Understanding Astigmatism - the Basics

What Is Astigmatism?

Most of the focusing power in the eye occurs along the front surface of the eye, at the cornea (the clear 'window' in the front of the eyeball). The ideal cornea has a symmetrically curved surface. Astigmatism is caused by a cornea that is not symmetrical.

Here's a good way to demonstrate the effects of astigmatism. Compare your reflection in the curved surface of a round soup spoon and with your reflection in an oval teaspoon.

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, 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. 

The center 3 mm portion of the cornea is the critical region for clear vision. Folks with small amounts of astigmatism usually have no symptoms because ordinary blinking helps the cornea maintain its symmetrical curve. 

Astigmatism is measured in diopters. More than 1 diopter typically requires correction with glasses. Soft contact lenses can also correct small amounts of astigmatism. Toric and rigid gas-permeable contact lenses can correct moderate amounts of astigmatism. More about contact lenses later. 

Astigmatism can run in families and often occurs in combination with other refractive problems such as nearsightedness or farsightedness.

What Causes Astigmatism?

Researchers don't fully understand why some people develop astigmatism and others don't. Some infants are born with astigmatism, however people also acquire it later in life. Most refractive problems tend to stabilize by age 25-30.  Possible explanations for acquired astigmatism include: 

• The weight of the upper eyelid resting on the eyeball
• Healed corneal lacerations (scarring)
• Blunt eye trauma (being hit in the eye)
• Scarring in the cornea from other causes like infections
• Changes in corneal shape following eye surgery

A rarer cause of astigmatism is keratoconus (KEHR-a-toh-kohn-nus), in which the cornea progressively becomes cone-shaped and thin. Keratoconus usually strikes around puberty, more commonly in women. The cornea is a sheet of transparent collagen, and eyes with keratoconus have abnormal collagen synthesis. Keratoconus may progress to the point where corneal transplantation is necessary. Corneal transplantation for keratoconus has a very high success rate -- rejection is uncommon.

Until now we have been talking about corneal astigmatism, but another form of astigmatism is lenticular astigmatism. It is caused by abnormalities in another curved structure inside the eye -- the crystalline lens that also focuses incoming rays.  A variety of metabolic diseases cause lenticular astigmatism. For example, high blood sugar levels can change the shape of the lens of the eye, so diabetics are at increased risk of astigmatism. The lens usually returns to normal when blood sugar levels are controlled through diet or insulin.