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Astigmatism

Published by Bupa's health information team, July 2008.

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

Astigmatism is when the cornea (the clear dome that covers the pupil) or the lens of your eye is not its usual perfectly round shape. Instead it's an oval shape, a bit like a rugby ball.

Astigmatism is very common - one in nine people in the UK have it to some degree. However, you will only need to have it corrected if your vision is affected. The higher your degree of astigmatism the poorer your vision will be.

• About astigmatism
• Symptoms
• Causes
• Diagnosis
• Treatment
• Further information
• Questions and answers
• Related topics
• Sources

About astigmatism

Astigmatism is a refractive error. This means there is an error in the amount your eye bends light rays. To understand what astigmatism is, it helps to understand how your eyes focus.

Normal vision

When you look at an object, light rays pass into your eye through the cornea then through the lens towards the retina at the back. In a healthy eye the lens and cornea focus the light rays on a small area of the retina so that you can see the object clearly.

Astigmatism

With astigmatism, the curve on the cornea or lens isn't its usual round shape. Instead, it's an oval shape. This means that when light enters the eye 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.

The shape of the cornea or lens determines whether you have regular or irregular astigmatism.

• Regular astigmatism is when the curve of the cornea or lens is equal throughout, making both sides of the oval shape the same.
• Irregular astigmatism is when the curve of the cornea or lens is different throughout. This means that each side of the oval shape is different.

Astigmatism is often accompanied by other vision conditions, such as short-sightedness (myopia) or long-sightedness (hyperopia).

Symptoms

Most people have some degree of astigmatism. However, only some will have it to such a degree to cause symptoms. These may include:

• blurred vision; this may happen when looking at objects close up and far away
• tired eyes
• headaches

Causes

Astigmatism is usually caused by a defect in the curve of your cornea, but it may also be due to an irregularly shaped lens, or it can be caused by both. This is usually something you are born with (inherited from your parents).

In addition to this, irregular astigmatism can be caused by an underlying eye condition, eye surgery or an injury. Eye conditions causing astigmatism include chalazion and keratoconus.

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 distort and change shape causing astigmatism, although this is rare.

Keratoconus is a relatively rare eye condition. The cornea becomes thin and stretched in the centre, and the thin part of the cornea bulges. This causes the cornea to become an irregular shape making vision distorted.

Surgery on your eye or an infection can cause scarring on your cornea. This changes the way light enters the eye and focuses on the retina. Also, stitches in your eye after surgery can distort your cornea causing astigmatism. This usually goes away once your wound has healed and your stitches have been removed.

Diagnosis

Your optometrist (a health professional who examines eyes, tests sight and dispenses glasses and contact lenses) will diagnose your astigmatism.

He or she will carry out several tests on your vision. This will include:

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

These tests will determine the best treatment for your astigmatism, and the prescription for contact lenses or glasses.

It's important to have eye tests with your optometrist every two years. They routinely diagnose problems with vision, like astigmatism, but can also help in diagnosing serious health conditions like high blood pressure and diabetes.

Treatment

Glasses and contact lenses

Glasses or contact lenses can be used 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.

Rigid contact lenses (hard or gas-permeable lenses) have traditionally been used to correct astigmatism. This is because they are rigid and hold their shape on your cornea. However, soft contact lenses have now been developed that can be used to correct astigmatism. Often called toric lenses, they are made to fit the shape of your cornea, with one half of the lens shaped to correct for more refractive error than the other half.

Orthokeratology

Orthokeratology uses contact lenses to re-shape the cornea to correct your astigmatism. Specially designed rigid gas-permeable contact lenses are worn at night. When you wake up in the morning and remove the lenses, the 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, the cornea is very elastic and will always return to its natural shape, so the lenses need to be worn regularly at night.

This treatment is only available to people with low degrees of astigmatism. If you are interested in orthokeratology, talk to your optometrist.

Refractive surgery

Techniques such as laser surgery (for example LASIK), radial keratotomy and corneal transplantation can be used to correct astigmatism. These procedures involve reshaping the cornea to allow light to focus correctly on the retina. Talk to your optometrist to find out more or ask them to refer you to an ophthalmologist.

Laser refractive surgery

There are several different types of laser surgery operations. They involve using a laser to even out the curve of your cornea to make it round rather than oval. This allows light rays to correctly focus onto your retina. The operation is carried out under local anaesthetic, so you are awake throughout the procedure.

This type of procedure can't be used for all degrees of astigmatism - talk to your ophthalmic surgeon to find out if it's suitable for you.

Laser surgery is still a relatively new procedure and the long-term outcomes are still not fully known.

Your surgeon will discuss how suitable laser surgery is for you and the risks and benefits during your consultation.

Radial keratotomy

Several tiny cuts are made in your cornea to flatten it. This is an older procedure and has largely been replaced by more modern laser-based techniques.

Corneal transplantation

This involves replacing your cornea with a healthy donor cornea, and usually only used for complex cases. The operation can be done under local anaesthetic or general anaesthetic - your ophthalmologist (a doctor specialising in eyes) will discuss the options with you.

Further information

• The College of Optometrists
  020 7839 6000
  www.college-optometrists.org

Related topics

• Long-sightedness
• LASIK
• Short-sightedness

Sources

• Astigmatism. GP Notebook. www.gpnotebook.co.uk, accessed 04 February 2008
• Titcomb L, Laser surgery for refractive errors. The Pharmaceutical Journal 2006. 276: 511-514
• Eye disorders. The Eyecare Trust. www.eye-care.org.uk, accessed 5 February 2008
• Blepharitis. Clinical Knowledge Summaries. http://cks.library.nhs.uk, accessed 10 April 2008
• What is Keratoconus? UK Keratoconus Self Help and Support Group. www.keratoconus-group.org.uk, accessed 5 February 2008
• Khaw PT and Elkington AR, ABC of Eyes. London: BMJ Publishing Group, 2002
• Lenses for astigmatism. The British Contact Lens Association. www.bcla.org.uk, accessed 11 February 2008
• Orthokeratology. The British OrthoKeratology Society. www.boks.org.uk , accessed 11 February 2008
• Orthokeratology. The College of Optometrists. www.college-optometrists.org, accessed 11 February 2008

This information was published by Bupa's health information team and is based on reputable sources of medical evidence. It has been peer reviewed by Bupa doctors. The content is intended for general information only and does not replace the need for personal advice from a qualified health professional.

Publication date: July 2008.