What is Astigmatism?
Astigmatism is a refractive error caused by a misshapen lens or cornea (the transparent covering over the iris, pupil, and lens) that prevents sufferers from seeing objects clearly from a distance or up close. Astigmatism may occur in varying degrees in each eye, and often accompanies myopia (nearsightedness) or hyperopia (farsightedness). Mild astigmatism is usually not noticeable, or causes only slight blurriness, while severe astigmatism causes objects to appear blurry at any distance. Approximately 80 percent of Americans have some degree of astigmatism, although many cases do not require correction.
A normal cornea is shaped like a perfect sphere. The eye’s lens is also curved in equal degree in all directions. But the corneas or lenses of people with astigmatism are not symmetrically curved. One side may be steeper than the other, making the cornea look more like a football than a basketball. Because of this, light entering the eye is not focused correctly on the retina, resulting in a blurred image. Light entering the eye from different directions is focused on different focal points, causing images to appear blurry.
In mild cases of astigmatism, symptoms are hardly noticeable, and treatment may not be necessary. In more severe cases,it can be difficult to see fine details, either up close or far away. People with severe astigmatism may suffer from headaches, eye fatigue, and fluctuating vision, especially while reading a book, staring at a computer screen, or looking off into the distance.
Types of Astigmatism
This condition is defined by whether it is caused by deformity of the lens (lenticular astigmatism) or of the cornea (corneal astigmatism) and by its effect on vision:
With Myopic astigmatism, one or both principal meridians of the eye are nearsighted. A meridian is a line that bisects a circle or a curve that bisects a sphere (such as an eyeball). For example, on a clock’s face, a line connecting 6 to 12 is a meridian. With Hyperopic astigmatism, one or both principal meridians of the eye are farsighted. With Mixed astigmatism, one principal meridian is nearsighted while the other is farsighted.
What Causes Astigmatism?
This condition may be congenital (present at birth), or it may develop after an injury to the eye or after eye surgery. A rare condition called keratoconus may also cause astigmatism. In keratoconus the cornea progressively thins out and becomes more cone-shaped, which results in astigmatism. Although most cases of visual distortion caused by keratoconus can be corrected with contact lenses, the condition is known to progress to stages that require surgery, including corneal transplants.
Astigmatism can easily be diagnosed after a standard eye exam with a refraction test. To measure the curvature of the cornea, a keratometer may be used in a keratometry exam. In cases in which fine detail of the shape of the cornea needs to be determined, a more sophisticated test called a corneal topography may be performed. If the patient is unable to respond normally during the refraction test—as may be the case with young children or persons with disabilities—their refraction can be measured by a test called a retinoscopy, which uses reflected light. During a retinoscopy the eye doctor uses an instrument called a retinoscope, which focuses light into the eye. The eye doctor looks at the light reflex in the pupil while placing different lenses in front of the eye.
In most cases astigmatism is best corrected by eyeglasses. Contact lenses, more specifically toric contact lenses, can be specially designed for people with astigmatism. Minor cases can be corrected with soft toric lenses. High degrees of astigmatism are better corrected with eyeglasses or RGP toric contact lenses. Toric contact lenses are more expensive than normal contacts because of the extra correction they provide.
Surgical treatments include LASIK Eye Surgery, PRK (photorefractive keratectomy), and astigmatic keratotomy (AK). LASIK reshapes the cornea by removing eye tissue. In astigmatic keratotomy, an older treatment, an eye surgeon makes incisions in the periphery of the cornea to change its shape. Orthokeratology, also known as Ortho-K or CRT, uses RGP contact lenses to gradually reshape the cornea. The reshaping of the cornea is not permanent, however, and the special contact lenses will need to be worn a few hours each day to maintain the new shape. Only mild astigmatism is treated with this method.
It is important to know that most cases of astigmatism are caused by the curvature of the cornea. Because of that, only surgery can effectively correct the condition. Eyeglasses and contact lenses are good for correcting vision while they are being worn, but they are not a cure.
Talking to Your Eye Doctor
Here are some questions to ask your eye doctor about astigmatism:
- What prescription do I have?
- Based on the degree of my astigmatism, what are my treatment options?
- Is there an adjustment period for this treatment option?
- If the initial treatment is not effective, what are my next options?
- How long will it be before I can see clearly?
- How long will it take for my symptoms to go away?
- How often should I have my prescription checked?
- Are there any new treatment options you recommend?
- What new symptoms should I watch for after treatment begins? If new symptoms develop, how quickly should I come in to see you?
- J. DiGirolamo, MD “The Big Book of Family Eye Care” (Basic Health Publications, Inc. 2011) 52-55; 62; 77-78; 200
- J. Anshel, MD “Smart Medicine for Your Eyes” (SquareOne Publishers, 2011) 161-163
- J. Weizer, MD; J. Stein, MD, MS “Reader’s Digest Guide to Eye Care” (Quantum Publishing, Ltd. 2009) 30