27

Plateau Iris

Joel S. Schuman, MD, FACS; Malik Y. Kahook, MD; Rachel L. Anderson, MD; and David L. Epstein, MD, MMM

PLATEAU IRIS CONFIGURATION AND PLATEAU IRIS SYNDROME

Clinical Features

In a minority of eyes that develop primary angle-closure glaucoma, the anterior chamber does not appear as shallow as we are accustomed to associate with angle-closure glaucoma, and the front surface of the iris lacks the overall convex curvature that usually involves the whole iris from pupil to angle. In angle-closure glaucoma associated with the so-called plateau iris or flat iris plane, the surface of the iris extending from the pupil toward the angle may appear nearly flat centrally; it may be convex only in the far periphery, where it conforms to the curvature of the angle wall and closes the angle. The profile of such an iris, if it could be seen in cross-section, can be conceived as resembling a plateau, as pointed out by Tornquist in 1958.¹

It is now recognized that the major anatomic factor that results in plateau iris is the presence of anteriorly positioned ciliary processes that push forward on the iris root and crowd the angle.² Additionally, the iris often appears to be inserted more anteriorly along the face of the ciliary body, closer to the sclera spur, further crowding the angle.

In most cases in which angle closure is associated with plateau iris, the angle closure can be eliminated as successfully by iridectomy as in the more common cases in which the whole iris is convex. This indicates that relative pupillary block and accumulation of aqueous humor in the posterior chamber must be a significant factor contributing to the convexity of the iris in both the common and the plateau iris cases, though, with plateau iris, the convexity is usually limited to the periphery, and widening of the angle is not as striking after iridectomy as in eyes in which the whole iris is convex. However, there are cases of plateau iris that are not apparent initially, because the additional presence of significant pupillary block may result in diffuse iris convexity, and the plateau iris only becomes evident after an iridectomy results in flattening of the iris contour.

Additionally, there may be a distinct phacomorphic component whereby the crystalline lens pushes the iris forward and crowds the angle, a factor that can be alleviated with cataract extraction. While the plateau configuration and the apposition of the ciliary processes against the peripheral iris may not resolve with cataract extraction, the anterior chamber does appear to deepen, and the angles may open.^3,4 In such cases, lens removal creates more space in the posterior chamber, and one can picture the entire iris plateau being able to rotate to some degree posteriorly, allowing the peripheral iris to move away from the trabecular meshwork.

Uncommonly, despite the creation of a patent iridectomy and elimination of any component of relative pupillary block, the angle of an eye with plateau iris remains occludable, and rarely, it can be caused to close by dilating the pupil (see Chapter 26), and an attack of angle-closure glaucoma may result. Even more rarely, angle-closure glaucoma can occur spontaneously, without artificially dilating the pupil.

We have preferred to distinguish the preoperative condition of the iris from the post iridectomy recurrence of angle-closure glaucoma simply by calling the preoperative condition plateau iris configuration and the rare postoperative recurrence of glaucoma the plateau iris syndrome, as was done in a study of these conditions reported by Wand and colleagues.5 The rarity of glaucoma due to plateau iris syndrome was also indicated in that report; only 8 cases could be identified among a large number of glaucoma patients in the Glaucoma Service of the Massachusetts Eye and Ear Infirmary and the referral practices of 2 of its members. It should be noted that, at present, the term plateau iris syndrome is often used in reference to a persistently closed or occludable angle, despite a patent iridectomy, regardless of any intraocular pressure (IOP) elevation or glaucoma after dilation.

