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In refractive
disorders, the eye focuses light rays incorrectly on the retina,
causing blurred vision.
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Understanding Refraction
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Normally, the cornea and lens bend (refract) incoming light rays to focus them on the retina. When there is a refractive error, the cornea and lens cannot focus light rays on the retina. Refractive errors can be corrected by eyeglasses or contact lenses.
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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 more rounded, the lens allows near objects to be focused. By becoming flatter, the lens allows objects farther away to be focused. When the cornea and lens cannot focus the image of an object sharply on the retina, it is called a refractive error.
Causes
The lens and cornea may not bend light rays to focus them on the retina correctly for several reasons. The eyeball may be too large for the refractive power of the cornea and lens. 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 refractive power of the cornea and lens, so light is focused behind the retina. This is called farsightedness (hyperopia). People who are farsighted have trouble clearly seeing anything close and far as they get older. Some people have an imperfectly shaped cornea (not perfectly round or spherical), which may cause objects to appear blurred at any distance. This is called astigmatism (see What Is Astigmatism? ). Sometimes people have a significant difference between the refractive errors of the eyes. This is called anisometropia.
As people reach their early 40s, the lens becomes increasingly stiff. The lens 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 Cataract). 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 seeing the chalkboard in school.
Everyone should have regular eye examinations by a family doctor, internist, ophthalmologist, or optometrist. A Snellen eye chart is used to determine sharpness of vision (visual acuity). Visual acuity is measured in relation to what a person with normal (unimpaired) vision sees. For example, a person with 20/60 vision sees at 20 feet what a person with normal vision sees at 60 feet. In other words, the person must be 20 feet away to read letters that a person with normal vision can read from 60 feet away. 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 Symptoms and Diagnosis of Eye Disorders: Visual Field Testing) 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 a small piece of plastic floating on the cornea (contact lens). Good vision correction is possible with both eyeglasses and contact lenses. For most people, the choice is a matter of appearance, convenience, cost, and comfort.
Eyeglasses:
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 and can also be coated with a substance that helps them resist scratches. Both glass and plastic lenses can be tinted or treated with a chemical that darkens them automatically when exposed 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 regions of the eyeglass lens.
Contact 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. Some people, particularly older people and people with arthritis, may have trouble handling contact lenses and placing them in their eyes.
For some people, contact lenses cannot correct vision as well as eyeglasses can. However, newer types of contact lenses have been developed to correct a wider range of refractive errors. For example, soft toric lenses correct astigmatism. For people who have both presbyopia and myopia and who want to wear contact lenses, contact lenses can be prescribed by using an approach called monovision. With monovision, one eye is corrected for reading and the other is corrected for distance vision. However, some people have difficulty adjusting to monovision.
Rigid contact lenses, which are usually gas-permeable, are thin disks made of hard plastic. Oxygen, which the cornea needs to function properly, 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 require some time for the eye to adapt to their presence and 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. Vision with rigid contact lenses is usually sharper than vision with soft contact lenses.
Soft hydrophilic (water-absorbing) contact lenses are made of flexible plastic. They are larger than rigid contact lenses and cover the entire cornea. Not all soft contact lenses allow oxygen to reach the cornea easily.
Because they are larger, soft contact lenses are easier to handle than are rigid contact lenses. 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 from the first wearing. Soft contact lenses require scrupulous care to prevent problems, because the risk of infection is higher with soft contact lenses than with rigid contact lenses.
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 weekly treatment with an enzyme. They are not disposable. Some contact lenses are disposable. They do not require cleaning, enzyme treatment, or disinfecting if used only for one day. Some lenses are used for 1 to 4 weeks and then thrown away. 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. However, the risk of infection is higher with contact lenses that are worn overnight.
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 Corneal Disorders: Corneal Ulcer). The risks can be greatly reduced by following the instructions of the eye doctor and the manufacturer and by using common sense.
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 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. 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.
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. The goal of refractive surgery is to decrease dependence on eyeglasses or contact lenses. Before deciding on such a procedure, people should have a thorough discussion with an ophthalmologist and should carefully consider their 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, which 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 diseases or connective tissue diseases, with a cone-shaped cornea (keratoconus), with severe dry eyes, who are taking certain drugs (for example, isotretinoin or amiodarone), and with a few exceptions, people younger than 18 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 a loose or well-anchored surface), the shape and thickness of the cornea (using pachymetry (see Symptoms and Diagnosis of Eye Disorders: Pachymetry), 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. Because the eye is not held still, the person must 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. About 95% of people do not need corrective lenses for distance vision. The people most likely to have 20/20 distance vision after surgery are those who have weak eyeglass prescriptions before refractive 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 may 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.
Laser
in situ Keratomileusis (LASIK):
LASIK, the most common refractive surgical procedure, is used to correct nearsightedness, farsightedness, and astigmatism. In LASIK, a very thin flap is created in the central part of the cornea with a laser or a cutting device called a microkeratome. The flap is lifted, and 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, and 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 or a loose corneal surface, may not be good candidates for LASIK.
Photorefractive
Keratectomy (PRK):
This procedure also uses an excimer laser to reshape the cornea. It is used primarily to correct moderate nearsightedness, astigmatism, and farsightedness. Unlike LASIK, no flap is created. The cells on the surface of the cornea are removed at the start of the procedure. As in LASIK, 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 (because the removed surface cells need to grow back), PRK can be performed on people who cannot have LASIK, such as those with loose corneal surface cells or thin corneas.
Other Refractive Surgery:
Other techniques are available that may have advantages over or different risks than LASIK and PRK. For people who are very nearsighted, a plastic lens can be placed inside the eye, in front of or behind the iris (phakic intraocular lens implantation). Sometimes the natural lens is removed, and the plastic lens is placed behind the iris (clear lensectomy with intraocular lens implantation). Clear lensectomy with intraocular lens implantation may be better for people with hyperopia and who need reading glasses (presbyopia). 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. Other risks of phakic intraocular lens implantation include cataract formation, glaucoma, and swelling of the cornea over time. Clear lensectomy should usually be avoided in young people who are very nearsighted because they have an increased risk of retinal detachment.
Intracorneal ring segments (INTACS) are used for people with mild nearsightedness without astigmatism. Small plastic arcs are implanted into the middle layer of the cornea near its outer edge. Because no tissue is removed during the procedure, the intracorneal ring segment procedure can be reversed by removing the small plastic arcs. Risks include astigmatism, undercorrection, overcorrection, infection, glare, and seeing halos.
Conductive keratoplasty (CK) is used for people with mild farsightedness without astigmatism or for people that only need to wear reading glasses (presbyopia). It is a quick surgical procedure that does not involve any cutting. Rather, several small laser spots are placed in the cornea. The laser spots cause contraction of the cornea in a ring pattern and change the shape of the cornea. There are few risks, but some people lose some of the effect over time, and a few develop astigmatism.
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 surface or outer part of the cornea parallel to the edge of the cornea. These cuts change the shape of the cornea to help lessen the corneal irregularities of astigmatism.
Last full review/revision December 2007 by Deepinder K. Dhaliwal, MD
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