Chapter 7 The Sensory Systems 269 The associated vision loss can progress from minor to severe and even to blindness within a few hours or days. Surgical techniques using lasers, air bubbles, or a freezing probe can be used to reattach the retina. In most cases, surgery can restore good vision. If unrepaired, a detached retina can cause loss of peripheral vision and, subsequently, loss of central vision. Vision Disorders A variety of common, but relatively minor, eye defects can impair vision. Many of these defects can be completely corrected with prescription lenses (Figure 7.7). These defects, known collectively as ametropia, are shown in Figure 7.8. For comparison, normal vision is shown in Figure 7.8A. Myopia Commonly known as nearsightedness, myopia (migh-OH-pee-uh) results from an elongated eyeball shape, which causes the lens to focus objects in front of the retina rather than directly upon it (Figure 7.8B). Nearby objects can be seen clearly, but distant objects appear blurry. Eye Injuries The structure of the face and eyes helps protect the eyes from injury. The bony socket in which the eyeball is encased, the eyelid, the eyebrows, and the eyelashes all provide a barrier to foreign objects. In fact, these features are so effective in protecting the eye that many eye injuries do not affect the eyeball itself, but rather the surrounding tissues and structures. Still, certain injuries can damage the eyeball, causing impairment or loss of vision. Usually, minor irritants are flushed from the eye through tear production. Irritating chemicals, however, should be flushed with large quantities of water. Fragments of glass or other solid particles that become lodged in the eye should be removed only by a medical professional. The cornea is well supplied with pain and touch receptors. Consequently, corneal injuries (abrasions or tears) are extremely painful. Fortunately, however, the cornea has an astonishing ability to self-repair most injuries resolve themselves within 24 hours. Trauma to the eye can cause a detached retina, in which the retina separates from the underlying support tissue. The detachment may be partial in the beginning, but without treatment it can rapidly progress to complete detachment. Common Vision Disorders Etiology Prevention Pathology Diagnosis Treatment Myopia elongated eyeball none nearsightedness vision tests corrective lenses, laser surgery Hyperopia shortened (flattened) eyeball none farsightedness vision tests corrective lenses, laser surgery Presbyopia stiffness of lens none age-related farsightedness vision tests corrective lenses, laser surgery Astigmatism irregular curvature of cornea or lens none blurred vision vision tests corrective lenses, laser surgery Amblyopia abnormal dominance of one eye none lazy eye vision tests placing a patch over the strong eye for several weeks or months Diplopia abnormal alignment of the eyes none double vision vision tests eye exercises, corrective lenses, surgery Strabismus muscles in one eye do not coordinate with those in the other eye none one or both eyes turn inward, outward, upward, or downward vision tests eye exercises, corrective lenses, surgery Colorblindness genetic disorder of cone cells in retina none inability to distinguish colors vision tests corrective lenses may help Nyctalopia disorder of rod cells in retina none difficulty seeing at night vision tests corrective lenses or surgery Figure 7.6 Goodheart-Willcox Publisher Copyright Goodheart-Willcox Co., Inc.