Glaucoma is a clinical condition resulting from increased intraocular pressure over a sustained period of time. It can be caused by excessive secretion of aqueous humor or impedance of the drainage of aqueous humor from the an-terior chamber. The internal tissues of the eye, particularly the retina, are nourished by the diffusion of oxygen and nutrients from the intraocular vessels. Blood flows normally through these vessels (including the capillaries and veins) when the hydrostatic pressure within the vessels exceeds the intraocular pressure. If the drainage of the aqueous humor is impeded, the intraocular pressure increases be-cause the layers of the eye do not allow the wall to expand. This increased pressure interferes with normal retinal nour-ishment and function, causing the retinal nerve fiber layer to atrophy (Fig. F24.1.1). There are two major types of glaucoma:  Open-angle glaucoma is the most common type of glaucoma and the leading cause of blindness among adults. The removal of aqueous humor is obstructed be-cause of reduced flow through the trabecular meshwork of the iridocorneal angle into the scleral venous sinus (canal of Schlemm).  Close-angle glaucoma (acute glaucoma) is much less common and is characterized by a narrowed irido-corneal angle that obstructs inflow of the aqueous humor into the scleral venous sinus. Usually it is associ-ated with a sudden, painful, complete blockage of the scleral venous sinus and can result in permanent blind-ness if not treated promptly. Visual deficits associated with glaucoma include blurring of vision and impaired dark adaptation (symptoms that indicate loss of normal retinal function) and halos around lights (a symptom indicating corneal endothelial damage). If the con-dition is not treated, the retina will be permanently damaged, and blindness will occur. Treatments are directed  toward lowering the intraocular pressure by decreasing the rate of production of aqueous humor or eliminating the cause of the obstruction of normal drainage. Recently,  carbonic anhydrase inhibitors that specifically inhibit carbonic an-hydrase isoenzyme CA-II, which plays an  important role in the production of aqueous humor in humans, are used as the pharmacologic treatment of choice. Dorzolamide and brinzolamide are two carbonic anhydrase inhibitors that are currently available as eyedrops to treat glaucoma.
 FOLDER 24.1 Clinical Correlation: Glaucoma
FIGURE F24.1.1 • Glaucoma. This image shows a view of the fundus of the left eye in a patient with advanced glaucoma. As a result of the increased intraocular pressure, retinal nerve fibers undergo atrophy and shrink in size. Note a pale optic disc in the center of the image with a less pronounced rim due to atrophy of nerve fibers. Enlargement of the optic nerve cup (central area of the optic disc) is also visible and a characteristic finding for glaucoma. Compare this image to a normal retina in Fig. 24.14. (Courtesy of Dr. Renzo A. Zaldivar.)  The meridional (or longitudinal) portion consists of the outer muscle fibers that pass posteriorly into the stroma of the choroid. These fibers function chiefly in stretching the choroid. It also may help open the iridocorneal angle and facilitate drainage of the aqueous humor.
 The radial (or oblique) portion consists of deeper muscle fiber bundles that radiate in a fanlike fashion to insert in the ciliary body. Its contraction causes the lens to flatten and thus focus for distant vision.
 The circular (or sphincteric) portion consists of inner muscle fiber bundles oriented in a circular pattern that forms a sphincter. It reduces the tension on the lens, causing the lens to accommodate for near vision.