Most of the focusing power in the eye occurs along the front surface, at the cornea (the clear 'window' in the front of the eyeball), or within the lens, which sits behind the cornea inside the eye. The ideal cornea has a symmetrically curved surface, like a basketball. Astigmatism is caused by a cornea or a lens that is not symmetrical, like a football.
The cornea is the transparent window in front of the colored part of the eye. It bends (refracts) light rays to focus the light onto the retina in the back of the eye. When the cornea is oblong shaped (like an oval rather than a round teaspoon), it is not possible for the light rays to be focused at a single point. As a result, people with significant astigmatism may have distorted or blurry vision.
Astigmatism is measured in diopters. More than 1 diopter typically requires correction with glasses or soft contact lenses.
Astigmatism can run in families and often occurs in combination with other refractive problems such as nearsightedness or farsightedness.
What Causes Astigmatism?
The cause of astigmatism is almost always unknown and not associated with an external or genetic cause.
A rare cause of higher levels of astigmatism is keratoconus (KEHR-a-toh-kohn-nus), in which the cornea progressively becomes cone-shaped and thin. Keratoconus usually appears around puberty or early adulthood. The cornea is made of transparent collagen fibers that hold the cornea shape in place much like a building is held up by steel beams. Eyes with keratoconus have weaker collagen fibers. This causes the cornea to bulge out and distort vision. Keratoconus may progress to the point where corneal rigid contact lenses no longer solve the vision problems and transplantation of the cornea is necessary. Corneal transplantation for keratoconus is a last resort and has a variable success rate. Fortunately, modern procedures can treat keratoconus and help patients avoid a cornea transplant.
Another form of astigmatism is lenticular astigmatism. It is caused by abnormalities in another curved structure inside the eye -- the natural internal crystalline lens that also focuses incoming rays. Blurred vision from lenticular astigmatism can be treated with eyeglasses and contact lenses.