Encyclopaedia


Astigmatism

Introduction

Astigmatism is a minor condition of the eye that causes blurred vision. It occurs when the cornea is not a perfectly curved shape. Most people who wear glasses have astigmatism.

The cornea and retina

The cornea is the transparent layer of tissue at the front of the eye. The cornea should be shaped like a football, but in cases of astigmatism it has an irregular curve, shaped more like a rugby ball. This means that the light rays that come into the eye do not focus properly on the retina and create a blurred image.

The retina is the light-sensitive tissue at the back of the eye where the light that enters the eye is converted into electrical signals. The optic nerve sends the signals to the brain, which interprets what is being seen. See Astigmatism - causes for more information about how the eye works.

Refractive errors

Astigmatism belongs to a group of related eye conditions known as refractive errors. Other common refractive errors include:

How common is astigmatism?

Astigmatism is a very common eye condition. It can be present at birth (congenital). However, it sometimes develops after an injury to the eye or as a side effect of an eye operation.

Outlook

In many cases, the symptoms of astigmatism are so mild that no treatment is needed to correct your vision. In cases where your vision is significantly affected by astigmatism, glasses or contact lenses can be used to correct it.

Laser treatment can be used to treat astigmatism in adults. However, it is unlikely that you will be able to receive laser treatment through the NHS.

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Symptoms

Symptoms of astigmatism 

The symptoms of uncorrected astigmatism include:

  • blurred vision
  • headaches
  • eye strain and fatigue (tiredness) – particularly after doing tasks that involve focusing on something for a long period of time, such as reading or using a computer
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Causes

To understand the causes of astigmatism, it is useful to understand how the eye works.

How the eye works

The eye is made up of three parts:

  • the cornea and lens are located at the front of the eye; they act like a camera lens, helping to focus the light coming into the eye
  • the retina is located at the back of the eye; it is a light-sensitive layer of tissue that senses light and colour and converts the light that enters the eye into electrical signals
  • the optic nerve sits at the back of the eye; it transmits the electrical signals from the retina to the brain, where they are converted into an image

Astigmatism usually occurs as a result of problems with the cornea.

The cornea

The cornea is a transparent layer of tissue that covers the front of the eye. It helps to protect the eye from damage. The cornea and lens are also responsible for focusing incoming light onto the retina to create a clear image.

To work properly, the cornea needs to be perfectly curved, like the top half of a football. In cases of astigmatism, the curve of the cornea is irregular, and is shaped more like a rugby ball.

When light hits an irregularly curved cornea, the light is not focused correctly onto the retina. Instead, the incorrect focus blurs the image, resulting in blurred vision.

It is not known why some people are born with astigmatism. The condition could possibly be hereditary (runs in families).

Other causes

Other possible causes of astigmatism include:

  • injuries to the cornea, such as an infection that scars the cornea 
  • changes to the cornea caused by eye surgery
  • keratoconus – a rare condition that causes the cornea to weaken, get thinner and change shape
  • some conditions that affect the eyelids
  • other conditions that affect the eyes
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Diagnosis

Regular eye tests

Most people who have astigmatism are born with the condition, so it is very important that your children have regular eye tests.

If your child is born with astigmatism, they may not realise there is anything wrong with their vision. If their vision is not regularly tested, conditions like astigmatism could be undiagnosed for many years. If a child has uncorrected astigmatism, they may have difficulty reading and concentrating at school.

When to get tested

Your baby will have their eyes checked at birth to ensure that they are healthy. Any concerns regarding your child's eyes can be raised with your GP or health visitor. A health visitor is a qualified nurse with extra training who helps families with babies and young children to avoid illness and stay healthy.

A health visitor may also ask about your child’s vision at another assessment – for example, if your child is having a general review of their wellbeing at around eight months old. Your health visitor can refer your child to an eye specialist at any time.

