closure, and provide visualization of the double-hump sign of plateau iris and the vessels of neovascular angle closure. In uncertain diagnoses, gonioscopy can provide visual clues to the presence of pigment dispersion, pseudoexfoliation, and elevated episcleral venous pressure. It is unlikely that novel imaging technology will replace gonioscopy for many of these essential functions.
Gonioscopy, however, is subject to light-induced miosis and inadvertent corneal compression, artifacts that are capable of unintentionally opening the angle. In addition, because gonioscopy can be difficult to master and interpret, its results may be diagnostically uncertain. Gonioscopy is therefore often neglected, potentially resulting in misdiagnosis. For these reasons, among others, alternative imaging technologies are attractive.
The advantages of AS-OCT include that it is noncontact and does not require a highly skilled technician. Upon the identification of the scleral spur with AS-OCT (or UBM) (Figure 1), various iridocorneal angle parameters can be measured (Figure 2). Potential benefits include efficient population screening in order to identify individuals at risk for angle closure, although this capability remains unproven. AS-OCT cannot penetrate posterior to the iris.
UBM, in contrast, penetrates deep enough to image the ciliary body (Figures 1, 3, and 4). The main drawbacks of UBM are that it must be performed by a skilled technician and that direct contact may alter the angle's width. The benefits of UBM include its utility in identifying nonpupillary block mechanisms for angle closure—mechanisms that may contribute to angle closure in the majority of Asian patients.4
Both UBM and AS-OCT can identify narrow and closed angles with reasonable performance.5 Neither, however, can reliably differentiate between appositional and synechial closure, an essential distinction prior to surgical intervention. As discussed later, the lack of a validated classification system containing a clear case definition for occludable angles prevents predicting which narrow angles are at significant risk of closure. Finally, the identification of the scleral spur can be ambiguous for both UBM and AS-OCT. Without the localization of this landmark, diagnostic uncertainty may remain.6
NONPUPILLARY BLOCK MECHANISMS FOR ANGLE CLOSURE
Nonpupillary block mechanisms include plateau iris, traditionally described as a steep peripheral iris prone to closure and refractory to laser peripheral iridotomy. Plateau iris appears to occur secondary to anterior rotation of the ciliary processes—a UBM finding—which results in buttressing of the peripheral iris against the trabecular meshwork (compare Figure 3 to Figure 4). Another UBM finding is the loss of the ciliary sulcus. Other anatomical variants that appear to predispose eyes to nonpupillary block mechanisms include prominent peripheral iris roll and variability in iris insertion.1,4
Another potential nonpupillary block mechanism is uveal effusion, a UBM finding that has been associated with 25% of acute primary angle closure cases in Asian eyes (Figure 5).7 An intriguing hypothesis describes the potential contribution of choroidal expansion and resultant posterior pressure to the etiology of angle closure in some cases.8,9
PREDICTIVE MODELS
A major goal of research on ocular biometry with respect to primary angle closure is the development of a predictive model that forecasts the likelihood of developing angle closure, thus providing guidance as to when to intervene surgically. Biometric variables that are potentially correlated with the risk for angle closure include anterior chamber depth, axial length, and the lens' thickness as well as UBM- and OCT-measured angle parameters and the nonpupillary block factors discussed earlier. Future predictive models may also include biometric variables with demographic risk factors (eg, age, race, gender).
Despite extensive investigation into such biometric parameters, the predictive capability of current models is limited.10 Only approximately 10% to 25% of untreated primary angle-closure suspects will develop angle closure.9,10 Without reliable prognostic indicators from clinical examination or biometry, it is uncertain which primary angle-closure suspects require treatment. Two large clinical trials of these suspects—one in Guangzhou, China,11 and one in Singapore12—should provide useful information regarding prognostic risk factors, the natural history of untreated eyes, and the efficacy of treatment. These studies have a similar design in which one eye is randomized to laser peripheral iridotomy and the other is followed without treatment.
CLASSIFICATION SYSTEMS
The major differences between several available classification systems for primary angle closure (Tables 1 and 2) are whether (1) a suspect is defined by the degree of the angle's narrowness or by appositional closure, (2) symptoms are considered, and (3) the underlying mechanism(s) for angle closure are considered.
The AAO's Preferred Practice Pattern for Angle Closure advocates a diagnostic scheme that defines primary angle closure suspects as having narrow but not closed angles.13 The Academy's position is that "laser iridotomy may be considered for patients with narrow angles who require repeated pupil dilation for treatment of other eye disorders." In contrast, a widely used research definition developed by Foster and colleagues14 defines primary angle-closure suspects as having at least three quadrants of apposition of the iris to the trabecular meshwork but without peripheral anterior synechiae, elevated IOP, or the other factors shown in Table 1. The rationale behind this system is that patients with up to three quadrants of apposition can probably be observed without treatment. Again, the results from the two major clinical trials described earlier should provide clarification.
It is no longer satisfactory to use the older, primarily symptom-based clinical classification system consisting of acute, subacute/intermittent, and chronic angle-closure categories. This scheme has little or no prognostic value, does not guide management, and does not distinguish between elevated IOP and glaucomatous optic neuropathy15 (Table 1). A more comprehensive approach advocates using a stage-based system in combination with a mechanism-based system4 (Table 2).
STATIC VERSUS DYNAMIC BIOMETRIC PARAMETERS
Much of the research to date on diagnostic tools for primary angle closure has focused on static biometric parameters. Researchers are beginning to recognize that many important parameters are dynamic and subject to physiologic fluctuation, including pupillary dilation and its recently described relationship to iris volume. Specifically, normal eyes have peripheral irides that thin with dilation, but AS-OCT analysis has demonstrated that the iris thins to a lesser degree with dilation in eyes that have primary angle closure.9,16 An additional, potential, dynamic physiological factor is choroidal swelling.8,9 Future predictive models may include dynamic variables, which may have a greater impact on forecasting the risk for angle closure than the static variables studied to date.
Jason A. Goldsmith, MD, MS, is an assistant professor with the John Moran Eye Center, Department of Ophthalmology and Visual Sciences, University of Utah, Salt Lake City. Dr. Goldsmith may be reached at (801) 587-3760; jason.goldsmith@utah.edu
