C LINICAL A SPECTS OF V ISION AND H EARING

Transcription

1 C LINICAL A SPECTS OF V ISION AND H EARING CHAPTER CONTENTS How Can Vision Become Impaired? Focusing Problems Decreased Transmission of Light Damage to the Retina Optic Nerve Damage: Glaucoma The Eye Examination How Can Hearing Become Impaired? Conductive Hearing Loss Sensorineural Hearing Loss The Ear Examination and Hearing Evaluation Managing Hearing Loss THE PLASTICITY OF PERCEPTION: Decrease in Cortical Function Due to Aging ACROSS THE SENSES: Deafness and Visual Attention SOME QUESTIONS WE WILL CONSIDER What are the major causes of impaired vision and hearing? (546, 564) Can a person be legally blind but have 20/20 vision? (551) How can diseases of the ear and eye be treated? (552, 567) Although it is obvious that the man in Figure 16.1 is examining the woman s eye, most people do not understand exactly what he is seeing or what he is looking for. Even though most Americans have had their eyes and ears examined because of problems with either vision or hearing or just as part of a routine physical examination, few people understand exactly what is going on during these examinations. One of the purposes of this chapter is to demystify what goes on during examinations of the eye and the ear. Before we can understand what eye and ear specialists look for during an examination, we must understand the major problems that can cause impairments in vision and hearing. We therefore begin this 545

2 Figure 16.1 Ophthalmologist examining patient. chapter by describing a number of the most common visual problems and how they are treated to improve or restore vision. After we understand the nature of the most common causes of visual problems, we will describe how a routine eye examination detects these problems. Following our discussion of vision, we take the same approach for hearing. 2. Light is blurred as it enters the eye. Scarring of the cornea or clouding of the lens blurs light as it enters the eye. Specific problems: corneal injury or disease, cataract. 3. There is damage to the retina. The retina can be damaged by disruption of the vessels that supply it with blood, by its separation from the blood supply, VISUAL IMPAIRMENT HOW CAN VISION BECOME IMPAIRED? Four major types of problems can cause poor vision (Figure 16.2): Focus and blur problems Retinal damage 1. Light is not focused clearly on the retina. Problems in focusing light can occur because the eyeball is too short or too long or because the cornea or the lens does not function properly. We will describe the following specific problems: myopia (nearsightedness), hyperopia (farsightedness), presbyopia ( old eye ), and astigmatism. Figure 16.2 Places in the eye where visual problems can occur. Optic nerve damage 546

3 and by diseases that attack its receptors. Specific problems we will discuss include macular degeneration, diabetic retinopathy, detached retina, and hereditary retinal degeneration. 4. There is damage to the optic nerve. The optic nerve can degenerate. When this degeneration is due to a pressure buildup inside the eyeball, the cause is glaucoma. In addition, degeneration can be caused by poor circulation, toxic substances, or the presence of a tumor. We will focus on glaucoma in our discussion. How can we deal with this problem? One way to create a focused image on the retina is to move the stimulus closer. This pushes the focus point further back (see Figure 2.10), and if we move the stimulus close enough, we can push the focus point onto the retina (Figure 16.3b). The distance at which the spot of light becomes focused on the retina is called the far point, and when our spot of light is at the far We begin by considering a problem that affects more people than all the others combined: an inability to adequately focus incoming light onto the retina. FOCUSING PROBLEMS In Chapter 2, we described the optical system of the eye the cornea and the lens which, if everything is working properly, brings light entering the eye to a sharp focus on the retina. We also described the process of accommodation, which adjusts the focusing power of the eye to bring both near and far objects into focus. We will now consider the conditions myopia, hyperopia, presbyopia, and astigmatism, four problems that affect a person s ability to focus an image on the retina. Myopia Myopia, or nearsightedness, is an inability to see distant objects clearly. The reason for this difficulty, which affects over 70 million Americans, is illustrated in Figure 16.3a: In the myopic eye, parallel rays of light are brought to a focus in front of the retina so that the image reaching the retina is blurred. This problem can be caused by either of two factors: (1) refractive myopia, in which the cornea and/or the lens bends the light too much, or (2) axial myopia, in which the eyeball is too long. Either way, light comes to a focus in front of the retina, so that the image on the retina is out of focus, and far objects look blurred. (See Chapter 15 for a discussion of how myopia develops.) Figure 16.3 Focusing of light by the myopic (nearsighted) eye. (a) Parallel rays from a distant spot of light are brought to a focus in front of the retina, so distant objects appear blurred. (b) As the spot of light is moved closer to the eye, the focus point is pushed back until, at the far point, the rays are focused on the retina, and vision becomes clear. Vision is blurred beyond the far point. (c) A corrective lens, which bends light so that it enters the eye at the same angle as light coming from the far point, brings light to a focus on the retina. Angle A is the same in (b) and (c). 547 Clinical Aspects of Vision and Hearing

