Refractive Error
LASIK improves vision be correcting refractive errors of the eye. A refractive error is an imperfection in how the eye focuses light, resulting in blurred vision.
For perfect vision, the eye must focus light to a single point on the retina, the light-sensitive inner lining of the back of the eye. The goal of LASIK and other refractive surgery is to enable the eye to properly focus light on the retina without the need for eyeglasses or contact lenses.
There are three main types of refractive errors: nearsightedness, farsightedness and astigmatism. Refractive errors and the lens powers or surgical treatments used to correct them are measured in units called diopters (D).
Nearsightedness
Nearsightedness (or myopia) is a refractive error in which light comes to a focus too early in the eye, before it reaches the retina. Myopia causes blurred distance vision; near vision usually remains clear.
Approximately 41.6 percent of Americans are nearsighted, according to the National Health and Nutrition Examination Survey conducted in 1999-2004.
Most people who undergo LASIK do so to correct myopia.
Farsightedness
In farsightedness (or hyperopia), the eye isn't powerful enough to bring light to a focal point on the retina. Hyperopia usually causes blurred near vision, but higher amounts of farsightedness can cause blurry vision at all distances.
Sometimes, the only symptoms of hyperopia are eye strain and headaches when reading or doing other near vision tasks. Young people in particular can compensate for mild amounts of hyperopia without glasses by exerting extra focusing effort.
One large study suggests 45 percent of Americans over age 40 have at least 0.75 D of farsightedness (Baltimore Eye Survey, 1991).
Farsightedness sometimes is confused with presbyopia, which is a normal age-related change in the eye that reduces near vision. Virtually everyone, even people with good vision, experiences presbyopia some time after age 40 and will need reading glasses or bifocals.
Farsightedness usually develops early in life, and a person who wears glasses for hyperopia eventually will need bifocals or other multifocal lenses when they get older and become presbyopic.
Astigmatism
Astigmatism — which sometimes is incorrectly called "stigmatism" — occurs when the cornea (the clear front surface of the eye) and/or the lens inside the eye has unequal curves, which prevents light from being focused to a single point on the retina. Astigmatism usually causes blurred vision at all distances.
Several studies suggest that approximately 30 percent of people worldwide are affected by astigmatism. It's common for someone who is nearsighted or farsighted to also have astigmatism.
There are two powers on a lens prescription for astigmatism: a sphere power and a cylinder power. The sphere power corrects myopia or hyperopia; the cylinder power corrects astigmatism. A third number, called the "axis," identifies the location of the cylinder power in the lens. The axis is a whole number between 1 and 180, corresponding to a protractor scale superimposed on the front of the eye.
Astigmatism usually develops early in life. However, certain eye conditions can cause astigmatism to increase in adults.
Keratoconus is an eye disease that causes the cornea to become thinner and bulge forward, creating high amounts of irregular astigmatism that cannot be corrected with eyeglasses. Options for keratoconus treatment include gas permeable contact lenses, hybrid contact lenses, and surgical treatments, including corneal collagen cross-linking (CXL) and Intacs intrastromal corneal ring segment (ICRS) implantation. In cases of severe keratoconus, a cornea transplant may be the only solution.
The development of cataracts also can cause an increase in astigmatism.
How Refractive Errors Are Detected
A comprehensive eye exam is the best way to find out if you have nearsightedness, farsightedness or astigmatism. During the exam, an eye test called a refraction will be performed to see if you have refractive errors that require treatment.
If a refractive error is diagnosed, your eye doctor can discuss available treatment options, including LASIK, to help you decide which type of vision correction is best for you.
Eye Exercises
You may have seen ads on television and the Internet for programs of eye exercises that say you can improve your vision without glasses, contact lenses or refractive surgery. Most eye care professionals agree that eye exercises cannot prevent or eliminate refractive errors.
Programs of eye exercises designed to eliminate your need for glasses should not be confused with vision therapy that is prescribed by some eye doctors to correct binocular vision problems such as poor eye teaming, focusing flexibility or depth perception. In some cases, supervised programs of vision therapy also are recommended for the treatment of amblyopia ("lazy eye") or minor vision problems after LASIK or other refractive surgery.
Be sure to consult with your eye doctor before purchasing any program of eye exercises.