Refractive Errors and Presbyopia FAQ
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Q. What are refractive errors?
A. Refractive errors are common optical defects that affect your eyes' ability to bend (or "refract") light and accurately focus it on the retina for clear vision. There are three main refractive errors: nearsightedness (myopia), farsightedness (hyperopia) and astigmatism.
Q. How many people have refractive errors?
A. Approximately 50 percent of people in the United States have one or more refractive errors.
Q. What causes refractive errors?
A. To some degree, refractive errors are hereditary. But environmental factors, such as how much reading a person does during childhood, may play a role in one's risk for myopia.
Q. How are refractive errors detected?
A. Refractive errors are detected during routine eye exams with the aid of an eye chart and one or more pieces of equipment that allow your optometrist or ophthalmologist to perform a test called a refraction. During this test, lenses are manually or automatically placed in front of your eyes to determine if you need corrective eyewear.
Q. What is astigmatism?
A. Astigmatism is blurred or distorted vision at all distances.
Q. What causes astigmatism?
A. Astigmatism typically is caused by an irregularly shaped cornea (the clear front surface of the eye). A common analogy used to describe astigmatism is that your eye is shaped more like a football than a baseball. A relatively uncommon condition called keratoconus can cause severe irregular astigmatism.
Q. How many people have astigmatism?
A. Astigmatism is the most common refractive error. According to the National Health and Nutrition Examination Survey (NHANES), approximately 36 percent of Americans age 20 and older have at least 1.00 diopter (D) of astigmatism in at least one eye.
Q. How is astigmatism corrected?
A. Most cases of astigmatism can be corrected with eyeglasses, contacts or refractive surgery, such as LASIK. Correction of astigmatism with contact lenses requires either gas permeable contacts or special soft lenses called toric contact lenses.
Q. What is nearsightedness?
A. Nearsightedness, or myopia, is the inability to see distant objects clearly. (You are "near-sighted," so things up-close are clear but things far away are blurred.)
Q. What causes nearsightedness?
A. One or more of the following physical factors causes myopia: 1) the cornea is too steep; 2) the lens inside the eye is too curved; 3) the eyeball is too long. Whichever factors are involved, the eye focuses light too quickly in front of the retina instead of directly on it.
Q. How many people have myopia?
A. According to NHANES, approximately 33 percent of Americans age 20 and older are nearsighted. Nearsightedness usually develops in childhood and stabilizes in early adulthood.
Q. How is nearsightedness treated?
A. Like astigmatism, myopia can be corrected with prescription eyeglasses, contact lenses and refractive surgery. Nearsightedness can also be corrected non-surgically with a special type of gas permeable (GP) contact lens fitting called orthokeratology (or "ortho-k"). In ortho-k, you wear specially designed GP lenses while you sleep, then remove them during the day. Ortho-k provides only temporary correction of myopia, however. The nearsightedness will return if you discontinue wearing the cornea-reshaping lenses at night.
Q. What is farsightedness?
A. Farsightedness, or hyperopia, is the inability to see clearly at any distance without excessive focusing effort. Usually, farsightedness affects near vision more than distance vision. But in severe hyperopia, vision is blurred at all distances.
Q. What causes farsightedness?
A. Hyperopia is caused by 1) the cornea being too flat; 2) the lens inside the eye being too flat; 3) the eyeball being too short; or a combination of these factors. The result is that the eye focuses light too slowly behind the retina instead of directly on it. Farsightedness can be present in childhood; it can also worsen later in life.
Q. How many people have hyperopia?
A. Farsightedness is less common than nearsightedness and astigmatism. According to NHANES, only 3.6 percent of Americans age 20 and older have +3.00 D or more of hyperopia. Some children are born farsighted, but their hyperopia may diminish during the school years as their eyes continue to develop. In a 2006 study of Australian schoolchildren ages 6 and 12 years, the prevalence of moderate hyperopia (defined as +2.00 D or greater) was 13.2 percent among 6-year-olds and 5.0 percent among 12-year-olds. In some cases, hyperopia will worsen later in life.
Q. How is farsightedness corrected?
A. Like myopia and astigmatism, hyperopia can be corrected with prescription eyeglasses, contact lenses and refractive surgery (such as LASIK and PRK).
Q. What is presbyopia?
A. Presbyopia is a normal, age-related change that reduces the eye's ability to focus on near objects after age 40. Though it is sometimes confused with farsightedness because, like hyperopia, it affects near vision. But the two conditions have different causes and therefore are not the same.
Q. What causes presbyopia?
A. Presbyopia is caused by a loss of flexibility in the lens inside the eye with age. To see near objects clearly, the lens has to change shape (accommodate). Over time, the lens loses that ability, and this change is called presbyopia, which means "aging eye."
Q. How many people are affected by presbyopia?
A. Some time after age 40, presbyopia affects everyone.
Q. How is presbyopia corrected?
A. For people who already wear prescription eyeglasses, presbyopia is corrected with bifocals or progressive lenses. For people who wear contact lenses or who do not need corrective lenses for good distance vision, the most common treatment is reading glasses. Presbyopia also can be treated with monovision contact lenses or refractive surgery. And people who wear eyeglasses for mild myopia can overcome vision problems caused by presbyopia by simply removing their glasses and relying on their nearsightedness to provide good reading vision.
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