Testing for Astigmatism

Astigmatism is a common refractive error that happens when parts of the eye are irregularly shaped, preventing light from focusing properly on the retina. As a result, people with this condition complain of blurred and distorted vision. This refractive error occurs in about one in three people and may occur in combination with near- or farsightedness.

Getting a comprehensive eye exam is the best way to diagnose astigmatism. An optometrist or ophthalmologist will perform a few tests to see how the eyes focus light and the power of any corrective lenses needed to improve vision. Astigmatism tests can also be found online, but an eye doctor's exam is needed for a precise diagnosis.

 What Is Astigmatism?

Astigmatism occurs when the cornea (a transparent layer in front of the pupil that focuses light into the eye) or lens (sits behind the pupil and helps light focus onto the retina) has an irregular shape. While a normal cornea has a spherical shape, the area in people with astigmatism will be oval-shaped.

In an eye without astigmatism, light bends (refraction) when entering the eye and goes through two different parts: first the cornea and then the lens. When either of them has an abnormal shape, the light will scatter instead of focus properly on the retina, which is responsible for sending lights as electrical impulses to the brain. As a result, the images at any distance will become blurry.

Symptoms of astigmatism include:

• Blurred vision
• Double vision
• Eyestrain or eye discomfort
• Headaches
• Fatigue
• Low night vision
• Squinting

The cause of astigmatism is unknown. People are born with astigmatism, but it may not be noticed until a child begins to attend school or learns to read. Some people may develop the condition as a child or young adult, or after an eye injury or surgery.

How Do Doctors Test for Astigmatism?

An optometrist or ophthalmologist will check your eyes with a few tests during an eye exam:

• Visual acuity test: This test measures how well you can see and is a routine part of eye exams. An eye care specialist will ask you to read letters on a card or chart 20 feet away. Visual acuity is expressed as a fraction: The top number refers to the distance you stand from the chart, which is often 20 feet, and the bottom number indicates the distance at which a person with normal eyesight could read the same line you correctly read. Normal distance visual acuity is 20/20, while 20/70 to 20/160 is considered moderate visual impairment and 20/200 to 20/400 is considered severe visual impairment. Studies have shown that both near and distance visual acuity are affected in astigmatism.
• Keratometry: This test is specifically used to check for astigmatism. During this test, your eye doctor will check your cornea using a keratometer, which measures the curvature of the cornea by focusing a circle of light on the cornea and measuring its reflection. Keratometry can also determine the amount and axis of astigmatism (where the astigmatism is located on the cornea).
• Corneal topography: This imaging generates a contour map of the cornea and provides even more detail of the cornea's shape. This test can also be used to fit contact lenses.
• Refraction: Your eye doctor will place multiple lenses in front of your eyes using an instrument called the phoropter to measure how they focus light. The specialist will use a retinoscope (a handheld, lighted instrument) or an automated instrument to evaluate the approximate focusing power of the eye. The power is refined to determine the lenses for clearest vision based on your responses.

What Do Test Results Mean?

Astigmatism is measured in units of diopters. A perfect eye will have 0 diopters, but most people have between 0.5 to 0.75 diopters of astigmatism. Patients will only need glasses or contact lenses if the measurement is 1.5 diopters or more.

The last two numbers in your prescription refer to astigmatism:

• Cylinder measures what degree of astigmatism you have, or how flat or irregular the shape of your cornea is.
• Axis is measured in degrees and ranges from 0 to 180.

Are Online Tests Reliable?

A number of online tests can help people check if they have astigmatism. They usually display shapes and lines and ask the test taker to stand a certain length away from the monitor. You will alternate covering your left and right eyes during these tests.

Online tests can be a first step toward identifying a potential problem with your vision. If you get a positive result at home, you should follow up with a visit to your eye doctor. To get a precise diagnosis of astigmatism, however, you have to be evaluated by an eye specialist. The result from an online test relies on the person's judgment and may not be accurate.

A Word From Verywell

Astigmatism is one of the most common eye problems, and many people may not even notice they have the condition when the symptoms are mild. For this reason, it's important to have your eyes checked on a regular basis to catch the condition early. If you notice astigmatism symptoms, you can use an online test to check your vision. However, you should still visit your eye doctor for a proper evaluation afterward. They can tell you definitively whether you have the condition and recommend the appropriate treatment.

The treatments for astigmatism are often affordable, including eyeglasses and contact lenses. People who want to fix it permanently can also discuss the possibility of getting surgical treatment with their doctor.