Porth's Essentials of Pathophysiology, 4e

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Disorders of Special Sensory Function: Vision, Hearing, and Vestibular Function

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segment, and a highly modified outer segment. The cell membrane of the outer segment is tightly folded to form membranous disks (rods) or conical shapes (cones) con- taining visual pigment. These disks are continuously synthesized at the base of the outer segment and shed at the distal end. Discarded membranes are phagocytized by the retinal pigment cells. If this phagocytosis is dis- rupted, as in a condition called retinitis pigmentosa , the sensory retina degenerates. An area near the center of the retina, called the macula lutea (i.e., “yellow spot”), is especially adapted for acute and detailed vision. 16 This area is composed entirely of cones. In the central portion of the macula, the fovea centralis (foveola), the blood vessels and inner- most layers are displaced to one side instead of resting on top of the cones. This allows light to pass unimpeded to the cones without passing through several layers of the retina. Many of these cones are connected one to one with ganglionic cells, an arrangement that favors high acuity. Retinal Blood Supply The blood supply for the retina is derived from two sources: the choriocapillaris (i.e., the capillary layer of the choroid) and branches of the central retinal artery (Fig. 38-7A). Oxygen and other nutrients are supplied by diffusion from blood vessels in the choroid. Because the choriocapillaris provides the only blood supply for the fovea centralis, detachment of this part of the sensory retina from the pigment epithelium causes irreparable loss of vision. The central artery, which is a branch of the ophthalmic artery, supplies the rest of the retina. Retinal veins follow a distribution paral- lel to the arterial branches and carry venous blood to

the central vein of the retina, which exits the back of the eye through the optic disk. Funduscopic examination of the eye with an ophthal- moscope provides the means for examining the retinal blood vessels and other aspects of the retina (Fig. 38-7B). Because the retina is an embryonic outgrowth of the brain and the blood vessels are to a considerable extent representative of brain blood vessels, the ophthalmo- scopic examination of the fundus of the eye permits the study and diagnosis of metabolic and vascular diseases of the brain as well as pathologic processes that are specific to the retina. Retinopathies Retinopathies, which involve the small blood vessels of the retina, are characterized by changes in vessel structure and the development of microaneurysms, neo- vascularization, hemorrhage, and retinal opacities. 24,25 Microaneurysms are outpouchings of the retinal vascu- lature. On ophthalmoscopic examination, they appear as minute, unchanging red dots associated with blood vessels. Microaneurysms tend to leak plasma, resulting in localized edema that gives the retina a hazy appear- ance. They may also bleed, but areas of hemorrhage and edema tend to clear spontaneously. However, they reduce visual acuity if they encroach on the mac- ula and cause degeneration before they are absorbed. Neovascularization involves the formation of new blood vessels. They can develop from the choriocapil- laris, extending between the pigment and the sensory layers of the retina; or from the retinal veins, extending between the sensory retina and the vitreous cavity and sometimes into the vitreous. These new blood vessels are fragile, leak protein, and are likely to bleed. Although

Vitreous chamber (contains vitreous humor)

Sclera

Branches of retinal vessels

Choroid

Macula

Optic disk

Retina

Fovea centralis

Macula lutea

Central retinal vein

Central retinal artery

Optic nerve

Posterior ciliary artery

Optic disk (blind spot)

Retinal blood vessels

A

B

FIGURE 38-7. (A) Retinal circulation showing the distribution of the central retinal and posterior ciliary arteries and the central retinal vein. (B) Funduscopic image of the normal retina (From Moore KL, Daley AF, Agur AMR. Clinically Oriented Anatomy. 6th ed. Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams &Wilkins; 2010:897.)