Astigmatism is a common and usually minor eye condition that causes blurred or distorted vision.
It occurs when the cornea or lens isn't a perfectly curved shape. Many people who wear glasses have some degree of astigmatism.
Astigmatism belongs to a group of related eye conditions known as refractive errors. Other common refractive errors include:
If you have astigmatism, it's likely you'll also have one of these conditions.
Left untreated, astigmatism can cause headaches, eye strain and fatigue (tiredness), particularly after doing tasks that involve focusing on something for long periods, such as reading or using a computer.
What causes astigmatism?
Astigmatism is usually the result of an irregular-shaped cornea or lens. The cornea is the transparent layer of tissue at the front of the eye.
The cornea should be regularly curved like the surface of a football, but in cases of astigmatism it has an irregular curve, more like the shape of a rugby ball. This means that light rays entering the eye aren't focused properly, creating a blurred image.
In most cases, astigmatism is present at birth. However, it sometimes develops after an eye injury or as a complication of an eye operation.
Types of astigmatism
There are two types of astigmatism – regular and irregular.
Regular astigmatism is where the cornea is curved more in one direction than the other. It's more common than irregular astigmatism and can be corrected with glasses or contact lenses.
Irregular astigmatism is where the curvature of the cornea isn't even across the surface of the eye. Instead of being curved mostly in one direction, it could be curved in multiple directions, or the curve could be steeper towards the bottom.
Irregular astigmatism is often the result of an eye injury that causes a scar to develop on the cornea. It can't be corrected with glasses, but it can be corrected with contact lenses.
Astigmatism is usually diagnosed after a routine eye test.
It's very important for you and your children to have regular eye tests because astigmatism sometimes goes undiagnosed for years, and it can affect your or your child's ability to read and concentrate.
In many cases, the symptoms of astigmatism are so mild that treatment isn't 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.
In many cases, astigmatism is mild and treatment isn't required
Causes of astigmatism
The vision problems caused by astigmatism are a result of defects within the eye.
These defects are usually present from birth.
How the eye works
The eye is made up of three functional components. They are the:
optical system (cornea and lens) – the cornea and lens are positioned at the front of the eye and act like the lenses of a camera, focusing the light coming into the eye to form an image on the retina
retina – the light-sensitive layer of tissue at the back of the eye that senses light and colour and converts them into electrical signals
optic nerve – the "cable" that transmits the electrical signals from the retina to the brain, where they're interpreted and understood
Astigmatism usually occurs as a result of problems with the cornea or lens.
The cornea and lens
The cornea is a transparent layer of tissue that covers the front of the eye and helps protect the eye from damage. The cornea and lens are also responsible for focusing incoming light on to the retina to create a clear image.
To work properly, the cornea needs to be perfectly curved, like the surface of a football. However, in astigmatism the cornea is shaped irregularly, like the surface of a rugby ball.
When light hits an irregularly curved cornea, it's not focused correctly on to the retina. The incorrect focus blurs the image, resulting in blurred vision.
Astigmatism caused by problems with the cornea is known as corneal astigmatism.
The same problem can also sometimes be caused by an irregular-shaped lens that bends light unevenly in the eye. This is known as lenticular astigmatism.
It's not known why some people are born with astigmatism, but there may be a hereditary element.
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 and keratoglobus – eye conditions that cause the cornea to bulge, get thinner and change shape
some conditions that affect the eyelids and distort the cornea
other conditions that affect the cornea or lens
Astigmatism is usually diagnosed after a routine eye test.
Regular eye tests
Most people who have astigmatism are born with the condition, so it's important your children have regular eye tests.
Children may not realise there's anything wrong with their vision. If their vision isn't regularly tested, conditions like astigmatism could remain undiagnosed for many years, which can cause a lazy eye.
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 examined at birth or shortly afterwards (within 72 hours) to check for any obvious defects.
They'll have a second eye examination when they're between 6 and 8 weeks old, which will usually be carried out by your GP.
Your child's vision may also be checked when they start school at around five years of age, but this varies depending on where you live.
If your child has suspected vision problems, they may be referred to an orthoptist. An orthoptist specialises in problems relating to eye movements and vision development. They usually work in local health clinics or hospital eye clinics.
Eyesight problems may also be investigated by an ophthalmologist (a medical doctor who specialises in diagnosing and treating eye conditions). They mainly work in hospitals and hospital eye departments.
You may also see an optometrist. Optometrists examine eyes, test sight, and prescribe glasses and contact lenses. They're 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.
Adults should have an eye test at least once every two years, unless advised otherwise by their optometrist.
Testing for astigmatism
A number of tests can be used if astigmatism is suspected. The two most common tests are the visual acuity test and the keratometer test.
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.
A device called a keratometer can measure the degree of corneal astigmatism. It measures how light is being focused by the cornea and can detect irregularities in the curve of the cornea.
In many cases, the symptoms of astigmatism are so mild that treatment to correct vision isn't required.
If treatment is needed, the type that's recommended will depend on the type of astigmatism you have (regular or irregular). Treatment will usually be in the form of corrective lenses (glasses or contact lenses) or laser eye surgery.
Corrective lenses work by compensating for the irregular curve of the cornea so incoming light passing through the corrective lens is properly focused on to the retina. The retina is the light-sensitive layer of tissue at the back of the eye.
Glasses and contact lenses are usually equally effective at treating astigmatism. The type of corrective lenses you decide to use will therefore depend on your personal preference and the advice of your optometrist.
There's no medical reason why children can't wear contact lenses, although the opinion of your optometrist will be important in deciding if they're suitable. Most children over 12 years of age will be able to wear them.
However, it's important 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.
If you choose to wear contact lenses, it's important to ensure good lens hygiene to prevent eye infections developing.
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 isn't usually available free of charge on the as it's not considered an essential medical treatment and other effective treatments are available, such as glasses and contact lenses.
However, in 2011 the National Institute for Health and Care Excellence (NICE) announced corrective laser surgery could be a treatment option on the to improve vision after previous eye surgery.
For more information, see NICE guidance: laser correction of refractive error following non-refractive ophthalmic surgery (PDF, 96.8kb).
If you decide to have laser eye surgery, you'll have one or more appointments at a specialist eye clinic.
During laser surgery, the outer layer of cells of the corneal surface is removed. A laser is then used to remove tissue and change the shape of the cornea and the cornea is left to heal. The treatment usually takes 20 to 30 minutes.
There are a number of different techniques used for laser surgery. Your specialist should discuss the pros and cons of each technique with you, including any risks involved. The risks of complications associated with eye surgery are low.