ANATOMIC FEATURES OF NORMAL EYES
The anatomic structures that regulate intraocular pressure (IOP) are contained in the anterior segment of the eye. Behind the rounded apex of the angle is the ciliary body, which produces aqueous humor and regulates its outflow. The position of the lens and the overlying iris determines the depth of the anterior chamber. The width of the chamber angle is defined by the point of the iris insertion on the ciliary body, the peripheral contour of the iris as it drapes around the lens, and the pupillary size. Finally, there is the corneoscleral trabecular meshwork, through which aqueous humor percolates to reach Schlemm’s canal, the collector channels, and the anterior ciliary veins of the limbal area. This region is not only the site of the prime pathologic changes responsible for the increased IOP associated with all of the glaucomas, but it is also the focus of most of the medical and surgical procedures designed to alleviate increased IOP ( Fig. 5-1 ). This knowledge has been the bedrock of the modern understanding of glaucoma.
Distinguishing between the two parameters of anterior chamber depth and angle width is useful. For example, an important diagnostic distinction between pupillary-block primary angle-closure glaucoma and malignant glaucoma is that though both conditions have occluded angles, the former condition presents with a relatively deeper central anterior chamber depth than does the latter. Conversely, a plateau iris configuration typically has a very deep central chamber but a narrow and potentially occludable angle. Distinguishing among the various components that contribute to the three-dimensional configuration of the angle has led to a detailed and novel scheme for gonioscopic grading.
The size and the shape of the eyeball are genetically determined. Important gonioscopic structural differences among racial groupings are beginning to be appreciated. These studies find a more anteriorly inserting iris root (and potentially occludable angle) in Asian patients. Such variations in angle structure may contribute to the reportedly higher incidence of primary and chronic angle-closure glaucoma among distinctive populations, such as Alaskan and Greenland Eskimos, Chinese populations both in China and in Malaysia, Japanese people, and people of Asian ancestry in South Africa.
The deep-chambered eye almost always has a wide angle, whereas the angle contour of the shallow-chambered eye tends to be narrow. When the angle formed between the iris and the surface of the trabecular meshwork is between 20° and 45°, the eye is said to have a wide angle. Angles smaller than 20° are termed narrow angles ( Fig. 5-2 ). The narrower the angle, the closer the iris comes to the meshwork and the more probable angle closure becomes. The major contribution of gonioscopy is distinguishing open-angle from angle-closure glaucoma.
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 NARROW-ANGLED 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. An exaggeration of this pupillary block is primary cause of angle-closure 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. This has been demonstrated both by ultrasonic biomicroscopy and by histology. Pupillary dilation may bunch up the peripheral iris and occlude the angle, often appearing in a characteristic ‘sine wave’ configuration. 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 ). And although cataract removal deepens both the central chamber depth and opens the angle in normal eyes, the angle in plateau iris remains unchanged following such surgery.
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. 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 the anterior chamber is deep, its convexity (or even bowing) in eyes with a shallow anterior chamber, or its peripheral concavity in eyes with high myopia or signs of pigment dispersion. After assessing the configuration of the peripheral iris, attention should be paid to the site of iris insertion – both its apparent and actual juncture in the angle. Indentation gonioscopy is particularly helpful in distinguishing iris–trabecular touch (apposition) from genuine adhesion. The level of iris insertion can be described in reference to structures within the angle recess – at the level of the upper trabecular meshwork and Schwalbe’s line; at the level of the filtering trabecular meshwork; just below the scleral spur; below the spur in the ciliary body; or deep posteriorly in the ciliary band. Anteriorly inserting irides, at the level of the spur or lower trabecular meshwork, may possibly be more common among Asians and in patients with hyperopia. Third, the examiner should estimate the angulation between the iris insertion and the slope of the inner cornea in the angle, in approximate steps of 10°. As discussed in Chapter 7, this systematic assessment of angle anatomy is the basis of the most detailed gonioscopic grading systems. Last, abnormalities such as neovascularization, hypoplasia, atrophy, and polycoria should be noted.