Astigmatism is caused by an irregular curvature of the eye's cornea or lens. If your cornea or lens isn't smooth and evenly curved, light rays aren't refracted properly. This is called a refractive error. Astigmatism is one type of refractive error, with blurred or distorted vision at near and far distances.
Astigmatism is very common. In most cases, people with astigmatism are born with this condition. The reason why corneal shape differs from person to person is unknown, but the likelihood of developing astigmatism is inherited.
Sometimes, astigmatism can develop after an eye disease, eye injury or surgery. It is a myth that astigmatism can develop or worsen from reading in low light or sitting very close to the television.