extent of PAS, in conjunction with the absence or presence of elevated IOPs, disc, and field changes, constitutes the parameters for PAC suspects, PAC and PACG, not all published accounts discriminate these findings, making precise correlation difficult.

For example, reference to subacute PACG with pupillary block in

the literature has also been referred to by the terms prodromal, intermittent, and subclinical glaucoma.248,299 This is a milder form of angle

closure in which symptoms may be modest or absent. Patients often report that they have experienced mild episodes of discomfort, blurred vision more often at night or with dim illumination, and their halo vision was often relieved by sleep or exposure to bright light. (By history alone it is possible to confuse subacute angle closure with transient ischemic attacks or other neurologic causes of intermittent visual loss.300)

Between episodes of subacute angle closure, IOP and outflow facility are generally normal and no PAS are present. This latter finding suggests that some of these eyes have ‘narrow’ or ‘occludable’ angles, with or without reaching the threshold of 270° of irido-trabecular touch which currently defines a PAC suspect. On rare occasions, subacute angle-closure glaucoma is seen as intermittent ocular hypertension.301 Patients with subacute angle-closure glaucoma may progress to either an acute attack or chronic angleclosure glaucoma; this suggests that the progressive embarrassment of the angle by PAS was not observed as the eye progressed into PAC or frank PACG.

The next category in common usage is chronic PACG with pupillary block, also referred to as creeping angle closure.191,302,303 This

entity is often misdiagnosed because it closely resembles PACG: patients are asymptomatic and have quiet eyes, yet manifest cupping and atrophy of the optic discs, and corresponding visual field loss. Intraocular pressure is moderately elevated, but often poorly responsive to medical treatment.303 Gonioscopy is the key to the diagnosis of chronic angle-closure glaucoma, revealing a very narrow angle with apposition between the iris and the trabecular meshwork over most of the circumference of the angle.Therefore, gonioscopy remains an indispensable element in the evaluation of any eye with elevated IOP, cupping, or visual field loss.

The apposition usually begins in the superior angle and progresses in both directions toward the 6 o’clock position.14,304,305

In Chinese patients, the superior and temporal aspects of the angle are most likely to develop synechiae early.306 When approximately two-thirds of the angle is occluded, IOP rises substantially. As a general rule, the more extensive the synechial closure, the higher the IOP and the more optic nerve damage there will be on presentation.307 The height of the IOP depends on both the extent of synechial closure and on the competency of the remaining unoccluded trabecular meshwork.

This category of ‘chronic PACG’ is virtually congruent with the current definition of PACG itself, with the designation chronic for the most part indicating that symptoms are rare.This is in fact the commonest presentation of PACG in the world.

2. PLATEAU IRIS

Barkan308 noted that 20% of eyes with ‘noncongestive’ forms of angle-closure glaucoma were atypical in that they had normal central anterior chamber depths, little bombé, and minimal pupillary block.

This unusual anterior segment configuration was further detailed and given the name plateau iris by Shaffer and Chandler.309,310 Wand

and co-workers311 then divided plateau iris into two entities that they called ‘plateau iris configuration’ and ‘plateau iris syndrome’.

Plateau iris syndrome refers to the development of angle closure, either spontaneously or after pharmacologic dilation, in an eye with a patent iridotomy; plateau iris configuration refers to an anteriorly displaced peripheral iris compromising the angle.20

Though this entity, with merit, has been classified as an ‘angleclosure glaucoma without pupillary block’ due to a ‘pushing mechanism’, it is now included as one of three basic PACG mechanisms, along with ‘pupillary block’ and ‘phacomorphic’ glaucomas. All three are part of a closely related differential diagnosis at the time of clinical presentation, and all three are amenable to the new anterior segment imaging technologies of UBM and/or AS-OCT, which can often help differentiate among them.

Plateau iris configuration

Plateau iris configuration consists of narrow angles or angle closure in an eye with a ‘normal’ central anterior chamber depth on slit-lamp examination, and a flat iris plane from pupil to periphery (as opposed to the peripheral forward bow of pupillary block (see Fig. 15-5B)). On gonioscopy, the iris appears flat from the pupillary margin to the periphery – a shape that Tornquist termed ‘plateau’312 – at which point it takes a sharp turn posteriorly before inserting into the ciliary body. This sharp turn creates a narrow angle recess and the potential for angle closure. The iris root is usually anteriorly displaced. In many cases the iris periphery has redundant folds with a particularly prominent roll by the iris insertion: the double hump or S-sign of a peripherally elevated roll of iris may be seen on gonioscopy.

This anterior segment configuration differs from the shallow central anterior chamber and iris bombé seen typically with pupillary block (see Fig. 15-5A).The double hump sign is best seen with Koeppe gonioscopy, where the forwardly positioned ciliary processes prevent the peripheral iris from falling backward in the supine position; it can also be demonstrated with the on–off indentation maneuver of compression gonioscopy at the slit lamp. Ultrasound biomicroscopy reveals anteriorly displaced ciliary processes that crowd or push the peripheral iris forward and thus prevent the peripheral iris from falling back after iridotomy.313 When the pupil dilates spontaneously or in response to pharmacologic agents, the iris can crowd into the angle and occlude the trabecular meshwork. If this happens often enough or over a wide enough portion of the angle, elevated IOP and/or PAS may result, with the potential to progress into PAC or PACG.