4 point, a myope can see it clearly. Although a person with myopia can see nearby objects clearly (which is why a myopic person is called nearsighted), objects beyond the far point are still out of focus (see the left column of Table 16.1). The solution to this problem is well known to anyone with myopia: corrective eyeglasses or contact lenses. These corrective lenses bend incoming light so that it is focused as if it were at the far point (Figure 16.3c). Notice that the lens placed in front of the eye causes the light to enter the eye at exactly the same angle as light coming from the far point in Figure 16.3b. Table 16.1 Comparisons of focusing problems associated with the far point and the near point Far Point (Farthest Distance for Clear Vision) Problem: In myopia, the far point is close to the eye, and vision is blurred beyond the far point. Near Point (Closest Distance for Clear Vision) Problem: In presbyopia, the near point moves away from the eye, and vision is blurred closer than the near point. Before leaving our discussion of myopia, let s consider the following question: How strong must a corrective lens be to give the myope clear far vision? To answer this question, we have to keep in mind what is required of a corrective lens: It must bend parallel rays so that light enters the eye at the same angle as a spot of light positioned at the far point. Figure 16.4 shows what this means for two different locations of the far point. When the far point is close, as in Figure 16.4a, we need a powerful corrective lens to bend the light in the large angle shown in Figure 16.4b. However, when the far point is distant, as in Figure 16.4c, we need only a weak corrective lens to bend the light in the small angle shown in Figure 16.4d. Thus, the strength of the corrective lens depends on the location of the far point: A powerful lens is needed to correct vision when the far point is close, and a weak lens is needed to correct vision when the far point is distant. When ophthalmologists or optometrists write a prescription for corrective lenses, they specify the strength of the lens in diopters, using the following relationship: number of diopters 1/far point in meters. Thus, a slightly myopic person with a far Figure 16.4 The strength of a lens required to correct myopic vision depends on the location of the far point. (a) A close far point requiring (b) a strong corrective lens. (c) A distant far point requiring (d) a weak corrective lens. 548

5 Figure 16.5 The number of diopters of lens power needed to correct myopic vision for different far points. Without a corrective lens, vision is blurred at distances greater than the far point. A far point of 10 cm represents severe myopia, and a far point of 100 cm represents mild myopia. point at 1 meter (100 cm) requires a 1-diopter correction (diopters 1/1 1.0). However, a very myopic person with a far point at 2/10 of a meter (20 cm) requires a 5-diopter correction (diopters 1/ ). This relationship between the distance of the far point and the required number of diopters of correction is shown in Figure Although glasses or contact lenses are the major route to clear vision for the myope, surgical procedures in which lasers are used to change the shape of the cornea have been introduced that enable people to experience good vision without corrective lenses. The first widely used laser procedure, photorefractive keratotomy (PRK), was introduced in the United States around In this procedure, a type of laser called an excimer laser, which does not heat tissue, sculpts the cornea to give it either less power (for myopia; Figure 16.6a) or more power (for hyperopia; Figure 16.6b). This procedure appears to be most effective for myopia. Recently, PRK has been largely replaced with another laser procedure, laser-assisted in situ keratomileusis (LASIK). This procedure also involves sculpting the cornea with an excimer laser, but before the cornea is sculpted, a small flap, less than the thickness of a human hair, is cut into the surface of the cornea. The flap is folded out of the way, the cornea is sculpted by the laser, and the flap is then folded back into place. This procedure results in faster healing and less discomfort than the PRK procedures. Cornea Lens (a) Lens Figure 16.6 In the laser photorefractive keratotomy operation, an excimer laser is used to reshape the cornea, as shown by the dashed lines. (a) Reducing the curvature of the cornea on the myopic eye reduces the focusing power of the cornea so that the focus point moves back. (b) Increasing the curvature of the cornea in the hyperopic eye increases the focusing power of the cornea so that the focus point moves forward. (b) 549 Clinical Aspects of Vision and Hearing

6 Hyperopia A person with hyperopia, or farsightedness, can see distant objects clearly but has trouble seeing nearby objects (Figure 16.7a). In the hyperopic eye, the focus point for parallel rays of light is located behind the retina, usually because the eyeball is too short. By accommodating to bring the focus point back to the retina, people with hyperopia are able to see distant objects clearly. Nearby objects, however, are more difficult for the hyperope to deal with, because moving an object closer pushes the focus point farther back. The hyperope s focus point, which is behind the retina for far objects, is pushed even farther back for nearby objects, so the hyperope must exert a great deal of accommodation to return the focus point to the retina. The hyperope s constant need to accommodate when looking at nearby objects (as in reading or doing closeup work) results in eyestrain and, in older people, headaches. Headaches do not usually occur in young people since they can accommodate easily, but older people, who have more difficulty accommodating because of a condition called presbyopia, which we will describe next, are more likely to experience headaches and may therefore require a corrective lens that brings the focus point forward onto the retina (Figure 16.7b). Presbyopia Figure 16.7 Focusing of light by the hyperopic (farsighted) eye. (a) Parallel rays from a distant spot of light are brought to a focus behind the retina, so that, without accommodation, far objects are blurred. Hyperopes can, however, achieve clear vision of distant objects by accommodating. (b) If hyperopia is severe, the constant accommodation needed for clear vision may cause eyestrain, and a corrective lens is required. A decrease in the ability to accommodate due to old age is called presbyopia, or old eye. This decrease in accommodation affects the location of the near point, the closest distance at which a person can still see an object in focus (see the right column of Table 16.1). As a person ages, the near point moves farther and farther away, as shown in Figure The near point for most 20-year-olds is at about 10 cm, but it increases to 14 cm by age 30, 22 cm at 40, and 100 cm at 60. This loss in the ability to accommodate occurs because the lens hardens with age, and the ciliary muscles, which control accommodation, become weaker. These changes make it more difficult for the lens to change its shape for vision at close range. Though this gradual decrease in accommodative ability poses little problem for most people before the age of 45, at around that age the ability to accommodate begins to decrease rapidly, and the near point moves beyond a comfortable reading distance. This is the reason you may have observed older people holding their reading material at arm s length. But the real solution to this problem is a corrective lens that provides the necessary focusing power to bring light to a focus on the retina. Astigmatism Imagine what it would be like to see everything through a pane of old-fashioned wavy glass, which causes some things to be in focus and others to be blurred. This describes the experience of a person with a severe astigmatism; a person with an astigmatism sees through a misshapen cornea, which correctly focuses some of the light reaching the retina 550

