The links to each individual chapter of the Color Atlas of Gonioscopy are available at the Chapters link, below.
The purpose of gonioscopy is to permit visualization of the iridocorneal angle (or simply “angle”). This is the area in which the trabecular meshwork lies and is therefore the part of the eye that is responsible for aqueous outflow. Before exploring gonioscopic techniques and findings, it is important to review the anatomy and function of the structures of the angle (1‑1 and 1‑2).
1-1 Sketch of the anterior chamber angle. The labeled structures (listed alphabetically) are: A. Ch., anterior chamber; Bo., Bowman’s layer; Chor., choroid; Cil. ep., ciliary epithelium; Cil. m., ciliary muscle (longitudinal); Cil. pr., ciliary process; Cil. r. + c., ciliary body (radial and circular muscles); Coll. v., collector veins; Cor. ep., corneal epithelium; Cor. w., corneal wedge; Cr., iris crypt; Desc., Descemet’s membrane; Desc. en., corneal endothelium (or Descemet’s endothelium); F, iris furrow; H, Hanover’s canal; Hy., hyaloid; Ir. ep., iris pigment epithelium; L. c., lens cortex; Lim. v., limbal vessels; M. c., major circle of iris; Non pig., nonpigmented ciliary epithelium; Ora, ora serrata; P, Petit’s canal; Pig., pigmented ciliary epithelium; P. Ch., posterior chamber; P-l. s., postlenticular space; Ret., retina; Schl., Schlemm’s canal; Schw., Schwalbe’s line; Sin., angle recess (or sinus); Sph., sphincter; S. sep., scleral septum; S. sp., scleral spur; Suprach. s., suprachoroidal space; Tr., trabecular meshwork; W, Wieger’s ligament; Z, zonules. (Because this sketch was drawn in the 1940s, some of the terms, such as Descemet’s endothelium, are different from those used today.)
When examined with the slit lamp, the iris is seen to have two main zones: a central pupillary zone and a peripheral ciliary zone (1‑3). A wavy border, the collarette, separates these areas. There are intermittent crypts, which can extend deep into the stoma, and concentric furrows, which become more prominent as the pupil dilates.
The iris is composed of an anterior stromal layer and a posterior epithelial layer. The stroma is vascular connective tissue that has no anterior epithelial covering. The musculature of the iris lies within the stroma. A 1 mm wide band of sphincter muscle rings the pupil. The myoepithelial cells of the dilator muscle are spread throughout the stroma from the iris root as far centrally as the sphincter. Blood vessels in the iris are mostly located in the stromal layer and have a radial orientation. They are frequently visible in lightly pigmented eyes. The greater circle of the iris is found in the ciliary body or in the root of the iris and is often partially visible in a gonioscopic examination.
Posteriorly, there are two epithelial layers. As in the ciliary body, the cells of these two epithelial layers are aligned apex to apex. The anterior layer has little pigmentation and is continuous with the outer (pigmented) layer of the ciliary body. The posterior layer is densely pigmented and faces the posterior chamber. This layer is continuous with the nonpigmented layer of ciliary epithelium.
The iris generally inserts at a variable level into the face of the ciliary body, which is posterior to the scleral spur. Less commonly, the iris will insert on, or anterior to, the scleral spur. The iris thins at the periphery near its insertion.
Ciliary Body Face
The ciliary body lies behind the iris. Its functions include the manufacture of aqueous humor, the control of accommodation, the regulation of aqueous outflow, the secretion of hyaluronate into the vitreous, and the maintenance of a portion of the blood-aqueous barrier. There are two major muscle groups in the ciliary body: the circular muscle fibers, which are responsible for accommodation, and the longitudinal muscle fibers, which control the outflow of aqueous by pulling open the trabecular meshwork and Schlemm’s canal.
1-2 Histopathologic slide of the chamber angle showing structures labeled in 1‑1. Hematoxylin and eosin stain. (Courtesy of the National Museum of Health and Medicine, Armed Forces Institute of Pathology.)
1-3 Normal iris. The peripheral ciliary zone is separated from the pupillary zone by the wavy collarette (large arrow). The narrow band of sphincter muscle can be seen around the pupil. This iris has many crypts (small arrow).
The ciliary body face is the portion of the ciliary body that borders on the anterior chamber. The degree to which the ciliary body face is visible depends on the level and angle of iris insertion. In some eyes, the ciliary body face is not visible, being completely obscured by iris.
