Astigmatism

Astigmatism is a problem of how the eye focuses light typically caused by a defect in the lens, resulting in distorted images.

The word astigmatism comes from the Greek "a" meaning "without" and "stigma" meaning "spot."

To see clearly, the eye must be able to focus light into a single plane at the surface of the retina. 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 plane of 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 from the plus sign.

• In an eye without astigmatism, the surface of the cornea is shaped like a sphere the way a ping-pong ball is, 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 the bottom of a spoon, where there are two different surface curves located 90 degrees apart. This is called an astigmatic or toric surface.

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, although 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 surface of the cornea 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 lenses or surgery.
• Irregular astigmatism is defined as the focus resulting from any corneal surface that is neither spherical nor regularly astigmatic. It 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 (the 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.

Most astigmatism does not have a recognized cause but merely is 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. Astigmatism can increase in amount during the growing years.

• There is some hereditary basis to most cases of astigmatism, and most people with astigmatism have it in both eyes in a symmetrical fashion.
• Astigmatism is often associated with myopia (nearsightedness) or hyperopia (farsightedness).
• 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, 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.

Additionally, people who use power tools without safety glasses are subject to the type of injuries that may cause acquired 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 the following:

• Eyestrain
• Eye fatigue
• Squinting
• Headaches

Astigmatism is detected by an ophthalmologist 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 easily and painlessly made during 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.

The goal of treating astigmatism is to improve vision and eye comfort. Treatment options are corrective lenses or refractive surgery.

Corrective lenses

Wearing corrective lenses is the simplest way to treat astigmatism by counteracting uneven curvatures of the cornea or lens. Corrective lenses may be worn as eyeglasses or contact lenses.

Refractive surgery

Refractive surgery improves vision and replaces the need to wear eyeglasses or contact lenses.

Types of refractive surgery for astigmatism include:

• Laser-assisted in situ keratomileusis (LASIK): The eye surgeon creates a thin flap in the cornea. An excimer laser is then used to change the shape of the cornea. The flap is then repositioned.
• Laser-assisted subepithelial keratectomy (LASEK): Instead of creating a flap, the cornea is loosened using alcohol. A laser is then used to alter the curvature of the cornea and then the loosened epithelium is repositioned.
• Photorefractive keratectomy (PRK): The epithelium is removed and allowed to grow back naturally, according to the cornea's new shape.
• Epi-LASIK: This is similar to LASEK, but this uses a mechanized blunt blade instead of alcohol to loosen the cornea.
• Small incision lenticule extraction (SMILE): This is a newer type of refractive surgery that reshapes the cornea by using a laser.

About 30% of all people have astigmatism. In the vast majority of those cases, 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 later occur.

Progressive astigmatism can occur with the following:

• Corneal trauma
• Repeated infections of the cornea
• Degenerative diseases such as keratoconus

The surgeries for astigmatism are relatively safe; however, some of the possible complications that may occur after refractive surgery include:

• Under correction or overcorrection of the initial refractive error
• Appearance of halos or starbursts around lights
• Dry eyes
• Eye infection
• Scarring
• Vision loss

The common types of astigmatism cannot be prevented.

Astigmatism is usually present from birth, which could be genetic or may develop after any eye trauma, disease, or surgery. It is not caused or worsened by reading in the dark or poor lighting; watching TV or other screens for too long; sitting too close to the screen or squinting. Maintaining a healthy, nutritious diet, avoiding excessive screen time or straining the eyes, and protecting the eyes from pollution and injuries can maintain overall eye health. However, there are no specific preventive measures for astigmatism.

The incidence of astigmatism due to trauma to the cornea can be decreased by attention to eye safety.