7 Figure 16.8 The near point as a function of age. The distance of the near point in centimeters is indicated on the scale at the bottom, and various ages are indicated by the vertical lines. Objects closer than the near point cannot be brought into focus by accommodation. Thus, as age increases, the ability to focus on nearby objects becomes poorer and poorer; eventually, past the age of about 50, reading becomes impossible without corrective lenses. but distorts other light. The normal cornea is spherical, curved like a round kitchen bowl, but an astigmatic cornea is somewhat elliptical, curved like the inside of a teaspoon. Because of this elliptical curvature, a person with astigmatism will see the astigmatic fan in Figure 16.9 partially in focus and partially out of focus. As in hyperopia, eyestrain is a symptom of astigmatism, because no matter how much the person accommodates to try to achieve clear vision, something is always out of focus. Fortunately, astigmatism can be corrected with the appropriate lens. DECREASED TRANSMISSION OF LIGHT The focusing problems described above are the most prevalent visual problems, as evidenced by the large number of people who wear glasses or contact lenses. Because these problems can usually be corrected, most people with focusing problems see normally or suffer only mild losses of vision. We will now consider situations in which disease or physical damage causes severe visual losses or, in some cases, blindness. But before we begin to discuss these problems, we will define what we mean by blindness. What Is Blindness? It is a common conception that a person who is blind lives in a world of total darkness or formless diffuse light. While this description is true for some blind people, many people who are classified as legally blind do have some vision, and many can read with the aid of a strong magnifying glass. According to the definition of blindness accepted in most states, a person is considered legally blind if, after correction with glasses or contact lenses, he or she has a visual acuity of 20/200 or less in the better eye. A visual acuity of 20/20 means that a person can see at 20 feet what a Figure 16.9 Left: Astigmatic fan chart used to test for astigmatism. Right: An astigmatic patient will perceive the lines in one orientation (in this case vertical) as sharp and the lines in the other orientation as blurred. (From Trevor-Roper, 1970.) 551 Clinical Aspects of Vision and Hearing

8 person with normal vision can see at 20 feet. However, a person with an acuity of 20/200 needs to be at a distance of 20 feet to see what a person with normal vision can see from a distance of 200 feet. When we define blindness in terms of visual acuity, we are evaluating a person s ability to see with his or her fovea (which, as we saw in Chapter 2, is the cone-rich area of the retina that is responsible for detail vision). While poor foveal vision is the most common reason for legal blindness, a person with good foveal vision but little peripheral vision may also be considered legally blind. Thus, a person with normal (20/20) foveal vision but little or no peripheral vision may be legally blind. This situation, which is called tunnel vision, results from diseases that affect the retina, such as advanced glaucoma or retinitis pigmentosa (a form of retinal degeneration), which affect peripheral vision but leave the foveal cones unharmed until the final stages, when central vision can also be affected. We begin our discussion of problems caused by disease or injury by considering some conditions that affect both peripheral and central vision because they affect the perception of light at the beginning of the visual process, as light enters the eye through the cornea and the lens. Corneal Disease and Injury The cornea, which is responsible for about 70 percent of the eye s focusing power (Lerman, 1966), is the window to vision because light first passes through this structure on its way to the retina. In order for a sharp image to be formed on the retina, the cornea must be transparent, but this transparency is occasionally lost when injury, infection, or allergic reactions cause the formation of scar tissue on the cornea. This scar tissue decreases visual acuity and sometimes makes lights appear to be surrounded by a halo, which looks like a shimmering rainbow. In addition, corneal disease and injury can also cause pain. Drugs, which often bring the cornea back to its transparent state, are the first treatment for corneal problems. If drugs fail, however, clear vision can often be restored by a corneal transplant operation. The basic principle underlying a corneal transplant operation is shown in Figure The scarred area of the cornea, usually a disk about 6 to 8 mm in diameter, is removed and replaced by a piece of cornea taken from a donor. For best results, this donor should be a young adult who died of an acute disease or of an injury that left the corneal tissue in good condition. In the past, a major problem with this operation was the necessity of transplanting the donor cornea within a few hours after the donor s death. Now, however, donor corneas are preserved by lowtemperature storage in a specially formulated solution. Of the over 10,000 corneal transplants performed every year, about 85 percent are successful. Remember, however, that a corneal transplant operation involves only a small piece of the eye there is no such thing as an eye transplant. Indeed, the problems involved in transplanting a whole eye are overwhelming. For one thing, the optic nerve and the retina are sensitive to lack of oxygen, so that, once the circulation is cut off, irreversible damage occurs within minutes, just as is the case for the brain. Thus, keeping the donor s eye alive presents a serious problem. And even if it were possible to keep an eye alive, there is the problem of connecting the 1 million optic nerve fibers of the donor s eye to the corresponding nerve fibers of the patient s optic nerve. At this point, whole eye transplants are purely science fiction. Figure Corneal transplant operation. The scarred part of the cornea has been removed, and the donor cornea is about to be sutured in place. 552

