Gonioscopic anatomy

Gross anatomy

Anatomic features of normal eyes

The anatomic structures that regulate intraocular pressure (IOP) are contained in the anterior segment of the eye.1 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.2 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.3,4

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.5,6

The size and the shape of the eyeball are genetically determined. Important gonioscopic structural differences among racial groupings are beginning to be appreciated.7,8 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 angleclosure glaucoma among distinctive populations, such as Alaskan

and Greenland Eskimos,9–11 Chinese populations both in China and in Malaysia,12–14 Japanese people,15,16 and people of Asian

ancestry in South Africa.17

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 angleclosure glaucoma.18