Astigmatism is an imperfection in the curvature of your eye’s cornea or lens.
It may be helpful to think of the normal eye as being shaped like a basketball. With astigmatism, it's shaped more like an American football. When the steepest curve runs vertically, with the football lying on its side, it's called:
- with-the-rule astigmatism
When the steepest curve runs horizontally, with the football sitting on its end, it's called:
- against-the-rule astigmatism
Two types of astigmatism
Normally, the cornea and lens are smooth and curved equally in all directions. This helps to focus light rays sharply onto the retina at the back of your eye. If your cornea or lens isn't smooth and evenly curved, light rays aren't refracted (bent) properly. Doctors call this a refractive error.
When your cornea has a distorted shape, you have corneal astigmatism. When the shape of your lens is distorted, you have lenticular astigmatism. In either case, your vision for both near and far objects is blurry or distorted. It's almost like looking into a fun house mirror in which you can appear too tall, too short, too wide or too thin.
People may have astigmatism along with other refractive errors, such as:
Adults with significant astigmatism may realize their vision isn't as good as it should be. Children with astigmatism symptoms may not be aware they have this condition. They are unlikely to complain about blurred or distorted vision.
In a normal eye, the cornea and lens focus light rays on the retina.
In astigmatism, images focus in front of and beyond the retina. Close and distant objects both appear blurry.
Uncorrected astigmatism can impact a child's ability to achieve in school and sports. It is crucial that children have regular eye exams. Get these exams to detect astigmatism and other vision problems as early as possible.
What causes astigmatism?
Astigmatism is caused by an irregular curvature of the eye's cornea or lens. If your cornea or lens isn't evenly curved, light rays aren't refracted properly. With astigmatism you have blurred or distorted vision at near and far distances.
Astigmatism is very common. Doctors don't know why corneal or lens shape differs from person to person. They do know that likelihood of developing astigmatism is inherited.
Astigmatism can develop after an eye disease, eye injury or surgery. It is a myth that astigmatism can develop or worsen from reading in low light or sitting very close to the television.
Astigmatism symptoms may include:
- blurry vision or areas of distorted vision
- squinting to try to see clearly, or
- eye discomfort
If you have these symptoms you may not necessarily have astigmatism. You should visit to your ophthalmologist. A complete eye exam will determine what is causing your symptoms.
Your eye doctor will test you for astigmatism as part of your comprehensive eye exam.
Your eye doctor will test your visual acuity. He or she will ask you to read letters on an eye chart. This will determine the clarity of your vision at certain distances.
A phoropter helps your eye doctor find out your prescription for glasses or contact lenses.
Astigmatism Prescription Measurement Machines
Your eye doctor may use several devices during your exam:
- A phoropter helps determine how to shape a lens to correct your vision. The doctor has you look through a series of lenses in front of your eyes, and asks which ones make your vision better. Based on your answers, the doctor determines the lenses that provide the clearest vision.
- Your doctor can also use an autorefractor to determine the corrective lenses you need. The autorefractor shines light into the eye and measures how it changes as it bounces off the back of the eye.
- A keratometer measures the curve of your cornea. Your eye doctor may also use corneal topography. This provides more information about the shape of the surface of the cornea.
These tests help your eye doctor to precisely diagnose and measure your astigmatism.
Usually, you can correct mild to moderate astigmatism with eyeglasses or contact lenses. Glasses or contacts correct astigmatism by compensating for uneven curves in your cornea and lens.
Rigid contacts (RGP, for rigid gas permeable, or GP, for gas permeable) used to be the only contact lenses for astigmatism. This is no longer true. Now, soft lenses called toric contact lenses can correct astigmatism. These lenses may be appropriate for some people. If you have severe astigmatism, rigid contacts or glasses may be a better option. Your eye doctor will discuss your lens options with you.
Surgery, including LASIK, may be an option for some people with astigmatism. Your ophthalmologist can discuss refractive surgery options with you.