9 Clouding of the Lens (Cataract) Like the cornea, the lens is transparent and is important for focusing a sharp image on the retina. Clouding of the lens, which is called a cataract, is sometimes present at birth (congenital cataract), may be caused by an eye disease (secondary cataract), or may be caused by injury (traumatic cataract), but the most common cause of cataract is old age (senile cataract). Cataracts develop, for reasons as yet unknown, in 75 percent of people over 65 and in 95 percent of people over 85. Although millions of people have cataracts, in only about 15 percent of the cases does the cataract interfere with a person s normal activities, and only 5 percent of cataracts are serious enough to require surgery the only treatment. The basic principle underlying a cataract operation is illustrated in Figure 16.11a. A small opening is made in the eye, and the surgeon removes the lens while leaving in place the capsule, the tissue that forms a baglike structure that helps support the lens. A method for removing the lens that has the advantage of requiring only a small incision in the eye is phacoemulsification (Figure 16.11b). In this procedure, a hollow tubelike instrument that emits ultrasound vibrations of up to 40,000 cycles per second is inserted through a small incision in the cornea. The vibrations break up the lens, and the resulting pieces are sucked out of the eye through the tube. Removal of the clouded lens clears a path so that light can reach the retina unobstructed, but, in removing the lens, the surgeon has also removed some of the eye s focusing power. (Remember that the cornea accounts for 70 percent of the eye s focusing power; the lens is responsible for the remaining 30 percent.) Although the patient can be fitted with glasses, these create problems of their own, because glasses enlarge the image falling on the retina by as much as 20 to 35 percent. If one eye receives this enlarged image and the other receives a normal image, the brain cannot combine the two images to form a single, clear perception. The intraocular lens, a plastic lens which is placed inside the eye where the original lens used to be, is the solution to this problem. The idea of implanting a lens inside the eye goes back 200 years, but the first workable design for an Pressure to push cataract out Pieces suctioned out Cataract removed (a) (b) Capsule Ultrasound breaks up lens Capsule Figure A cataract operation. (a) The cataract (the clouded lens) is removed through an incision in the cornea. (b) The phacoemulsification procedure for removing the cataract. Highfrequency sound vibrations break up the lens, and the pieces are sucked into the tube. After the lens is removed, an intraocular lens is inserted. intraocular lens was not proposed until Although lenses introduced in the 1950s were not very successful, recent developments in plastics have resulted in small ultralightweight lenses, like the one shown in Figure 16.12, and installing an intraocular lens is now a routine part of most cataract operations. Notice that the lens is placed in the same location as the clouded lens that was removed, just above the capsule, which the surgeon was careful to leave in place when removing the cataract. The presence of the capsule helps hold the intraocular lens in place. 553 Clinical Aspects of Vision and Hearing

10 Wire loop Iris (a) (b) Cornea Wire loop Lens being inserted through pupil Intraocular lens (behind iris) Figure Installing an intraocular lens in the eye after the cataract has been removed. (a) The lens is inserted through an incision in the cornea. Notice that it is being inserted through the pupil so that it will be positioned where the original lens was, behind the iris and just above the capsule. (b) Frontal view, showing the lens in place behind the iris. The small wire loops hold the lens in place. The retina receives nourishment from the retinal circulation and from the pigment epithelium on which it rests. All four conditions described below cause a loss of vision because of their effects on the retinal circulation and on the relationship between the retina and the pigment epithelium. Diabetic Retinopathy Before the isolation of insulin in 1922, most people with severe diabetes, a condition in which the body doesn t produce enough insulin, had a life expectancy of less than 20 years. The synthesis of insulin (which won the 1923 Nobel Prize for its discoverers) greatly increased the life expectancy of diabetics, but one result of this greater life expectancy has been a great increase in an eye problem called diabetic retinopathy. Of the 10 million diabetics in the United States, about 4 million show some signs of this problem. Figure shows what happens as the disease progresses. At first, the capillaries swell, and although most cases of diabetic retinopathy stop here, a large number of diabetics suffer vision losses even when the disease stops at this point. The disease s further progression, which occurs in a small percentage of patients, involves a process called neovascularization. Abnormal new blood vessels are formed (Figure 16.13b), which do not supply the retina with adequate oxygen and which are fragile and so bleed into the vitreous humor (the jellylike substance that fills the eyeball); this bleeding interferes with the passage of light to the retina. Neovascularization can also cause scarring of the retina and retinal detachment (see below). DAMAGE TO THE RETINA (a) Figure Blood vessels in diabetic retinopathy. (a) In early stages of the disease, the blood vessels swell and leak slightly. (b) In later stages, in a process called neovascularization, abnormal new blood vessels grow on the surface of the retina. (b) 554

