Most nearsightedness (myopia) is caused by a natural change in the shape of the eyeball that makes it egg-shaped rather than round. This causes light to focus in front of the retina rather than directly on the retina. Nearsightedness is usually inherited. A child is likely to be nearsighted if one or both parents are nearsighted.

See more information about eye anatomy and function.

Many people have believed for a long time that too much close work, such as reading or sitting too close to the television, causes nearsightedness. One study suggested that people who have jobs that involve large amounts of reading do indeed have higher degrees of nearsightedness.²

Other than the natural change in the shape of the eyeball, there are a few rare causes of nearsightedness. These are:

• Pathological myopia, in which the back of the eyeball continues to grow longer after reaching normal adult size.
• Secondary myopia, which may develop as a result of many conditions, including premature birth and a number of eye diseases.
• Pseudomyopia, or sudden nearsightedness that rapidly gets worse because of an underlying cause, such as uncontrolled diabetes.

In some cases, nearsightedness may be caused by eye problems such as cataracts and keratoconus. Premature birth may increase the likelihood that a baby or young child may become nearsighted, especially if the baby has retinopathy of prematurity (ROP).

People who are very nearsighted (severely myopic) have an increased risk of glaucoma and retinal detachment. Retinal detachment happens because the retina may be stretched if you have severe myopia.