In the deep-chambered, wide-angled eye, the lens is held by the zonular ligaments, approximately centered in the ring made by the ciliary body. The iris originates at the inner anterior border of the ciliary body and lies with minimum contact on the anterior lens surface of the eye. An increase in IOP in such an eye must be caused by an obstruction to trabecular aqueous egress or an increase in the rate of aqueous production.The latter is rarely, if ever clinically encountered.

Anatomic features of narrowangled eyes

In contrast, the lens of the shallow-chambered, narrow-angled eye is well anterior to the ciliary body ring, and the iris is held more snugly against a much larger area of the posterior iris surface. Therein results a physiologic or relative pupillary block. In such an eye, a somewhat higher pressure is required in the posterior chamber to push aqueous humor through this tight iris–lens apposition than is necessary for the looser apposition of the wide-angled eye.19 An exaggeration of this pupillary block is primary cause of angleclosure glaucoma.The slight excess pressure in the posterior chamber lifts the iris root forward and may be adequate to push the iris against the trabecular meshwork in some eyes (see Fig. 7-1C and D). If the angle is sufficiently narrow, and the iris base sufficiently distensible, the iris is forced against the surface of the trabecular meshwork, blocking aqueous flow into Schlemm’s canal, and an attack of angle-closure glaucoma ensues. The alternative route for aqueous egress provided by a patent iridotomy can completely reverse this propensity for angle occlusion by the iris base, as demonstrated dramatically with ultrasonic biomicroscopy (Fig. 5-3).

An important narrow-angle configuration is the anatomic abnormality associated with plateau iris. In this condition the peripheral iris is displaced anteriorly into the angle by anomalously positioned and rotated ciliary processes behind the iris root.20 This has been demonstrated both by ultrasonic biomicroscopy21 and by histology.22 Pupillary dilation may bunch up the peripheral iris and occlude the angle, often appearing in a characteristic ‘sine wave’ configuration.23 Pupillary block plays only a small role in the mechanism; laser iridotomy neither appreciably increases the central anterior chamber depth nor reverses the cramping of the angle (Fig. 5-4).24 And although cataract removal deepens both the central chamber depth

(B)

Fig. 5-3  (A) Bowing of iris into angle during attack of pupillary-block angle-closure glaucoma. (B) Following laser iridectomy the contour of the iris has become flat, falling away from the angle.

(From Pavlin CJ, Foster FS: Ultrasound in biomicroscopy in glaucoma. In Ritch R, Shields MB, Krupin T, editors: The glaucomas, 2nd edn, St Louis, Mosby, 1996.)

and opens the angle in normal eyes,25 the angle in plateau iris remains unchanged following such surgery.26

Gonioscopic anatomy and microscopic interpretation

Pupil and iris

It is best to start gonioscopy by looking at the pupil for rapid orientation. The anterior lens surface can be observed for focal opacifications (glaukomflecken) of the anterior lens and for posterior synechiae.This position is also excellent for viewing the white dandruff-like flecks of exfoliation on the pigment at the posterior edge of the pupil, which is typical of exfoliative syndrome.27,28 Iridodonesis is present to a small degree in some deep-chambered normal eyes and is easily observed if of a pathologic degree.

The first of the three major iris features the examiner should carefully evaluate is the contour of the iris, noting its flatness when