Astigmatism refers to the incorrect curvature of the cornea and any vision loss experienced as a result.
The surface of the eye is called the cornea.
The abnormal curve of the cornea means that when light enters the eye, it is not correctly focused on the retina, resulting in an unclear image.
Astigmatism may also be caused by an irregularly shaped lens, located behind the cornea.
It can affect children and adults. It is usually congenital, or present at birth, but it can develop after an eye operation or an injury to the eye.
It is one of a group of eye conditions called refractive errors. These happen when the cornea or lens is not perfectly smooth and evenly curved.
Other refractive errors include near- or far-sightedness and presbyopia, which happens with aging.
Refractive errors affect around half of all adults in the United States (U.S.) aged 20 years and over.
Here are some key points about astigmatism. More information is in the main article.
- Astigmatism is a type of refractive error.
- It is a common condition.
- It is caused by an abnormal curve of the cornea or lens.
- Laser surgery can often correct astigmatism.
The following are common signs and symptoms of astigmatism:
- blurred or distorted vision at all distances
- excessive squinting
- eye strain, especially when the eye has to focus for long periods, as in reading from paper or a computer monitor
- difficulty driving at night
A person with these symptoms may not have astigmatism, but it is a good idea to have an eye test.
Astigmatism can be due to an abnormal curvature of the cornea or the lens.
The cornea is a transparent layer of tissue that covers the front of the eye. It transmits and focuses light into the back of the eye while protecting the eye from infection and damage.
A perfectly curved cornea can bend, or refract, light properly as it enters the eye.
If the cornea does not curve perfectly, the light that hits it will not correctly refract, and the retina at the back of the eye will receive an imperfect image.
The person will have blurred vision in that eye.
In a person with astigmatism, the cornea is often oval- shaped rather than perfectly round.
If the curve is like an oblong, the light rays will focus on two points in the retina instead of one.
Exactly why some people are born with a cornea that does not curve properly is unclear, but there may be a genetic component.
A higher percentage of infants who are born preterm have astigmatism, compared with those 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 cause a condition known as irregular astigmatism.
This is similar to corneal astigmatism, but it affects 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, or retina, imperfectly. Most patients with lenticular astigmatism have a cornea with a normal shape.
Astigmatism can be tested for using the visual acuity test.
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, so regular eye tests are important.
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 that 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, or 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.
For children, the American Optometric Association recommend eye tests:
- At 6 months
- At 3 years
- before first grade
- every 2 years after that.
For high-risk children, an eye exam is recommended every year.
If the astigmatism is mild, the doctor may suggest no treatment at all.
Otherwise, corrective lenses are the usual approach.
Corrective lenses for astigmatism
Corrective lenses bend the incoming light rays in a way that compensates for the error caused by faulty refraction.
In this way, images are properly received onto the retina. These may be in the form of glasses or contact lenses. However, glasses may be better for children under the age of 12 years.
Anyone using contact lenses must be aware of good lens hygiene, to reduce the risk of eye infection.
Orthokeratology involves wearing a specially fitted, rigid contact lenses, for example, overnight, to reshape the cornea. This does not permanently improve vision, but the person may find that they can see better all day after wearing these.
There is a wide range of spectacles to choose from.
Laser eye surgery
Laser surgery is an option for the treatment of astigmatism but comes with risks.
Some people with astigmatism can be treated with laser eye surgery, the most common of which is laser in situ keratomileusis (LASIK).
In LASIK, the doctor uses a device called a keratome to make a thin, round hinged cut into the cornea.
The surgeon lifts the flap, 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.
Other laser options are:
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 can cause moderate to severe pain.
Laser epithelial keratomileusis (LASEK): Alcohol is used to loosen the surface of the cornea before operating.
A much thinner layer is affected, making the eye less vulnerable to damage or injury, compared with PRK. However, it is usually more painful than LASIK.
A laser is used to change the shape of the cornea.
LASEK may be a better option for a patient with a thin cornea.
Who should avoid laser surgery?
Laser eye surgery may not be suitable if:
- the patient is aged under 18 years.
- the patient's vision is still changing, for example, in older people. Vision should be stable for at least one year before laser surgery.
- the patient has diabetes, as surgery may worsen abnormalities in the eye caused by diabetes.
- a woman is pregnant or breast-feeding, because fluctuating hormones may cause the result to be inaccurate.
- the person has an immune condition, such as rheumatoid arthritis, lupus, or HIV, because it may be harder to recover after surgery.
- the person has another existing eye condition, such as cataracts and glaucoma, as these will need treating first.
- the person is taking certain medications such as Accutane or oral prednisone.
Risks of laser eye surgery
Risks can include:
- Correction errors: The surgeon removes the wrong amount of tissue, and the patient's vision worsens.
- Regression: Vision defects recur after surgery
- Visual loss: Some people's vision might worsen after surgery
- Dry eyes: This is a common problem after laser eye surgery.
In most countries, the risk of complications is low.
Patients should ensure that their surgeon is qualified and experienced and that they carry out an accurate assessment beforehand.