Refractive Disorders
 See the figure Understanding Refraction.
The eye normally creates a clear image because the cornea and lens bend (refract) incoming light rays to focus them on the retina. The shape of the cornea is fixed, but the lens changes shape to focus on objects at various distances from the eye. By becoming thicker, the lens allows near objects to be focused; by becoming flatter, the lens allows objects farther away to be focused. A refractive error occurs when the cornea and lens cannot focus the image of an object sharply on the retina.
Causes
The lens and cornea may not bend light correctly for several reasons. The eyeball may be too large for the optical power of the focusing system. Because of this, light is focused in front of (rather than directly on) the retina, and the person has trouble clearly seeing distant objects. This is called nearsightedness (myopia). In some people, the eyeball is too small for the optical power of the focusing system, so light is focused behind the retina. This is called farsightedness (hyperopia). People who are farsighted have trouble clearly seeing anything close. Some people have an imperfectly shaped cornea, which may cause objects to appear blurred from any distance. This condition is called astigmatism (see Section 20, Chapter 225).
As people reach their early 40s, the lens becomes increasingly stiff; it does not change shape easily, so it cannot focus on nearby objects, a condition called presbyopia. If a person has had a lens removed to treat cataracts but has not had a lens implant, objects look blurred from any distance (see Section 20, Chapter 231). The absence of a lens (as a result of birth defect, eye injury, or eye surgery for cataract) is called aphakia.
Symptoms and Diagnosis
A person who has a refractive error may notice that vision is blurred. For example, a child who becomes nearsighted may have difficulty with schoolwork.
Everyone should have regular eye examinations by a family doctor, internist, ophthalmologist (a physician who specializes in diagnosing and treating eye diseases and performing eye surgery), or optometrist (a nonphysician specialist in eye defects and refractive errors). A Snellen eye chart is used to determine visual acuity. Visual acuity (sharpness of vision) is measured in relation to what a person with normal vision sees. For example, a person with 20/60 vision sees at 20 feet what a person with normal vision sees at 60 feet. Although refractive errors usually occur in otherwise healthy eyes, testing generally also includes assessments unrelated to refractive error, such as a test of the visual fields (see Section 20, Chapter 225) and eye movements. The eyes are tested together and individually.
Treatment
The usual treatment for refractive errors is to wear corrective lenses. However, certain surgical procedures and laser treatments that change the shape of the cornea also can correct refractive errors.
Corrective Lenses
Refractive errors can be corrected with glass or plastic lenses mounted in a frame (eyeglasses) or with small pieces of plastic placed directly over the cornea (contact lenses). Good vision correction is possible with both eyeglasses and contact lenses; for most people, the choice is a matter of appearance, convenience, and comfort.
Plastic lenses for eyeglasses are lighter but tend to scratch; glass lenses are more durable but are more likely to break. Plastic lenses are more commonly used because they are thinner; they can also be coated with a chemical that helps them resist scratches. Both glass and plastic lenses can be tinted or treated with a chemical that darkens them automatically on exposure to light. Lenses can also be coated to reduce the amount of potentially damaging ultraviolet light that reaches the eye.
Bifocals contain two lenses--an upper lens that corrects the view of distant objects and a lower lens that corrects the view of nearby objects, as in reading. However, people also need to focus at middle distances, such as when viewing a computer screen. Trifocals meet this need by adding a lens for middle distance. Continuously variable lenses (progressive add lenses) also permit focusing at middle distances and have a cosmetic advantage in that there is no line or sharp division between the lenses.
Many people think contact lenses are more attractive than eyeglasses, and some think that vision is more natural with contact lenses. However, contact lenses require more care than eyeglasses, and, rarely, they can damage the eye. For some people, contact lenses cannot correct vision as well as eyeglasses can. However, newer types of contact lenses have been developed to allow correction of a wider range of refractive errors. For example, soft toric lenses allow correction of astigmatism. Some people, particularly older people and people with arthritis, may have trouble handling contact lenses and placing them in their eyes.
Rigid contact lenses, which include hard and gas-permeable contact lenses, are thin disks made of hard plastic. Oxygen, which the cornea needs to survive, does not pass easily through the plastic of the older style hard contact lenses. Gas-permeable contact lenses, which are made of plastics such as newer silicone compounds, permit more oxygen to reach the cornea. Rigid contact lenses can be used to correct irregularities in the cornea (astigmatism).
