Plateau iris

Except in the rare younger patients (age 20-30s), some degree of pupillary block may also be present. Because a component of pupillary block may exist in patients with plateau iris configuration, laser iridotomy should be performed as the first intervention.3 If the angle remains capable of closure despite a patent iridotomy, the condition is termed plateau iris syndrome.

The exact prevalence of plateau iris is unknown. Its diagnosis should be suspected when angle closure occurs in a young myopic patient. Plateau iris is an important cause of angle-closure glaucoma in patients younger than 40 years and is seen most commonly in women and often with a family history of primary angle-closure (relative pupillary block) glaucoma. Barkana et al.4 reviewed retrospectively the data regarding angle closure in high myopes and found plateau iris syndrome in three patients of 17938 patients with high myopia more than -6.0 Diopters.

The racial predisposition to progressive angle closure was analyzed by Hagadus et al.,5 who found that Caucasian eyes have a lesser predisposition to angle closure as compared to Hispanic eyes (46.9% vs 72.7%) even after a laser iridotomy. The frequency of open angles after LI (laser iridotomy) was similarly higher among Caucasians than among Hispanics (75% vs 50%). Choi JS et al6 analyzed the risk factors of peripheral anterior synechiae progression after laser iridotomy. Over a 3 year period the rate of PAS progression was higher in eyes exhibiting plateau iris.

The effect of mydriatics agents on the anterior segment anatomy was assessed by Sekhar et al.,7 who described a case of plateau iris syndrome who demonstrated a post mydriatic rise in IOP in one eye but not in the other eye. They suggested that plateau iris may be caused by difference in the extent of closure of the angle circumferentially over the 3600 of the angle.

Clinical presentation3

•Patients with plateau iris tend to be female, in their 30-50s with a myopic refractive error. They may present with angle closure, either spontaneously or after pupillary dilation. Usually the diagnosis of plateau iris configuration is made on routine examination. Plateau iris syndrome may be recognized post laser iridotomy when the angle remains persistently narrow in an eye after iridotomy.

•Slit lamp examination of patients with plateau iris usually shows a normal central anterior chamber depth with a flat or slightly convex iris surface.4 On gonioscopy, the angle is extremely narrow or closed, with a sharp drop-off of the peripheral iris (sine wave configuration). When indentation gonioscopy is performed, the double-hump sign is seen. The more peripheral hump is determined by the ciliary body propping up the iris root, and the more central hump represents the central third of the iris resting over the anterior lens surface. The space between the humps represents the space between the ciliary processes and the endpoint of contact of the iris to the anterior lens capsule. In such eyes much more force is required to open the angle on indentation gonioscopy as compared to cases with pupillary block angle closure.

Plateau iris configuration vs plateau iris syndrome8

Plateau iris configuration is defined as an anatomically narrow angle as determined by gonioscopy in the presence of a flat iris plane and a normal central ACD. This configuration occurs because of large or anteriorly inserted ciliary processes which hold the iris in apposition with the trabecular meshwork and prevent posterior movement of the iris. Relative pupillary block is also believed to play a significant role and laser iridotomy is considered effective in opening up the angle.

Plateau iris syndrome is characterized by the development of angle closure in an eye with a plateau iris configuration even after a patent iridotomy. The level of the iris stroma in relation to the angle structures, referred to as the height of the plateau, differentiates two subtypes of plateau iris syndrome.

1.In the complete syndrome, the angle is occluded to the upper trabecular meshwork or the Schwalbe line and intraocular pressure (IOP) rises.

2.In the incomplete syndrome, the iris occludes the angle to the mid level, leaving the upper portion of the filtering meshwork open and IOP unchanged. This latter situation is far more common and is clinically important because these patients can develop peripheral anterior synechiae (PAS) and synechial angle closure years after a successful iridotomy. Therefore, patients with an open angle after iridotomy should not be assumed to be cured. The angle can narrow further with age, and angle closure can occur years later.

Differential diagnosis

Pseudoplateau iris8,9

In pseudoplateau iris, the anterior displacement of the peripheral iris is not caused by an enlarged or anteriorly positioned ciliary body. Cysts of the iris and/or ciliary body neuroepithelium most often are responsible. This condition is distinguished easily from plateau iris because the angle is usually closed in only one quadrant or, if cysts are multiple, at several focal loculations. Ultrasound biomicroscopy (UBM) is extremely helpful in identifying the underlying mechanism responsible for pseudoplateau iris. The plateau iris also needs to be distinguished from angle closure associated with extensive peripheral anterior synechiae and an incomplete iridotomy.

Imaging studies10-14

Ultrasound biomicroscopy

UBM uses high frequency ultrasound (50 MHz) to produce high resolution images of the anterior segment (30-50 μ) This contact procedure is non invasive and can visualize the cornea, iris, angle, anterior chamber, posterior chamber and ciliary body with accuracy. The configuration of the iris surface, iris root insertion, iridocorneal angle, and ciliary processes can be seen in detail to confirm the clinical findings of plateau iris. The UBM characteristics include forward positioned pars plicata,

Fig. 1. UBM picture showing anteriorly positioned ciliary processes pushing up the peripheral iris. This has caused a narrowing of the angle recess in the presence of a relatively flat central iris configuration.

Fig. 2. Second eye of the same patient, again showing an anteriorly placed ciliary process and a closed angle in the presence of a relatively flat central iris configuration.

with a closed ciliary sulcus. This anterior position pushes the root of iris into the trabecular meshwork causing angle closure (Figs 1 and 2). This structural support also prevents the posterior movement of the iris following YAG iridotomy.Any element of pupillary block also may be confirmed with Ultrasound Biomicroscopy (UBM) imaging. Pilocarpine leads to an opening up of the angle.

