Nearsightedness, or myopia, as it is medically termed, is a vision condition in which close objects are seen clearly, but objects farther away appear blurred. Nearsightedness occurs if the eyeball is too long or the cornea (the clear front cover of the eye) has too much curvature. As a result, the light entering the eye isn’t focused correctly and distant objects look blurred.
Nearsightedness is a very common vision condition affecting nearly 30 percent of the U.S. population. Some research supports the theory that nearsightedness is hereditary. There is also growing evidence that it is influenced by the visual stress of too much close work.
Generally, nearsightedness first occurs in school-age children. Because the eye continues to grow during childhood, it typically progresses until about age 20. However, nearsightedness may also develop in adults due to visual stress or health conditions such as diabetes.
A common sign of nearsightedness is difficulty with the clarity of distant objects like a movie or TV screen or the chalkboard in school. A comprehensive optometric examination will include testing for nearsightedness. An optometrist can prescribe eyeglasses or contact lenses that correct nearsightedness by bending the visual images that enter the eyes, focusing the images correctly at the back of the eye. Depending on the amount of nearsightedness, you may only need to wear glasses or contact lenses for certain activities, like watching a movie or driving a car. Or, if you are very nearsighted, they may need to be worn all the time.
Laser procedures are also a possible treatment for nearsightedness in adults. They involve reshaping the cornea by removing a small amount of eye tissue. This is accomplished by using a highly focused laser beam on the surface of the eye.
For people with higher levels of nearsightedness, other refractive surgery procedures are now available. These procedures involve implanting a small lens with the desired optical correction directly inside the eye, either just in front of the natural lens (phakic intraocular lens implant) or replacing the natural lens (clear lens extraction with intraocular lens implantation). These procedures are similar to one used for cataract surgery patients, who also have lenses implanted in their eyes (intraocular lens implants).
How is nearsightedness diagnosed?
Testing for nearsightedness may use several procedures in order to measure how the eyes focus light and to determine the power of any optical lenses needed to correct the reduced vision.
As part of the testing, letters on a distance chart are identified. This test measures visual acuity, which is written as a fraction such as 20/40. The top number of the fraction is the standard distance at which testing is performed, twenty feet. The bottom number is the smallest letter size read. A person with 20/40 visual acuity would have to get within 20 feet to identify a letter that could be seen clearly at forty feet in a “normal” eye. Normal distance visual acuity is 20/20, although many people have 20/15 (better) vision.
Using an instrument called a phoropter, an optometrist places a series of lenses in front of your eyes and measures how they focus light using a hand held lighted instrument called a retinoscope. The doctor may choose to use an automated instrument that automatically evaluates the focusing power of the eye. The power is then refined by patient’s responses to determine the lenses that allow the clearest vision.
This testing may be done without the use of eye drops to determine how the eyes respond under normal seeing conditions. In some cases, such as for patients who can’t respond verbally, or when some of the eye’s focusing power may be hidden, eye drops may be used. They temporarily keep the eyes from changing focus while testing is performed.
Using the information obtained from these tests, along with the results of other tests of eye focusing and eye teaming, your optometrist can determine if you have nearsightedness. He or she will also determine the power of any lens correction needed to provide clear vision. Once testing is complete, your optometrist can discuss options for treatment.
How is nearsightedness treated?
Persons with nearsightedness have several options available to regain clear distance vision. They include:
- contact lenses
- laser and other refractive surgery procedures
Eyeglasses are the primary choice of correction for persons with nearsightedness. Generally, a single vision lens is prescribed to provide clear vision at all distances. However, for patients over about age 40, or children and adults whose nearsightedness is due to the stress of near vision work, a bifocal or progressive addition lens may be needed. These multifocal lenses provide different powers or strengths throughout the lens to allow for clear vision in the distance and also clear vision up close.
A large selection of lens types and frame designs are now available for patients of all ages. Eye glasses are no longer just a medical device that provides needed vision correction, but can also be a fashion statement. They are available in a wide variety of sizes, shapes, colors and materials that not only correct for vision problems but also may enhance appearance.
For some individuals, contact lenses can offer better vision than eyeglasses. They may provide clearer vision and a wider field of view. However, since contact lenses are worn directly on the eyes, they require regular cleaning and care to safeguard eye health.
Nearsightedness can also be corrected by reshaping the cornea using a laser beam of light. Two commonly used procedures are photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK).
In PRK, a laser is used to remove a thin layer of tissue from the surface of the cornea in order to change its shape and refocus light entering the eye. There is a limit to how much tissue can safely be removed and therefore the amount of nearsightedness that can be corrected.
LASIK does not remove tissue from the surface of the cornea, but from its inner layers. To do this, a section of the outer corneal surface is cut and folded back to expose the inner tissue. Then a laser is used to remove the precise amount of corneal tissue needed to reshape the eye, and then the flap of outer tissue is placed back in position to heal. The amount of nearsightedness that LASIK can correct is limited by the amount of corneal tissue that can be removed in a safe manner.
People who are highly nearsighted or whose corneas are too thin to allow the use of laser procedures now have another option. They may be able to have their nearsightedness surgically corrected by implanting small lenses in their eyes. These intraocular lenses look like small contact lenses and they provide the needed optical correction directly inside the eye.
People with nearsightedness have a variety of options to correct their vision problem. In consultation with your optometrist, you can select the treatment that best meets you visual and lifestyle needs.
source: www.aoa.org 10/08