What Is Astigmatism?

Astigmatism is the most common vision problem. It is caused by an error in the shape of the cornea or lens of the eye. Normally, the cornea and lens are smooth and curved equally in all directions, helping to focus light rays sharply onto the retina at the back of the eye. However, if the cornea or lens isn't smooth and evenly curved, it can change the way light passes or is refracted onto the retina, resulting in a so-called refractive error

Corneal Astigmatism

The cornea is a transparent layer of tissue that covers the front of the eye. A perfectly curved cornea bends, or refracts, light as it enters the eye so that the retina receives a perfect image. In a person with corneal astigmatism, the cornea is oval-shaped rather than perfectly round, with the result that the light rays will focus on two points on the retina instead of one.

Lenticular Astigmatism

Lenticular astigmatism, which is less common than corneal astigmatism, occurs when the lens has variations that cause images to reach the retina imperfectly. They may focus either in front of or beyond the retina, causing blurred vision of both far and near objects. Most people with lenticular astigmatism have a normal-shaped cornea. What Causes Astigmatism?

It is not known what causes astigmatism, but it is thought to be an inherited condition. It is often present at birth, but may develop later in life, sometimes after eye disease, injury or surgery.

Who is at Risk for Astigmatism?

The risk of developing astigmatism may be higher if there is a family history of astigmatism, scarring of the cornea, excessive shortsightedness or farsightedness, or certain types of eye surgery.

What are the Symptoms of Astigmatism?

The symptoms of astigmatism may differ from person to person, and some people have no symptoms at all. Generally, the symptoms include blurry or distorted vision for both close and far distances, difficulty with night vision, eyestrain or discomfort, headaches, and squinting to try to see clearly. Having any of these symptoms may not necessarily mean that you have astigmatism, but they do indicate the need for a visit to your optometrist.

While adults with a higher degree of astigmatism may realise that their vision isn't as good as it should be, children who have astigmatism may not be aware of it, and are unlikely to complain about blurred or distorted vision. Uncorrected astigmatism can impact a child's ability to achieve in school and sports, so regular eye examinations are advisable.

How is Astigmatism Diagnosed?

Various tests and instruments are used by the optometrist to conduct a comprehensive eye examination. During a visual acuity assessment, you will be asked to read letters from a chart to determine the clarity of your vision at certain distances.

The curvature of the cornea is measured with a keratometer, and corneal topography may be done to provide additional information about the shape of the cornea. Using a phoropter, which has corrective lenses of different strengths, the lens that is appropriate for the correction of your vision is found.

What Do the Measurements Mean?

Astigmatism is measured in diopters. A perfect eye with no astigmatism has 0 diopters. Most people have between 0.5 to 0.75 diopters of astigmatism, but this does not usually affect their vision. People with a measurement over 0.75 dioptres typically need to have their astigmatism corrected in order to maintain clear vision. Of the three numbers on the prescription for glasses or contact lenses, the last two refer to astigmatism.

"Spherical" indicates whether you are shortsighted or farsighted. A plus sign indicates that you are farsighted, a minus sign that you are shortsighted. The higher the number, the higher the refractive error, and the stronger your prescription.

"Cylinder" measures what degree of astigmatism you have.

"Axis" is measured in degrees, and refers to where on the cornea the astigmatism is located. Axis numbers go from 0 to 180.

How is Astigmatism Treated?

Mild cases of astigmatism without shortsightedness or farsightedness may not need to be treated at all. Almost all other cases can usually be corrected with glasses or contact lenses, which counteract the uneven curvature of the cornea or lens by bending the light rays in a way that compensates for the error caused by faulty refraction.

Orthokeratology (Ortho-K) is a treatment that uses rigid contact lenses to temporarily correct the irregular curvature of the cornea. These lenses are worn for limited periods of time, usually during sleep, and removed during the day. While the improved vision is only temporary and the cornea will return to its irregular shape when the lenses are not being used, many people appreciate having clear vision without glasses or contact lenses during the day.

Refractive surgery to reshape the cornea may be recommended for some cases of astigmatism. Your optometrist will refer you to an ophthalmic surgeon who will explain the various procedures, as well as their benefits and risks.

A discussion with your optometrist will help you to choose the best option for your vision and lifestyle needs.