Chapter 7 The Sensory Systems 271 distinguish colors. Red-green colorblindness, the inability to distinguish red from green, is a common form of this disorder. Colorblindness is an inherited condition. For a son to be colorblind, he needs to inherit the gene for colorblindness only from his mother. By contrast, for a daughter to be colorblind, she must inherit the gene from both parents. This explains why men have a higher incidence of colorblindness compared to women. Nyctalopia In nyctalopia (nik-tahl-OH-pee-uh), or night blindness, the rods in the retina do not function optimally, making it difficult for a person to see well at night. Nyctalopia is associated with aging, but several other disorders may also affect night vision. Eye Diseases This section briefly describes eye diseases that commonly require professional care. The etiology, strategies for prevention, pathology, diagnosis, and common treatments for these common eye diseases and disorders are summarized in Figure 7.9. Conjunctivitis Commonly known as “pinkeye,” conjunctivitis (kuhn-junk-ti-VIGH-tis) is a highly contagious inflammation of the conjunctiva. Symptoms include redness, pain, swelling, and mucus discharge. Although it is usually caused by a viral infection, conjunctivitis can be treated with antibiotics if caused by a bacterial infection. Cataracts A cataract (KAT-uh-rakt) is the development of a progressive clouding of the transparent lens of the eye, causing obstruction of light. The result is blurred vision, poor night vision, yellowing of colors, and “halos” around lights. Cataracts are associated with aging and frequently occur in people older than 70 years of age. Exposure to bright sunlight can speed the development of cataracts. Cataracts are treated with laser surgery. MEMORY TIP You may have heard middle-aged individuals joke that they are “getting old” because they need reading glasses. The word presbyopia comes from presby/o, a combining form meaning “old,” and -opia, opia a suffix that means “the condition of having a visual defect.” The term was adopted in the late eighteenth century, when people in their forties were considered old! The suffix -opia is used in conjunction with other combining forms to describe various eye hyperopia, and amblyopia, for Astigmatism Irregular curvature of the cornea or lens causes astigmatism (ah-STIG-ma-tizm), another common eye disorder. The result is blurred vision. Depending on the nature and extent of the curvature, vision may be proportionally affected. Corrective lenses can partially or completely correct astigmatism. Amblyopia Amblyopia (am-blee-OH-pee-uh), or lazy eye, usually appears during childhood when one eye is extremely dominant and the other eye—the lazy eye—develops poor vision. If uncorrected, the lazy eye can become blind. Treatment generally consists of covering the “good” eye, requiring the extrinsic muscles of the lazy eye to function. Diplopia Diplopia (di-PLOH-pee-uh), or double vision, results when one eye is misaligned, causing two images of the object to be perceived simultaneously. Treatment may involve wearing a temporary patch over the affected eye, corrective lenses, or surgery. Strabismus In a person with strabismus (strah-BIZ-mus), one or both eyes drift in different directions due to malfunctioning of the extrinsic muscles of that eye. The condition is treated with eye exercises, corrective lenses, or surgery. Colorblindness Colorblindness affects the cone cells on the retina, impairing an individual’s ability to P middle-aged “getting g g presbyopia presby/o g g g eighteenth people ! -opia conjunction g disordersdisorders—myopia,ia,pomy— hyperopia amblyopia examexample.le.p Copyright Goodheart-Willcox Co., Inc.