Astigmatism: Causes, types, symptoms, and treatments

Astigmatism is a common condition in which the surface of the eye, or cornea, is not curved in the normal way, leading to blurred vision.

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.

Fast facts on 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
Astigmatism refers to the incorrect curvature of the cornea and any vision loss experienced as a result.

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.

Diagnosis

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.

Adults should have an eye test every two years, and more often if they have chronic conditions, like diabetes.

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Causes

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.

Astigmatism testing
Astigmatism can cause blurry vision.

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.

It is unclear exactly why some people are born with a cornea that does not curve properly, 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.

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