What is Astigmatism?

Reviewed by Sonia Kelley, OD, MS on 7/21/2023

Astigmatism is a refractive error that can affect a person’s vision at multiple distances. Refractive errors are a group of eye irregularities that affect how the eye focuses light. In the case of astigmatism, the irregularity is found in the cornea and/or the natural lens of the eye.

For vision to be clear , incoming light rays must focus directly on the retina, which is the light-sensitive tissue that lines the back of the eye. The cornea and lens of the eye work together to focus the light perfectly, like a highly focused spotlight on a stage.

In an eye with astigmatism, imperfections in the cornea, the lens, or both cause the light to disperse rather than focus. So instead of spotlighting the center of the stage, light scatters to several focal points around it.

In addition to having blurry vision, astigmatic people often have trouble seeing in low-light conditions. This can make night driving difficult and potentially dangerous.

Astigmatism can be categorized in a few ways. The first way to organize the refractive error is by cause. This separates it into three types:

• Corneal astigmatism– The cornea, which is the clear, frontmost layer of the eye, has irregularities in its curvature. This is the most common type.
• Lenticular astigmatism– The lens, which sits behind the pupil and iris (the colored part of the eye), has imperfections in its curvature, positioning, or angle in which it refracts light.
• Retinal astigmatism– The macula (the center of the retina) is in a slanted position, which makes it difficult for light to land on it perfectly.

When the main meridians of the eye are 90 degrees apart — known as regular astigmatism — eye doctors often separate it into two categories:

• With-the-rule astigmatism– The cornea’s curvature is steeper along the vertical axis. This makes the cornea appear like a football when it’s lying on its side, or like a sphere that’s being pinched together from the top and bottom. This type is the most common.
• Against-the-rule astigmatism– The cornea has steeper curvature along the horizontal axis, so it looks like a football standing up, or a sphere that’s being pinched together from the sides.

When you look at an average cornea and lens from a side view, they should both have a smooth round curvature like a ping pong ball. But the profile view of an astigmatic cornea looks more like the back of a spoon — the curvature isn’t uniform across the eye.

It’s this wonky curvature that results in distorted or blurred vision at multiple distances. But doctors aren’t quite sure what causes the corneal or lenticular irregularities to develop in the first place. Most often, the imperfection is present at birth. However, there have also been cases that developed after an eye procedure like cataract surgery.

People with mild astigmatism may not notice any symptoms, which makes it tough to diagnose without an eye exam.

Those who do have symptoms often experience:

• Distorted or blurry vision
• Eye strain and fatigue
• The need to squint in order to see clearly
• Headaches
• Glare or halos that appear around lights
• Light sensitivity
• Symptoms that worsen at night or in low-light conditions

Regular eye exams help ensure that any vision or eye health issues you have are identified and treated properly.

Yes, most types of astigmatism can be corrected. In fact, there are several correction options, depending on your personal preference.

The most common vision correction methods are eyeglasses and contact lenses. However, people looking for a more permanent option may benefit from refractive surgery to correct astigmatism.

Glasses to Correct Astigmatism

For mild cases without another refractive error present, your eye doctor may recommend wearing eyeglasses during certain activities, such as driving at night.

Glasses to correct astigmatism have cylindrical lenses, while other refractive errors are corrected with spherical lenses. Cylindrical lenses have a different prescription at different meridians in the lens. Spherical lenses have the same corrective power throughout the entire lens.

READ MORE: How Do Glasses Help You See?

Contact Lenses for Astigmatism

If you need full-time astigmatism correction, you may prefer contacts over eyeglasses. Contacts for astigmatism are called toric contact lenses.

Because it has a specific point of correction, a toric contact lens is designed to rotate to the perfect position when placed on the eyeball. This ensures the point of correction is lined up precisely with the place of imperfection on the cornea.

Surgery to Fix Astigmatism

If you want a permanent alternative to glasses and contacts, laser eye surgery to correct astigmatism may fit the bill. Some of the most common procedures include:

• Laser-assisted in situ keratomileusis (LASIK)– This procedure consists of reshaping the cornea using a laser. An incision is made in the cornea’s outer layer to create a flap. Once the reshaping by laser is complete, the flap is put back into place and heals. The surgery takes about 30 minutes or less to complete.
• Photorefractive keratectomy (PRK)– This procedure is very similar to LASIK surgery in that lasers are used to reshape the cornea. However, rather than creating a flap, the cornea’s outer layer is completely removed during PRK surgery. Over time, the outer layer will grow back. Because a flap is not created, this procedure is more suited for people who have dry eye or thin corneas.
• Radial keratotomy (RK)– This procedure involves making tiny cuts in the cornea to help flatten its irregular curvature.
• Small incision lenticule extraction (SMILE)– In this procedure, the eye surgeon uses a femtosecond laser to form a tiny disc of eye tissue called a lenticule. The lenticule is removed from the eye through a small peripheral incision, which effectively reshapes the cornea. This procedure avoids the complications involved with the creation of a corneal flap.

Before making any decisions about astigmatism correction, it’s best to talk to your eye doctor about the benefits and potential risks. They can advise you on the most effective way to correct your astigmatism and put you on the road to clear vision.

SOURCES

1. Astigmatism. Cleveland Clinic. November 2022.
2. Astigmatism.StatPearls. May 2023.
3. Astigmatism. Cedar Sinai. August 2022.
4. Astigmatism. National Eye Institute. June 2019.
5. Astigmatism Glasses. Vision Center. December 2022.
6. Cylindrical Lens. Academic Accelerator. Accessed July 2023.
7. Astigmatism. UVA Health. Accessed July 2023.
8. Contact Lenses for Astigmatism. All About Vision. February 2019.
9. LASIK Eye Surgery. Cleveland Clinic. February 2023.
10. LASIK vs. PRK: Which Vision Correction Surgery Is Right for You?University of Michigan Medicine. December 2017.
11. Radial Keratotomy Correction.StatPearls. April 2023.
12. SMILE Eye Surgery. Cleveland Clinic. June 2023.