Astigmatism is a term used for the irregular shape of the surface of the eye 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.
A cornea without astigmatism has a perfectly round shape like the surface of a ball.
With astigmatism, the surface of the eye is shaped more like a football.
It focuses light at two places on the back of the eye, and this causes blur.
Astigmatism may also be caused by an irregularly shaped lens, located behind the cornea inside the eye.
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 (AOA) recommends 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.
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 to check.
Astigmatism happens when there is an irregular curvature of the cornea, the lens, or both.
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.
In a person with astigmatism, the cornea is often egg-shaped with two different curves rather than perfectly round. This is sometimes known as corneal astigmatism.
Because of the two different curves, the light rays will focus on two points on the retina instead of one. This causes blurry vision and sometimes double vision, if the astigmatism is severe.
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.
If the astigmatism is mild, the doctor may suggest no treatment at all.
Otherwise, corrective lenses are the usual approach, and some people may benefit from laser surgery.
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 projected onto the retina.
These may be in the form of glasses or contact lenses. A normal prescription for near- or far-sightedness includes sphere power, to correct the vision.
Lenses for astigmatism will need:
- a spherical power, to correct the near or far-sightedness
- a "cylinder" lens power, to correct the astigmatism
- an axis designation that describes the positioning of the cylinder correction
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, or corneal refractive therapy
This 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.
Alongside corneal astigmatism, the most common type, there are other forms astigmatism can take.
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.
Other ways of classifying astigmatism
Astigmatism can also be categorized according to other refractive errors that are present.
Myopic astigmatism happens when astigmatism is combined with near-sightedness and the two curves are focused in front of the retina.
Hyperopic astigmatism is when far-sightedness is combined with astigmatism and the two curves are focused behind the retina.
Mixed astigmatism is when one curve is far-sighted and the other is near-sighted.
Astigmatism can also be regular or irregular.
If it is regular, the two curves are at a 90-degree angle to each other, but if they are irregular, the angle is not 90 degrees.
Irregular astigmatism can result from trauma, surgery, or an eye condition known as keratoconus, where the cornea gradually becomes thinner.
Some people with astigmatism can be treated with laser eye surgery, the most common of which is laser in situ keratomileusis (LASIK).
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): A thin layer of cornea is removed by the surgeon and a laser is used to change the shape of the cornea. The corneal tissue is then replaced.
A much thinner layer is affected, making the eye less vulnerable to damage or injury, compared with PRK. This surgery is sometimes preferred if someone has a thin cornea and is unable to have Lasik. However, it is usually more painful than LASIK.
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.