In astigmatism, the front surface of the eye (cornea) is not curved in the normal way. The primary symptom of astigmatism is blurred vision.
Because of the abnormal curve of the cornea, when light rays enter the eye, they do not focus correctly on the retina, resulting in a blurred image.
Astigmatism may also be caused by an irregularly shaped lens, which is located behind the cornea.
Astigmatism can occur in children and adults; it is fairly common and not contagious. It is usually congenital - present at birth - but can develop after an eye operation or an injury to the eye.
This condition belongs to a group of eye conditions called refractive errors; these include myopia (short-sightedness), hyperopia (long-sightedness), and presbyopia (aging of the lens in the eye).
These conditions are common, around half of all adults in the United States aged 20 and older have refractive errors in their eyes.
Contents of this article:
Fast facts on astigmatism
Here are some key points about astigmatism. More detail and supporting information is in the main article.
- Astigmatism is a type of refractive error
- Astigmatism is a common condition in the population
- It is caused by an abnormal curve of the cornea or lens
- Laser surgery can often correct astigmatism
Causes of astigmatism
Astigmatism is very common. Regular eye tests are important for diagnosis.
Astigmatism can be due to an abnormal curvature of the cornea (corneal) or the lens (lenticular):
The cornea is a clear (transparent) layer of tissue that covers the front of the eye. As well as transmitting and focusing light into the eye, it protects the eye from infection and damage. The cornea needs to have a perfect curve in order to bend (refract) light properly as it enters the eye.
If the cornea does not curve perfectly, the light that hits it will not refract properly and the retina at the back of the eye will receive an imperfect image. The person will have blurred vision from that eye - astigmatism.
People with astigmatism commonly have an oblong-shaped (oval-shaped) cornea rather than a perfect sphere. A ping-pong ball is a perfect sphere, while an American football or a rugby ball has an oblong shape. If the cornea's curve is like an oblong, the light rays will focus on two points in the retina, rather than just one.
Experts are not sure why some people are born with a cornea that does not curve properly. Twin studies indicate that there may be a genetic component to astigmatism.
A higher percentage of babies born prematurely have astigmatism, compared with babies that are born closer to their due date.
Certain types of surgery or eye injuries that cause scarring of the cornea may cause astigmatism.
Keratoconus is a degenerative disorder of the eye where the cornea gradually thins and changes to a more conical shape; this can also cause astigmatism.
This is similar to corneal astigmatism but exists in the lens rather than the cornea.
Instead of a perfect curve, the lens has variations that cause images to reach the back of the eye (retina) imperfectly. Most patients with lenticular astigmatism have a normally shaped cornea.
Symptoms of astigmatism
The following are common symptoms of astigmatism:
- Blurred or distorted vision at all distances
- Excessive squinting
- Eye strain - occurs more often when the eye has to focus for long periods, as in reading from paper or a computer monitor
Diagnosis of astigmatism
A visual acuity test is commonplace when having an eye exam.
An eye specialist may use the following tools to examine the eyes:
- Visual acuity test - this involves reading letters on a chart. The letters become progressively smaller on each line.
- Astigmatic dial - a chart showing a series of lines which make up a semi-circle. People with perfect vision will see the lines clearly, while those with astigmatism will see some more clearly than others.
- Keratometer (ophthalmometer) - this device measures the reflected light from the surface of the cornea. It measures the radius of the curvature of the cornea and can assess the degree of abnormal curvature.
- Corneal topography - this process gives more information about the shape and curve of the cornea.
The importance of regular eye tests
Astigmatism is very common. Many children who are born with astigmatism will not realize they have it until they have an eye test.
Reading and concentrating at school may be affected if a child has undiagnosed astigmatism. Therefore, regular eye tests are important.
The American Optometric Association recommend that children have an eye exam at age 6 months, age 3 years, before first grade, and every 2 years after that. For high-risk children, an eye exam is recommended every year. Adults should have an eye test every couple of years, and more often if they have chronic conditions, like diabetes.
Treatments for astigmatism
If the astigmatism is very mild, the doctor will commonly suggest no treatment at all.
Corrective lenses for astigmatism
Corrective lenses bend the incoming light rays in a way that compensates for the error caused by faulty refraction so that images are properly received onto the retina. Whether the corrective lenses are in the form of glasses or contact lenses is up to the patient - they are equally effective.
Experts say that children can wear contact lenses as long as they are careful about using them properly - this includes not wearing them for too long and cleaning them correctly, which might be difficult to achieve if the child is under the age of 12.
People of any age who use contact lenses need to be aware of good lens hygiene. Otherwise, there is a significant risk of eye infection.
People with astigmatism may be prescribed rigid gas permeable lenses, which are made from plastics, silicone and/or fluoropolymers. These lenses are hard and hold their shape, but allow oxygen to pass freely to the eye. Individuals with more severe astigmatism may prefer these. Soft contact lenses are commonly used for more minor astigmatism.
Laser eye surgery
Some people with astigmatism can be treated with laser eye surgery, the most common of which is LASIK.
- Photorefractive keratectomy (PRK) - some of the outer protective layer of the cornea is removed. An excimer laser changes the shape of the cornea by removing tissue. When the cornea heals, it usually has a more even and spherical curve. This procedure can be moderately to very painful.
- LASEK (Laser epithelial keratomileusis) - alcohol is used to loosen the surface of the cornea before it is operated on; a much thinner layer is affected, making the eye less vulnerable to damage or injury, compared with PRK. A laser is then used to change the shape of the cornea. LASEK may be a better option for a patient whose cornea is thin. This procedure is usually less painful than PRK, but slightly more painful than LASIK.
- Laser in situ keratectomy (LASIK) - the doctor uses a device called a keratome to make a thin, round hinged cut into the cornea. The flap is then lifted, and an excimer laser sculpts the shape of the cornea under the flap. LASIK causes less pain than the other procedures, and the patient will recover their vision within a few days; because of this, LASIK is usually the preferred laser treatment option.
LASEK and PRK are better procedures than LASIK in cases where the cornea is thin.
Laser eye surgery may not suitable if:
- The patient is under the age of 18.
- Vision is still changing - the vision of some older people may still be changing. Experts say that a person's vision should be stable for at least 1 year before undergoing laser surgery.
- Patients with diabetes - in some cases, laser surgery may worsen abnormalities in the eye caused by diabetes.
- Pregnant or breast-feeding mothers - during pregnancy and breast-feeding, hormone fluctuations still exist within the eye, making it more difficult to carry out surgery accurately.
- People with some immune conditions - people with rheumatoid arthritis, lupus, or HIV, for example, may find it harder to recover after surgery.
- People with other existing eye conditions - they will need to have other conditions treated first before becoming eligible candidates for laser eye surgery. Examples include cataracts and glaucoma.
- People taking certain medications - if the patient is taking medications, such as Accutane or oral prednisone, they should not undergo laser eye surgery.
Risks of laser eye surgeryRisks can include:
- Correction error - the surgeon may have taken out the wrong amount of tissue and the patient's vision worsens.
- Regression - vision defects can recur after surgery
- Visual loss - some people's vision might become slightly worse after surgery
- Dry eyes - dry eye is very common after laser eye surgery
In most countries, the risk of complications from laser eye surgery is very small. It depends on how experienced the surgeon is, and how accurately the patient was assessed beforehand.