11 One technique for stopping neovascularization is called laser photocoagulation, in which a laser beam of high-energy light is aimed at leaking blood vessels. The laser photocoagulates, or seals off, these vessels and stops the bleeding. A procedure called panretinal photocoagulation has been used with considerable success. In this technique, the laser scatters 2,000 or more tiny burns on the retina, as shown in Figure The burns do not directly hit the leaking blood vessels, but, by destroying part of the retina, they decrease the retina s need for oxygen, so that the leaking blood vessels dry up and go away. If laser photocoagulation is not successful in stopping neovascularization, a procedure called a vitrectomy, shown in Figure 16.15, is used to eliminate the blood inside the eye. In this operation, which is done only as a last resort, a hollow tube containing a guillotine-like cutter takes in the vitreous humor and chops it into pieces small enough to be sucked out of the eye through the tube. When the vitreous humor and blood are removed, they are replaced with a salt solution. This procedure removes the blood inside the eye and often prevents further bleeding. Macular Degeneration Imagine your frustration if you could see everywhere except where you were looking, so that every time you looked at something you lost sight of it. That is exactly what happens if a region of the retina called the macula is damaged. The macula is an area about 5 mm in diameter that surrounds and includes the cone-rich fovea (itself only slightly larger than one of the periods on this page). If the macula degenerates, blindness results in the center of vision (Figure 16.16). This condition is extremely debilitating because, although peripheral vision remains intact, the elimination of central vision makes reading impossible. There are a number of forms of macular degeneration, but the most common is called age-related macular degeneration because it occurs, without obvious reason, in older people. In its mild form, there is a slight thinning of the cone receptors and the formation of small white or yellow lumps on the retina. This form of macular degeneration usually progresses slowly and may not cause serious visual Figure Laser photocoagulation in the treatment of diabetic retinopathy. The picture illustrates the technique of panretinal photocoagulation. Each dot represents a small laser burn on the retina. Figure Vitrectomy. The hollow needle inserted into the eyeball first sucks out the liquid inside the eye and then fills the eyeball with a salt solution. problems. In 5 to 20 percent of the cases, however, small new blood vessels, similar to those in diabetic retinopathy, grow underneath the macular area of the retina. These new blood vessels form very rapidly over a period of only one or two months and leak fluid into the macula, killing the cone receptors. 555 Clinical Aspects of Vision and Hearing

12 Figure Macular degeneration causes a loss of central vision. Until recently, there was no treatment for agerelated macular degeneration. However, a study by the National Eye Institute indicates that, if the problem can be caught at an early stage in some patients with the more severe form of the disease, laser photocoagulation can stop or greatly reduce leakage of the newly formed vessels. Detached Retina Detached retina, a condition in which the retina becomes separated from the underlying pigment epithelium (Figure 16.17), has occurred in a number of athletes because of traumatic injuries to the eye or the head. Sugar Ray Leonard, the former welterweight boxing champion, retired temporarily from boxing because of a detached retina. He returned to boxing a number of years later amid much discussion about whether returning to the ring was worth the risk of losing his sight in one eye. As it turned out, Leonard won both the fight and the gamble with his sight, apparently escaping without further damaging his eye. A detached retina affects vision for two reasons: (1) For good image formation, the retina must lie smoothly on top of the pigment epithelium, and (2) when the retina loses contact with the pigment epithelium, the visual pigments in the detached area are separated from enzymes in the epithelium necessary for pigment regeneration. When the visual pigment can no longer regenerate, that area of the retina becomes blind. The treatment for a detached retina is an operation to reattach it. The basic idea behind this operation is to cause the formation of scar tissue inside the eye that will attach itself to the retina and anchor it in place. This process is accomplished by applying either a cooling or a heating probe to exactly the right place on the outside of the eyeball. Figure 16.17b shows the procedure used to determine where to apply the probe. While looking into the eye with a special viewing device, the surgeon presses on the outside of the eyeball, which causes an indentation that can be seen inside the eye. The surgeon presses at a number of points, until the indentation inside the eyeball matches the location of the tear or hole in the retina, where the detachment originated. Once the point where the detachment has occurred is located, it is marked on the outside of the eyeball, and that point is cooled or heated to create an 556

13 Figure (a) A detached retina. (b) Procedure for reattaching the retina. To locate the site of detachment, a probe pushes the eyeball from outside while the surgeon, at S, looks into the eye. Once the site of the detachment is located, the outside of the eye is marked, and a cooling or heating probe is applied at the marked point. inflammatory response. The retina must then be pushed flush with the wall of the eyeball. This is accomplished by placing a band around the outside of the eyeball that creates a dumbbell-shaped eye. Then, with the retina pressed against the wall of the eye, the inflammation causes scarring that welds the retina back onto the pigment epithelium. If the area of detached retina is not too big, there is a 70 to 80 percent chance that this procedure will work. In most cases, it restores vision, although vision is sometimes not restored even though the retina is successfully reattached. The larger the detached area, the less likely it is that this operation (or others, which we will not describe here) will work. Sometimes, if a retinal tear can be caught at an early stage, before fluid has gotten through it and caused the retina to detach, it is possible to prevent detachment by surrounding the tear with laser burns. This is a quick procedure that can be carried out in the ophthalmologist s office and requires no surgery. Hereditary Retinal Degeneration The most common form of hereditary retinal degeneration is a disease called retinitis pigmentosa, a degeneration of the retina that is passed from one generation to the next (although not always affecting everyone in a family). We know little about what actually causes the disease, although one hypothesis is that it is caused by a problem in the pigment epithelium. A person with retinitis pigmentosa usually shows no signs of the disease until reaching adolescence. At this time, the person may begin to notice some difficulty in seeing at night, since the disease first attacks the rod receptors. As the person gets older, the disease slowly progresses, causing further losses of vision in the peripheral retina. Then, in its final stages, which may occur as early as a person s 30s or as late as the 50s or 60s (depending on the strain of the disease), retinitis pigmentosa also attacks the cones, and the result is complete blindness. OPTIC NERVE DAMAGE: GLAUCOMA A leading cause of blindness in the United States is glaucoma, which causes nerve fibers in the optic nerve to degenerate and therefore prevents the nerve impulses generated by the retina from being transmitted to the brain. Although the end result of glaucoma is damage to the optic nerve, the source of the problem is at the front of the eye. We can understand how damage to the front of the eye affects the optic nerve by looking at the cross section of the eye in Figure 16.18a. Under normal conditions, the aqueous humor (the liquid found in the space between the cornea and the lens), which is continuously produced at A, passes between the iris and the lens following the path indicated by the arrows; it then drains from the eye at B. In glaucoma, the drainage of aqueous humor is partially blocked. Closed-angle glaucoma is a rare form of glaucoma in which a pupillary block (Figure 16.18b) constricts the opening between the iris and the lens and causes a pressure buildup that pushes the iris up, thereby closing the angle between the 557 Clinical Aspects of Vision and Hearing

