What Are Refractive Errors

What Are Refractive Errors?

For our eyes to be able to see, light rays must be bent or "refracted" so they can focus on the retina, the nerve layer that lines the back of the eye. The cornea and the lens refract light rays. The retina receives the picture formed by these light rays and sends the image to the brain through the optic nerve. A refractive error means that the shape of your eye doesn't refract the light properly, so that the image you see is blurred.

While refractive errors are called eye disorders, they are not diseases. In a normal eye, the cornea and lens focus light rays on the retina.

What are the different types of refractive errors?

Myopia (nearsightedness)
A myopic eye is longer than normal, so that the light rays focus in front of the retina. Close objects look clear but distant objects appear blurred. In myopia, distant objects are blurry because the eye is too long, and images focus in front of the retina instead of on it. Myopia is inherited and is often discovered in children when they are eight to twelve years old. During the teenage years, when the body grows rapidly, myopia gets worse. Between the ages of 20 and 40, there is usually little change.

If the myopia is mild, it is called low myopia. Severe myopia is known as high myopia. If you have high myopia, you have a higher risk of detached retina. It is important to have regular eye examinations by an Eye Care Professional to watch for any changes in the retina. If the retina does detach, a surgical operation is the only way to repair it.

Hyperopia (farsightedness)
A hyperopic eye is shorter than normal. Light from close objects, such as the page of a book, cannot focus clearly on the retina. In hyperopia, the eye is too short for images to focus on the retina, so close objects are blurry.

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

Astigmatism (distorted vision)
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. Astigmatism distorts or blurs vision for both near and far objects. It's almost like looking into a funhouse mirror in which you appear too tall, too wide or too thin. You can have astigmatism in combination with myopia or hyperopia.

Presbyopia (aging eyes)
When you are young, the lens in your eye is soft and flexible. The lens of the eye changes its shape easily, allowing you to focus on objects both close and far away. After the age of 40, the lens becomes more rigid. Because the lens can't change shape as easily as it once did, it is more difficult to read at close range. This perfectly normal condition is called presbyopia. You can also have presbyopia in combination with myopia, hyperopia or astigmatism.

How are refractive errors corrected?

Eyeglasses
Glasses are an easy method to correct refractive errors. They can also help protect your eyes from harmful light rays, such as ultraviolet (UV) light rays. A special coating that screens out UV light is available when you order your glasses. Bifocals are glasses that are used to correct presbyopia. They have a correction for reading on the bottom half of the lens and another for seeing 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 distance, you can buy over-the-counter reading glasses to correct presbyopia. No exercise or medication can reverse presbyopia. You will probably need to change your prescription from time to time between the ages of 40 and 60, because your lens will continue to lose flexibility.

Contact lenses
There are now a wide variety of contact lenses available. The type that is best for you depends on your refractive error and your lifestyle. If you want to wear contact lenses, discuss the various options with your ophthalmologist. You may have heard of a process called orthokeratology to treat myopia. It uses a series of hard contact lenses to gradually flatten the cornea and reduce the refractive error. Improvement of sight from orthokeratology is temporary. After the use of the lenses is discontinued, the cornea returns to its original shape and myopia returns.

Refractive surgery
Radial keratotomy is a surgical operation to improve myopia by changing the curve of the cornea over the pupil. The surgeon makes several deep incisions (keratotomies) in the cornea in a radial, or spoke-like, pattern. The incisions flatten out the cornea and shorten the distance light rays must travel to the retina. In radial keratotomy, incisions are made in the cornea to change its shape and improve myopia. In one large study of RK, 64% of people did not wear glasses or contacts five years after their surgery--and 36% did, at least for some situations. People with mild myopia have even more satisfactory results. Reading glasses may be necessary for older adults, because RK does not alter the normal aging process of the eye. Complications at the time of surgery are rare but can be serious. After RK, the cornea heals slowly, and concerns remain about the side effects of this delayed corneal healing. There may be:

Fluctuating vision, especially the first few months after surgery;
A weakened cornea, more vulnerable to rupture if hit directly;
Infection;
The need for additional refractive surgery;
Difficulty fitting contact lenses;
Glare or starburst around lights;
Temporary pain.
Photorefractive keratectomy (PRK) In the past few years a new type of laser surgery has been investigated to correct myopia. In a process called photorefractive keratectomy (PRK), the excimer laser precisely sculpts the surface of the cornea. There are several thousand people in the United States who have had PRK through research studies overseen by the Food and Drug Administration. For mild to moderate myopia, PRK and RK have similar results.
What is the best method of correcting refractive errors?

There is no "best" method for correcting refractive errors. The most appropriate correction for you depends on your eyes and your lifestyle. You should discuss your refractive errors and your lifestyle with your ophthalmologist to decide on which correction will be most effective for you