Rigid contact lenses usually need to be worn for up to a week before they feel comfortable for a prolonged period. The contact lenses are worn for a gradually increasing number of hours each day. Although rigid contact lenses may be uncomfortable at first, they should not be painful. Pain indicates an improper fit.
Soft hydrophilic (water-absorbing) contact lenses are made of flexible plastic. They are larger than rigid contact lenses and cover the entire cornea. Soft contact lenses allow oxygen to reach the cornea easily.
Soft contact lenses are easier to handle than are rigid contact lenses, because they are larger. They are also less likely than rigid contact lenses to fall out or to allow dust and other particles to get trapped underneath. In addition, soft contact lenses are usually comfortable on the first wearing. They do, however, require scrupulous care to prevent problems.
Most contact lenses must be removed and cleaned every day (daily wear). Most contact lenses must be disinfected each night and cleaned of protein and calcium deposits. Some require treatment with an enzyme weekly. Some contact lenses are replaced every day. They do not require cleaning, enzyme treatment, or disinfecting. Some are used for 1 to 4 weeks. Others are not disposable. Some regular or disposable soft contact lenses are designed so that they may be kept in the eye during sleep for a number of days (extended wear). Most can be kept in place for up to 7 days, but newer contact lenses are available that can be kept in place for up to 30 days.
The risk of serious infections increases when swimming with contact lenses and if contact lenses are cleaned with homemade saline solution, saliva, tap water, or distilled water. Sleeping while wearing any type of soft contact lens also increases the risk of serious infections. The risk of infection increases for every night a person sleeps in soft contact lenses. The best way to reduce the risk of infection is to not sleep in contact lenses unless necessary. If a person experiences discomfort, excessive watering of the eye, vision changes, or eye redness, the contact lenses should be removed immediately. If the symptoms do not resolve quickly, the person should contact an eye doctor.
Wearing contact lenses poses a risk of serious, vision-threatening, painful complications, including the formation of ulcers on the cornea. Ulcers can be caused by an infection, which can lead to a loss of vision (see Section 20, Chapter 230). The risks can be greatly reduced by following the instructions of the eye doctor and the manufacturer and by using common sense.
Surgery for Refractive Errors
Surgical and laser procedures (refractive surgery) can be used to correct nearsightedness, farsightedness, and astigmatism. These procedures are used to reshape the cornea so that it is better able to focus light on the retina. These procedures usually correct vision about as well as eyeglasses and soft contact lenses do. Before deciding on such a procedure, a person should have a thorough discussion with an ophthalmologist and should carefully consider his own needs and expectations, along with the risks and benefits.
The best candidates for refractive surgery are people who cannot tolerate contact lenses and those who enjoy activities, such as swimming or skiing, that are difficult to do with eyeglasses or contact lenses. Many people undergo this surgery for convenience and cosmetic purposes. However, refractive surgery is not recommended for all people with refractive errors. For example, people whose eyeglass or contact lens prescription has changed in the past year and those with autoimmune or connective tissue diseases, with signs of keratoconus, who are taking certain drugs (for example, isotretinoin or amiodarone), and with a few exceptions, people younger than 21 years of age, usually should not have laser refractive surgery.
The doctor determines the exact refractive error (eyeglass prescription) before surgery. The eyes are thoroughly examined, and special attention is paid to the surface cells of the cornea (including whether the cornea has loose or well anchored surface skin), the cornea's shape and thickness (using pachymetry (see Section 20, Chapter 225)), the pupil size in light and dark, the intraocular pressure, the optic nerve, and the retina. Refractive surgical procedures are generally brief and cause little discomfort. Eye drops are used to numb the eye. The eye is held still, but the person must also not move the eye during the procedure. Usually, a person can go home soon after the procedure.
After refractive surgery, most people have distance vision that is good enough to do most things well (for example, driving or going to the movies), although not everyone has perfect 20/20 vision without eyeglasses after the procedure. If a person has a weak eyeglass prescription before refractive surgery, he is most likely to have 20/20 distance vision after surgery. Even if they do not wear eyeglasses for distance vision, most people older than 40 still need to wear eyeglasses for reading after refractive surgery.