If your child has suspected vision problems, they may be referred to an orthoptist. An orthoptist specialises in problems relating to the development of vision in children. They usually work in local health clinics or hospital eye clinics. You may also see:

  • an ophthalmologist – a medical doctor who specialises in eye conditions and their treatment; they mainly work in hospitals and hospital eye departments 
  • an optometrist – optometrists examine eyes, test sight and prescribe glasses and contact lenses; they are trained to recognise eye conditions and sight defects

If your child's vision has been checked and is normal, they should continue to have regular eye checks about once every year.

In some areas, screening is also carried out when your child is between four and five years old, either in school or at a local clinic. You can ask your health visitor what is available in your area.

Adults should have an eye test at least every two years, unless advised otherwise by their optometrist.

Testing for astigmatism

A number of tests can be used to diagnosis astigmatism. They are briefly described below.

Visual acuity test

A visual acuity test can be used to assess your or your child’s ability to focus on objects at different distances. It usually involves reading letters on a chart called a Snellen chart. The letters become progressively smaller on each line of the chart.

Keratometer

A device called a keratometer can measure the degree of astigmatism. It measures how light is being focused by the cornea (transparent layer of tissue at the front of the eye), and it can detect irregularities in the curve of the cornea.

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Treatment

In many cases, the symptoms of astigmatism are so mild that treatment to correct your vision is not required. If treatment is necessary, it usually involves wearing glasses or contact lenses.

Corrective lenses

Corrective lenses work by compensating for the irregular curve of the cornea (transparent layer of tissue at the front of the eye), so that the incoming light passing through the corrective lens is properly focused onto the retina. The retina is the light-sensitive layer of tissue at the back of the eye.

Usually, glasses and contact lenses are equally as effective in treating astigmatism. Therefore, the type of corrective lenses you decide to use will depend on your personal preference and the advice of your optometrist. Your optometrist is the healthcare professional who examines your eyes, tests your sight, and prescribes and provides any glasses or contact lenses.

There is no clinical reason why children cannot wear contact lenses, although the opinion of your optometrist will be important in deciding if they are suitable. Most children over 12 years old will be able to wear them. However, it is important that your child is able to use their lenses correctly. They must be able to follow any instructions related to their lenses, such as how long to keep them in, and when to clean them.

Contact lenses

If you choose to wear contact lenses, it is important to keep good lens hygiene to prevent eye infections.

There are three different types of contact lenses:

  • rigid contact lenses
  • gas permeable contact lenses
  • soft contact lenses

These are described in more detail below.

Rigid contact lenses

Rigid contact lenses are made from a combination of plastic and glass. The disadvantage of rigid contact lenses is that they prevent oxygen from getting into the eye. A lack of oxygen can make your eye produce new blood vessels, which may obscure your vision.

These are an old-fashioned type of lens, and may not be commonly used for astigmatism.

Gas permeable contact lenses

Gas permeable contact lenses use a type of plastic that allows oxygen to pass through the lens and into your eye. These materials are known as gas permeable or oxygen permeable.

Soft contact lenses

Soft contact lenses are made from a mixture of water and polymer, which is known as a hydrogel.

The plastic itself is not gas permeable, but the water in the lens allows the oxygen to enter your eye. In some soft lenses, the water evaporates quickly, so you can only wear them for a short time before they need to be replaced. However, newer materials, such as silicone gel, are oxygen permeable, so you can wear them for longer.

There are several varieties of soft contact lenses, such as:

  • lenses you wear for one day, then throw away after taking them out in the evening
  • lenses you wear every day for one month, taking the lenses out every evening and putting them in again in the morning 
  • lenses that you wear continuously for one month, then replace

Laser eye surgery

Laser surgery involves using lasers (narrow beams of light) to remould the tissue of the cornea to change its curve.

Laser surgery to correct astigmatism is not usually available free on the NHS. This is because it is not considered an essential medical treatment, and alternative effective treatments are available, such as glasses and contact lenses.

