Health Topics A-Z

What Is Astigmatism?

Astigmatism is the most common refractive problem responsible for blurry vision. 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 perfectly round surface. Anything other than perfectly round contributes to abnormal corneal curvature - this is astigmatism. 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 layer 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 3mm 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 a round shape. On the other hand, astigmatism measuring greater than 1 diopter typically requires correction.

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 errors such as nearsightedness or farsightedness.

What Causes It?

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 errors 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 stigmatism is caused by a condition called 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. Eyes with keratoconus have abnormal collagen synthesis and the cornea is a sheet of transparent collagen. Keratoconus may progress to the point where corneal transplantation is necessary. Good news! 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 is called lenticular astigmatism. It is caused by abnormalities in another circular structure inside of the eye - a crystalline lens that focuses on incoming rays. A variey 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.