Astigmatism is a type of refractive error that affects how the eye focuses light. In order for clear vision to occur, rays of light must pass through the eye and land in a single point on the retina. The shape of the cornea, the clear front cover of the eye, directs light toward the retina. When the cornea is abnormally shaped (astigmatism), light does not come to a focal point at the back of the eye. As a result, vision is blurred.
Types of Astigmatism
There are three primary types of astigmatism:
- Myopic astigmatism occurs when either the vertical or horizontal meridian of the eye is nearsighted. A meridian can be described as a line that runs from 6 to 12 o’clock or from 9 to 3 o’clock.
- Hyperopic astigmatism occurs when either the vertical or horizontal meridian is farsighted.
It is possible for both the vertical and horizontal meridians to be nearsighted are farsighted. It is also possible for one to be nearsighted and the other to be farsighted. This is referred to as mixed astigmatism.
There are also regular and irregular types of astigmatism. Regular astigmatism, in which the meridians of the eye are at perpendicular 90-degree angles, is the most common type. Irregular astigmatism is diagnosed when the meridians of the eye are not perfectly vertical and horizontal (not perpendicular).
Symptoms of Astigmatism
The most common symptom of astigmatism is distorted or blurred vision at all distances. However, younger patients may not be able to accurately describe their vision. Instead, they may demonstrate symptoms such as rubbing their eyes, squinting, or complaining of eye or head pain, especially after visual tasks such as reading.
Causes of Astigmatism
Astigmatism is a common structural abnormality in which the cornea is improperly shaped. Normally, this front cover of the eye has a rounded curve like a baseball. In astigmatism, the cornea is more oval, like an egg or a football. There is no clear cause for regular astigmatism. Irregular astigmatism may be caused by an injury to the eye. The following may also cause this eye condition:
- Lid swellings such as chalazia
- Corneal scars
Astigmatism is diagnosed during a comprehensive eye exam.
- Visual acuity testing is performed to measure how clearly a person sees a letter chard located several feet away. Results of this test look something like 20/20 or 20/40. The first number represents the 20-foot standardized testing distance. The second number represents the smallest letter size a patient can see clearly. A person with 20/20 vision has normal distance acuity. A person with 20/40 vision would need to be within 20 feet of an object to see it clearly when they should be able to observe it from a 40-foot distance.
- A phoropter is an instrument that resembles glasses. The patient looks through a series of different lenses and reports which offers clearer vision. This test determines how to correct vision.
- A lighted instrument called an autorefractor may be used to observe the way in which light bounces off the back of the eye.
- The shape of the cornea may be observed using corneal topography or a keratometer.
Can astigmatism be reversed?
Astigmatism is the result of an irregularly shaped cornea. There is no proven way to reverse asymmetrical shape without surgery.
How common is astigmatism?
Astigmatism is one of the most common refractive errors to be diagnosed each year. It is estimated that 1 in 3 people has this condition. Many of the patients who develop this refractive error are children. While some young people do outgrow the abnormal curvature of the cornea, many require eyeglasses to correct blurred vision.
Treatment of Astigmatism
Treatment options for astigmatism include:
- Eyeglasses that contain a precise lens prescription which compensates for the curvature of the cornea. Eyeglasses may also contain prescription to correct presbyopia at the same time.
- Contact lenses are an alternative to eyeglasses when a wider field of view is desired. In order to correct astigmatism, the patient will likely need rigid gas-permeable contacts or toric soft contact lenses.
- Laser eye surgery such as LASIK or PRK may improve vision by manually correcting the shape of the cornea. The two procedures are similar in outcome. However, LASIK only manipulates tissue at the inner layer of the cornea. PRK removes both superficial tissue and tissue at the inner layer of the cornea.
Can astigmatism improve without treatment?
It is not likely that astigmatism will improve without treatment. In most cases, astigmatism gets worse as the eyes age. This can happen even if eyeglasses or contact lenses are worn to correct the refractive error.
Is astigmatism common in children?
Along with other refractive errors, astigmatism is relatively common in children. According to a study funded by the National Eye Institute, approximately 10% of preschool children have some degree of astigmatism. The same study revealed that approximately 23% of children aged 6 to 12 months have this condition.
In another study of just over 2,500 school-aged children (ages 5 to 17), researchers found astigmatism of 1.0 diopter or greater in 28% of participants. However, there is evidence that many children who develop astigmatism at a young age outgrow the condition.
Eye exams are important for children of all ages to identify vision problems that may keep them from learning and fully engaging in life.