Refractive Errors

A refractive error is an error in the focusing of light by the eye and a frequent reason for reduced visual acuity.

Symptoms

The four most common refractive errors are

  • myopia, or nearsightedness

  • hyperopia, or farsightedness

  • astigmatism

  • presbyopia

It is possible to have more than one of these.

Myopia (nearsightedness)
A myopic eye is longer than normal or has a cornea that is too steep, so that the light rays focus in front of the retina. Close objects look clear, but distant objects appear blurred.

Hyperopia (farsightedness)
A hyperopic eye is shorter than normal. Light from close objects cannot focus clearly on the retina. The words on a page will seem blurry, or it will be difficult to see to thread a needle.

Astigmatism (distorted vision)
Astigmatism distorts or blurs vision for both near and far objects. It's almost like looking into a fun house mirror in which you appear too tall, too wide or too thin.

The cornea is the clear front window of the eye. A normal cornea is round and smooth, like a basketball. When you have astigmatism, the cornea curves more in one direction than in the other, like a football. It is possible to have astigmatism in combination with myopia or hyperopia.

Presbyopia (aging eye)
You may start to notice presbyopia around the age of 40, when you begin to hold reading materials farther away from your face in order to see them more clearly. This familiar event is often the first sign of presbyopia, which, if left uncorrected, can cause eye fatigue and headaches.

Treatment

There are several alternatives to correct your vision. You should discuss your vision and your lifestyle with your eye doctor to decide which of these will be best for you.

Eyeglasses or contact lenses are the most common methods of correcting refractive errors. They work by refocusing light rays on the retina, compensating for the shape of your eye.

Refractive surgery is also an option to correct or improve your vision. These surgical procedures are used to adjust your eye's focusing ability by reshaping the cornea, or front surface of your eye.

Eyeglasses
Glasses are an easy way to correct refractive errors. They also can help protect your eyes from harmful light rays, such as ultraviolet (UV) light rays.

Bifocals are glasses used to correct presbyopia. They have a correction for reading on the bottom half of the lens and another for seeing at a distance on the top. Trifocals are lenses with three different lens corrections in one set of eyeglasses.

If you don't need correction for seeing at a distance, you can receive a prescription for reading glasses or buy them over the counter to correct presbyopia.

You will probably need to change your prescription from time to time between the ages of 40 and 60, because your focusing ability will continue to decline.

Contact Lenses
Contact lenses are thin plastic discs that sit directly on the eye. There are a wide variety of contact lenses available. The best type for you depends upon your eyes and your lifestyle. If you want to wear contact lenses, discuss the various options with your ophthalmologist. Read more about contact lenses.

Refractive Surgery
Surgery can reduce or eliminate the need for glasses or contact lenses. Talk to your ophthalmologist about the potential for success, as well as the potential for complications. It is essential to make an informed decision about any surgical procedure.

LASIK (Laser In Situ Keratomileusis)
LASIK is a combined microsurgical and laser procedure to correct myopia, hyperopia and astigmatism. In LASIK, the surgeon uses a highly specialized instrument (microkeratome) to cut a thin flap in the cornea. This flap is folded back, and the excimer laser reshapes the cornea. Then, the flap is replaced and allowed to heal back into position. Read more about LASIK.

Astigmatic Keratotomy (AK)
Astigmatic keratotomy is a microsurgical procedure to correct astigmatism. The surgeon makes deep cuts in the cornea (usually one or two) in a curved pattern. These flatten the areas of the cornea that are too steeply curved.

Photorefractive Keratectomy (PRK)
An excimer laser is used to reduce myopia and astigmatism in a procedure called photorefractive keratectomy (PRK). Using an invisible, high-energy light, the laser sculpts the cornea.

Tests/Diagnosis

Your eye care professional will conduct an eye exam with the help of an instrument called a phoropter. This contains a number of lenses of varying power. By having an individual look at an eye chart through these lenses, the optometrist or ophthalmologist can estimate the amount of refractive error. A retinoscope is also used to shine a light into the patient's eye and view the reflection from the retina in the back of the eye.

The process of showing a patient these lenses of progressively stronger or weaker power is called refraction.

Refractive errors are just one potential cause of blurred vision, so it is important to have an examination right away if you are experiencing symptoms.

Causes/Risk Factors

Myopia is inherited. It often discovered in children whey they are between 8 and 12 years old. During the teenage years, when the body grows rapidly, myopia may become worse. Between the ages of 20 and 40, there is usually little change.

People with severe myopia have a higher risk of detached retina. Talk to your eye doctor about the warning signs of retinal detachment if you are very nearsighted. A surgical procedure is the only way to repair a detached retina. It is important to have regular eye examinations by an ophthalmologist to watch for changes in the retina.

Like nearsightedness, farsightedness is usually inherited. Babies and young children tend to be slightly hyperopic. As the eye grows and becomes longer, hyperopia decreases.

Presbyopia is a normal part of aging.

When you are young, the lens in your eye is soft and flexible. The lens can change its shape easily, allowing you to focus on objects both near and far away.

After age 40, the lens becomes more rigid and cannot change shape as easily as it once did. As a result, it is more difficult for the eye to focus clearly on close objects. Reading and performing other close-up tasks such as threading a needle can become very difficult