Andrew A. Dahl, MD, is a board-certified ophthalmologist. Dr. Dahl's educational background includes a BA with Honors and Distinction from Wesleyan University, Middletown, CT, and an MD from Cornell University, where he was selected for Alpha Omega Alpha, the national medical honor society. He had an internal medical internship at the New York Hospital/Cornell Medical Center.
Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.
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 focussed 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.
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.
In people with astigmatism, either the corneal or lens shape is distorted, causing multiple images on the retina. This causes objects at all distances to appear blurry. Many people have a combination of either myopia or hyperopia with astigmatism.