Astigmatism means that light cannot be focussed on the retina using a simple spherical lens. It may be regular (correctable with a cylindrical lens) or irregular (only correctable with a gas permeable contact lens).
Astigmatism is caused by variations in the curvature of the front and back surfaces of the cornea and cyrstalline lens. In young adults, astigmatism is often caused by the front surface of the eye being steeper in the vertical meridian than in the horizontal meridan (with the rule astigmatism). With age, the shape of the cornea may gradually change and the horizontal meridian becomes steeper (against the rule astigmatism). In either case, the surface of the cornea resembles the surface of a rugby ball.
Usually, patients require glasses with a cylindrical lens in to treat the astigmatism. However, it may be reduced surgically by careful placement of the incisions at the time of cataract surgery, toric intraocular lenses implanted at the time of cataract surgery or excimer laser.