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Astigmatism is a refractive error of the eye in which there is a difference in degree of refraction in different meridians. It is typically characterized by an aspherical, non-figure of revolution cornea in which the corneal profile slope and refractive power in one meridian is greater than that of the perpendicular axis. Astigmatism causes difficulties in seeing fine detail, and can often be corrected by glasses with a cylindrical lens (i.e. a lens that has different radii of curvature in different planes), contact lenses, or refractive surgery.

Astigmatism occurs when either the cornea or the lens of the eye is not a figure of revolution or, in other words, is not perfectly smooth or round. As a result, the eye has different focal points in different planes. For example, the image may be clearly focused on the retina in the horizontal (sagittal) plane, but not in front of the retina in the vertical (tangential) plane.

In some cases vertical lines (e.g., walls) may appear to the patient to be leaning over like the tower of Pisa.

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