Astigmatism is a common condition in which either the cornea or the crystalline lens does not curve in the typical way. Corrective lenses can often treat astigmatism, but surgery is also an option.

Astigmatism is one of a group of eye conditions called refractive errors. Other refractive errors include nearsightedness (myopia), farsightedness (hyperopia), and presbyopia, which happens with aging.

Refractive errors are common. In fact, the National Eye Institute estimates that more than 150 million people in the United States have a refractive error. Research suggests that astigmatism is the most common type of refractive error in both children and adults worldwide.

In this article, we detail the symptoms and causes of astigmatism and explain how eye doctors can treat it.

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The following are common signs and symptoms of astigmatism:

  • blurred or distorted vision at all distances
  • headaches
  • squinting in order to see clearly
  • eyestrain, especially when the eye has to focus for long periods
  • difficulty seeing or driving at night

A person with these symptoms may not have astigmatism, but it is advisable to have an eye exam to check.

Types

Different types of astigmatism include:

  • Corneal astigmatism: This is the most common type of astigmatism.
  • Lenticular astigmatism: This type of astigmatism occurs due to changes in the eye’s lens.
  • Irregular astigmatism: In this type, the curvature of the cornea is uneven.

Astigmatism can also occur with myopia, hyperopia, or both:

  • Myopic astigmatism: Myopic astigmatism happens when astigmatism combines with myopia, and the two curves in the cornea or the lens — the curves from top to bottom and side to side — are focused in front of the retina.
  • Hyperopic astigmatism: This occurs when hyperopia combines with astigmatism, and the two curves are focused behind the retina.
  • Mixed astigmatism: This is when one curve produces symptoms of hyperopia and the other produces symptoms of myopia.

Astigmatism can also be regular or irregular:

Regular astigmatism

If astigmatism is regular, the two curves are at a 90-degree angle to each other, and the curvature of each is uniform.

Irregular astigmatism

In irregular astigmatism, the two curves are not at 90-degree angles to each other, and the curvature of each is not uniform or even. Irregular astigmatism can often result from trauma, surgery, or keratoconus, which is the thinning of the cornea.

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 onto the back of the eye.

In a person with astigmatism, the cornea is often egg- or football-shaped and curves differently from top to bottom than it does from side to side, rather than being perfectly round. As a result, the light will focus on two points on the retina instead of one, causing blurry vision and, sometimes, double vision.

It is unclear why some people are born with a cornea that does not curve properly, but there may be a genetic component. Infants born preterm have a higher risk of astigmatism than those born closer to their due date.

Certain types of surgery or eye injuries that cause corneal scarring may also cause irregular astigmatism. Keratoconus is another potential cause of irregular astigmatism. In this degenerative disorder of the eye, the cornea gradually thins and changes to a more conical shape.

Lenticular astigmatism is typically the result of an irregularly shaped lens in the eye.

The early detection and treatment of astigmatism are important.

An eye specialist may use the following techniques when examining the eyes:

  • Visual acuity test: A visual acuity test checks how well a person can read letters or characters at a distance. It typically involves reading lines of letters on a chart. The letters become progressively smaller on each line.
  • Keratometry: A keratometer is a device that measures the reflected light from the surface of the cornea. This provides an eye doctor with information about the shape and curvature of the cornea.
  • Corneal topography: Corneal topography is a type of imaging technology that provides measurements of the cornea alongside images. This offers a much more detailed assessment than keratometry.
  • Refraction: Refraction assesses how the eyes focus light, and it involves placing a series of lenses in front of the eyes. An eye doctor will ask which lenses make a person’s vision better.

For most children, the American Optometric Association (AOA) recommends eye exams:

  • at 6–12 months
  • at 3–5 years
  • before first grade
  • every year after first grade

Children with a high risk of astigmatism may benefit from more regular checkups.

Adults should have an eye exam every 2 years until the age of 65 years, when they should have an eye exam every year.

Adults with an increased risk of eye problems will need to see the eye doctor annually, regardless of age. These individuals include people with a history of eye conditions and those with chronic conditions that can affect the eyes, such as diabetes.

If the astigmatism is mild, the doctor may suggest no treatment at all. Otherwise, corrective lenses are the usual approach, although some people may prefer to opt for laser surgery.

Corrective lenses for astigmatism

Corrective lenses can help correctly project images onto the retina. These may be in the form of glasses or contact lenses.

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 astigmatism

If a person has presbyopia, their lenses will require additional, or add, power to treat this.

Orthokeratology, or corneal refractive therapy

Orthokeratology involves wearing a specially fitted, rigid contact lens during sleep to reshape the cornea. This does not permanently improve vision, but the person may find that they can see better for at least several hours after wearing it.

Some people with astigmatism may wish to undergo laser eye surgery to correct their vision. The most common procedure is laser in situ keratomileusis (LASIK).

LASIK

During this procedure, the surgeon uses a keratome device or femtosecond laser to create a thin flap in the cornea. They then lift the flap and use a laser to sculpt the shape of the cornea under the flap. Afterward, the surgeon folds the flap back into place where it will heal.

LASIK initially causes dry eyes and changing vision, but these side effects, along with any others, usually disappear within a month.

Other laser options include photorefractive keratectomy (PRK) and laser epithelial keratomileusis (LASEK).

PRK

In PRK, a surgeon removes some of the outer protective layer of the cornea. This can cause moderate-to-severe pain. A laser then changes the shape of the cornea by removing tissue. The surgeon will place a bandage contact lens on the eye during healing to manage pain.

LASEK

In LASEK, the surgeon removes a thin layer of tissue from the cornea. They then use a laser to change the shape of the cornea before replacing the corneal tissue.

Who should avoid laser surgery?

Laser eye surgery may not be suitable for a person who:

Risks

The risks of surgery can include:

  • dry eyes
  • light sensitivity
  • glare or halos around lights
  • double or blurry vision
  • over- or undercorrection, which may result in a need for corrective lenses after surgery
  • regression, in which vision impairments reoccur after surgery
  • vision loss

Overall, the risk of complications is low. A 2020 study of 31,921 LASIK procedures found that the incidence of adverse effects was only 1.3%. The incidence of serious adverse effects was even lower at 0.4%.

Astigmatism happens when the cornea has an abnormal shape, leading to blurred vision in the affected eye or eyes.

People are often born with astigmatism. However, it can sometimes occur later in life due to, for example, an eye injury or previous eye surgery.

Eyeglasses or contact lenses are often effective treatments for astigmatism. Surgery is also an option for eligible individuals.

Many people with astigmatism may not be aware that they have it until they have an eye exam. For this reason, it is important to undergo regular eye exams to identify and treat any eye problems.