Refractive Error FAQ
Q. What are refractive errors?
A. Refractive errors are imperfections in how the eye focuses light. Usually, refractive errors cause noticeably blurred vision. The three main types of refractive errors are nearsightedness, farsightedness and astigmatism.
Q. How are refractive errors corrected?
A. Refractive errors usually are corrected with prescription eyeglasses or contact lenses. LASIK and other refractive surgery also can be used to correct refractive errors.
Q. How are refractive errors detected and measured?
A. An eye exam is the best way to determine if you are nearsighted, farsighted or have astigmatism. Your eye care professional will perform an eye test called a refraction to detect and measure refractive errors. The degree of refractive errors and the power of lenses to correct them are measured in units called diopters (D).
Q. What is nearsightedness?
A. Nearsightedness (also called myopia), is a refractive error where light is focused too early in the eye, before it reaches the retina. Myopia generally caused blurred distant vision (you remain "near-sighted"). Approximately 40 percent of Americans have some degree of nearsightedness. (National Health and Nutrition Examination Survey, 1999-2004).
Q. What is farsightedness?
A. Farsightedness (also called hyperopia), is a refractive error where the eye is not powerful enough to focus light onto the retina. Hyperopia usually causes near vision to be blurred, but higher amounts of farsightedness can cause blurry vision at all distances. Other symptoms of uncorrected farsightedness include headaches and eye strain when reading. Approximately 45 percent of Americans over age 40 have at least 0.75 D of farsightedness (Baltimore Eye Survey, 1991).
Q. Is farsightedness the same as presbyopia?
A. No. Though both conditions typically cause near vision problems, hyperopia can occur at any age, whereas presbyopia is a normal age-related loss of near focusing ability that affects people over age 40, requiring them to wear bifocals, other multifocal lenses or reading glasses to restore their reading vision. People who are farsighted also will become presbyopic later in life.
Q. What is astigmatism?
A. Astigmatism (sometimes incorrectly called "stigmatism") is a refractive error where the front surface of the eye (cornea) and/or the lens inside the eye has asymmetric curves, which prevents light from being focused to a single point on the retina. Astigmatism generally causes blurred or distorted vision at all distances. Approximately 30 percent of people worldwide have some degree of astigmatism, according to several studies.
Q. Can a person have more than one type of refractive error?
A. Yes. It is common for someone with nearsightedness or farsightedness to also have some degree of astigmatism. A lens prescription for astigmatism includes two powers: a sphere power and a cylinder power. The sphere power corrects myopia or hyperopia; the cylinder power corrects astigmatism. It's uncommon, but in cases of "mixed" astigmatism, nearsightedness and farsightedness can exist in the same eye: one meridian of the eye is nearsighted and the meridian 90 degrees from it is farsighted.
Q. Do eye diseases affect refractive errors?
A. In some cases, yes. Keratoconus is an eye disease that causes the cornea to become thinner and bulge forward, resulting in high amounts of irregular astigmatism that cannot be corrected with glasses. Options for keratoconus treatment include gas permeable or hybrid contact lenses, corneal collagen cross-linking (CXL) and Intacs corneal implants. In severe cases, a corneal transplant is sometimes required. Cataracts also can cause changes in refractive errors.
Q. Can eye exercises prevent or correct refractive errors?
A. No studies have shown convincing evidence that eye exercises can prevent or reverse refractive errors. In some cases, certain eye "exercises" might even be harmful.
Eye exercises advertised on television and the Internet for the correction of refractive errors should not be confused with legitimate vision therapy that sometimes is prescribed by eye doctors to correct binocular vision problems such as poor eye teaming or depth perception.
Consult your eye doctor before performing any unsupervised eye exercises.