The Eye’s Refractive Errors
Refractive Errors and Vision Correction Surgery Dallas Fort Worth
Myopia is the most common form of refractive error and affects nearly 30% of the U.S. population. If you are nearsighted, you can take off your glasses or contacts and see objects clearly when held at a close range, but objects in the distance will appear blurred.
Myopia is caused either by an eye that is too long, a cornea that is too steep, or a combination of these two causes. Most nearsighted people feel that they have "bad" eyes. This is not really true. A nearsighted eye is usually healthy. It just has an abnormal focal length. This abnormal focal length can be corrected with glasses or contact lenses, which change the focus or power of the eye thus correcting the vision.
Of all myopic people, about 90% have corrections less than -6.00 diopters. The categories below indicate the severity levels for myopia:
- Mild Myopia <-3.00 diopters
- Moderate Myopia -3.00 to -6.00 diopters
- High Myopia -6.00 to -9.00 diopters
- Extreme Myopia >-9.00 diopters
Almost everyone feels that his or her myopia is severe because of how dramatically dependent he or she is on glasses or contact lenses. However, only one in ten myopic people are actually in the high or extreme categories.
In order to surgically correct the myopic eye, the focusing power of the cornea must be reduced. This can be accomplished by flattening the central portion of the cornea with a refractive procedure such as PRK or LASIK.
Hyperopia, or farsightedness, is the opposite of myopia. In this condition, the image of concern is focused behind the retina, creating blurry vision. The hyperopic person cannot see clearly up close or at distance.
Hyperopia occurs when the eye is too short or the cornea is too flat. Just as in myopia, hyperopia can be corrected with contact lenses or glasses. If hyperopia is to be corrected by a surgical procedure, the focusing power of the cornea must be increased. This is accomplished by steepening the cornea. As you can imagine, producing a steepening effect on the cornea with a laser is more difficult than flattening the cornea.
Astigmatism occurs when the cornea has an irregular shape. A normal cornea is shaped like the surface of a basketball, a round surface. A cornea in an eye with astigmatism is shaped like a football, or like the back surface of a spoon. This condition often occurs along with either myopia or hyperopia and causes light to focus in more than one point on the retina, producing blurry vision.
Astigmatism is also measured in diopters. Of all myopic people, 50% or more have astigmatism as well. Most of these people have corrections of less than 1 diopter. The table below shows the categories of severity for astigmatism:
- Mild Astigmatism <1.00 diopters
- Moderate Astigmatism 1.00 to 2.00 diopters
- High Astigmatism 2.00 to 4.00 diopters
- Extreme Astigmatism > 4.00 diopters
Presbyopia, "aging vision," is probably the most difficult refractive error to understand. Inside the eye, the lens changes shape in order to focus objects at various distances from the eye. If an object is far away, the lens becomes skinny, producing less focusing power. If the object is close, the lens becomes fat, with more focusing power. Between the ages of 42 to 45, most people begin to lose the ability to focus up close, thus requiring reading glasses.
If you are nearsighted and become presbyopic, you can take off your glasses or contacts and still see up close. This issue becomes important if you are considering refractive surgery such as LASIK. In most cases, LASIK is performed to correct your vision for distance focusing. If you are 45 or older and undergo LASIK, you will need to plan on wearing reading glasses for close work. This is a complicated issue and you will have the opportunity to discuss this topic with your physician prior to your surgery. Recent advances in lens options for cataract surgery have enabled our surgeons to address presbyopia correction when performing cataract surgery.