Refractive Errors
A refractive error is an imperfection in how the eye focuses light. Usually, refractive errors cause a noticeable decrease in clarity of vision (visual acuity).
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. Refractive errors prevent this from occurring without corrective eyewear.
There are three main types of refractive errors: nearsightedness, farsightedness and astigmatism.
Refractive errors can be corrected with prescription eyeglasses, contact lenses and LASIK or other refractive surgery. The power of lenses or surgical techniques used to correct refractive errors is measured in units called diopters (D).
Nearsightedness
In nearsightedness (myopia), light comes to a focus too early within the eye, before it reaches the retina. Myopia causes distant objects to be blurred; near objects remain clear.
According to a large study conducted in 1999-2004, approximately 41.6 percent of Americans are nearsighted (National Health and Nutrition Examination Survey).
Lenses prescribed to correct myopia are called minus (-) lenses. These lenses move the focus point of the eye backward, onto the retina.
Farsightedness
In farsightedness (hyperopia), the eye is not powerful enough to bring light to a focal point on the retina. Hyperopia usually causes near vision to be blurred, but higher amounts of farsightedness can cause blurry vision at all distances.
Young people with mild hyperopia may not notice blurred near vision, but may experience eye fatigue, headaches and other discomfort symptoms when reading or performing other near vision tasks.
One large study suggests 45 percent of Americans over age 40 have at least 0.75 D of farsightedness (Baltimore Eye Survey, 1991).
Lenses prescribed to correct farsightedness are called plus (+) lenses. These lenses add focusing power to the eye so light can be focused on the retina for clear, comfortable vision.
Farsightedness sometimes is confused with presbyopia, which is a normal age-related change in the eye that reduces near vision. Virtually everyone, including people with good vision, will experience presbyopia some time after age 40.
A person who is farsighted will also develop presbyopia when they get older. Presbyopia is treated with multifocal eyeglasses or reading glasses.
Astigmatism
Astigmatism (sometimes incorrectly called "stigmatism") occurs when the front surface of the eye (cornea) and/or the lens inside the eye has unequal curves, which prevents light from being focused to a single point on the retina. Because of this, astigmatism causes blurred vision at all distances.
Astigmatism can occur along with nearsightedness or farsightedness. Several studies suggest that approximately 30 percent of people worldwide are affected by astigmatism.
A prescription for astigmatism includes two powers: a sphere power (to correct any nearsightedness or farsightedness) and a cylinder power (to correct astigmatism). The prescription also includes an "axis" designation, which is a number between 1 and 180 that identifies the rotational location of the cylinder power in the lens.
Astigmatism generally is stable throughout a person's lifetime. However, certain eye conditions can cause a significant increase in astigmatism.
Keratoconus is an eye disease that causes the cornea to become thinner and bulge forward, resulting in high amounts of irregular astigmatism. New keratoconus treatment options are showing promise in stabilizing the condition and thereby avoiding the need for a cornea transplant.
Cataracts also can cause changes in astigmatism.
Eye Exams
To determine if you have a refractive error, schedule a comprehensive eye exam with an optometrist or ophthalmologist. During the exam, your doctor will perform an eye test called a refraction to determine if you have any nearsightedness, farsightedness or astigmatism.
If a refractive error is diagnosed, ask your eye doctor to discuss all treatment options to determine whether eyeglasses, contact lenses or refractive surgery is the best choice for you.
Preventing or Reversing Refractive Errors
Many people wonder if eye exercises can prevent or reverse refractive errors. Generally, vision therapy of this type is prescribed to correct binocular vision problems such as poor eye teaming, focusing flexibility or depth perception.
Though some companies advertise eye exercises designed to improve vision without glasses, contacts or surgery, most eye care professionals say these "treatments" are ineffective and a waste of time and money. Consult your eye doctor for details.