Laser surgery is usually carried out on an outpatient basis. This means that you will not have to stay in hospital overnight. Instead, you will have one or more appointments at a specialist eye clinic. The treatment usually takes 20 to 30 minutes.

There are three different types of laser surgery:

  • photorefractive keratectomy (PRK)
  • laser epithelial keratomileusis (LASEK)
  • laser in situ keratectomy (LASIK)

These are described below.

Photorefractive keratectomy (PRK)

During photorefractive keratectomy (PRK), a small amount of the cornea's surface is removed. A laser is used to remove tissue to change the shape of the cornea. The surface of the cornea is then left to heal. PRK is not as common as the other two types of surgery.

Laser epithelial keratomileusis (LASEK)

Laser epithelial keratomileusis (LASEK) is a similar procedure to PRK, but it involves using alcohol to loosen the surface layer of cells of the cornea before it is moved out of the way. A laser is then used to change the shape of the cornea. After treatment has finished, the surface layer of cells will grow back naturally after a few days.

Laser in situ keratectomy (LASIK)

Laser in situ keratomileusis (LASIK) is similar to LASEK, but here a flap of tissue is created often using a specialised kind of laser called a femtosecond laser. The flap is lifted up to expose the underlying corneal tissue, which is then treated with laser reshaping.

The flap is then repositioned on the eye. LASIK is often the preferred laser treatment option because it causes very little pain and your vision starts to recover within a few hours. Full recovery of vision can take a little longer, however.

LASEK and PRK are more painful procedures than LASIK. It can take up to a week for your vision to recover. Full vision recovery typically takes around three to four weeks after LASEK or PRK.

LASIK treatment can only be carried out if your cornea is thick enough. If your cornea is too thin, the risk of complications and side effects, such as a reduction in your vision, is too high. LASEK and PRK may be possible if your cornea is not thick enough for LASIK surgery.

When laser eye surgery can be used

People under 21 years old should not have laser surgery of any type. This is because their vision may still be changing and it is unwise to alter the structures of the eyes at this stage.

Your vision can also change even when you are over 21. Before carrying out laser treatment, your eye care clinic should check your glasses or contact lens prescription to confirm that your vision has been stable for at least two years.

Laser surgery may also not be suitable for you if:

  • you have diabetes, which can cause abnormalities in the eyes that can be made worse by laser surgery to the cornea
  • you are pregnant or breastfeeding – your body will contain hormones that cause slight fluctuations within the eye, making precise surgery too difficult
  • you have a condition that affects your immune system (the body’s natural defence system) – conditions such as HIV and AIDS or rheumatoid arthritis may affect your ability to recover after surgery
  • you have other problems with your eyes, such as glaucoma (an eye condition that affects vision) or cataracts (where the lens of the eye becomes cloudy); these conditions will need to be treated before laser surgery to avoid the risk of any damage

Risks of laser eye surgery

As with all surgical procedures, laser surgery carries some risks. Possible complications of laser surgery include:

  • your vision may get worse if the amount of tissue taken from your cornea is more or less than intended; this is known as a correction error
  • the surface of your cornea can begin to grow into the other layers of the cornea; this is known as epithelial ingrowth and it can cause problems with your vision that may need to be corrected with more surgery 
  • your cornea becomes too thin and your vision is reduced or lost; this is called ectasia
  • your cornea becomes infected; this is called microbial keratitis

All of the above risks are very rare. For example, in 2006, the National Institute of Health and Clinical Excellence (NICE) found that after laser surgery there was around a 1 in 100 chance of one of these complications occurring. One recent study into LASIK surgery for astigmatism had no complications after six months for the 137 eyes treated.

Although information about the risks of LASEK and PRK is not as comprehensive as it is for LASIK, the risk factors are thought to be similar.

More recent research into laser eye surgery has also found the risks of complications to be low. Ask your surgeon to explain the possible risks before your operation.

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The information on this page has been adapted by NHS Wales from original content supplied by NHS Choices.

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