Although most outflow of aqueous occurs through the trabecular meshwork, approximately 10% is by nonconventional routes, primarily through the ciliary body face into the suprachoroidal space (Bill and Phillips, 1971), but also through the root of the iris. Uveoscleral outflow is pressure-independent. Cholinergic agents, such as pilocarpine, compact the fibers in the ciliary body and decrease uveoscleral outflow. Anticholinergic drugs, such as atropine, increase nonconventional outflow through the ciliary body face (Bill and Phillips, 1971). In some eyes with severe compromise of trabecular outflow, anti-cholinergic medications may lower intraocular pressure, while cholinergic drugs may, paradoxically, increase intraocular pressure. The prostaglandin F2a drugs appear to promote a marked increase in nonconventional outflow through the ciliary body face (Gabelt and Kaufman, 1989) and are now routinely used as first-line agents in glaucoma therapy.
The scleral spur is composed of a ring of collagen fibers that run parallel to the limbus. It marks the posterior border of the trabecular meshwork. The spur projects slightly into the anterior chamber and is seen as a white to yellowish line in most eyes. The longitudinal muscle of the ciliary body attaches to the scleral spur and opens the trabecular meshwork by pulling on the spur. On histopathologic slides, the scleral spur can be located by following the longitudinal muscle of the ciliary body forward to its point of attachment (1‑4). The structural integrity supplied by the scleral spur may prevent the ciliary muscle from causing Schlemm’s canal to collapse (Moses and Grodzki, 1977).
1-4 The scleral sulcus, in which the trabecular meshwork lies, is clearly demonstrated in this histopathological specimen stained with the Masson trichrome stain. The scleral sulcus is bordered anteriorly by Schwalbe’s line (white arrow) and posteriorly by the scleral spur (black arrow). The longitudinal muscle (LM) of the ciliary body attaches to the scleral spur. The separation between sclera and ciliary body (*) is an artifact. (Courtesy of the National Museum of Health and Medicine, Armed Forces Institute of Pathology.)
The trabecular meshwork is located between the scleral spur and Schwalbe’s line. Most of the trabecular meshwork sits within the scleral sulcus (1‑4). Approximately 90% of aqueous outflow is through the trabecular meshwork. This flow is pressure-dependent, increasing as intraocular pressure increases. Aqueous humor flowing through the trabecular meshwork enters Schlemm’s canal and from there flows into the scleral, episcleral, and conjunctival venous systems. For aqueous to exit the eye by this route, the intraocular pressure must be higher than the episcleral venous pressure. At pressures below episcleral venous pressure (8 to 15 mm Hg), all aqueous outflow must be via nonconventional routes (1‑5) (Pederson, 1986).
The trabecular meshwork consists of three layers (1‑6). Closest to the aqueous is the uveal meshwork, which consists of endothelium-coated collagen beams separated by large (25 to 75 μm) spaces (1‑7). The uveal meshwork extends from the ciliary body in the angle recess to Schwalbe’s line and covers the ciliary body face, the scleral spur, and the trabecular meshwork. In most eyes the uveal meshwork is colorless and is either not visible or is seen only as a glistening veil in the angle of young patients. In some eyes the uveal meshwork is dense and pigmented, giving a rough appearance to the trabecular meshwork and occasionally obscuring portions of the scleral spur. The uveal meshwork does not provide any resistance to aqueous outflow. Iris processes appear as thicker strands in front of the uveal meshwork and extend from the periphery of the iris to the trabecular meshwork (Chapter 5).
1-5 Uveoscleral and trabecular (conventional) outflow as a function of the intraocular pressure. Below episcleral venous pressure all outflow is through uveoscleral and other nonconventional means. C, outflow facility; IOP, intraocular pressure; Pe, episcleral venous pressure. (Reprinted with permission from Macmillan Publishers Ltd. Pederson JE. Ocular hypotony. Eye.1986;105:220_226.)
1-6 The three layers of the trabecular meshwork (uveal, corneoscleral, and juxtacanalicular) are shown in this cutaway illustration. (Reprinted with permission from R. Rand Allingham. Shields’ Textbook of Glaucoma. 2005. Courtesy of Lippincott Williams & Wilkins.)