A recent ultrasound study of anterior chamber depth challenges the long-held clinical perspective that the central anterior chamber depth is normal with the plateau iris configuration.This study found that, surprisingly, the anterior chamber depth of eyes with plateau iris was abnormally shallow; not only when compared to normal eyes, but also in comparison to eyes with pupillary block glaucoma.314

Plateau iris configuration is more common than recognized, although the condition is probably not common per se. It is one of the more frequent causes of otherwise rare angle closure in older children or young adults.315

In many cases, plateau iris configuration is accompanied by some degree of pupillary block, which exacerbates the problem. Therefore, it is often not possible on clinical grounds to differentiate between a pupillary block PAC and a plateau iris configuration embarrassing the angle until after an iridotomy has been performed. In conditions precipitated by pupillary block, iridotomy will cause the iris to fall back and the peripheral chamber to deepen; whereas in plateau iris, the peripheral angle remains unchanged. However, iridotomy often can prevent the development of, or progression of,

elevated IOP with its potential for progressive PAC or glaucoma; this suggests that pupillary block, indeed, does play at least some role in some cases in the development of this angle closure. If iridotomy does not widen the angle substantially following iridotomy it is assumed there was only minimal pupillary block preoperatively.

After the iridotomy, these eyes should be examined periodically for signs of elevated IOP, peripheral synechiae and glaucomatous damage. Pupillary dilation should be undertaken with care, even after iridotomy. Mydriasis is best accomplished with one drop of either 0.5% tropicamide or 2.5% phenylephrine (unless examination of the retinal periphery is mandatory), since the effects of these agents can be readily reversed with dipiperazole. It is probably best to use dipiperazole (if available) routinely after dilation in these cases.

Patients with plateau iris configuration (and plateau iris syndrome) should be warned to avoid agents which have a potential for dilating the pupil (see Box 15-2).These include any medication with anticholinergic activity such as antihistamines, phenothiazine

antianxiety agents, antidepressants, and drugs used for incontinence and for diarrhea.316–319

Plateau iris syndrome

Plateau iris syndrome is defined as angle closure in the presence of plateau iris configuration following a patent iridotomy. Whereas earlier studies implicated a peculiar anatomic configuration of the peripheral iris which allowed it to bunch in the angle and occlude the trabecular meshwork with pupillary dilation, it is now under-

stood that angle closure occurs because the ciliary processes are rotated forward,320,321 as amply demonstrated on UBM (Fig. 15-11).

(Note the inherent limitation of AS-OCT, which cannot consistently image tissue behind the iris plane – hence UBM is the mainstay technology for diagnosing this condition.) Ultrasound biomicroscopy has also revealed several patients with multiple ciliary body cysts associated with plateau iris syndrome – now called pseudoplateau iris (see below).322

Plateau iris syndrome is often seen in young patients aged 30–40 years old, and occurs equally among men and women. This syndrome presents with or without symptoms occurring days, weeks, months, or years after iridotomy or other intraocular surgery. This can occur spontaneously or after pharmacologic dilation of the pupil. If not diagnosed and treated properly, repeated episodes of angle closure can progress to PAC or PACG: increasing numbers of PAS, persistent elevation of IOP, and ultimate glaucomatous damage.323

Lowe and Ritch324 proposed an ‘incomplete’ type of plateau iris syndrome that is more common than the ‘complete’ form of the syndrome. In the incomplete plateau iris syndrome, the iris is not as far forward as in the complete syndrome; in these patients,

the IOP does not rise with dilation, but PAS may form over time. Gonioscopy and ultrasound show large and/or anteriorly

positioned ciliary processes that seem to push the iris against the trabecular meshwork.325,326 Such a configuration has also been

detected with an absent ciliary sulcus and long ciliary processes.103

Earlier investigators thought that plateau iris syndrome was relatively common, occurring in 6–20% of eyes with angle closure.327,328

However, these estimates were based on post-iridectomy provocative tests that were done without gonioscopic documentation of angle closure. Plateau iris syndrome is apparently seen in most populations; but ethnic variability, as with all the PACGs, is apparent. For example, in Samoans, plateau iris may be relatively common.329 And in a series of PACG eyes from India, evidence of plateau iris was found by UBM in 40% of eyes whose angles opened after iridotomy, and in two-thirds of eyes whose angle did not open after iridotomy.330

Although it is possible to identify plateau iris before iridotomy by noting on gonioscopy the typical configuration of a peripherally flat and anterior iris insertion yielding little angle detail with compression, the diagnosis is usually not made until after a successful iridotomy; at that point, little significant opening of the chamber angle and elevated IOP (especially after pharmacologic dilation) may signal its presence. Therefore, gonioscopy should always be performed after peripheral iridotomy. Obviously, if there are extensive PAS, then the diagnosis cannot be substantiated by this criterion. A UBM study is usually definitive in making the diagnosis.

The differential diagnosis of plateau iris syndrome includes extensive PAS due to any cause; imperforate or occluded iridotomy with persistent pupillary block; multiple cysts of the iris or ciliary body; ciliary block glaucoma (aqueous misdirection or malignant glaucoma); open-angle glaucoma with anatomically narrow angles (as with increasing cataract formation); and the effect of chronic topical corticosteroids or cycloplegic agents.

The sequence of laser procedures for plateau iris syndrome is not fixed. Usually the first treatment for plateau iris syndrome is a laser iridotomy. However, if UBM imaging is available, a peripheral iridoplasty may be the initial treatment, since iridotomy often does not change the anterior segment anatomy.331 In practice though, because of the possibility of some component of pupillary block, an iridiotomy is eventually performed.

In a common treatment scenario, once the syndrome is confirmed post-iridotomy, several miotic agents can be tried to minimize pupillary dilation (e.g., pilcocarpine 1–2% 2–4 times a day, or pilocarpine 4% ophthalmic ointment (Pilogel™) at bedtime).Argon laser peripheral iridoplasty (gonioplasty) can also be undertaken (see Ch. 31 for details). If the response to miotics is inadequate, or if the patient is unable or unwilling to use them long term,