Astigmatism

Published by Bupa's Health Information Team, September 2010.

This factsheet is for people who have astigmatism, or who would like information about it.

Astigmatism is when the cornea (the clear dome that forms the front of the eye) isn't a perfectly round shape. Instead it's an oval shape, a bit like a rugby ball. This means that light entering the eye isn't focused properly, which causes blurred vision.

About astigmatism

Astigmatism is a type of refractive error. This means the way that your eye bends and focuses light rays is uneven.

When you look at an object, light rays pass into your eye through the cornea and the lens towards the retina at the back of your eye. If your eyes are completely regular in shape, the cornea is round and focuses the light rays evenly on a small area of the retina, so that you can see the object clearly.

With astigmatism, the curve on your cornea is oval shape. This means that when light enters your eye from certain directions, it's not bent in the correct way to focus sharply on the retina. This creates a blur on the retina rather than a clearly focused image and the object that you're looking at will seem distorted or stretched slightly.

There are two types of astigmatism - regular and irregular. Regular astigmatism is when the curve of your cornea is equal throughout, making both sides of the oval the same shape. Irregular astigmatism is when the curve of your cornea is different throughout. This means that each side of the oval is a different shape.

If you have astigmatism you may also have other visual problems, such as short-sightedness (myopia) or long-sightedness (hyperopia).

Symptoms of astigmatism

Most people have astigmatism, though it's usually mild and doesn't need treatment. However, some people have more severe astigmatism which causes symptoms. These may include:

• blurred or distorted vision - this may happen when looking at objects close-up and faraway
• tired eyes
• headaches

Causes of astigmatism

Astigmatism is usually caused by a defect in the curve of your cornea that you're born with. It can also be caused by an irregular shape of the lens in your eye.

Astigmatism can also be caused by a number of eye conditions, sometimes as a result of surgery or infection. Some of the main causes are listed below.

• Stitches put into your eye during surgery can distort your cornea causing astigmatism. This usually gets better once your wound has healed and your stitches have been removed.
• Surgery to your eye, injury or an infection can cause scarring on your cornea. This changes the way light enters your eye and focuses on the retina.
• A chalazion is a lump caused by a blocked gland on the inside of your eyelid. It can put pressure on your cornea forcing it to change shape, which causes astigmatism, although this is rare.
• Keratoconus is a rare eye condition where the cornea becomes thin and stretched in the centre. This means that the thin part of the cornea bulges, causing distorted vision.
• Some types of surgery for retinal detachment

Diagnosis of astigmatism

Astigmatism is usually diagnosed during a routine eye examination. Your eye test will probably be carried out by an optometrist, a registered health professional who examines eyes, tests sight and dispenses glasses and contact lenses. He or she will carry out several tests on your vision. The main tests are listed below.

• A visual acuity test, which will test the focusing power of your eyes at different distances.
• A measurement of the refractive error correction needed in your eyes, using a process called refraction.

These tests will help your optometrist to choose the best treatment for you, and the correct prescription for contact lenses or glasses.

It's important to have regular eye tests. This means having an eye test every two years if you're over the age of 65 or have other health problems that may affect your sight such as diabetes. If your eyes are healthy and you're younger than 65, you may need to have a test less often. Your optometrist can give you advice about when to be tested. A regular eye test can help to diagnose any visual problems and health conditions like high blood pressure and diabetes.

Treatment of astigmatism

Glasses and contact lenses

You can wear glasses or contact lenses to correct astigmatism - your optometrist will discuss both options with you.

Depending on how severe your astigmatism is, you may only need to wear your glasses when you're doing specific tasks or you may need them all the time.

You can wear rigid (hard or gas-permeable lenses) or soft contact lenses to treat astigmatism. Rigid lenses work because they hold their shape and correct your cornea. They may last for a year or more before they need replacing.

Soft contact lenses for astigmatism, often called toric lenses, are made to fit the shape of your cornea, with one half of the lens shaped to correct for the more refractive error than the other half. You will need to replace these every two to four weeks.

Orthokeratology

Orthokeratology uses contact lenses to re-shape your cornea and correct your astigmatism. You wear specially designed rigid gas-permeable contact lenses at night. When you wake up in the morning and remove the lenses, your cornea has been re-shaped giving you perfect vision. This means you don't need to wear glasses or contact lenses during the day.

However, your cornea is very elastic and will always return to its natural shape, so you will need to wear the lenses regularly. This usually means wearing the contact lenses every night or at least every two or three nights.

This treatment is only available if you have a low degree of astigmatism. If you're interested in orthokeratology, talk to your optometrist.

Surgery

There are a number of different types of surgery that can treat astigmatism. Most of the procedures involve reshaping your cornea to allow light to focus correctly on the retina. Some of the main types of surgery are listed below.

• Laser refractive surgery. This is where your surgeon will use a laser to even out the curve of your cornea, to make it round rather than oval.
• Radial keratotomy. This is where your surgeon will use several tiny cuts in your cornea to flatten it.
• Corneal transplantation. This kind of surgery is only done if you have severe problems with your sight and if other treatments don't work. It's where your surgeon replaces your cornea with a healthy one from a donor.
• Toric intraocular lens implant. If you're having cataract surgery, it may be possible to replace the natural lens of your eye with an artificial one that will focus the light in a way that corrects your astigmatism (a toric lens).

Talk to your optometrist to find out more about the different types of surgery available, or ask for a private referral to an ophthalmologist.

For answers to frequently asked questions on this topic, see FAQs.

For sources and links to further information, see Resources.