Your eye has two parts that focus images:
The cornea is the clear, dome-shaped front surface of your eye.
The lens is a clear structure about the size and shape of an M&M’s candy.
In a normally shaped eye, each of these focusing elements has a perfectly smooth curvature, like the surface of a marble. A cornea and lens with such curvature bend (refract) all incoming light to make a sharply focused image directly on the retina, at the back of your eye.
A refractive error
If your cornea or lens isn’t evenly and smoothly curved, light rays aren’t refracted properly, and you have a refractive error.
Nearsightedness usually occurs when your eyeball is longer than normal or your cornea is curved too steeply. Instead of being focused precisely on your retina, light is focused in front of your retina, resulting in a blurry appearance for distant objects.
Other refractive errors
In addition to nearsightedness, other refractive errors include:
Farsightedness (hyperopia). This occurs when your eyeball is shorter than normal or your cornea is curved too little. The effect is the opposite of nearsightedness. In adults, both near and distant objects are blurred.
Astigmatism. This occurs when your cornea or lens is curved more steeply in one direction than in another. Uncorrected astigmatism blurs your vision.
Certain risk factors may increase the likelihood of developing nearsightedness, such as:
Genetics. Nearsightedness tends to run in families. If one of your parents is nearsighted, your risk of developing the condition is increased. The risk is even higher if both parents are nearsighted.
Environmental conditions. Some studies support the idea that a lack of time spent outdoors may increase the chances of developing myopia.
Nearsightedness is associated with a variety of complications from mild to severe, such as:
Reduced quality of life. Uncorrected nearsightedness can affect your quality of life. You might not be able to perform a task as well as you wish. And your limited vision may detract from your enjoyment of day-to-day activities.
Eyestrain. Uncorrected nearsightedness may cause you to squint or strain your eyes to maintain focus. This can lead to eyestrain and headaches.
Impaired safety. Your own safety and that of others may be jeopardized if you have an uncorrected vision problem. This could be especially serious if you are driving a car or operating heavy equipment.
Financial burden. The cost of corrective lenses, eye exams and medical treatments can add up, especially with a chronic condition such as nearsightedness. Vision reduction and vision loss also can affect income potential in some cases.
Other eye problems. Severe nearsightedness puts you at an increased risk of retinal detachment, glaucoma, cataracts and myopic maculopathy — damage in the central retinal area. The tissues in long eyeballs are stretched and thinned, causing tears, inflammation, new blood vessels that are weak and bleed easily, and scarring.