The American Academy of Ophthalmology Preferred Practice Patterns and the American Optometric Association Clinical Practice Guidelines agree for both primary open-angle glaucoma and PACG that gonioscopy is an essential part of the evaluation of glaucoma patients, yet recent studies suggest that gonioscopy is likely underperformed in these patients.7
Gonioscopy was performed in fewer than 50 percent of patients in the five years preceding glaucoma surgery.7,13
In an open angle, the most posterior structure visible is the ciliary body and appears as a grayish-brown structure next to the iris root. Moving anteriorly, the next most posterior structure is the scleral spur. This structure is often white in appearance but can also appear light gray in some individuals. The next structure is the trabecular meshwork, which can be subdivided into anterior and posterior. The posterior portion of the trabecular meshwork filters aqueous into Schlemm’s canal. The amount of pigment visible in the trabecular meshwork can increase over time, darkening its initial light gray color. The most anterior structure in the angle is Schwalbe’s line, which may also be light gray with pigment.
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Various classification systems exist to describe the angle appearance; however, they are not uniform in their designations and may cause confusion if not accurately interpreted. Some require estimation or measurement of the degrees of angle opening, with an angle measuring 10 to 20 degrees defined as narrow.4 The most common classification is to identify the most posterior structure seen for all angles, noting any iris abnormalities (synechiae, approach).
Types of angle closure
The most common form of primary angle closure is thought to arise from pupillary block11 in which aqueous forces the pupil forward. In this scenario, apposition of the lens and the posterior iris at the pupil leads to blockage of the aqueous flow from the posterior chamber to the anterior chamber. As the pressure in the posterior chamber increases, the peripheral and midperipheral iris is pushed forward, blocking trabecular meshwork drainage.14
Another mechanism of primary angle closure is plateau iris in which the iris root is displaced anteriorly, creating a fold in the angle and displacing the peripheral iris into the trabecular meshwork.
Secondary angle closure may arise from pathological enlargement and anterior displacement of the lens, pushing the iris forward to narrow the angles in phacomorphic glaucoma.14,15 Other etiologies include neovascularization, membrane obstruction of the angle, and developmental abnormalities.14
Another contribution to recognize in the development of narrow or closed angles is the role of prescription and over-the-counter medications. Many widely prescribed and readily available antidepressants and antihistamines can increase the risk of ACG by inducing iris dilation or anteriorly displace the lens-iris diaphragm.16
One medication to note is topiramate (Topamax, Janssen), used in the treatment and management of epilepsy and migraines and more recently for weight loss, along with many off-label uses. Topiramate can cause swelling of the ciliary body and lens, as well as uveitis, myopia, and bilateral angle closure.14,16,17 The management of topiramate-induced angle closure consists of immediate discontinuation of the medication along with medical IOP reduction.16,17
Although it is difficult to be precise about the mechanism of narrow angle in all eyes,15 it is important to recognize it may be the result of a combination of a number of factors.
Gonioscopy is the standard for visualizing and diagnosing angle closure. It is important to evaluate all angles carefully. If the most posterior structure visible is the posterior trabecular meshwork, the angle is described as narrow. If only the anterior trabecular meshwork is visible, the angle is typically open 10 degrees or less and is likely to close. If greater than 180 degrees of the angle is found to be narrow, that patient is considered to be at risk for angle closure.
When performing gonioscopy as an initial assessment, it is important to balance the pressure of the lens and not inadvertently perform indentation gonioscopy. In that case, a narrow or closed angle may be mistakenly perceived as open.
Additional factors when performing gonioscopy include the level of illumination, both in the exam room and the light entering the pupil from the slit lamp.18 Illumination from a computer screen or eyechart may cause the pupil to constrict, thereby pulling the iris out of the angle. In addition, using a small parallelepiped square to assess the inferior angle first will allow for a more accurate assessment. The lack of a fixation target will serve to reduce the role of accommodation in pupil constriction, although it may create some additional complications when the lens is first applied.
As mentioned, it is important to assess all angles because peripheral anterior synechiae may be more likely to develop earlier in the superior and temporal angles due to anatomical structure.