Astigmatism is a refractive error caused by a curvature flaw in the cornea. LASIK and PRK laser eye surgery can treat astigmatism and wavefront LASIK and wavefront PRK can treat irregular astigmatism. A patient from Camarillo asked me a question this week that is one of the most common questions I am asked. “What,” she asked, “is astigmatism?” She said she had been told she has astigmatism her whole life, but she never really understood what it was.
Astigmatism occurs in the cornea of the eye. The cornea is the clear, domed cap of the front of the eye, much like the crystal on a watch. Astigmatism, along with nearsightedness and farsightedness, is a refractive error. Refractive errors in the cornea, whether astigmatism, nearsightedness, or farsightedness, are due to flaws in the shape of the cornea. In the case of astigmatism, the shape flaw in the cornea is that it has different curvatures in different axes instead of being uniformly curved.
The word “astigmatism” comes from the Greek roots “a” — meaning “not” — and “stigma” — meaning “focused to a point like spot.
The best way to understand how astigmatism works is to understand how the cornea focuses light. The cornea focuses light by being curved. In fact, all curved, clear structures focus light. If you think of a window pane, it is clear, but flat. You can look right through it and light is not focused because the window pane is not curved. A raindrop, on the other hand, is clear and curved. If you look through a raindrop, you can see it focusing light.
The cornea focuses light because it is both clear and curved. Ideally, all the light rays passing through the cornea should be focused to the same exact point — on the retina in the back of the eye. If the cornea has different curvatures in different places, then it will focus light different depending where the light rays pass through the cornea. For this reason, the light rays will not all be focused to the same exact spot on the retina and the image will be blurred — and blurred both at near and far away.
The best way to visualize astigmatic curvature of the cornea is to think of a football shape. If you hold a football tip to tip in each hand, it will have a flatter curvature in the horizontal and a steeper curvature in the vertical. A football has astigmatism. With a basketball, by contrast, no matter how you rub your hand on it, the curvature is always exactly the same. A basketball does not have astigmatism — it is said, instead, to be “spherical.”
If, like a football, the steep and flat axes of astigmatism are perpendicular to each other, then the cornea is said to have “regular” astigmatism. If the axes are not perpendicular straight lines as in a football shape, then the cornea has “irregular” astigmatism.
LASIK and PRK laser eye surgery both can very exactingly correct astigmatism. Customized wavefront LASIK and customized wavefront PRK are able to treat irregular as well as regular astigmatism.
See Also
Laser In Situ Keratomileusis for Moderate and High Myopia and Myopic Astigmatism
A German study looking at the effects of LASIK laser vision correction for nearsightedness and astigmatism.
Laser In Situ Keratomileusis for Myopia and Astigmatism: Safety and Efficacy. A Report by the American Academy of Ophthalmology
A synopsis prepared by the American Academy of Ophthalmology about the use of LASIK for correcting nearsightedness and astigmatism.
Wavefront-guided LASIK With the Zyoptix 3.1 System for the Correction of Myopia and Compound Myopic Astigmatism with 1-year Follow-Up
A study looking at the outcomes of wavefront LASIK treatment of nearsightedness and astigmatism, including changes in astigmatism and higher order aberrations.
Gaussian Light Beams with General Astigmatism
A technical review of the optical principles of astigmatism
Astigmatism
The U.S. National Library of Medicine discussion of astigmatism.
Astigmatism
A website discussion of astigmatism by the American Optometric Association.
Astigmatism
A discussion by Dr. Gary Heiting about the 3 most common types of astigmatism.
Astigmatism and Your Eyes
A Web-MD informational discussion for the lay reader about astigmatism
Open Your Eyes
A blog from the Canadian Association of Optometrists about astigmatism
What is LASIK
FDA website discusses treatment of astigmatism with LASIK
FDA Panel recommends approving VISX excimer for astigmatism
An article discussing the original FDA approval for the VISX excimer laser for conventional correction of astigmatism using PRK