Jeffrey M. Liebmann, M.D.

Robert Ritch, M.D.

New imaging technologies are revolutionizing our understanding and treatment of a wide variety of ocular disorders. Confocal scanning laser ophthalmoscopy, ultrasound biomicroscopy (UBM), confocal scanning laser polarimetry, color Doppler imaging of ocular blood flow, and optical coherence tomography are providing important information regarding disease pathophysiology, diagnosis, progression, and treatment. This article, on the role of UBM in the diagnosis and management of angle-closure glaucoma, is the first in a series of articles which will evaluate these new modalities.

Ultrasound biomicroscopy was developed by Dr. Charles Pavlin and his colleagues in Toronto. The commercial version is produced by Humphrey Instruments (Zeiss-Humphrey). High frequency transducers permit improved resolution, approaching 50 microns in the living eye. Scanning is performed with the patient in the supine position, using a 20 mm eye cup inserted between the lids to hold the methylcellulose or normal saline coupling medium. The real-time video display is similar to traditional B-scan ultrasonography.

Angle-closure glaucoma is an anatomic disorder comprising a final common pathway of iris apposition to the trabecular meshwork resulting from various abnormal relationships of anterior segment structures. These in turn result from one or more abnormalities in the relative or absolute sizes or positions of anterior segment structures or posterior segment forces which alter anterior segment anatomy. Prolonged appositional angle-closure leads to trabecular meshwork dysfunction and peripheral anterior synechiae or acute angle-closure glaucoma. The two most common anatomic causes of primary angle closure are relative pupillary block and plateau iris.

Relative pupillary block is the most common cause of angle-closure glaucoma and is responsible for more than 90% of cases. In pupillary block, flow of aqueous from its site of production by the ciliary epithelium in the posterior chamber to the anterior chamber is limited because of resistance to aqueous flow through the pupil in the region of iridolenticular contact. Since the iris is an otherwise flaccid structure, it assumes an anteriorly bowed, or convex configuration in response to this pressure gradient (Figure 1). Laser iridotomy eliminates the pressure differential between the anterior and posterior chambers and relieves the iris convexity. Following iridotomy, the central iris is closely adherent to the anterior lens capsule, while the peripheral iris assumes a flat configuration from its point of departure from the lens capsule to its insertion into the ciliary body (Figure 2).

In plateau iris, a large or anteriorly positioned pars plicata supports the iris root in proximity to the trabecular meshwork, forcing the peripheral iris into the angle (Figure 3). The anterior chamber is usually of medium depth and the iris surface slightly convex. On gonioscopy, the iris root angulates forward and then centrally. With indentation gonioscopy, the ciliary processes prevent posterior movement of the peripheral iris, resulting in a configuration in which the slit beam follows the curvature of the iris to its deepest point at the periphery of the lens where the ciliary processes begin, then rises again over the ciliary processes before dropping peripherally (S sign). Greater force is needed to open the angle than in pupillary block because the ciliary processes must be displaced, and the angle does not open as widely. UBM provides a method for substantiating the presence of continued appositional closure after iridotomy for pupillary block or of occludability under conditions of physiologic dilation in a darkened room.

The key to successful management of the angle-closure glaucomas is accurate diagnosis. Besides indentation gonioscopy, clinicians in the past often relied upon a variety of provocative tests. Many of these, including the prone dark-room and dilation provocative tests were considered positive on the basis of a rise in IOP of 8 mmHg or more, accompanied by gonioscopically confirmed angle-closure. However, most gonioscopy was not done under darkroom conditions. At the present time, provocative tests have largely fallen into disuse with the universal advent of laser iridotomy. We have also only recently discovered that a very large number of angles which are open under light conditions are occludable in the dark.

Failure to diagnose angle-closure is often an important factor in eyes with labile or poorly controlled IOP. When one is attempting to determine whether or not a narrow angle is occludable, gonioscopy should always be performed in a completely darkened room using the smallest square of slit-lamp illumination possible which will enable a view of the angle. The difference in the angle in light and dark conditions may be much greater than expected and can be demonstrated by UBM. UBM is also extremely useful for explaining the nature of the disease and the rationale of treatment to patients who may be confused between open-angle and angle-closure glaucomas and different types of laser surgery.

An objective assessment of angle configuration can be accomplished by comparing the UBM appearance of the angle under dark and light conditions. Within minutes of dimming the examination room illumination, the pupil dilates and bows anteriorly, narrowing the angle and causing angle-closure in susceptible patients. he most important anatomic landmark in the evaluation is the scleral spur, which can be seen an the innermost point of the line separating the ciliary body and the sclera. The trabecular meshwork is located directly anterior to this structure. Because the scleral spur can be easily visualized on UBM, the presence of iridotrabecular apposition or significant angle narrowing is readily detectable.

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