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What causes nearsightedness?

The exact cause of nearsightedness is not completely understood, but several factors contribute to this refractive error, which is characterized by clear eyesight up close but blurry distance vision.

Nearsightedness, or myopia, occurs when an eyeball grows too long or the cornea becomes too steeply curved. The result is that light entering the eye doesn't come to a clear focus point on the retina, which is required for clear vision at all distances.

Researchers who study nearsightedness have identified at least two key risk factors.

The first is genetics. More than 150 myopia-prone genes have been identified in recent years. One such gene alone may not cause the condition, but people who carry several of these genes have a much higher risk of becoming nearsighted.

Nearsightedness — along with these genetic markers — can be passed along from one generation to the next. When one or both parents are nearsighted, there’s a greater chance that their children will develop myopia.

The other major risk factor for nearsightedness is focusing the eyes up close for extended periods of time, such as when reading or staring at the screens of computers, tablets and smartphones. 

Because myopia causes a person's distance vision to become blurry, squinting is the hallmark sign that a person is becoming nearsighted. Squinting reduces the size of the opening through which light enters the eye. This creates a "pinhole camera effect" which makes distant objects look a little clearer.

Unfortunately, frequent squinting leads to another common symptom of myopia: headaches.

Early detection of nearsightedness is important. If discovered early, myopia control measures can be started to slow the progression of nearsightedness. This is important because high levels of myopia can increase the risk of developing serious eye problems later in life, such as glaucoma and retinal detachment.

Even if you don't know exactly what causes nearsightedness, myopia is easily detected during a routine eye exam by an eye doctor. To reduce your child's risk of high myopia later in life, schedule an eye exam at the beginning of each school year with an eye doctor near you.

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