1-7 Pillars of the uveal trabecular meshwork are seen in this scanning electron micrograph. Note the large intervening spaces which do not provide resistance to aqueous outflow. (Courtesy of Carmen Rummelt and Volker Rummelt, MD, University of Erlangen–Nürnberg.)
The corneoscleral meshwork lies deep to the uveal meshwork. It is the central layer that extends from the scleral spur to the anterior wall of the scleral sulcus. It is a layer of five to nine sheets of endothelium-coated collagen fibers perforated by holes of 5 to 50 μm (Flocks, 1956). This layer, like the uveal meshwork, does not offer significant resistance to aqueous outflow.
The deepest layer of the trabecular meshwork is the juxtacanalicular tissue, the last layer that aqueous crosses before entering Schlemm’s canal. The juxtacanalicular tissue has trabecular endothelium on one side and Schlemm’s endothelium on the other. Between these endothelial layers is a loose connective tissue. This juxtacanalicular tissue provides the most resistance to aqueous outflow. The aqueous must travel through the endothelium of Schlemm’s canal to enter the canal. There are no direct routes of any significance between endothelial cells into Schlemm’s canal. Sondermann’s canals have been described in the past as being direct passages through the juxtacanalicular tissue to Schlemm’s canal, but there is doubt that such passages actually exist.
Aqueous outflow occurs primarily through the posterior portion of the trabecular meshwork—which is the portion that overlies Schlemm’s canal. With time, this posterior portion of the meshwork usually becomes pigmented, whereas the anterior meshwork usually remains relatively nonpigmented.
The endothelial cells in the trabecular meshwork differ from corneal endothelial cells in that they are larger with less prominent cell borders (1-8) (Spencer et al, 1968). A function of endothelial cells is to digest phagocytized foreign material. After engulfing foreign material some endothelial cells undergo autolysis or migrate away from the trabecular meshwork into Schlemm’s canal (Grierson and Chisholm, 1978). With age or repeated insult the endothelial cell count decreases, as does aqueous outflow.
Schlemm’s canal, which is a tube 190 to 350 μm in diameter at the base of the scleral sulcus, collects aqueous and drains it into the venous system (Hoffmann and Dumitrescu, 1971). Occasionally, the canal is a plexus rather than a single, discrete vessel. On the trabecular side of Schlemm’s canal, there are many vacuoles through which aqueous traverses the endothelial cells. The vacuoles and the prominent nuclei of the endothelial cells lining the trabecular side of the canal give it a roughened appearance (1‑9) (Tripathi, 1968). On the scleral side of Schlemm’s canal, the endothelium is much smoother and is intermittently perforated by 25 to 35 aqueous collector channels.
Schlemm’s canal is not a rigid structure, although it does contain septa, which provide some support. At high intraocular pressures the canal collapses and resistance to aqueous outflow increases. The longitudinal muscle of the ciliary body can open Schlemm’s canal by pulling on the scleral spur. Cholinergic drugs decrease resistance to outflow through this action.
1-8 Scanning electron micrograph of trabecular endothelial cells with large nuclei and indistinct cell borders. (Courtesy of Carmen Rummelt and Volker Rummelt, MD, University of Erlangen–Nürnberg.)
1-9 Schlemm’s canal, demonstrating the roughened endothelial surface on the trabecular meshwork side of the canal and the smoother surface on the corneoscleral side of the canal. Aqueous passes through the endothelium and into Schlemm’s canal by way of the vesicles. CW, corneoscleral wall; D, diverticulum; PT, pericanalicular connective tissue; N, nucleus of cell; SC, Schlemm’s canal; V, vesicle. (Reprinted with permission from Frederick A. Jakobiec. Ocular Anatomy, Embryology and Teratology, “Functional Anatomy of the Anterior Chamber Angle” by Ramesh C. Tripathi. 1982. www.lww.com.)
Schwalbe’s line occurs in a 50 to 150 μm transition zone (zone S) between the trabecular meshwork and the corneal endothelium (1‑10). It is the anterior border of the trabecular meshwork and the posterior border of Descemet’s membrane. There is also a transition from the scleral curvature to the steeper corneal curvature at Schwalbe’s line, which can cause a settling of pigment in this area.
1-10 Schwalbe’s line, demonstrating transition from trabecular meshwork endothelium (TM) to corneal endothelium (C). (Courtesy of Carmen Rummelt and Volker Rummelt, MD, University of Erlangen–Nürnberg.)