The anterior chamber is bordered anteriorly by the cornea and posteriorly by the iris diaphragm and the pupil. The anterior chamber angle, which lies at the junction of the cornea and the iris, includes the following 5 structures (Figs 2-11 through 2-14):
The depth of the anterior chamber averages 3.0 mm but is deeper in aphakia, pseudophakia, and myopia and shallower in hyperopia. In the normal adult eye, the anterior chamber is deepest centrally and reaches its narrowest point slightly central to the angle recess.
The anterior chamber is filled with aqueous humor, which is produced by the ciliary epithelium in the posterior chamber. The fluid passes through the pupil aperture and drains by the trabecular pathway (ie, through the trabecular meshwork into the Schlemm canal) and the uveoscleral pathway (ie, the root of the iris and the ciliary body face, into the suprachoroidal space). The uveoscleral pathway, thought to be influenced by age, accounts for up to 50% of aqueous outflow in young people. BCSC Section 10, Glaucoma, discusses the anterior chamber and aqueous humor in detail. High-resolution ultrasound biomicroscopy provides detailed 2-dimensional views of the anterior segment of the eye and is performed in vivo (see Fig 2-12), allowing the clinician to view the relationship of the structures in the anterior segment under different pathologic conditions (Video 2-1).
Figure 2-11 Structures of the anterior chamber angle. 1, Peripheral iris: a, insertion; b, curvature; c, angular approach. 2, Ciliary body band. 3, Scleral spur. 4, Trabecular meshwork: a, posterior; b, mid; c, anterior. 5, Schwalbe line. (*), Corneal optical wedge.
A, Ultrasound biomicroscopy composite image of the anterior segment, including the anterior chamber (AC). The iris is slightly convex, indicating mild pupillary block. The corneoscleral junction (CS jct), ciliary processes, and posterior chamber (PC) region are clearly imaged. The angle is narrow but open. Iris–lens contact is small. B, Ultrasound biomicroscopy image showing normal angle structures. CB = ciliary body; CP = ciliary processes; SS = scleral spur.
(Part A courtesy of Charles Pavlin, MD; part B courtesy of Ken K. Nischal, MD.)
Figure 2-13 Trabecular meshwork. A, Electron micrograph with en face view of the trabecular meshwork from the anterior chamber. Note the decreasing space between trabecular beams in the deeper tissue planes. B, Layers of the trabecular meshwork: uveal, corneoscleral, and juxtacanalicular. The point of highest resistance to outflow is at the juxtacanalicular layer. The outlet channel traverses the limbus and drains into an aqueous vein.
(Part A reproduced with permission from Bowling B. Kanski’s Clinical Ophthalmology: A Systematic Approach. 8th ed. Oxford: Elsevier Limited; 2016:306. Part B modified with permission from Shields MB. Textbook of Glaucoma. 3rd ed. Baltimore: Williams & Wilkins; 1992.)
Imaging the anterior chamber angle.
Courtesy of Hiroshi Ishikawa, MD.
Access all Section 2 videos at www.aao.org/bcscvideo_section02.
The internal scleral sulcus accommodates the Schlemm canal externally and the trabecular meshwork internally. The Schwalbe line, the peripheral limit of the Descemet membrane, forms the anterior margin of the sulcus; the scleral spur is its posterior landmark. The scleral spur receives the insertion of the longitudinal ciliary muscle, contraction of which opens up the trabecular spaces.
Figure 2-14 Anterior chamber angle, ciliary body, and peripheral lens. Note the triangular shape of the ciliary body. The ciliary muscle fibers (CM) appear red in contrast to the connective tissue. Note the longitudinal fibers inserting into the scleral spur (SS), which is clearly delineated from the ciliary muscle in the region of the trabecular meshwork (TM). The ciliary processes (CP) and ciliary stroma (CS) are lined by the double-layered ciliary epithelium (CE). The lens (L) is artifactually displaced posteriorly. (Masson trichrome stain ×8.) C = cornea; I = iris; S = sclera.
(Courtesy of Thomas A. Weingeist, PhD, MD.)
Myofibroblast-like scleral spur cells with contractile properties are disposed circumferentially within the scleral spur. They resemble mechanoreceptors, receive sensory innervation, and are connected by elastic tissue to the trabecular meshwork. In experiments, stimulation with vasoactive intestinal polypeptide (VIP) or calcitonin gene–related peptide (CGRP) causes an increase in outflow facility. Individual scleral spur cells are innervated by unmyelinated axons, the terminals of which contact the cell membranes of the spur cells without an intervening basal lamina. The nerve fibers in this region are immunoreactive for neuropeptide Y, substance P, CGRP, VIP, and nitrous oxide; therefore, they are mediated by sympathetic, sensory, and pterygopalatine nerve pathways. There are no cholinergic fibers in this region.
Myelinated nerve fibers extending forward from the ciliary region to the inner aspect of the scleral spur yield branches to the meshwork and to club-shaped endings in the scleral spur. These endings have the morphologic features of mechanoreceptors found elsewhere in the body, such as in the carotid artery. The endings are incompletely covered by a Schwann cell sheath and make contact with extracellular matrix materials such as elastin. Various functions have been proposed for these endings, including proprioception to the ciliary muscle, which inserts into the scleral spur; signaling contraction of the scleral spur cells; and baroreception in response to changes in intraocular pressure.
Tamm ER, Braunger BM, Fuchshofer R. Intraocular pressure and the mechanisms involved in resistance of the aqueous humor flow in the trabecular meshwork outflow pathways. Prog Mol Biol Transl Sci. 2015;134:301–314.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.