Polikoff et al.15 studied the various UBM parameters in cases of plateau iris syndrome and assess the effect of laser iridotomy in such eyes. They concluded that

LI did not alter the anterior segment anatomy probably because of fixed anterior insertion of the iris and ciliary body in plateau iris configuration. Matsunaga et al.16 studied the anterior segment anatomy in three groups of patientsrelative pupillary block, plateau iris configuration and peripheral anterior synechiae with the use of indentation UBM gonioscopy using a newly deigned cup and found that the angle opened significantly with indentation in eyes with relative pupillary block. Ritch et al.17 evaluated the long term success rates of ALPI in the management of plateau iris syndrome and concluded that ALPI was successful in eliminating residual angle closure after laser iridotomy during the six-year follow-up period even though a small number of patients might require retreatment. A contrasting study regarding the ACD measurement with the use of UBM was presented by Mandell et al.,18 who retrospectively reviewed the Biometric analysis of the UBM measurements of the ACD. The mean ACD in patients with pupillary block was 2.17 ± 0.3 mm, 2.04 ± 0.3 mm in plateau iris, 3.0 mm in normal individuals (p = 0.0001). They concluded that ACD associated with plateau iris is shallower than normal and also shallower than pupillary block. A UBM study done on Indian eyes by Garudadri CS et al.19 after laser iridotomy, showed that 66.66% (22 of 33 eyes) of eyes with narrow angles had anteriorly placed ciliary processes after laser iridotomy.

Anterior segment OCT

This new emerging imaging modality is being used more often in the evaluation of anterior segment anatomy in cases of plateau iris syndrome. The chief advantages of this technique being very user friendly and can provide precise information in a non invasive way besides shortening the image acquisition time. The role of OCT in the evaluation of anterior segment anatomy was evaluated by Leung et al.,20 who demonstrated crowding of the peripheral iris in a case of plateau iris syndrome which subsequently opened after ALPI.

Treatment

Laser iridotomy21 to bypass the pupillary block component is the primary modality in patients with plateau iris configuration or syndrome. However, pilocarpine may produce iris thinning and facilitate angle opening in some cases.

A patent iridotomy may be therapeutic in reducing risks of angle closure. However, in some patients, laser iridotomy22 may not significantly alter the anterior chamber depth or anatomy. Even after a successful iridotomy produces what appears to be a well-opened angle, periodic gonioscopy remains crucial because these patients may have incomplete plateau iris syndrome or the angle may narrow further with age because of the enlargement of the lens or gradual formation of peripheral anterior synechiae. Although usually recognized in the post iridotomy period, plateau iris syndrome may develop years later. Patients with plateau iris configuration should not be assumed to be cured, even though plateau iris syndrome does not develop immediately.

If persistent iridotrabecular apposition is present despite a patent iridotomy, the diagnosis is consistent with plateau iris syndrome, and peripheral laser iridoplasty (ALPI)23 is indicated. Argon laser peripheral iridoplasty is the procedure of choice

to effectively open an angle that remains occluded after successful laser iridotomy. The procedure consists of placing laser burns on the surface of the peripheral iris to contract the iris stroma between the site of the burn and the angle. The result is iris stromal tissue contraction and compaction that physically widens the angle and prevents the apposition of the peripheral iris against the trabecular meshwork.

Laser settings in ALPI500μ spot size, 200 mW initial power which can be increased in 40 mW increments, duration 0.5 seconds. The end point should consist of adequate peripheral stromal contraction. All four quadrants can be treated in the initial procedure. The use of Abraham lens during the procedure is recommended. The power should be reduced if there is charring of iris or pigment release, formation of gas bubbles, production of a pop sound

The post ALPI care should consist of topical corticosteroids for five days and IOP should be checked 1-2 hours after the procedure to note any post procedure IOP spike. Gonioscopy should be performed after one week to assess the effect of the procedure. However, even after successful opening of the angle, regular gonioscopic examination remains crucial.

A study by Ritch et al.17 has documented the long term success of ALPI (six years) in eliminating the residual appositional closure as well as the need of filtration surgery.

If angle closure persists despite iridotomy and sufficient peripheral laser iridoplasty, miotic therapy may be used to prevent angle closure, or a surgical intervention in the form of trabeculectomy, or tube-shunt implantation surgery may be needed to allow bypass of aqueous flow and to control IOP.

Miotic agents24 cause the pupillary sphincter to contract, mechanically pulling the iris away from the trabecular meshwork and opening the anterior chamber angle. In addition, these agents also have an IOP-lowering effect by stimulating contraction of the ciliary muscle and thereby increasing trabecular outflow of aqueous humor. Induced myopia, pupillary constriction, brow ache, and retinal detachment are potential adverse effects of this therapy. The concentration of pilocarpine used is 1-2% 3-4 times a day and the incidence of side effects can be decreased with the use of artificial drops instilled after the instillation of pilocarpine drops. Pilocarpine is efficacious because it has been shown to cause peripheral iris thinning and opens up the angle in plateau iris syndrome. Pavlin et al.24 evaluated the efficacy of pilocarpine with the use of UBM biometry in cases of plateau iris syndrome. They found that pilocarpine causes significant peripheral iris stromal thinning (253 ± 48 μ vs 338 ± 34 μ in light conditions, p = 0.0002) and is effective in opening the angle. UBM can be used to document this and can be used prior to long term treatment with this drug.

Prognosis

Patients with plateau iris have a good prognosis in general; however, regular fol- low-up examinations are needed because angle-closure may develop years after successful iridotomy or iridoplasty.

References

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