Porth's Essentials of Pathophysiology, 4e

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Nervous System

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reflex test performed to check for abnormalities in the back of the eye (posterior segment) and opacities in the visual axis, such as cataracts or corneal opacity. 45 Visual examinations should then be performed during all well-child visits. These should include age-appropriate evaluation of visual acuity, ocular alignment, and ocular media clarity (cataracts, tumors). The treatment of children at risk for development of amblyopia must be instituted well before the age of 6 years to avoid the suppression phenomenon. Surgery for congenital cataracts and ptosis should be done early. Severe refractive errors should be corrected. In children with strabismus, the alternate blocking of the vision in one eye and then the other with a patch forces the child to use both eyes for form discrimination. The duration of occlusion of vision in the good eye must be short (2 to 4 hours per day) and closely monitored, or deprivation amblyopia can develop in the good eye as well. ■■ The eyeball, or optic globe, which is a complex sensory organ that provides the sense of vision, is protected posteriorly by the bony structures of the orbital cavity and anteriorly by the eyelids. The eyeball is composed of three layers: an outer layer, which includes the white, opaque sclera and the transparent cornea; a middle vascular layer called the uvea, which includes the choroid, ciliary body, and iris; and an inner neural layer, which includes an outer pigment epithelium and the inner, light-sensitive retina, which is connected to the optic nerve. ■■ A conjunctiva lines the inner surface of the eyelids and covers the eyeball to the junction of the cornea and sclera. Conjunctivitis, also called red eye or pink eye, may result from bacterial or viral infection, allergic reactions, or the injurious effects of chemical agents, physical agents, or radiant energy. ■■ Keratitis, or inflammation of the cornea, can be caused by infections, hypersensitivity reactions, ischemia, defects in tearing, or trauma.Trauma or disease that involves the stromal layer of the cornea heals with scar formation and permanent opacification.These opacities interfere with the transmission of light and may impair vision. ■■ The lens is a biconvex, transparent, flexible structure that can change shape to allow precise focusing of light on the retina. Refraction refers to the ability of the lens to bend light rays in order to focus an object on the retina. In hyperopia, SUMMARY CONCEPTS

or farsightedness, the image falls behind the retina, and in myopia, or nearsightedness, it falls in front of the retina. Accommodation is the process by which a clear image is maintained as the gaze is shifted from a far object to a near object. Presbyopia is a change in the lens that occurs because of aging such that the lens becomes thicker and less able to change shape and accommodate for near vision. A cataract is characterized by increased lens opacity. ■■ The retina covers the inner aspect of the posterior two thirds of the eyeball and is continuous with the optic nerve. It contains the photoreceptors for vision: the rods, for black and white discrimination, and the cones, for color vision.The retinopathies are visual disorders involving the small blood vessels of the retina, which can result from a number of local and systemic disorders, including diabetes mellitus and hypertension. Retinal detachment involves separation of the sensory receptors from their blood supply; it causes blindness unless reattachment is accomplished promptly. Macular degeneration, which is a leading cause of blindness in the elderly, is characterized by loss of central vision resulting from destructive changes in the central fovea. ■■ Glaucoma is a chronic, degenerative visual field loss usually associated with an increase in intraocular pressure due to impaired outflow of aqueous humor from the anterior chamber of the eye. Glaucoma is commonly classified as open-angle glaucoma, in which impaired outflow of aqueous humor is due to dysfunction of the drainage system, and angle-closure glaucoma, in which occlusion of the anterior chamber angle impairs access to the drainage system. ■■ Eye movement, which is controlled by the extraocular muscles, provides for alignment of the eyes and binocular vision. Strabismus, which involves abnormalities in coordination of eye movements, can be caused by weakness or paralysis of the extraocular muscles (paralytic strabismus), inappropriate length or insertion of the extraocular muscles (nonparalytic strabismus), or accommodation disorders. The neural pathways for vision develop during infancy. Amblyopia is a condition of diminished vision that results from inadequately developed CNS circuitry due to visual deprivation or abnormal binocular interactions during the period of visual immaturity.