Astigmatism, derived from Greek meaning "without spot," refers to a common refractive condition that decreases acuity. Astigmatism has both a magnitude and a direction, or axis, unlike spherical refractive error, which lacks orientation. Because of the deleterious effect of astigmatism on vision, modern ophthalmologic practice has developed multiple methods to treat astigmatism at both the corneal and the lenticular level.
Pathophysiologically, astigmatism results from asymmetry, typically in the anterior cornea, but it can also be caused by the posterior cornea or lens. The asymmetry means that light is focused differently between the 2 meridians. As a result, the image does not focus at a single point.
Some level of detectable astigmatism is present in as many as 95% of eyes, although the prevalence of clinically significant astigmatism varies depending on the definition of significance. Almost half of the population has at least 0.5 D of astigmatism, 10% has at least 1 D, and 8% has 1.5 D or more. The prevalence of significant astigmatism has been reported to vary from 7.5% to 75%. Astigmatism also seems to increase with age.
Patients experience astigmatism as blurring of vision, often with streaks of light in dark conditions. Depending on the axis, astigmatism also can cause tilting of the image. In general, patients will find 1 D or more of astigmatism to be significant, although some may find lesser amounts to be bothersome. One study suggests that as many as 20% of eyes with cataract have at least 1.5 D of corneal astigmatism.
In addition to its magnitude and direction, astigmatism may also be classified based on its symmetry. Regular astigmatism is symmetric around the visual axis and classically shows a "bow tie" on corneal topography. The power of each meridian is successive as the meridians travel 180 degrees. Astigmatism that does not show symmetry or otherwise conform to a geometric appearance is irregular.
Int Ophthalmol Clin. 2013;53(1):65-78. © 2013 Lippincott Williams & Wilkins