Astigmatism

Irregular cornea or lens distorting focus at all distances

Overview

Astigmatism is a refractive error in which the cornea or lens is not evenly curved, so light entering the eye is focused differently along different meridians. A normal cornea is shaped roughly like part of a sphere, whereas an astigmatic cornea is toric, more like the side of a rugby ball. The result is blur or distortion at all distances, sometimes with ghosting, eye strain, and halos or streaks around lights at night. Some degree of astigmatism is very common, and it frequently occurs together with myopia or hyperopia. Irregular astigmatism, as in keratoconus or after corneal scarring, cannot be fully corrected by ordinary lenses. Regular astigmatism is corrected with cylindrical lens powers in spectacles or toric contact lenses, and can also be addressed by laser refractive surgery or toric intraocular lenses. The English astronomer George Airy described his own astigmatism and had cylindrical lenses made to correct it in the 1820s.