Refractive disorders

Normally, the eye 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. The shape of the eyeball further helps to create a clear image on the retina.

People who are farsighted (hyperopic) have trouble seeing anything close, and those who are nearsighted (myopic) have trouble focusing on distant objects. As people reach their early 40s, the lens becomes increasingly stiff, so that it can't focus on nearby objects, a condition called presbyopia. If a person has had a lens removed to treat cataracts but hasn't had a lens implant, objects look blurred at any distance; the absence of a lens is called aphakia. An imperfectly shaped cornea may cause visual distortion from astigmatism.

Everyone should have regular eye examinations by a family doctor, internist, ophthalmologist, or optometrist. The eyes are tested together and individually. Vision testing usually also includes assessments unrelated to refractive error, such as a test of the ability to see colors.

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). 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 more likely to break. Both types 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 nearsightedness and a lower lens that corrects farsightedness.

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 glasses, they may damage the eye, and they can't correct vision for some people as well as eyeglasses can. The elderly and people with arthritis may have trouble handling contact lenses and placing them in their eyes.

Hard (rigid) contact lenses are thin disks made of hard plastic. Gas-permeable lenses, made of silicone and other compounds, are rigid but permit better oxygen transport to the cornea. Soft hydrophilic contact lenses made of flexible plastic are larger and cover the entire cornea. Most soft, nonhydrophilic lenses are made of silicone.

Elderly people generally find soft lenses easier to handle because they're larger. They're also less likely than rigid lenses to fall out or to allow dust and other particles to get trapped underneath. Plus, soft contact lenses are usually comfortable on the first wearing. They do, however, require scrupulous care.

People need to wear their first pair of rigid contact lenses for up to a week before it feels comfortable for a prolonged period. The lenses are worn for a gradually increasing number of hours each day. Although lenses may be uncomfortable at first, they shouldn't be painful. Pain indicates an improper fit.

Most contact lenses must be removed and cleaned every day. As an alternative, a person can use disposable lenses-some of which are replaced every week or two, others of which are replaced every day. Using disposable lenses avoids the need to clean and store lenses because each lens is regularly replaced with a fresh one.

Wearing any type of contact lenses poses a risk of serious, painful complications, including corneal ulceration from an infection, which can lead to a loss of vision. The risks can be greatly reduced by following the instructions of the manufacturer and the eye doctor and by using common sense.

All reusable contact lenses must be sterilized and disinfected; enzyme cleaning is no substitute for sterilizing and disinfecting. The risk of serious infections increases from cleaning contact lenses with homemade saline solution, saliva, tap water, or distilled water and from swimming while wearing contact lenses.

A person shouldn't wear soft contact lenses-including daily wear, extended wear, or disposable types-to bed at night, unless there's a special reason for doing so. If a person experiences discomfort, excessive tearing, vision changes, or eye redness, the lenses should be removed immediately. If the symptoms don't resolve quickly, the person should contact an eye doctor.

Surgery and laser therapy

Certain surgical and laser procedures (refractive surgery) can be used to correct nearsightedness, farsightedness, and astigmatism. However, these procedures don't usually correct vision as well as glasses and contact lenses do. Before deciding on such a procedure, a person should have a thorough discussion with an ophthalmologist and should carefully consider the risks and benefits.

The best candidates for refractive surgery are people whose vision can't be corrected by eye-glasses or contact lenses and people who can't tolerate wearing them. However, many people undergo this surgery for convenience and cosmetic purposes, and many are satisfied with the outcome.

Radial and astigmatic keratotomy: Keratotomy is a surgical procedure used.to treat nearsight-edness and astigmatism. In radial keratotomy, the surgeon makes small radial (or wheel spoke) cuts in the cornea. Usually, four to eight cuts are made. In astigmatic keratotomy, which is used to correct naturally occurring astigmatism and astigmatism after cataract surgery or corneal transplant, the surgeon uses perpendicular cuts. Because the cornea is only 112 millimeter in thickness, 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 cornea, so it can better focus incoming light on the retina. This change improves vision, and about 90 percent of those who have the surgery can function well and drive without their glasses or contact lenses. Some-times, a second or third touch-up procedure is needed to improve vision sufficiently.

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 can't be treated effectively, a surgeon tries to avoid doing too much correction at anyone time. As mentioned, undercorrection can be addressed by a second or third touch-up procedure.

The most serious complication is infection, which develops in far less than 1 percent of cases. When it does develop, it must be treated with antibiotics.

Photorefractive Keratectomy: This laser surgical procedure reshapes the cornea. Photorefractive keratectomy uses a highly focused beam of light to remove small amounts of the cornea and thus change its shape. As with the surgical procedures, changing the shape of the cornea better focuses light onto the retina and improves vision.

Although laser surgery appears promising for correcting poor vision, there are problems with it. For example, the recovery period is longer and more painful than with other refractive surgery procedures. However, the risks are similar to those for radial and astigmatic keratotomy.

