Disease: Astigmatism

Astigmatism introduction

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.

What is the definition of astigmatism?

In order to see clearly, the eye must be able to focus light into a single plane at the retinal 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 at two different planes, causing blurred vision. An optical system (or eye) without astigmatism is called spherical and has only one focus for all rays of light. An optical system with astigmatism is one in which 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 are the different types of astigmatism?

There are various classification systems for astigmatism, based on the anatomical source of the astigmatism, the regularity/ irregularity of astigmatism, or the direction of astigmatism.

Most astigmatism in the human eye has its source within the cornea, although there are irregularities of the lens that can lead to astigmatism, known as lenticular astigmatism.

Most corneal astigmatism is regular, meaning that the cornea is most curved (steepest) 90 degrees away from the surface of the cornea that is the least curved (flattest) and that the transition from most curved to least curved surface occurs in a regular manner. Regular astigmatism can be corrected with glasses, soft lenses, rigid lenses, or refractive surgery.

Irregular astigmatism is defined as arising from any corneal surface that is neither spherical nor regularly astigmatic. Irregular astigmatism cannot be corrected with glasses or soft contact lenses.

A 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 to the cornea) lies in or close to the vertical meridian, similar to the surface of a football lying on it 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 on end.

What causes astigmatism?

Most astigmatism does not have a recognized cause, but merely is an anatomical imperfection in the shape of the cornea, in which 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.

Who is at risk for 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 the type of injuries that may cause acquired astigmatism.

What are symptoms of astigmatism?

In an eye with astigmatism, vision is blurred due to the inability of the optical elements of the eye to focus objects into a sharp image on the retina. In astigmatism, a point of light is focused and perceived as a line, rather than as a point. 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. Sometimes uncorrected astigmatism can lead to eye strain, eye fatigue, squinting, or headaches in addition to blurring and distortion of vision at all distances.

What are signs of astigmatism?

Astigmatism is detected by your ophthalmologist by either checking you for the need for glasses (refraction) or actually measuring the curvature of the front of the cornea by using a keratometer or corneal topographer.

What tests are used to diagnose astigmatism?

The diagnosis of astigmatism is easily and painlessly made during the course of a complete eye examination. This would include a refraction (measuring the eyes for the need for glasses), an examination with a slit lamp, and measurements of the curvature of the corneas using various forms of light and computer algorithms.

What is the treatment for astigmatism?

Many patients with mild astigmatism have no symptoms and don't require 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. Wavefront guided LASIK or photorefractive keratectomy (PRK) can also reduce irregular astigmatism. Various considerations involving ocular health, refractive status, and lifestyle frequently determine whether one option may be better than another.

What causes astigmatism?

Most astigmatism does not have a recognized cause, but merely is an anatomical imperfection in the shape of the cornea, in which 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.

Who is at risk for 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 the type of injuries that may cause acquired astigmatism.

What are symptoms of astigmatism?

In an eye with astigmatism, vision is blurred due to the inability of the optical elements of the eye to focus objects into a sharp image on the retina. In astigmatism, a point of light is focused and perceived as a line, rather than as a point. 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. Sometimes uncorrected astigmatism can lead to eye strain, eye fatigue, squinting, or headaches in addition to blurring and distortion of vision at all distances.

What are signs of astigmatism?

Astigmatism is detected by your ophthalmologist by either checking you for the need for glasses (refraction) or actually measuring the curvature of the front of the cornea by using a keratometer or corneal topographer.

What tests are used to diagnose astigmatism?

The diagnosis of astigmatism is easily and painlessly made during the course of a complete eye examination. This would include a refraction (measuring the eyes for the need for glasses), an examination with a slit lamp, and measurements of the curvature of the corneas using various forms of light and computer algorithms.

What is the treatment for astigmatism?

Many patients with mild astigmatism have no symptoms and don't require 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. Wavefront guided LASIK or photorefractive keratectomy (PRK) can also reduce irregular astigmatism. Various considerations involving ocular health, refractive status, and lifestyle frequently determine whether one option may be better than another.

Source: http://www.rxlist.com

Many patients with mild astigmatism have no symptoms and don't require 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. Wavefront guided LASIK or photorefractive keratectomy (PRK) can also reduce irregular astigmatism. Various considerations involving ocular health, refractive status, and lifestyle frequently determine whether one option may be better than another.

Source: http://www.rxlist.com

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