Astigmatism

Astigmatism is when your cornea (the clear dome that forms the front of your eye) isn't a perfectly round shape. Instead it's an oval shape, a bit like a rugby ball.

About astigmatism

If you have astigmatism, the way that your eye bends and focuses light rays is uneven. This is known as a refractive error. Refractive errors cause blurred vision and are the most common reason for seeing an optometrist (a registered health professional who examines eyes, tests sight and dispenses glasses and contact lenses). If you have astigmatism you may also have other refractive visual problems, such as short-sightedness (myopia) or long-sightedness (hyperopia).

When you look at an object, light rays pass into your eye through your cornea and lens, towards the retina at the back of your eye. If your eyes are completely regular in shape, the cornea is round and bends the light rays evenly onto a small area of your 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 rather than a clearly focused image and the object that you're looking at will seem distorted or stretched slightly.

Symptoms of astigmatism

Most people have a form of 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 far away
• eye strains
• headaches

Causes of astigmatism

Astigmatism is usually caused by an abnormal oval curvature of your cornea. However, sometimes, astigmatism can be caused by an unequal bending of light by the lens inside your eye.

Some other possible causes of astigmatism are listed below.

• Stitches put into your eye after surgery can cause your cornea to wrinkle, resulting in astigmatism. This usually gets better once your wound has healed and your stitches have been removed.
• Surgery to your eye may 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. This can get better if the chalazion is treated.
• Keratoconus. This is a condition where the structure of your cornea changes from its normal shape to a cone shape.
• An injury to your eye may cause damage to your cornea, changing its shape.

Diagnosis of astigmatism

Astigmatism is usually diagnosed during a routine eye examination, carried out by an optometrist. He or she may use one or more of the following tests to find out if you have astigmatism.

• A visual acuity test. This involves reading letters on a distance chart.
• Keratometry. This measures the curve of your cornea.
• Refraction. A series of lenses are placed in front of your eyes and your optometrist will measure how well they focus.
  
• Corneal topography. This is occasionally used to give more detail about the shape of your cornea. It scans your eye and creates a three-dimensional image.

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. 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

Wearing glasses or contact lenses can correct astigmatism – your optometrist will discuss both options with you. Your glasses are likely to have a special lens prescription to adjust for your astigmatism.

You can wear rigid gas-permeable contact lenses or soft contact lenses to treat astigmatism.

Rigid lenses work because they hold their shape and correct your cornea. They may last for up to a year before they need replacing and you will need to remove and clean them daily.

Soft contact lenses for astigmatism, often called toric lenses, are made to fit the shape of your cornea. One half of a toric lens is shaped with a steeper angle to correct your astigmatism. You will need to replace these either daily, or every two or four weeks. Soft contact lenses also need to be removed and cleaned after each period of wear.

See our frequently asked questions for more information.

Orthokeratology

Orthokeratology uses specially designed rigid gas-permeable contact lenses to reshape your cornea and correct your astigmatism. You wear the lenses at night and remove them during the day. 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.

This treatment is only available if you have mild astigmatism. Talk to your optometrist if you're interested in orthokeratology.

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. Your surgeon will use a laser to even out the curve of your cornea, to make it round rather than oval. For more information see our frequently asked questions.
• Radial keratotomy. Your surgeon will make several small cuts in your cornea to flatten it.
• Corneal transplantation. 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.

Produced by Dylan Merkett, Bupa Health Information Team, September 2012.

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

For sources and links to further information, see Resources.