Blurred or distorted vision caused by an irregularly shaped cornea or lens
Astigmatism is a common eye condition most often caused by a misshaped or irregular cornea. Astigmatism is a refractive error which results in blurred or distorted vision. In most cases, Astigmatism is correctable with glasses or contact lenses.
What Is a Refractive Error?
A refractive error is an irregularity in the way light passes through the eye. Normally, light rays enter the eye and are focused at a single point on the retina, the layer of light-sensitive tissue at the back of the eye. The cornea (clear portion at the front of the eye) and the lens inside the eye bend or refract the light rays so that they focus properly. The retina receives the images formed by the focused light rays and transmits them to the brain through the optic nerve.
In an eye with a refractive error, the light rays do not bend properly to achieve a single focus point on the retina. Instead, light rays either focus in front of the retina, behind the retina, or do not focus at a single point. Refractive errors, which include nearsightedness, farsightedness, and astigmatism, result from a defect in the shape of the eye.
How Does Astigmatism Affect Vision?
For normal, undistorted vision, the cornea should be smooth and equally curved in all directions. When astigmatism is present, the cornea is warped and curved more in one direction than the other. In other words, the cornea is shaped more like a football than a basketball.
Normally light enters the eye and is focused precisely on the retina. With astigmatism, the warped cornea causes the light rays to bend improperly. They are not refracted equally in all directions and one focus point on the retina is not attained. Some light rays are not focused on the retina but are focused in front of or behind the retina. The result of multiple focal points is distorted vision. Objects appear somewhat indistinct and slanted. Vision with astigmatism can be compared to what one sees when looking at a mirror in a fun house or when looking through a glass bottle, with images appearing too tall, too thin, too wide or blurred.
What Causes Astigmatism?
Astigmatism is most often caused by a defect in the curvature or the cornea, but may also occur from an unequal bending of light by the lens inside the eye. Astigmatism is usually inherited and may be present at birth. Astigmatism can occur in conjunction with nearsightedness or farsightedness and usually remains unchanged throughout life.
Astigmatism can also result from corneal scars or surgery. An eye injury may cause the cornea to scar, changing the path of light as it enters the eye. Some patients experience surgically induced astigmatism after eye surgery. Tightly tied sutures can cause a slight wrinkling of the cornea. The patient will experience distorted vision or astigmatism until healing or loosening of the sutures relieves the problem.
What Are the Symptoms of Astigmatism?
A person with astigmatism normally experiences difficulty focusing on both near and distant objects. Patients may also experience headaches and eye fatigue.
How Is Astigmatism Diagnosed?
Astigmatism is diagnosed with a routine eye exam. A visual acuity test is performed to determine the focusing power of the eye at different distances. A process called refraction is used to measure the refractive error of the eye and determine the prescription for corrective lenses.
How Is Astigmatism Treated?
A small amount of astigmatism is common and does not need correction. However, in cases where the problem is more severe, glasses and contact lenses are used to correct vision. To correct astigmatism, glasses or contact lenses are ground to neutralize the defective curvature of the cornea. Hard or gas permeable contact lenses generally improve astigmatism better than soft contact lenses. In some cases, however, soft contact lenses may be helpful.
Astigmatism can also be treated with refractive surgery and laser vision correction. Radial keratotomy, corneal transplant and laser surgery are procedures used to correct the path of light as it enters the eye. The cornea is reshaped to allow light to focus properly on the retina. With refractive surgery and laser vision correction, the eye regains its proper focusing ability and astigmatism is corrected.