What Is an Astigmatism?
- In order to see clearly, the eye must be able to focus light into a single plane onto the back of the eye at the retina.
- The word astigmatism comes from the Greek "a" meaning "without" and "stigma" meaning "spot." In astigmatism, a point (or spot) of light is focused at two different planes, causing blurred vision.
- An optical system (or eye) without astigmatism has only one focus for all rays of light. An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. For example, if an optical system with astigmatism is used to form an image of a plus sign, the vertical and horizontal lines will never be in focus at the same time, since they are in sharp focus at two distinctly different distances.
- In an eye without astigmatism, the surface of the cornea is shaped like a Ping-Pong ball, where all the curves are the same. This is called a spherical surface. In an eye with astigmatism, the surface of the cornea is shaped more like a football, where there are two different surface curves located 90 degrees apart. This is called a toric surface.
What Causes an Astigmatism?
- Most astigmatism does not have a recognized cause other than merely an anatomical imperfection in the shape of the cornea, where the front curvature of the cornea is toric, rather than spherical.
- A small amount of astigmatism is considered normal and does not represent a disease of the eye. This type of astigmatism is extremely common and frequently is present at birth or has its onset during childhood or young adulthood.
- There is some hereditary basis to most cases of astigmatism, and most people with astigmatism have it in both eyes.
- Astigmatism is often associated with myopia (nearsightedness) or hyperopia (farsightedness).
- Astigmatism can increase in amount during the growing years.
- In regular astigmatism, the meridians in which the two different curves lie are located 90 degrees apart. Most astigmatism is regular. In irregular astigmatism, the two meridians may be located at something other than 90 degrees apart or there are more than two meridians.
- A scar in the cornea, resulting from an injury or infection, or a disease called keratoconus may also cause astigmatism. This type of astigmatism is usually irregular.
What Are the Symptoms of an Astigmatism?
In an eye with astigmatism, vision is blurred due to the inability of the optical elements of the eye to focus a point object into a sharply focused image on the retina. Astigmatism makes it difficult to see fine details, both close-up or at a distance. Small amounts of astigmatism may not be noticed at all. Other astigmatism symptoms and signs are
- eye fatigue,
- squinting, or
- headaches in addition to blurring and distortion of vision at all distances.
What Are the Exams and Tests to Diagnose Astigmatism?
The diagnosis of astigmatism is easily made during the course of a complete eye examination.
Astigmatism is detected by your eye doctor (ophthalmologist) by either checking your need for glasses (refraction) or actually measuring the curvature of the front of the cornea by using a keratometer or corneal topographer.
What Is the Treatment for an Astigmatism?
Many patients with mild astigmatism have no symptoms from this and require no treatment. If there is regular astigmatism and it causes blurred vision, the astigmatism can be compensated for satisfactorily with eyeglasses or contact lenses. If myopia or hyperopia are also present, the glasses or contact lenses can also correct that condition. If the astigmatism is irregular or of a high degree, glasses or a soft contact lens may not fully correct the astigmatism and a hard contact lens may be necessary to allow the eye to see normally. Neither glasses nor contact lenses permanently correct the curvature abnormality. Modern refractive surgery, which reshapes the surface of the eye with a laser, can also be used to reduce or eliminate the astigmatism. Various considerations involving ocular health, refractive status, and lifestyle frequently determine whether one option may be better than another.
What Is the Prognosis for an Astigmatism?
A significant percentage of the population has astigmatism. For the vast majority of those with astigmatism, the condition does not change much after the age of 25. The presence of astigmatism as a child or young adult does not signify that an eye disease will occur.
Is It Possible to Prevent an Astigmatism?
The common types of astigmatism cannot be prevented. The incidence of astigmatism due to trauma to the cornea can be decreased by attention to eye safety.
Medically reviewed by William Baer, MD; Board Certified Ophthalmology
"Visual impairment in adults: Refractive disorders and presbyopia"