14 Figure (a) Arrows indicate the flow of aqueous humor in the normal eye. The aqueous humor is produced at A and leaves the eye at B. In open-angle glaucoma, the aqueous humor cannot leave the eye because of a blockage at B. (b) In closed-angle glaucoma, the raised iris hinders the flow of aqueous humor from the eye. An iridectomy cutting a hole in the iris can provide a way for the aqueous humor to reach B. cornea and the iris and blocking the area at B where the aqueous humor leaves the eye. In open-angle glaucoma, which is the most common form of the disease, the eye looks normal (Figure 16.18a), but the drainage area at B is partially blocked, so that it is more difficult for the aqueous humor to leave the eye. The blocks that occur in both closed- and open-angle glaucoma result in a large resistance to the outflow of aqueous humor, and since the aqueous humor continues to be produced inside the eye, the intraocular pressure the pressure inside the eyeball rises. This increase in intraocular pressure presses on the head of the optic nerve at the back of the eye. This pressure cuts off circulation to the head of the optic nerve, which results in the degeneration of the optic nerve fibers that causes blindness. The increase in pressure that occurs in closedangle glaucoma usually happens very rapidly and is accompanied by pain. The treatment for this type of glaucoma is an operation called an iridectomy, in which a small hole is created in the iris with a laser (Figure 16.18b). This hole opens a channel through which the aqueous humor can flow and releases the pressure on the iris. With the pressure gone, the iris flattens out and uncovers the area at B so that aqueous humor can flow out of the eye. Intraocular pressure increases more slowly in open-angle glaucoma, so the patient may be unaware of any symptoms. In many cases, visual loss is so gradual that much of the patient s peripheral vision is gone before its loss is noticed. For that reason, ophthalmologists strongly recommend that people over 40 have their eyes checked regularly for glaucoma, since early detection greatly enhances the chances of effective treatment by medication. In 5 to 10 percent of the cases of open-angle glaucoma, medications do not decrease the pressure, and an operation becomes necessary. The goal of this operation is to cut an opening in the wall of the eyeball that creates a new route for fluid to leave the eye. THE EYE EXAMINATION So far, we have described some of the things that can go wrong with the eye and how these problems are treated. In this part of the chapter, we will describe the procedures used to uncover some of these problems. Before describing the eye examination, we will consider who examines the eyes. 558

15 Who Examines Eyes? Three types of professionals are involved in eye care: ophthalmologists, optometrists, and opticians. 1. An ophthalmologist is an M.D. who has completed undergraduate school and four years of medical school, which provide general medical training. In order to become an ophthalmologist, a person needs four or more years of training after graduation from medical school to learn how to treat eye problems medically and surgically. Some ophthalmologists receive even further training and then specialize in specific areas, such as pediatric ophthalmology (practice limited to children), diseases of the cornea, retinal diseases, or glaucoma. Most ophthalmologists, however, treat all eye problems, as well as prescribing glasses and fitting contact lenses. 2. An optometrist has completed undergraduate school and, after four years of additional study, has received a doctor of optometry (O.D.) degree. Optometrists can examine eyes and fit and prescribe glasses or contact lenses. In some states, optometrists have won the right to include medical treatment using drugs for some eye conditions. Surgery, however, is still done exclusively by ophthalmologists. 3. An optician is trained to fabricate and fit glasses and, in some states, contact lenses, on the prescription of an ophthalmologist or an optometrist. What Happens During an Eye Exam? The basic aims of an eye exam are (1) to determine how well the patient can see, (2) to correct vision if it is defective, (3) to determine the causes of defective vision by examining the optics of the eye and checking for eye diseases, and (4) to diagnose diseases that the patient may not even be aware of. To accomplish these aims, an examination by an eye specialist usually includes the following. Medical History The first step in an eye exam is to take a medical history. This history focuses on any eye problems that the patient may have had in the past, on any current eye problems, and on any general medical problems that may be related to the patient s vision. Visual Acuity This is the familiar part of the eye exam, in which you are asked to read letters on an eye chart like the one in Figure The old version of the eye chart, which most people are familiar with, had a large E at the top. This new version results in more accurate measurements of acuity because there are the same number of letters on each line and the spacing between the letters is proportional to the sizes of the letters. The top row of letters is the 20/400 line. This means that a person with normal vision should be able to see these letters from a distance of 400 feet. Since the eye chart is usually viewed from about 20 feet, people with normal vision see these letters easily. When asked to read the smallest line he or she can see, the patient usually picks a line that is easily read. With a little encouragement, however, most patients find that they can see lines smaller than the one they originally picked, and the examiner has the patient read smaller and smaller lines until letters are missed. The smallest line a person can read indicates his or her visual acuity, with normal vision defined as an acuity of 20/20. A person with worse than normal acuity say, 20/40 must view a display from a distance of 20 feet to see what a person with normal acuity can see at 40 feet. A person with better than normal acuity say, 20/10 can see from a distance of 20 feet what a person with normal vision can see only at 10 feet. The visual acuity test described above tests only foveal vision, since the patient looks directly at each letter, so the image of that letter falls on the fovea. Thus, as mentioned earlier, a person who scores 20/20 on a visual acuity test may still be classified as legally blind if he or she has little or no peripheral vision. Testing peripheral vision is usually not part of a routine eye exam, but when peripheral vision problems are suspected, a technique called perimetry is used, in which the patient is asked to indicate the location of small spots of light presented at different locations in the periphery. This test locates blind spots (called scotomas) that may be caused by retinal degeneration, detachment of the retina, or diseases such as glaucoma. 559 Clinical Aspects of Vision and Hearing

