Astigmatism occurs when the cornea is shaped more like an oblong than its normal spherical shape. In most astigmatic eyes, the oblong or oval shape causes light rays to focus on two points in the back of your eye, rather than on just one. This is because an astigmatic cornea has a steeper curve and a flatter one.
In regular astigmatism, the meridians in which these two different curves lie are located 90 degrees apart. In irregular astigmatism, the two meridians may be located at something other than 90 degrees apart; or there are more than two meridians.
Regular astigmatism is usually easy to correct (by wearing prescription glasses or contacts) but irregular astigmatism can be complicated and more difficult to correct, depending on the extent of the irregularity and its cause. Usually astigmatism is a hereditary condition. Many people are born with an oblong cornea and the resulting vision problems may get worse over time.
Astigmatism may also result from eye injury that has caused scarring on the cornea, from certain types of eye surgery or from keratoconus, a disease that causes gradual thinning of the cornea.