Anterior Chamber
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).
VIDEO 2-1 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.
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.
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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.