Nearsightedness (Myopia)

What Is It?

Nearsightedness, also called myopia, is a condition in which a person has difficulty seeing objects in the distance, although vision of close objects is good.

In some cases, nearsightedness is an inherited condition caused by an abnormally long eye, as measured from front to back. Because there is an increased distance between the cornea (the clear "window" that covers the front of the eye) and the retina (the light-sensitive layer at the back of the eye), images tend to focus in front of the retina, rather than on the retina itself.

In other cases, nearsightedness is the result of a mismatch between the length of the eye and the ability of the eye's lens to focus an image in the correct location. Again, this causes images to focus in front of the retina, resulting in nearsightedness.

Currently, nearsightedness is the most common form of vision problem in the United States, affecting an estimated 25 percent of all Americans. In many cases, genetic factors play a role in the condition, so that several generations of the same family suffer from the problem.

Symptoms

Symptoms of nearsightedness can include:

• Difficulty seeing distant objects, such as blurred vision when watching television or a movie or when trying to read a billboard, a traffic sign or a teacher's notes on a chalkboard
• Squinting
• Headaches triggered by squinting
• Poor school performance — this is often the first clue in young children, who rarely complain about vision problems

Diagnosis

After reviewing your symptoms, the doctor will perform a thorough eye examination, including tests of visual acuity (how well you can see).

To measure visual acuity, doctors use a ratio in which the top number represents the patient's vision, and the bottom number represents the vision of someone who sees perfectly. For example, a person is said to have 20/20 vision if he or she can see at 20 feet what a person with perfect vision is expected to see at 20 feet. On the other hand, a mildly nearsighted person may see at 20 feet what a person with perfect vision can see at 30 feet. This is called 20/30 vision. A more severely nearsighted person might have a ratio of 20/40 or 20/100.

In older children and adults, visual acuity often is measured by having the person read letters from an eye chart (usually the Snellen eye chart) that is displayed at a precisely measured distance from where the person stands. A hand-held chart, called the Rosenbaum card, is also available. In young children and people who cannot read, the doctor can use alternative charts that display objects, animals or the capital "E" facing in different directions.