Astigmatism is a common vision condition that occurs when either the front surface of the eye, the cornea is slightly irregular in shape (corneal astigmatism), or the crystalline lens located within the eye just behind the cornea, is irregular in shape (lenticular astigmatism). Various objects have been used to describe this distorted shape. If we think of the normal cornea as being spherical like the side of a basketball or soccer ball, then the cornea that has an astigmatic shape might be considered as being like the shape of the side on an American football, or a teaspoon. The curvature is different in each orientation or direction. This irregular shape of the cornea prevents light from focusing properly on the photoreceptive cells located in the retina in the back of the eye. As a result, vision may be blurred at all distances.
The name astigmatism is derived from "a", meaning without, and "stigma", meaning point, because in the condition of astigmatism, light is not focused to a point.
In the most common type of astigmatism, regular astigmatism, the curvature of the cornea or crystalline is greater in one direction than it is 90 degrees away from that direction. There is also a condition known as irregular astigmatism when the curves of greater and lesser curvature are not 90 degrees apart, but this is usually the result of corneal diseases such as Keratoconus or from corneal injury. Regular astigmatism is the natural occurring form and is one of the most common vision problems that people have. The result is that objects appear brighter or sharper focus in one direction than they are in another direction 90 degrees away. Vertical lines may appear sharper than horizontal lines, or vice versa. The lines that our out of focus might be in any direction. This direction corresponds to the direction in the cornea or crystalline lens that is either flatter or steeper than the curvature of cornea or crystalline lens 90 degrees away. The direction of blur is called the axis of astigmatism and the amount is called the cylinder.
Someone with uncorrected astigmatism might see a figure of radial lines of equal darkness in a distorted manner, such as the figure pictured to the left. If we consider the lines as pointing to numbers on a clock, if all the lines were the same darkness, a person with astigmatism with axis in the direction of 4:00 to 10:00 would see the lines less clear in the 4:00 to 10:00 direction than the lines in the 2:00 to 8:00 direction. (The illustration to the left is from the website The Joy of Visual Perception at www.yorku.ca/eye). When the lens prescription is written to correct the condition, the direction of astigmatism, the axis, is expressed in degrees of a semicircle, with the lines in the 3:00-9:00 o'clock direction expressed as the 180 degree axis, the lines in the 12:00-9:00 o'clock direction expressed as axis 90 degrees and the lines around the clock dial in between expressed as other degrees between 0 and 180. The distortion in the cornea or crystalline lens in astigmatic eyes causes light to focus either in front or in back of the retina. Corrective cylinder lenses focus all the light in the same place precisely on the retina, as it is in a normal eye.
Symptoms of astigmatism vary. Large amounts of astigmatism may cause blurriness at all distances, but smaller amounts of astigmatism may cause very little blur, and the main symptom may be eyestrain or headaches, especially when using the eyes for critical vision tasks such as reading or using a computer.
Astigmatism is corrected with glasses containing cylinder lenses or contact lenses. The generic name for contact lenses that have cylinder correction to correct astigmatism is toric contact lens. Refractive surgery corrects astigmatism by reshaping the corneal curvature.
