Porth's Essentials of Pathophysiology, 4e

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

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of an asymmetric bowing of the cornea, but it also can result from defects in the cornea, lens, or retina. Both hyperopia and myopia, as well as astigmatism, can be corrected with the appropriate lens. Refractive corneal surgeries such as laser in situ keratomileusis (LASIK), photorefractive keratectomy, and radial kera- totomy can be performed to correct the corneal curva- ture in persons with myopia. 17 Disorders of Accommodation The ability of the eye to adjust its focus from far to near objects by changing the shape of its lens is called accommodation. Accommodation is associated with convergence of the eyes (focusing the vision on a near point), pupillary constriction, and thickening of the lens through contraction of the ciliary muscle of the eye. 16 Contraction of muscle fibers in the ciliary body is controlled mainly by parasympathetic stimulation of the oculomotor nerve (CN III). Paralysis of the cili- ary muscle resulting in pupillary dilation, with loss of accommodation, is called cycloplegia. Pharmacologic cycloplegia through the use of a topical anticholinergic agent is sometimes used to aid in refractive examination of the eye, particularly in small children. The term presbyopia refers to a decrease in accommo- dation that occurs because of aging. 16 The normal prolif- erative process in the lens nucleus that occurs throughout life causes the lens to thicken and its capsule to become less elastic, so that the range of focus or accommodation is diminished. As a result, the person begins to notice the inability to read small print or discriminate close objects. This is usually worse in dim light, on arising in the morning, or when the person is fatigued. Cataracts A cataract is a lens opacity that interferes with the trans- mission of light to the retina. Aging is the most common cause of cataracts, and most people over age 60 have some degree of lens opacity. Although aging is the most important cause of cataracts, other factors such as hered- ity, environmental influences, systemic diseases (e.g., dia- betes mellitus), drugs, and trauma may be involved. 15,18 Long-term exposure to sunlight (ultraviolet radiation) has been associated with increased risk of cataract for- mation. Corticosteroids have been implicated as caus- ative agents in cataract formation. Both systemic and inhaled corticosteroids have been cited as risk factors. 19 Clinical Manifestations. The manifestations of cata- racts depend on the extent of opacity and whether the defect is bilateral or unilateral. Cataracts are usually bilateral (with the exception of traumatic or congenital cataracts). Age-related cataracts, which are typically the result of opacification of the lens nucleus (nuclear scle- rosis), are characterized by increasingly blurred vision and visual distortion. 15,20 Dilation of the pupil in dim light improves vision. In addition to decreased visual acuity, cataracts tend to cause light entering the eye to be scattered, thereby producing glare or the abnormal presence of light in the visual field (see Fig. 38-1C).

Diagnosis and Treatment. Diagnosis of cataracts is based on ophthalmoscopic examination and the degree of visual impairment on the Snellen vision test. On oph- thalmoscopic examination, a cataract may appear as a gross opacity filling the pupillary aperture, or as an opacity silhouetted against the red background of the fundus. Other tests that determine the potential ability to see well after surgery, such as electrophysiologic test- ing in which the response to visual stimuli is measured electronically, may be done. There is no effective medical treatment for cataract. Although strong bifocal lenses, magnification, appropri- ate lighting, and visual aids may help, surgery is the only treatment for cataract. 15,20 It is commonly performed on an outpatient basis with the use of local anesthesia. The cataract lens is typically fragmented into fine pieces, which then are aspirated from the eye, and an intraocu- lar lens implanted. Monofocal intraocular lenses that correct for distance vision are available. Although eye- glasses may still be needed for near vision, the recent introduction of multifocal intraocular lenses has reduced the need for both distance and near vision corrective eyeglasses. cataracts, which are present at birth or appear shortly thereafter, and acquired cataracts, which occur later and are usually related to a specific cause. 15,21 About one third of childhood cataracts are hereditary; another third are secondary to metabolic or infectious processes or associated with a variety of syndromes; and the final third results from undetermined causes. The development of congenital or infantile cataracts depend on the total dose of the agent and the stage of development at the time of exposure. It is during the last trimester of fetal life, genetically or environmentally influenced malformation of the superficial lens fibers can occur. Congenital lens opacities may occur in children of diabetic mothers. The presence of bilateral cataracts is often inherited and associated with other disease conditions, so genetic testing is required. Most congenital cataracts are not progressive and are not dense enough to cause significant visual impair- ment. 15 However, cataracts that are dense, central, and larger than 2 mm in diameter require surgical manage- ment on an urgent basis. If not treated within the first 2 months of life, visual deprivation produces irreversible loss of vision 15,21 (i.e., ambylopia, to be discussed). Disorders ofThe Retina The retina is the most complex structure of the eye. It receives visual images produced by the optical system of the eye and converts the light energy into an electri- cal signal that is transmitted by way of the optic nerve to the visual cortex, where the image is perceived. 16,22,23 Disorders of the retina and its function include disorders of the retinal vessels such as retinopathies that cause Childhood Cataract. Childhood cataracts can be divided into two groups: congenital (infantile)