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 this case by the eye, in order to focus it. 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:
- Short sight (myopia).
- Long sight (hypermetropia).
- Age-related long sight (presbyopia).
- Astigmatism (a refractive error due to an unevenly curved cornea).
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.
Light rays come off an object in all directions, as they result from the light around us from sun, moon and artificial light bouncing back off the object. The part of this bounced light that comes into the eye from an object needs to be focused on a small area of the retina. If this doesn't happen, what we look at will be blurred.
The cornea and lens have the job of focusing light. The cornea does most of the work, as it (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 central 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 become fatter. 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 is curved, a little like a rugby ball. When this curve is too great, or curves 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 but are spread. This lack of 'point focus' means that images received by the brain are blurred.
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 (both depth-ways and sideways). However, like the other refractive errors, the end result is the same: there is blurring of vision which can impair eyesight. The two eyes are usually not the same (ie if both eyes have astigmatism they may not be affected in the same line of vision or to the same degree). If the condition is mild, the brain will compensate for the difference between the two eyes, although this can cause eye strain and headaches. 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. It can result from an injury, scar or operation to the eye, particularly if the corneal surface is damaged. It can also result from anything pressing persistently on the surface of the cornea (such as a large lump on the eyelid) which pushes it out of shape.
Astigmatism can run in families and seems to be more common in premature or low-birth-weight babies. This may be because the cornea did not have enough time to develop properly in the womb.
Problems with the structure of the cornea can cause astigmatism. About 1 in 5 people with Down's syndrome have this problem and have 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, astigmatism 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.
Other symptoms can include difficulty seeing one colour against another (contrast), and distorted images, such as lines which lean to one side. Severe astigmatism can cause double vision.
Astigmatism often occurs along 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 details.
Are there any complications with astigmatism?
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 details.
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 are often the best option for astigmatism. Toric lenses are used. Contact lenses sit right on the surface of the eye. Many different types of contact lenses are available. Lenses may be soft or rigid gas-permeable. They 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 prescription.
Contact lenses tend to be more expensive than glasses. They require more care and meticulous hygiene. They are more suitable for older teenagers and adults, rather than very young 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. Laser eye surgery is expensive but offers the chance to restore normal sight permanently. The procedure is generally painless.
Complete and permanent resolution of the astigmatism is possible in a number of people. Others have a significant improvement even though perfect vision is not achieved, and glasses or contact lenses may still be needed.
A small number of people develop complications. Some develop hazy vision, a problem with night vision, or problems with bright light haloes in their peripheral (edge) vision.
Many private companies advertise laser eye surgery. Before embarking upon this type of treatment you should do some research. You only have one pair of eyes and you need to find the best treatment for you. This may not be the cheapest. 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 all similar, typically taking about ten minutes per eye and aiming to reshape the cornea by using the laser to remove a very thin layer of corneal tissue. The reshaping of the 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 thin flap of the cornea.
- This helps to flatten the cornea so that the light rays can be focused further back and on to the retina.
- The flap is then replaced and sticks spontaneously to the underlying cornea. The flap serves as a natural bandage, keeping the eye comfortable as it heals. Healing occurs relatively quickly.
- This is the most popular and common type of laser eye surgery.
- Vision recovery time is said to be around 24 hours.
For people who are not suitable for LASIK® the following two options are sometimes offered.
- PRK stands for Photo-Refractive Keratectomy.
- During PRK®, instead of creating a corneal flap as in LASIK®, the surgeon completely removes the extremely thin outer layer of the cornea, using an alcohol solution, a 'buffing' device or a blunt surgical instrument. The underlying cornea is then reshaped with a laser. A new epithelial layer grows back within five days.
- The healing time in PRK® is faster than in LASEK®.
- LASEK stands for LAser Sub-Epithelial Keratomileusis.
- The LASEK® procedure involves taking a thinner flap of corneal epithelium than in LASIK®. The cornea underneath is then treated as in LASIK® and the thinner flap is repositioned and held in place with a bandage contact lens.
- The hinged flap made in LASEK® surgery is much thinner than the corneal flap created in LASIK® (which contains both epithelial and deeper stromal tissues).
- The LASEK® technique lessens the likelihood of removing too much cornea. There is also slightly less risk of developing dry eyes afterwards.
- Patients with a naturally thin cornea may be more suited to this treatment.
- LASEK® can be a better option for patients with a high degree of myopia, which requires more tissue removal from the central cornea.
- LASEK® tends to be more painful and discomfort can last longer than with LASIK®. Visual recovery time can be up to a week.
- In some cases, the thin flap created during LASEK® is not strong enough to be replaced and will be removed completely as it would have been in PRK.
- The alcohol solution used during LASEK® can irritate and slow the healing process immediately after surgery.
Side-effects of all laser surgery may include blurred vision, over-correction or under-correction of short-sight, eye infection and dry eyes.
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 the eye (glaucoma), macular degeneration, or a family history of these conditions, you should check to see what your optician recommends about regular check-ups.
If you fall into a high-risk group, you should have at least a two-yearly (biennial) eye examination if you are over 50 years of age, and a yearly (annual) one if you are over 60 years of age.
If you have more than one risk factor then an eyesight check is recommended at least every three years once you reach 40 years of age.
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 years, an eye test is needed every 10 years. Between the ages of 41 and 55 years, it is recommended that you see an optician every five years. At any age between 56 and 65 years, two-yearly checks are needed, dropping to annual checks in low-risk people who are aged 65 years or more.
Dr Katrina Ford
Dr Mary Lowth
Dr Colin Tidy