Refractive Errors

Myopia | Hyperopia | Astigmatism | Presbyopia

The cornea is the transparent tissue that covers the front of the eye. (Read about "The Eye") It provides a physical barrier that protects the eye. It is also the part of the eye that helps focus light to create an image on the retina. The retina is the paper-thin tissue at the back of the eye that starts the translation of light into vision. If the shape of the cornea isn't right, the image on the retina will be distorted. Since refraction is the bending and focusing of light, imperfections in the focusing power of the eye are called refractive errors.

The Food and Drug Administration (FDA) and the American Academy of Ophthalmology (AAO) say there are four primary types of refractive errors:

• Myopia - In myopia or nearsightedness, people have more difficulty seeing distant objects as clearly as near objects. AAO says that in myopia, the problem is that the eye is too long and images focus in front of the retina instead of on the retina. AAO says myopia is usually inherited and is often discovered in children under age 12, often during routine eye exams. (Read about "Eye Exams") Between the ages of 20 and 40, there is usually little change.
• Hyperopia - Persons with hyperopia or farsightedness, have more difficulty seeing near objects as clearly as distant objects. A hyperopic eye is too short for images to focus on the retina, so close-up objects are blurry.
• Astigmatism - Astigmatism is a distortion of the image on the retina caused by irregularities in the cornea or lens of the eye. Combinations of myopia and astigmatism or hyperopia and astigmatism are common, according to FDA.
• Presbyopia - This is the one most people notice as they get older. When we're young, the lens of the eye is soft and flexible. But as we age, it becomes more rigid. This makes it more difficult to read at close range. AAO says you can have presbyopia in combination with myopia, hyperopia or astigmatism.

Glasses and contacts

Glasses or contact lenses can compensate for the eye's imperfections. AAO points out that glasses, when they have a special coating, can also help protect your eyes from harmful light rays, such as ultraviolet (UV) light rays. (Read about "Eyes and Sun") Sometimes the entire lens of the glasses is the same prescription. Sometimes it's divided; for example, bifocals can have a correction for reading on the bottom half of the lens and another for seeing distance on the top. People may also combine prescriptions to read a computer screen through the top portion of a lens. (Read about "Computers and Health")

Contact lenses are another option for correcting vision. FDA says there are three basic options: hard contacts (which do not allow oxygen transmission through the lens), rigid gas-permeable contacts (which do allow oxygen transmission through the lens) and soft contacts. Soft lenses, which are the most popular, can be disposable or planned-replacement. Lenses can also be prescribed for either daily wear or for extended wear. Extended wear lenses may be left in the eyes while the wearer is both awake and asleep. There are advantages and risks to all types of lenses. FDA cautions that people should never wear their contact lenses for longer periods than recommended by their eye care professional. You can discuss the many different options, as well as necessary precautions, with your doctor or eye care professional. This is essential because the National Eye Institute (NEI) says that corneal infections, although relatively infrequent, are the most serious complication of contact lens wear. (Read about "Keratitis")

In addition to removable lenses, FDA has more recently approved a plastic lens that is permanently implanted into the eye to correct moderate to severe nearsightedness. FDA says there are restrictions and potential complications with this type of lens that should be carefully discussed with your doctor.

FDA has also approved a contact lens that is worn overnight and removed during the day. During the day, nearsightedness is corrected or reduced, according to FDA. Again, there are restrictions and potential complications with this type of lens that should be carefully discussed with your doctor.

Surgical options

Surgery, called refractive surgery, can also be used to correct the focusing power of the eye. There are a number that are used. Perhaps the best known is LASIK.

LASIK stands for Laser-Assisted In Situ Keratomileusis. FDA says LASIK permanently changes the shape of the cornea, using an excimer laser.

LASIK is done using a topical anesthetic. During the procedure, a flap is cut in the cornea. FDA says a hinge is left at one end of this flap. The flap is folded back revealing the stroma, the middle section of the cornea. Pulses from a computer-controlled laser vaporize a portion of the stroma and the flap is replaced.

LASIK can significantly improve vision in people with refractive problems, but it is not for everyone. FDA says the following conditions are among those that can make someone a poor candidate for the procedure:

• if your prescription has changed in the past year
• if you're younger than 20
• if you have certain diseases, including lupus, diabetes, herpes, glaucoma or dry eyes (Read about "Lupus" "Diabetes" "Herpes" "Glaucoma" "Dry Eye")
• if you are active in contact sports
• if you have large pupils or thin corneas

There are other surgical treatments that can be used on refractive errors. Procedures that have been approved by the FDA include:

• Photorefractive keratectomy (PRK) is a procedure that involves removing the very top layer of the cornea. A computer controlled laser is used to alter the shape of the cornea. The surface layer then grows back during the healing process. The Eye Surgery Education Council (ESEC) says PRK can be used to correct myopia, hyperopia and astigmatism.
• LASEK is a close cousin of LASIK, according to ESEC and may be a possibility for people who can't have traditional LASIK. A thinner flap is cut into the cornea, which is why it is possible for some people who can't have LASIK performed, because of thin corneas, to undergo LASEK.
• Laser thermal keratoplasty (LTK) is a procedure to treat hyperopia. It only has a temporary effect and the correction that is accomplished will fade over time, according to ESEC. A laser is used to heat part of the cornea in a circular pattern and cause it to contract and change shape. Patients must be over 40 to be considered.
• Conductive keratoplasty (CK) is another way to heat the fibers in the cornea and cause them to shrink slightly. Using a metal probe, radiofrequency energy is applied to the surface of the cornea in a circular pattern. This changes the cornea's shape, giving it a steeper angle. This allows light to better focus on the retina at the back of the eye and gives clearer images of objects at a distance. Like LTK, it is a temporary fix and patients must be over 40 to be considered, according to FDA.

The Federal Trade Commission (FTC) suggests that people considering LASIK or another type of refractive surgery get as much information as possible beforehand. Among other things, they should ask their doctor how long he or she has been doing LASIK, how much improvement they can expect, what are the risks or potential complications they face and what specific procedures do patients need to follow both before and after surgery in order to reduce the risk of complications.

FDA also warns that dry eye can be a complication of refractive surgery. According to FDA, dry eye not only causes discomfort, but can reduce visual quality due to intermittent blurring and other visual symptoms. FDA says that for some people, the condition may be permanent, and intensive drop therapy and use of plugs may be needed. (Read about "Dry Eye")

Related Information:

    Cataracts

    Macular Degeneration

    Glossary of Vision Terms