16 In addition to using the eye chart to test far vision, it is also customary to test near vision, especially in older patients who may be experiencing the effects of presbyopia. This testing is done by determining the smallest line of a card like the one in Figure that the patient can see from a comfortable reading distance. Refraction A score of 20/60 on a visual acuity test indicates worse than normal acuity but does not indicate what is causing this loss of acuity. Acuity could be decreased by one of the diseases described earlier or by a problem in focusing: myopia, hyperopia, presbyopia, or astigmatism. If the problem lies in the focusing mechanism of the eye, it is usually easily corrected by glasses or contact lenses. Refraction is the procedure used to determine the power of the corrective lenses needed to achieve clear vision. The first step in refraction is a retinoscopy exam, an examination of the eye with a device called a retinoscope. This device projects a streak of light into the eye that is reflected into the eye of the examiner. The examiner moves the retinoscope back and forth and up and down across the eye, noticing what the reflected light looks like. If the patient s eye is focusing the light correctly, the examiner sees the whole pupil filled with light, and no correction is necessary (in this case, the patient will usually have tested at 20/20 or better in the visual acuity test). If, however, the patient s eye is not focusing the light correctly, the examiner sees a streak of light move back and forth across the pupil as the streak of light from the retinoscope is moved across the eye. To determine the correction needed to bring the patient s eye to 20/20 vision, the examiner places corrective lenses in front of the eye while still moving the streak of light from the retinoscope back and forth. One way of placing these lenses in front of the eye is to use a device like the one shown in Figure Figure Not Available Figure A card for testing close vision. The patient s close vision is determined by the smallest line that he or she can read from a comfortable reading distance. Figure A device for placing different corrective lenses in front of the patient s eyes. Different lenses are placed in front of the eye during the retinoscopy exam and again as the patient looks at the eye chart. 560

17 This device contains a variety of lenses that can be changed by turning a dial. The examiner s goal is to find the lens that causes the whole pupil to fill up with light when the retinoscope is moved back and forth. This lens brings light to a focus on the retina and is usually close to the one that will be prescribed to achieve 20/20 vision. The retinoscopy exam results in a good first approximation of the correct lens to prescribe for a patient, but the ultimate test is what the patient sees. To determine this, the examiner has the patient look at the eye chart and places lenses in front of the patient s eyes to determine which one results in the clearest vision. When the examiner determines which lens results in 20/20 vision, he or she writes a prescription for glasses or contact lenses. To fit contact lenses after determining the prescription, the examiner must match the shape of the contact lens to the shape of the patient s cornea. Refraction is used to determine the correction needed to achieve clear far vision. Using a procedure we will not describe here, the examiner also determines whether a correction is needed to achieve clear near vision. This determination is particularly important for patients over 45 years old, who may experience reading difficulties due to presbyopia. External Eye Exam In an external eye exam, the examiner uses a variety of tests to check the condition of the external eye. The examiner checks pupillary reaction by shining light into the eye, to see if the pupil responds by closing when the light is presented and by opening when the light is removed. The examiner also checks the color of the eye and the surrounding tissues. Red eye may indicate that an inflammation is present. The movement of the eyes is checked by having the patient follow a moving target, and the alignment of the eyes is checked by having the patient look at a target. If the eyes are aligned correctly, both eyes will look directly at the target, but, if the eyes are misaligned, one eye will look at the target, and the other will veer off to one side. Slit-Lamp Examination The slit-lamp examination checks the condition of the cornea and the lens. The slit lamp, shown in Figure 16.21, projects a narrow slit of light into the patient s eye. This light can be precisely focused at different places inside the eye, and the examiner views this sharply focused slit of light through a binocular magnifier. This slit of light is like the sharp edge of a knife that cuts through the eye. What does the examiner see when looking at the cutting edge of light from the slit lamp? By focusing Figure A patient being examined with a slit lamp. The examiner is checking the condition of the lens and the cornea by viewing the slit of light through a binocular magnifier. 561 Clinical Aspects of Vision and Hearing