 

Refractive disorders

Normally, the eye 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. The shape of the eyeball further helps to create a clear image on the retina. People who are farsighted (hyperopic) have trouble seeing anything close, and those who are nearsighted (myopic) have trouble focusing on distant objects. As people reach their early 40s, the lens becomes increasingly stiff, so that it cant focus on nearby objects, a condition called presbyopia. If a person has had a lens removed to treat cataracts but hasnt had a lens implant, objects look blurred at any distance; the absence of a lens is called aphakia. An imperfectly shaped cornea may cause visual distortion from astigmatism. Everyone should have regular eye examinations by a family doctor, internist, ophthalmologist, or optometrist. The eyes are tested together and individually. Vision testing usually also includes assessments unrelated to refractive error, such as a test of the ability to see colors. 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). 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 more likely to break. Both types 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 nearsightedness and a lower lens that corrects farsightedness. 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 glasses, they may damage the eye, and they cant correct vision for some people as well as eyeglasses can. The elderly and people with arthritis may have trouble handling contact lenses and placing them in their eyes. Hard (rigid) contact lenses are thin disks made of hard plastic. Gas-permeable lenses, made of silicone and other compounds, are rigid but permit better oxygen transport to the cornea. Soft hydrophilic contact lenses made of flexible plastic are larger and cover the entire cornea. Most soft, nonhydrophilic lenses are made of silicone. Elderly people generally find soft lenses easier to handle because theyre larger. Theyre also less likely than rigid lenses to fall out or to allow dust and other particles to get trapped underneath. Plus, soft contact lenses are usually comfortable on the first wearing. They do, however, require scrupulous care. People need to wear their first pair of rigid contact lenses for up to a week before it feels comfortable for a prolonged period. The lenses are worn for a gradually increasing number of hours each day. Although lenses may be uncomfortable at first, they shouldnt be painful. Pain indicates an improper fit. Most contact lenses must be removed and cleaned every day. As an alternative, a person can use disposable lenses-some of which are replaced every week or two, others of which are replaced every day. Using disposable lenses avoids the need to clean and store lenses because each lens is regularly replaced with a fresh one. Wearing any type of contact lenses poses a risk of serious, painful complications, including corneal ulceration from an infection, which can lead to a loss of vision. The risks can be greatly reduced by following the instructions of the manufacturer and the eye doctor and by using common sense. All reusable contact lenses must be sterilized and disinfected; enzyme cleaning is no substitute for sterilizing and disinfecting. The risk of serious infections increases from cleaning contact lenses with homemade saline solution, saliva, tap water, or distilled water and from swimming while wearing contact lenses. A person shouldnt wear soft contact lenses-including daily wear, extended wear, or disposable types-to bed at night, unless theres a special reason for doing so. If a person experiences discomfort, excessive tearing, vision changes, or eye redness, the lenses should be removed immediately. If the symptoms dont resolve quickly, the person should contact an eye doctor. Surgery and laser therapy Certain surgical and laser procedures (refractive surgery) can be used to correct nearsightedness, farsightedness, and astigmatism. However, these procedures dont usually correct vision as well as glasses and contact lenses do. Before deciding on such a procedure, a person should have a thorough discussion with an ophthalmologist and should carefully consider the risks and benefits. The best candidates for refractive surgery are people whose vision cant be corrected by eye-glasses or contact lenses and people who cant tolerate wearing them. However, many people undergo this surgery for convenience and cosmetic purposes, and many are satisfied with the outcome. Radial and astigmatic keratotomy: Keratotomy is a surgical procedure used.to treat nearsight-edness and astigmatism. In radial keratotomy, the surgeon makes small radial (or wheel spoke) cuts in the cornea. Usually, four to eight cuts are made. In astigmatic keratotomy, which is used to correct naturally occurring astigmatism and astigmatism after cataract surgery or corneal transplant, the surgeon uses perpendicular cuts. Because the cornea is only 112 millimeter in thickness, 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 persons visual acuity. The surgery flattens the cornea, so it can better focus incoming light on the retina. This change improves vision, and about 90 percent of those who have the surgery can function well and drive without their glasses or contact lenses. Some-times, a second or third touch-up procedure is needed to improve vision sufficiently. 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 cant be treated effectively, a surgeon tries to avoid doing too much correction at anyone time. As mentioned, undercorrection can be addressed by a second or third touch-up procedure. The most serious complication is infection, which develops in far less than 1 percent of cases. When it does develop, it must be treated with antibiotics. Photorefractive Keratectomy: This laser surgical procedure reshapes the cornea. Photorefractive keratectomy uses a highly focused beam of light to remove small amounts of the cornea and thus change its shape. As with the surgical procedures, changing the shape of the cornea better focuses light onto the retina and improves vision. Although laser surgery appears promising for correcting poor vision, there are problems with it. For example, the recovery period is longer and more painful than with other refractive surgery procedures. However, the risks are similar to those for radial and astigmatic keratotomy.