The main symptom of astigmatism is blurred vision. It occurs because the cornea at the front of the eye is unevenly curved. Eyesight problems, such as astigmatism, are also known as refractive errors. Astigmatism is a common condition that can be corrected by glasses or contact lenses, or cured with laser eye surgery.
This leaflet is part of our series on refractive errors
What is a refractive error?
A refractive error is an eyesight problem. Refractive errors are a common reason for reduced level of eyesight (visual acuity).
Refraction refers to the bending of light. In terms of the eye, a refractive error means that the eye cannot focus light on to the retina properly. This usually occurs either due to abnormalities in the shape of the eyeball, or because age has affected the workings of the focusing parts of the eye.
There are four types of refractive error:
- Myopia (short sight).
- Hypermetropia (long sight).
- Astigmatism (a refractive error due to an unevenly curved cornea).
- Presbyopia (age-related long sight).
In order to understand refractive errors fully, it is useful to know how we see.
When we look at an object, light rays from the object pass through the eye to reach the retina. This causes nerve messages to be sent from the cells of the retina down the optic nerve to the vision centres in the brain. The brain processes the information it receives, so that in turn, we can see.
The light rays have to be focused on a small area of the retina; otherwise, what we look at is blurred. The cornea and lens have the job of focusing light. The cornea partly bends (refracts) the light rays which then go through the lens, which finely adjusts the focusing. The lens does this by changing its thickness. This is called accommodation. The lens is elastic and can become flatter or more rounded. The more rounded (convex) the lens, the more the light rays can be bent inwards.
The shape of the lens is varied by the small muscles in the ciliary body. Tiny string-like structures called the suspensory ligaments are attached at one end to the lens, and at the other to the ciliary body. This is a bit like a trampoline with the middle bouncy bit being the lens, the suspensory ligaments being the springs, and the ciliary muscles being the rim around the edge.
When the ciliary muscles in the ciliary body tighten, the suspensory ligaments slacken, causing the lens to fatten. This happens for near objects. For looking at far objects, the ciliary muscle relaxes, making the suspensory ligaments tighten, and the lens thins out.
More bending (refraction) of the light rays is needed to focus on nearby objects, such as when reading. Less bending of light is needed to focus on objects far away.
What is astigmatism?
The cornea at the front of the eye is not perfectly rounded but has a very slight curve, a little like a rugby ball. When this curve is too great, or pointing in the wrong direction, astigmatism occurs. Light rays coming through the cornea and lens are not focused on to one sharp spot on the retina. This lack of 'point focus' means that images received by the brain are blurred.
So astigmatism is a bit more complicated than either short sight (myopia) or long sight (hypermetropia), as there is a problem with the focus of light in two different directions. However, like the other refractive errors, the end result is the same, ie it can reduce eyesight. Although astigmatism is very common (about 9 in 10 people have some degree of astigmatism), it does not always cause a problem.
What causes astigmatism?
Astigmatism is usually present at birth but it can result from an injury, scar or operation to the eye, if the corneal surface is damaged. It can also come about as a result of anything pressing persistently on the surface of the cornea such as a large lump on the eyelid.
Astigmatism can run in families and seems to be more common in premature or low birth weight babies, perhaps because the cornea does not have enough time to develop properly.
Finally, problems with the structure of the cornea can cause astigmatism. Some of these are present from birth. About 1 in 5 people with Down's syndrome have a significant astigmatism. Other corneal disorders develop throughout life. The most common of these is a condition called keratoconus. This can cause significant astigmatism, as well as short sight (myopia), and corneal scarring.
What are the symptoms of astigmatism?
For most people, this is a very mild, minor problem which may not even be noticed. However, with more advanced astigmatism, there can be a variety of symptoms including blurred vision, light sensitivity (photophobia), eye strain and fatigue (especially after long periods of concentration, such as when using a computer), and headaches.
You may have difficulty seeing one colour against another (contrast) or you may experience distorted images, such as lines which lean to one side. Severe astigmatism can cause double vision.
Astigmatism usually occurs with either short sight (myopia), long sight (hypermetropia) or age-related long sight (presbyopia). (See separate leaflets called Short Sight - Myopia, Long Sight - Hypermetropia and Age-related Long Sight (Presbyopia) for more information.)
Are there any complications?
