Understanding Astigmatism - the Basics

What Is Astigmatism?

Astigmatism is the most common refractive problem responsible for blurry vision. You may also hear astigmatism called a refractive error.

Most of the eyeball's focusing power occurs along the front surface of the eye, involving the tear film and cornea (the clear 'window' along the front of the eyeball). The ideal cornea has a symmetrical curved surface. Astigmatism is caused by a cornea that is not symmetrical.

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

The cornea is the transparent window over the colored part of the eye. It bends (refracts) light rays and helps focus the light onto the retina in the back of the eye so people can see. When the cornea is oblong shaped, it causes light rays to focus on two different points on the retina, instead of just one. 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 most people acquire it later in life. Most refractive problems tend to stabilize by age 25-30.  Some 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.

Up 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 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.