Porth's Essentials of Pathophysiology, 4e

968

Nervous System

U N I T 1 0

Cornea

Anterior chamber

Pupil

Iris

Canal of Schlemm

Trabecular meshwork

Posterior chamber

Ciliary body

Lens

A

pressure), opthalmoscopic visualization of the optic nerve, and visual field testing. The elevation in intraocular pressure in persons with open-angle glaucoma is usually treated pharmacologically or, in cases where pharmacologic treatment fails, by increasing aqueous outflow through a surgically created pathway. Drugs used in the long-term management of glaucoma fall into five classes: β -adrenergic antagonists, prostaglandin analogs, α -adrenergic agonists, carbonic anhydrase inhibitors, and cholinergic agonists. 34,37 Most glaucoma drugs are applied topically as eye drops. Topical β -adrenergic antagonists, which are thought to lower intraocular pressure by decreasing aqueous humor production, are usually the drugs of first choice. Prostaglandin analogs, which increase the outflow of aqueous humor through the iris root and ciliary body, are highly effective first-line or adjuvant agents. 34 The α -adrenergic agonists cause an early decrease in production of aqueous humor by constricting the vessels supplying the ciliary body. Carbonic anhydrase inhibitors reduce the secretion of aqueous humor. Cholinergic drugs, which exert their effects by increasing the effects of acetylcholine (a postganglionic neurotransmitter for the parasympathetic nervous system), increase aqueous outflow through contraction of the ciliary muscle and pupillary constriction. When a reduction in intraocular pressure cannot be maintained through pharmacologic methods, laser or surgical trabeculoplasty may become necessary. 34 With laser trabeculoplasty, the microburns created by the laser treatment scar rather than penetrate the trabecular meshwork, a process thought to enlarge the outflow channels by increasing the tension exerted on the trabecular meshwork. Cryotherapy, diathermy, and FIGURE 38-12. Optic nerve head in glaucoma.The anterior part of the optic nerve is decompressed (“optic cupping” arrows), and the blood vessels crossing the margin of the optic nerve head are displaced to the nasal side.The fundus appears dark because the eye of an African-American patient contains numerous pigmented melanocytes in the choroid. (From Klintworth GK.The eye. In: Rubin R, Strayer DS, eds. Pathology; Clinicopathologic Foundations of Medicine. 6th ed. Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams & Wilkins; 2012:1410).

B

C

FIGURE 38-11. (A) Normally, aqueous humor, which is secreted in the posterior chamber, gains access to the anterior chamber by flowing through the pupil. In the angle of the anterior chamber, it passes through the canal of Schlemm into the venous system. (B) In open-angle glaucoma, the outflow of aqueous humor is obstructed at the trabecular meshwork. (C) In angle-closure glaucoma, the aqueous humor encounters resistance to flow through the pupil. Increased pressure in the posterior chamber produces a forward bowing of the peripheral iris, so that the iris blocks the trabecular meshwork.