Astigmatism is an eye disorder in which the cornea (the clear tissue covering the front of the eye) is abnormally curved, causing out-of-focus vision. It is commonly treated with glasses, contact lenses, or refractive surgery.
To see clearly, the eye must focus light on a single plane at the retina’s surface. The word astigmatism comes from the Greek “a” meaning “without” and “stigma,” meaning “spot.” In astigmatism, a point (or spot) of light is focused on two different planes, causing blurred vision. An optical system (or eye) without astigmatism is called “spherical.” It has only one plane of focus for all rays of light. An optical system with astigmatism is when rays that propagate in two perpendicular planes have different foci. For example, suppose an optical system with astigmatism is used to form a plus sign image. In that case, the vertical and horizontal lines will never focus simultaneously since they are in sharp focus at two distinctly different distances from the plus sign.
In an eye without astigmatism, the cornea’s surface is shaped like a sphere where a ping-pong ball is, where all the curves are the same. This is called a spherical surface. In an eye with astigmatism, the cornea’s surface is shaped more like a spoon’s bottom. There are two different surface curves located 90 degrees apart. This is called an astigmatic or toric surface.
Different types of Astigmatism
There are various classification systems for astigmatism based on the anatomical source of astigmatism, the regularity/ irregularity of astigmatism, or the direction of astigmatism.
Most astigmatism in the human eye has its source within the cornea. However, there are irregularities of the lens that can lead to astigmatism, known as lenticular astigmatism.
Most corneal astigmatism is regular, signifying that the cornea is most curved (steepest) 90 degrees away from the cornea’scornea’s surface that is the least curved (flattest) and that the transition from most curved to the least curved surface occurs regularly. Regular astigmatism can be corrected with glasses, toric soft lenses, rigid lenses, or refractive surgery.
Irregular astigmatism is defined as the focus resulting from any corneal surface that is neither spherical nor regularly astigmatic. Irregular astigmatism cannot be corrected with glasses or soft contact lenses.
Historical classification of astigmatism differentiates ” with the rule” astigmatism from “against the rule” astigmatism. In “with the rule” astigmatism, the steepest curvature (most curved part of the corneal surface) lies in or close to the vertical meridian, similar to the surface of a spoon lying on its side. In “against the rule” astigmatism, the steepest (most curved) part of the cornea is in or close to the horizontal meridian, similar to the surface of a football standing upright.
In people with astigmatism, either the corneal or lens shape is distorted, causing multiple retina images. This causes objects at all distances to appear blurry. Many people have a combination of either myopia or hyperopia with astigmatism.
Causes of Astigmatism
Most astigmatism does not have a recognized cause but merely an anatomical imperfection in the cornea’s shape. 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 astigmatism is prevalent and frequently is present at birth or has its onset during childhood or young adulthood. There is some genetic basis for most astigmatism cases. Most people with astigmatism have it in both eyes in a symmetrical fashion. 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 found at something other than 90 degrees apart or more than two meridians.
A scar in the cornea, resulting from an injury or infection, may also cause astigmatism. Astigmatism can be caused by ocular surgery, including cataract surgery and corneal transplantation. Certain diseases of the eye, such as keratoconus or pellucid degeneration, will cause irregular astigmatism.
Individuals with a family history of high degrees of astigmatism or keratoconus are at risk for astigmatism. People who use power tools without safety glasses are subject to injuries that may cause acquired astigmatism.
Symptoms of 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 point image on the retina. Astigmatism makes it difficult to see fine details, both close-up and at a distance. Small amounts of astigmatism may not be noticed at all. Sometimes uncorrected astigmatism can lead to eyestrain, eye fatigue, squinting, or headaches in addition to blurring and
Signs of Astigmatism
An ophthalmologist detects astigmatism by either checking for the need for glasses (refraction) or measuring the curvature of the front of the cornea by using a keratometer or corneal topography machine. These tests are noninvasive and painless.
The diagnosis of astigmatism is quickly and painlessly made during the course of a complete eye examination. This would include refraction (measuring the eyes for the need for glasses), an examination with a slit lamp, and measurements of the corneas’ curvature using various forms of light and computer algorithms.
Treatment for Astigmatism
Many patients with mild astigmatism have no symptoms from this and require no treatment. If there is regular astigmatism and causes blurred vision, astigmatism can be compensated satisfactorily with eyeglasses or contact lenses. If myopia or hyperopia is also present, the glasses or contact lenses can also correct that condition. If the astigmatism is an irregular or high degree, drinks or a soft contact lens may not be entirely correct for astigmatism. A hard (rigid) contact lens may be necessary to allow the eye to see usually. Neither glasses nor contact lenses permanently correct the curvature abnormality.
Modern refractive surgery, which reshapes the eye see’s surface with a laser, can also reduce astigmatism. Wavefront-guided LASIK or photorefractive keratectomy (PRK) can also minimize irregular astigmatism. Various considerations involving ocular health, refractive status, and lifestyle frequently determine whether one option may be better.
Prognosis for Astigmatism
About 30% of all people have astigmatism. In the vast majority of those, the condition does not change much after the age of 25. As a child or young adult, the presence of astigmatism does not signify that an eye disease will later occur. Progressive astigmatism can occur with corneal trauma, repeated infections of the cornea, and degenerative diseases such as keratoconus.
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.