Think of the normal eye as shaped like a round ball, with the cornea and lens curved equally all the way around. This smooth shape is what helps focus light onto the retina. With astigmatism, the eye is shaped like an egg with mismatched curves. This means light isn’t refracted properly and vision is impaired.
If the cornea is distorted, it is known as corneal astigmatism; a misshaped lens is known as lenticular astigmatism. In both cases, both near and distance vision is blurred or distorted. Astigmatism is often accompanied by nearsightedness or farsightedness.
There is no clear reason as to what causes this shape irregularity, but the likelihood of having it is inherited. Astigmatism primarily presents at birth; however, it can also develop after an eye disease, injury, or surgery. If you’ve heard it can develop from reading in dim light or sitting too close to the television, don’t believe it — these are only myths.