So What Is Astigmatism?
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Clearing Up Astigmatism: Understanding astigmatism and how to correct it by Gary Heiting, OD Astigmatism is a very common vision problem—and perhaps the most How to Correct Astigmatism misunderstood. Even the word trips up a lot of people: “My eye doctor says I have a stigmatism.” (No, you have astigmatism, not a stigmatism.) Astigmatism is easily corrected with eyeglasses and can also be corrected The word “astigmatism” is from the Greek roots a- (“without”) and stigma specialty contact lenses. Many people (“point”). These word roots give a very good clue as to what astigmatism find glasses provide the most convenient, is—and what it does. economical and effective solution to their astigmatism. So What Is Astigmatism? When choosing glasses to correct astigmatism, it’s important to examine Like nearsightedness and farsightedness, astigmatism is a refractive error—meaning the lens prescription carefully to avoid it’s a condition that causes a deviation from the amount of bending (“refracting”) of unpleasant surprises with lens thickness. light rays required to create a distinct focal point on the retina for clear vision. A lens that corrects astigmatism has two Instead of focusing light rays to a single focal point directly on the retina, an eye with powers—called the sphere and the cylinder uncorrected astigmatism creates two separate images in different locations. powers. One of the principal meridians of the lens contains only the sphere power; but the other contains both the sphere and cylinder power combined. This combined power can significantly increase the thickness of the lens in that meridian. Astigmatism and Safe Driving What Causes It? Even small amounts of uncorrected astigmatism can cause significant eye The clear front surface of the eye (the cornea) is responsible for about 70 percent strain, make road signs less legible, and of the focusing power of the eye. The remainder is provided by the crystalline lens, affect reaction time when driving. located directly behind the iris and pupil. Everyone of driving age should have Most astigmatism is caused by unequal curvature of different meridians of the cornea. routine eye exams to see if they have mild (If you look directly at an eye and imagine the face of a clock superimposed on the astigmatism that could affect their comfort cornea, a line that connects the 12 and 6 would be one meridian, a line connecting and safety behind the wheel—including the 1 and 7 would be another, and so on.) The most and least curved meridians of people who can pass a drivers vision the cornea—which usually are 90 degrees apart—are called the principal meridians. screening test without glasses. People with mild astigmatism often are amazed by No one knows for sure why some people develop astigmatism, but it’s very common. how much better they see when wearing Recent studies suggest more than one third of Americans age 20 and older have a prescription eyeglasses for driving. clinically significant amount of astigmatism in at least one eye. Astigmatism-correcting glasses for night Symptoms of Astigmatism driving should have clear lenses with anti- reflective coating; for daytime driving, Astigmatism often causes objects at all distances to prescription polarized sunglasses provide appear blurry or distorted. Other common symptoms the best vision and comfort. include eye strain and headaches—especially when driving, using a computer, or reading for prolonged Gary Heiting, OD, has more than 20 years periods. Uncorrected astigmatism in children also can experience as a clinical optometrist and is senior contribute to problems in the classroom. editor of AllAboutVision.com..