The most common form of angle-closure glaucoma is relative pupillary block. In relative pupillary block, aqueous pressure behind the iris plane forces the iris anteriorly. Elimination of this pressure gradient can be achieved with laser iridotomy. A second, less common form of angle-closure glaucoma is known as plateau iris syndrome. In plateau iris, the iris is forced into the angle by the presence of an abnormally placed, anterior ciliary body. Because this form of angle-closure is not due to fluid pressure gradient, it responds poorly to laser iridotomy. If undiagnosed, recurrent angle-closure may develop. Other forms of angle-closure of interest to clinicians include lens-induced angle-closure, iris cysts, iris tumors, ciliary body rotation due to effusion, dark room provocative testing during ultrasound biomicroscopy, malignant glaucoma.
Pupillary Block Angle-Closure Glaucoma
Relative pupillary block is the most common form of angle-closure glaucoma. Resistance to aqueous movement from its site of production by the ciliary epithelium within the posterior chamber through the pupil produces a pressure gradient across the iris which forces the iris anteriorly, into the trabecular meshwork, thereby closing the angle. This give the iris its typical convex configuration on ultrasound biomicroscopy (left).
The most important anatomic landmark, particularly in the evaluation of the angle-closure glaucomas 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. If aqueous humor, which has no reflectivity and is therefore black on ultrasound biomicroscopy, has access to the meshwork, the angle is open.
Following laser iridotomy, aqueous has free access to the anterior chamber and the pressure gradient is eliminated. The iris assumes a flat (planar) configuration and the angle opens (right).
The Effect of Illumination on Angle Configuration
The reasons why some individuals with narrow angles develop angle- closure while others do not is not clearly understood. One factor which can affect angle configuration is the amount of illumination, which alters pupillary size. Under normal conditions, the miotic response to light causes the angle to open (left).
If the room illumination is dimmed during scanning of the patient shown above, pupillary dilation may cause the peripheral iris to crowd the and become apposed to the trabecular meshwork, causing angle closure (right).
Plateau Iris Syndrome
Not all angle-closure is caused by relative pupillary block. In plateau iris syndrome, the physical presence of an anteriorly placed ciliary body forces the peripheral iris into the angle. Iridotomy relieves the contribution of pupillary block component to the angle narrowing, but not the closure related to the abnormal ciliary body position. Laser iridoplasty is often useful under these circumstances.
Lens Intumescence or Anterior Subluxation
A large, intumescent lens or forward lens movement due to zonular laxity or dehiscence may cause mechanical crowding of the angle.
Focal forms of angle-closure may be induced by cystic or solid masses involving the iris or the ciliary body. Iridociliary cysts are characterized by an echolucent interior.
Ciliary Body or Iris Tumor
Tumors or infiltration of the iris or ciliary body may also cause angle-closure and are characterized by uneven internal echoes when compared to cystic structures.
Malignant glaucoma, an infrequent cause of postoperative angle-closure glaucoma, can result from aqueous misdirection or from annular ciliary body detachment. In either case, anterior rotation of the ciliary body about its insertion into scleral spur may cause a secondary angle-closure glaucoma.