Porth's Essentials of Pathophysiology, 4e

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Nervous System

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soft contact lens, particularly overnight or without proper disinfection. 14 It also may occur in non–contact lens wear- ers after exposure to contaminated water or soil. Acanthamoeba keratitis is characterized by severe pain, redness of the eye, and photophobia. Diagnosis is confirmed by scrapings and culture with specially pre- pared medium. In the early stages of infection, epithe- lial debridement may be beneficial. Treatment includes intensive use of topical antimicrobials. However, the organism may encyst within the corneal stroma, mak- ing treatment more difficult. Keratoplasty may be neces- sary in advanced disease to arrest the progression of the infection. Disorders of the Lens The lens is a remarkable structure, which like that in a camera, functions to bring images into focus on the retina. The lens is an avascular, transparent, biconvex structure, the posterior side of which is more convex than the anterior side. 2,15,16 It is positioned just posterior to the iris and is held in place by suspensory ligaments known as the zonules , which are composed of numerous fibrils that arise from the ciliary body (see Fig. 38-3B). The pull of the zonular fibers and lens capsule is nor- mally under tension, causing the lens to have a flat- tened shape for distant vision. Relaxation of the fibers allows the lens to assume a more spherical shape for near vision. There are no pain fibers, blood vessels, or nerves in the lens. The lens has three principal components: the lens cap- sule, a subcapsular epithelium, and lens fibers. The lens capsule is a transparent elastic structure that envelops the entire lens. The subcapsular epithelium is a cuboidal layer of cells that is present on the anterior surface of the lens. The lens fibers, which constitute the bulk of the lens, continue to produce new fibers throughout life, with the older fibers being compressed into the central nucleus of the lens. This causes the lens to gradually become larger and less elastic with age. Refraction refers to deflection or bending of light as it passes from one transparent medium to another of dif- ferent density. When parallel light rays pass through the center of a lens, their direction is not changed; however, the divergent rays passing peripherally through a lens are bent (Fig. 38-5A). The refractive power of a lens is usually described as the distance (in meters) from its surface to the point at which the rays come into focus on the retina (i.e., focal length). Usually, this is reported as the recipro- cal of this distance (i.e., diopters). 16 For example, a lens that brings an object into focus at 0.5 m has a refractive power of 2 diopters (1.0/0.5 = 2.0). The closer the object, the more divergent the peripheral rays, and the stronger and more precise the focusing system must be. In the eye, the major refraction of light begins at the convex corneal surface. Further refraction occurs as light moves from the posterior corneal surface to the aqueous humor, from the aqueous humor to the anterior lens surface, from the anterior lens surface to the poste- rior lens surface, and from the posterior lens surface to the vitreous humor.

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Disorders of Refraction A perfectly shaped eyeball and cornea result in opti- mal visual acuity, producing a sharp image in focus at all points on the surface of the retina. Unfortunately, individual differences in eyeball length result in inap- propriate focal image formation. If the anterior-poste- rior dimension of the eyeball is too short, the point of focus will fall behind the retina. This is called hypero- pia, or farsightedness 16 (see Fig. 38-5B). In such cases, the accommodative changes of the lens (to be discussed) can bring distant images into focus, but near images become blurred. If the anterior-posterior dimension of the eyeball is too long, the focus point falls in front of the retina. This condition is called myopia, or nearsight- edness 16 (see Fig. 38-5C). Persons with myopia can see close objects without a problem because accommoda- tive changes in their lens bring near objects into focus, but distant objects are blurred. A refractive defect of the corneal surface, or astigmatism, is usually the result FIGURE 38-5. (A) Accommodation.The solid lines represent rays of light from a distant object, and the dotted lines represent rays from a near object.The lens is flatter for the former and more convex for the latter. In each case, the rays of light are brought to a focus on the retina. (B) Hyperopia corrected by a biconvex lens, shown by the dotted lines. (C) Myopia corrected by a biconcave lens, shown by the dotted lines.