If following iridectomy, the IOP is found to increase with dilation, then plateau iris syndrome should be considered; however, the diagnosis needs to be established by performing gonioscopy and noting appositional angle closure. Other causes of IOP increase, spontaneously or from application of mydriatics, after iridectomy for angle-closure glaucoma include the following:

• Failure of the iridectomy to make a hole through all layers of the iris
  
• Residual peripheral anterior synechiae (see Chapter 25)
  
• Presence of multiple cysts of the ciliary body (see Chapter 35)
  
• Anterior lens subluxation or intumescence (“phacomorphic” glaucoma), which mechanically narrows the angle
  
• Presence of open-angle glaucoma (see Chapter 26)
  
• Malignant glaucoma

Slit-lamp biomicroscopy and gonioscopy can establish these diagnoses. Only if there is conspicuous axial shallowing of the anterior chamber associated with the postoperative increase of IOP and closure of the angle does one need to consider a diagnosis of malignant glaucoma (see Chapter 30). However, this distinction is important because the topical treatment of plateau iris syndrome and the treatment of malignant glaucoma are totally different. Plateau iris syndrome may be precipitated by cycloplegics and relieved by pilocarpine, whereas malignant glaucoma may be precipitated or aggravated by miotics and relieved or benefited by cycloplegics.

The conspicuous axial shallowing of the anterior chamber in malignant glaucoma (see Chapter 30) is the principal feature that should distinguish this condition from plateau iris syndrome. However, there is an important point in a rare case in which there is some uncertainty due to shallowness of the anterior chamber at the time of recurrent glaucoma after angle-closure glaucoma previously associated with plateau iris. The point we believe to be significant is the relationship of ciliary processes to lens equator, as viewed (when possible) through the iris coloboma (or the widely dilated pupil) with the aid of Koeppe lens gonioscopy. According to our observations so far, in malignant glaucoma, the lens equator tends to extend behind and peripheral to the tips of the ciliary processes, so that the lens equator is in a position to push ciliary processes and iris forward (see Chapter 29), whereas in the plateau iris syndrome, we have not seen the lens extend peripherally beyond the tips of the ciliary processes.

DIAGNOSIS

Plateau iris may be suspected before laser iridectomy based upon the iris profile on gonioscopy and from the disparity of the axial/peripheral chamber depths on slit-lamp examination. The typical finding seen on indentation gonioscopy has been termed the double-hump sign, with a central elevation representing the portion of the iris supported by the lens, a mid-peripheral iris depression near the lens periphery, and a peripheral roll of iris that is supported by the iris processes (which can create loci of peripheral iris elevation). The most peripheral iris surface then steeply angulates posteriorly toward its insertion.⁶

To help confirm the clinical suspicion of plateau iris, ultrasound biomicroscopy can be performed (Figure 27-1). The typical features to look for are occludable angles (particularly in the dark), ciliary processes that push up against the iris root while closing the ciliary sulcus, and a relatively flat iris profile. The iris may be seen inserting anteriorly with respect to the face of the ciliary body.

Although a true plateau iris syndrome is rare, after laser iridectomy, the pupils should be dilated in each patient to rule out this condition (see Chapter 26) before reassuring the patient about his or her cure from the procedure. Each patient should have sequential unilateral mydriatic provocative tests (eg, 4 to 6 weeks post laser). Measurement of IOP and gonioscopy should be performed before and after dilation. As discussed in Chapter 26, an additional benefit of this routine is to gain experience with the interpretation of gonioscopy in the presence of a dilated pupil because, often in narrow-angle eyes, a peripheral iris roll may partially obscure the viewing of a functionally open angle when the pupil is dilated.

CLINICAL COURSE

In patients who have a true plateau iris syndrome, the risk for the future could involve symptomatic attacks usually of subacute angle closure or, more commonly, asymptomatic progressive chronic angle closure with peripheral anterior synechiae formation. Undiagnosed plateau iris can cause creeping angle closure—a term that is not favored because it indicates no mechanism and constitutes more than one entity after iridectomy.

In the mechanism of true plateau iris syndrome, the angle becomes progressively crowded and occluded by the iris with increasing pupillary dilation. Such full-blown attacks of angle-closure glaucoma from maximum mydriasis are, therefore, more common after pharmacological pupillary dilation in the ophthalmologist’s office than spontaneously (where subacute attacks are more common). The potential for such an iatrogenic event makes it imperative that the results of routine unilateral mydriatic provocative testing be systematically studied in each patient after laser iridectomy, and the patient then appropriately classified at that time (not subsequently after an acute event).

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