18 the light at different levels inside the cornea and lens, the examiner can detect small imperfections places where the cornea or the lens is not completely transparent that cannot be seen by any other method. These imperfections may indicate corneal disease or injury or the formation of a cataract. Tonometry Tonometry measures intraocular pressure, the pressure inside the eye, and is therefore the test for glaucoma. Nowadays, an instrument called a tonometer is used to measure intraocular pressure, but before the development of this device, it was known that large increases of intraocular pressure, which accompany severe cases of glaucoma, cause the eye to become so hard that this hardness could be detected by pushing on the eyeball with a finger. There are several types of tonometers, which measure the intraocular pressure by pushing on the cornea. The Schiotz tonometer is a hand-held device that consists of a small plunger attached to a calibrated weight. The weight pushes the plunger and indents the cornea. If the intraocular pressure is high, the plunger causes a smaller indentation than if the intraocular pressure is normal. Thus, intraocular pressure is determined by measuring the indentation of the cornea. (Though this procedure may sound rather painful, it is not, because the examiner applies a few drops of anesthetic to the cornea before applying the tonometer.) The applanation tonometer, shown being applied to a patient s cornea in Figure 16.22, is a more sophisticated and accurate instrument than the Schiotz tonometer. After a few drops of anesthetic are applied to the cornea, the flat end of a cylindrical rod, called an applanator, is slowly moved against the cornea by the examiner, who watches the applanator s progress through the same magnifiers used for the slit-lamp exam (Figure 16.22). The examiner pushes the end of the applanator against the cornea until enough pressure is exerted to flatten a small area on the cornea s curved surface. The greater the force that must be exerted to flatten the cornea, the greater the intraocular pressure. Ophthalmoscopy So far, we have looked at the outside of the eye (external eye exam), examined the lens and cornea (slit-lamp exam), and measured the intraocular pressure (tonometry), but we have yet to look at perhaps the most important structure of all: the retina. Since there is a hole (the pupil) in the front of the eye, it should be simple to see the retina; we only have to look into the hole. Unfortunately, it s not that simple; if you ve ever looked into a person s pupil, you realize that it s dark in there. In order to Figure An applanation tonometer being applied to a patient s cornea. 562

19 see the retina, we must find some way to light up the inside of the eye. This is accomplished by the ophthalmoscope, which was first developed by Hermann von Helmholtz, of the Young-Helmholtz theory of color vision, in The principle underlying Helmholtz s ophthalmoscope is shown in Figure A light off to the side is directed into the patient s eye with a halfsilvered mirror. The half-silvered mirror reflects some of the light and transmits the rest, so that an examiner positioned as shown in Figure can see through the mirror and into the patient s eye. Actual ophthalmoscopes are much more complicated than the one diagrammed here, since they include numerous lenses, mirrors, and filters, but the basic principle remains the same as that of the original ophthalmoscope designed by Helmholtz in Figure is a patient s-eye view of an examination with an ophthalmoscope, although the examiner is actually very close, as shown in Figure Figure shows a close-up of what the ophthalmologist Figure Patient s-eye view of an ophthalmoscopic exam. Figure The principle behind the ophthalmoscope. Light is reflected into the patient s eye by the half-silvered mirror. Some of this light is then reflected into the examiner s eye (along the dashed line), allowing the examiner to see the inside of the patient s eye. Figure Close-up view of the head of the optic nerve and the retinal circulation as seen through an ophthalmoscope. 563 Clinical Aspects of Vision and Hearing

20 sees if the patient has a normal retina. The most prominent features of this view of the retina are the optic disk, the place where the ganglion cell fibers leave the eye to form the optic nerve, and the arteries and veins of the retina. In this examination, the ophthalmologist focuses on these features, noting any abnormalities in the appearance of the optic disk and the retinal circulation. For example, the ophthalmologist may detect the presence of diabetic retinopathy by noticing a number of very small blood vessels (neovascularization). In fact, all the retinal injuries and diseases described above cause some change in the appearance of the retina, which can be detected by looking at the retina with an ophthalmoscope. Our description of an eye examination has covered most of the tests included in a routine exam. The examiner may decide to carry out other tests if a problem is suggested by the routine tests. For example, a technique called fluorescein angiography is used to examine more closely the retinal circulation in patients with diabetic retinopathy. A fluorescent dye is injected intravenously into the arm, and when this dye reaches the retina, it sharply outlines the retinal arteries and veins, as shown in Figure Only by this technique can we observe the leakage of fluid that occurs in the abnormal neovascularized blood vessels that accompany diabetic retinopathy. Determining the location of the leakage identifies areas that are to be treated with photocoagulation. Other tests, which we will not describe here, include the electroretinogram, which measures the electrical response of the rod and cone receptors and is therefore useful in diagnosing such retinal degeneration as retinitis pigmentosa, and the cortical evoked potential, which measures the electrical response of the visual cortex and is useful for diagnosing vision problems caused by head injuries or tumors. See Summary Table 16.1 for an overview of the material we have discussed so far. HEARING IMPAIRMENT Image Not Available In our consideration of the clinical aspects of vision, we saw that visual functioning can be impaired because of problems in delivering the stimulus to the receptors, because of damage to the receptors, and because of damage to the system that transmits signals from the receptors toward the brain. An analogous situation exists in hearing, as we will see by considering the various causes of hearing impairment. HOW CAN HEARING BECOME IMPAIRED? Figure Fluorescein angiograph of a normal eye. In this view, the head of the optic nerve is on the far right, just outside the picture. The fovea is in the dark space near the middle of the picture. In the normal eye, the blood vessels stand out in sharp contrast to the background. (Photograph courtesy of Eye and Ear Hospital of Pittsburgh.) In considering the question How can hearing become impaired? it is important to distinguish between impairments in the auditory system and what effects these impairments have on a person s hearing. A hearing impairment is a deviation or change for the worse in either the structure or the functioning of the auditory system. A hearing handicap is the disadvantage that a hearing impairment causes in a person s ability 564