What is an astigmatism?

DEAR DOCTOR K:

At my last visit, my eye doctor told me I have astigmatism. He told me what that means, but I still don’t really understand what it is. Please explain it to me.

DEAR READER:

Astigmatism means that the eye’s cornea has an irregular shape, which causes vision problems. Astigmatism is very common; I am among the several billion people who have it. Fortunately, it’s easy to correct.

Let’s start with a little eye anatomy. The cornea is the clear, dome-like window at the front of the eye. Rays of light pass through the cornea and then through the lens, which focuses images onto the retina. The retina captures the images, then sends them to the brain via the optic nerve. I’ve put an illustration of eye anatomy below:

How We See:

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Rays of light pass through the cornea, the anterior chamber, and then through the lens, which focuses images. The lens is nourished by the aqueous humor, a clear, watery solution that circulates from the posterior chamber into the anterior chamber and helps maintain normal pressure.

Light reaches the retina after it passes from the lens through the vitreous humor, a clear gel that fills most of the eyeball. The retina has light-sensitive cells that capture images, which are then sent to the brain via the optic nerve. At the retina’s center is the macula, a small region that provides sharp, central vision.

The cornea is normally round. In people with astigmatism, the cornea may be shaped like a football or oval. This causes light to scatter instead of focus on a single point as it passes through the cornea.

Blurred vision is the most common symptom of astigmatism. Some people describe it as double vision but in only one eye. You may develop eyestrain, squinting and headaches from working to focus on near or distant objects.

Many people have some degree of astigmatism; in fact, it is rare to find a perfectly formed eye. Though people are born with astigmatism, it is often not noticed until a child attends school or learns to read. Astigmatism tends to remain stable, but it may slowly worsen over time. An eye doctor can diagnose astigmatism using a standard eye examination.

Glasses are a good, reliable choice for correcting astigmatism. Patients who prefer contact lenses may be limited to hard contact lenses. Soft lenses are sometimes an option, but they aren’t as effective for severe astigmatism.

Another option is corrective surgery. The most common type used to correct astigmatism is LASIK (laser-assisted in situ keratomileusis).

In LASIK, the doctor makes a small incision on the surface of the eye to create a small flap of tissue, which is lifted up. Then the doctor uses a cool laser beam to correct the irregular shape of the cornea. Recovery is usually quick and painless.

When LASIK surgery was introduced, I took a wait-and-see attitude. The surgery clearly produced short-term improvement in vision, with minimal risks. But I was concerned about the possibility that over the longer term the surgery would become less effective, or there would be adverse effects. Today, after nearly 30 years of its use, I’m much more confident in recommending this procedure to my patients.

It’s the unusual person, though, who needs surgery to correct astigmatism. Glasses have always worked for me, as did contact lenses.

I think eyeglasses are one of the most important medical discoveries — right up there with antibiotics, X-rays and vaccines. So many children and young adults begin to lose clear vision due to myopia, astigmatism and other common eye problems. Without eyeglasses, society would be much less happy and productive.