Astigmatism in only one eye may cause lazy eye (amblyopia) if present from birth. The affected eye does not 'learn' how to see because the brain ignores the signals it receives. Amblyopia can be treated with eye patching if diagnosed early enough, before the vision pathways in the brain are fully developed. (See separate leaflet called Amblyopia for more information.)
What is the treatment for astigmatism?
In many cases the symptoms of astigmatism are so mild that no treatment is needed. If vision is more significantly affected, glasses, contact lenses or surgery can correct the vision.
The simplest, cheapest and safest way to correct a regular astigmatism is with glasses. The lenses of the glasses adjust the direction of the incoming light rays, correcting the uneven curve of the cornea. There is an enormous choice of spectacle frames available, to suit all budgets. An irregular astigmatism cannot be corrected by a lens. Glasses are seen as more acceptable these days, and younger people may even regard them as a fashion accessory.
These do the same job as glasses and may be the best option for correcting some types of astigmatism. Many different types of contact lenses are available. Lenses may be soft or rigid gas-permeable. Toric lenses are used to correct astigmatism. Contact lenses can be daily disposable, extended wear, monthly disposable, or non-disposable. Your optician can advise which type is most suitable for your eyes and your astigmatism prescription. Contact lenses often work out more expensive than glasses and they require more care, and meticulous hygiene. They are suitable for older teenagers and adults, rather than for children.
Surgery is an option for some people to cure their astigmatism and any associated short or long sight. Generally, these operations are not available on the NHS, and so are a private option. Laser eye surgery is expensive but offers the chance to restore normal sight permanently. The procedure is generally painless.
There is the potential for complete and permanent resolution of the refractive error in a number of people. Others may have improved but not perfect vision and glasses or contact lenses may still be needed.
However, a small number of people develop complications after surgery to correct astigmatism. Some people develop hazy vision, a problem with night vision, or problems with bright light haloes in their peripheral vision.
Many private companies aggressively market their laser eye surgery. Before embarking upon this type of treatment you need to do some research. Try to go with personal recommendations, preferably a recommendation by an NHS eye surgeon (ophthalmologist). It is important that you know your facts - the failure rate, the risk of complications, level of aftercare and what the procedure involves, before submitting yourself to an irreversible, costly treatment.
Several types of laser surgery have been developed. These include: LASIK®, PRK® and LASEK®. They are similar because the basic idea is to reshape the cornea using the laser to remove a very thin layer. The reshaped cornea allows the refraction of the eye to be corrected.
- LASIK stands for Laser-Assisted In situ Keratomileusis. This is the most popular form of laser eye surgery.
- The laser is used to lift and remove a very thin layer of the cornea. This helps to alter the shape of the cornea (making it more regular), so that the light rays can be focused to a single point on the retina.
- Any associated short sight (myopia) or long sight (hypermetropia) can also be corrected at the same time.
- PRK stands for Photo-Refractive Keratectomy. It is an older surgical operation, which has mostly been replaced by newer techniques.
- LASEK stands for LAser Sub-Epithelial Keratomileusis. It is an improved form of PRK with some similarities to LASIK.
- Most of the outer layer of the cornea (the epithelium) is left intact.
- The LASEK procedure tends to be more painful, and discomfort can last longer than with LASIK.
There are other methods available - most are variants of laser surgery. Implantable contact lenses (refractive lens exchange) and corneal grafts are options in very severe or specialised cases of astigmatism.
How often do I need an eyesight test?
This depends on your age, your family history and any pre-existing medical conditions.
People at high risk of sight problems need more frequent eyesight checks. If you have diabetes, raised pressure in your eye (glaucoma), macular degeneration, or a family history of these conditions, you should check to see what your optician recommends about regular check-ups. As a guide, if you fall into the high-risk group, you should have at least a yearly (annual) eye examination if you are over 60 years of age. If you are over 50 years of age it should be every two years, and over 40 years, with risk factors, then an eyesight check is recommended at least every three years.
Low-risk people with no symptoms of an eyesight problem, do not need to have their eyes tested so frequently. If you fall in this group and are aged between 19 and 40, an eye test is needed every ten years. Between the ages of 41 and 55, it is recommended that you see an optician five-yearly. At any age between 56 and 65, two-yearly checks are needed, dropping to annual checks in low-risk people who are 65 or more.
Dr Katrina Ford
Dr Colin Tidy
Dr Olivia Scott