The cornea and lens of your eye helps you focus. A refractive error occurs when the eye cannot clearly focus. The result of refractive errors is blurred vision, which is sometimes so severe that it causes visual impairment.
Refractive errors are vision problems that happen when the shape of the eye keeps you from focusing well. The cause could be the length of the eyeball (longer or shorter), changes in the shape of the cornea, or aging of the lens.
Common refractive errors are:
- myopia (nearsightedness): difficulty in seeing distant objects clearly;
- hyperopia (farsightedness): difficulty in seeing close objects clearly;
- astigmatism: distorted vision resulting from an irregularly curved cornea, the clear covering of the eyeball;
- Presbyopia: inability to focus on close objects as a result of aging
The most common symptom is blurred vision. Other symptoms may include double vision, haziness, glare or halos around bright lights, squinting, headaches, or eye strain.
Refractive errors cannot be prevented, but they can be diagnosed by an eye examination and treated with corrective glasses, contact lenses or refractive surgery.
If you have a refractive error, such as nearsightedness (myopia), farsightedness (hyperopia), astigmatism or presbyopia, refractive surgery is a method for correcting or improving your vision. This surgical procedure is used to adjust your eye’s focusing ability by reshaping the cornea. Other procedures involve implanting a lens inside your eye. The most widely performed type of refractive surgery is LASIK (laser-assisted in situ keratomileusis), where a laser is used to reshape the cornea.
For people who are nearsighted, certain refractive surgery techniques will reduce the curvature of a cornea that is too steep so that the eye’s focusing power is lessened. Images that are focused in front of the retina, due to a longer eye or steep corneal curve, are pushed closer to or directly onto the retina following surgery.
Farsighted people will have refractive surgery procedures that achieve a steeper cornea to increase the eye’s focusing power. Images that are focused beyond the retina, due to a short eye or flat cornea, will be pulled closer to or directly onto the retina after surgery.
LASIK (laser-assisted in situ keratomileusis) is an outpatient refractive surgery procedure used to treat nearsightedness, farsightedness and astigmatism. A laser is used to reshape the cornea to improve the way the eye focuses light rays onto the retina at the back of the eye.
With LASIK, an ophthalmologist (Eye M.D.) creates a thin flap in the cornea using either a blade or a laser. The surgeon folds back the flap and precisely removes a very specific amount of corneal tissue under the flap using an excimer laser. The flap is then laid back into its original position where it heals in place.
For people who are nearsighted, LASIK is used to flatten a cornea that is too steep. Farsighted people will have LASIK to achieve a steeper cornea. LASIK can also correct astigmatism by shaping an irregular cornea into a more normal shape.
It is important that anyone considering LASIK have realistic expectations. LASIK allows people to perform most of their everyday tasks without corrective lenses. More than 90 percent of people who have LASIK achieve somewhere between 20/20 and 20/40 vision without glasses or contact lenses.
You should be comfortable with the possibility that you may need a second or that you might need to wear glasses for certain activities, such as reading or driving at night. Also, you should be aware that LASIK cannot correct presbyopia, the age-related loss of close-up focusing power.