Complications include overcorrection, undercorrection, excessive inflammation, infection, double vision, sensitivity to bright light, glare and halos around lights, difficulty with seeing or driving at night, wrinkling of the cornea, and deposition of cells or other material in the cornea. Rarely, even with eyeglasses, a person may have worse vision after refractive surgery. Because treating undercorrection is usually easier than treating overcorrection, surgeons prefer not to overcorrect. If undercorrection or overcorrection occurs, further correction can usually be done.
LASIK (Laser in situ Keratomileusis): LASIK, the most common refractive surgical procedure, is used to correct nearsightedness, farsightedness, and astigmatism. In LASIK, a very thin flap is cut in the central part of the cornea with a knife called a microkeratome. Pulses from an excimer laser vaporize tiny amounts of corneal tissue under the flap to reshape the cornea. The flap is then laid back in place and heals over several days. LASIK causes little discomfort during and after surgery. Vision improvement is rapid; many people are able to go back to work within 1 to 3 days. People who have any conditions that preclude refractive surgery, as well as those who have thin corneas, loose corneal surface skin, and large pupils, may not be good candidates for LASIK.
Photorefractive Keratectomy (PRK): This procedure uses an excimer laser to reshape the cornea. It is used primarily to correct moderate nearsightedness, mild astigmatism, and farsightedness. Computer-controlled pulses of highly focused ultraviolet light remove small amounts of the cornea and thus change its shape to better focus light onto the retina and improve vision without eyeglasses. This procedure usually takes less than 1 minute per eye. Although there is more discomfort and longer healing time than with LASIK, PRK can be performed on people who cannot have LASIK, such as those with loose corneal surface cells or thin corneas.
Radial and Astigmatic Keratotomy: These are surgical procedures used to treat nearsightedness and astigmatism by making very deep cuts in the cornea with a small blade.
In radial keratotomy, the surgeon makes small radial (or wheel spoke) cuts in the cornea. Usually, four to eight cuts are made. Because the cornea is only ½ millimeter thick, the depth of the cuts must be determined precisely. The surgeon determines where to make each cut after analyzing the shape of the cornea and the person's visual acuity.
The surgery flattens the central part of the cornea, so it can better focus incoming light on the retina. This change improves vision without eyeglasses, and about 90% of those who have the surgery can function well and drive without eyeglasses or contact lenses. Sometimes, a second or third procedure (enhancement) is needed to sufficiently improve vision without eyeglasses. For some people who have radial keratotomy, the vision without eyeglasses fluctuates slightly throughout the day, and for some, the effect of the surgery may increase over years. Since the development of laser refractive procedures with lower risks and better results, radial keratotomy is rarely performed now.
Astigmatic keratotomy is used to correct naturally occurring astigmatism and astigmatism that occurs after cataract surgery or corneal transplantation. In this procedure, the surgeon makes one or two curved or straight deep cuts in the peripheral part of the cornea parallel to the edge of the cornea.
No surgical procedure is risk free, but the risks from radial and astigmatic keratotomy are small. The major risks are overcorrection and undercorrection of the vision problem. Because overcorrection usually cannot be treated effectively, a surgeon tries to avoid doing too much correction at one time. Undercorrection can be treated by a second or third procedure. Vision can fluctuate with changes in atmospheric oxygen, such as at high altitudes. The most serious complication is infection, which is rare. When it does develop, it must be treated with antibiotics.
Other Refractive Surgery: Other techniques are available that may have advantages over or different risks than LASIK. For people who are very nearsighted, surgical procedures in which a plastic lens is placed inside the eye, in front of the iris (phakic intraocular lens implantation), between the iris and the natural lens (implantable contact lens), or behind the iris after the natural lens has been removed (clear lensectomy with intraocular lens implantation) may be best. Because these techniques make an opening into the eye, there is a very small risk (but significantly higher than for LASIK) of severe infection inside the eye.
Intracorneal ring segments are used for people with mild nearsightedness without astigmatism. Small plastic arcs are implanted into the middle layer of the cornea. Because no tissue is removed during the procedure, the intracorneal ring segment procedure can be reversed by removing the small plastic arcs.
Laser thermal keratoplasty (LTK) is used for people with mild farsightedness without astigmatism. It is a quick surgical procedure that does not involve any cutting; several small laser burns are made in the cornea. There are few risks, but some people lose some or most of the effect over time.
Laser epithelial keratomileusis (LASEK), a modification of the LASIK technique, may be used for nearsightedness, farsightedness, or astigmatism. Like PRK, LASEK is better than LASIK for people with thin corneas.
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