Astigmatism is a disorder in which the surface of the eye, or cornea, is not normally curved, resulting in blurred vision.
The irregular curve of the cornea means it is not correctly centered on the retina when light enters the eye, resulting in an ambiguous image.
An irregularly shaped lens, situated behind the cornea, can also cause astigmatism.
Children and adults may be affected. It is normally congenital, or present at birth, but can develop after an eye surgery or an eye injury.
It is one of a group of disorders in the eye called refractive errors. This arise when the cornea or lens isn’t completely smooth and curved evenly.
Other refractive errors include near- or far-sightedness and presbyopia, which happens with aging.
Refractive errors affect approximately half of all adults aged 20 years and over in the United States ( U.S.).
Important facts about astigmatism
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.
What is astigmatism?
Astigmatism is a term used to describe the irregular shape of the surface of the eye called the cornea.
The abnormal curve of the cornea means that when the light enters the eye, it is not correctly focused on the retina, resulting in a blurred image.
A cornea without astigmatism has a perfectly round shape, much like a ball ‘s surface.
With astigmatism, the surface of the eye is shaped more like soccer.
It focuses light on the back of the eye in two places, and this causes blur.
Astigmatism may also be caused by an irregularly shaped lens, which is located behind the cornea inside the eye.
Many children born with astigmatism will not realize that they have it until they have an eye test.
Reading and concentrating at school can be affected if the child has undiagnosed astigmatism, so regular eye tests are important.
An eye specialist may use the following tools to examine the eye:
- 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
Astigmatism may not be present in a person with these symptoms, but it is a good idea to get an eye test.
Astigmatism occurs when the cornea, lens, or both are irregularly curvatured.
A translucent layer of tissue that protects the front of the eye is the cornea. It transmits light into the back of the eye and focuses it while shielding the eye from infection and injury.
A perfectly curved cornea as it reaches the eye will bend, or refract, light properly.
In an individual with astigmatism, the cornea is always egg-shaped rather than completely round, with two distinct curves. Often this is called corneal astigmatism.
The light rays will concentrate on two points on the retina instead of one because of the two different curves. This causes fuzzy vision and, if the astigmatism is serious, sometimes double vision.
Exactly why certain people are born with a cornea that does not curve properly, is unknown, but a genetic component could be present.
Compared with those born prior to their due date, a larger percentage of babies who are born preterm have astigmatism.
Some forms of surgery or damage to the eyes that cause corneal scarring can cause astigmatism.
Keratoconus is a degenerative eye disease in which the cornea slowly diminishes and changes to a more conical form. This can trigger a disorder referred to as irregular astigmatism.
The doctor may recommend no treatment at all if the astigmatism is mild.
Otherwise, the normal solution is the corrective lenses and certain people can benefit from laser surgery.
For astigmatism, corrective lenses
Corrective lenses bend the incoming light rays in such a way that the error caused by defective refraction is compensated for. Thus images are correctly projected onto the retina.
It may be in the form of contact lenses or glasses. To correct the vision, a standard prescription for near- or far-sightedness requires spherical control.
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
For children under the age of 12 years, glasses might be safer.
To of the risk of eye infection, anyone using contact lenses must be conscious of good lens hygiene.
Orthokeratology, or corneal refractive therapy
This requires, for example, wearing a specially fitted, rigid contact lens overnight to reshape the cornea. This does not permanently improve vision, but after wearing these, the person can find they can see better the whole day after.
There is a huge selection of spectacles to choose from.
Aside from the most common type of corneal astigmatism, there are other types that astigmatism can take.
This is similar to corneal astigmatism but it affects not the cornea but the lens.
The lens has variations that allow images to imperfectly hit the back of the eye, or retina, instead of a smooth curve. The majority of patients with lenticular astigmatism have a normal-shaped cornea.
Other ways of classifying astigmatism
Astigmatism can also be classified according to other established refractive errors.
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 astigmatism is associated with distance-sightedness and the two curves are concentrated behind the retina.
Mixed astigmatism is when one curve is far-sighted and the other is near-sighted.
Even astigmatism may be regular or irregular.
The two curves are at a 90-degree angle to each other if they are normal, but the angle is not 90 degrees if they are irregular.
Trauma, surgery, or an eye condition known as keratoconus, where the cornea gradually becomes thinner, may result in irregular astigmatism.
Laser eye surgery, the most common of which is laser in situ keratomileusis (LASIK), can treat certain people with astigmatism.
LASIK: The doctor makes a small, round hinged cut into the cornea using a device called a keratome.
The surgeon raises the flap, and under the flap, an excimer laser sculpts the shape of the cornea.
LASIK induces less discomfort than most treatments, and after a few days, the patient will regain his or her vision.
Some choices for lasers are:
Photorefractive keratectomy (PRK): This eliminates some of the cornea ‘s outer protective layer. By removing tissue an excimer laser changes the appearance of the cornea.
It typically has a more regular and spherical curve as the cornea heals. This can cause pain that is mild to extreme.
Laser epithelial keratomileusis (LASEK): The surgeon removes a thin corneal layer and uses a laser to alter the form of the cornea. It then removes the corneal tissue.
Compared with PRK, a much thinner layer is affected, making the eye less vulnerable to damage or injury. When anyone has a thin cornea and is unable to have Lasik, this surgery is often preferred. Usually it is more painful though 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.
The risks of surgery can include:
- Refractive 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.
The risk of complications is low in most countries.
Patients should ensure that their surgeon is trained and knowledgeable and that an accurate examination is conducted in advance.