Astigmatism is usually caused by an irregularly shaped cornea; instead of the cornea having a symmetrically round shape (like a basketball), it is shaped more like a football, with one meridian being significantly more curved than the meridian perpendicular to it.
In an eye with astigmatism, light fails to come to a single focus on the retina to produce clear vision. Instead, multiple focus points occur, either in front of the retina or behind it (or both).
Like nearsightedness and farsightedness, astigmatism is a refractive error, meaning it is not an eye disease or eye health problem; it's simply a problem with how the eye focuses light.
There are three main types of astigmatism:
1. Myopic astigmatism -- one or both of the principal meridians of the eye are nearsighted
2. Hyperopic astigmatism -- one or both of the principal meridians of the eye are farsighted.
3. Mixed astigmatism -- one principal meridian is farsighted, and the other is nearsighted.
A Toric Intraocular Lens (IOL) is an artificial lens used to correct corneal astigmatism and restore distance vision after removal of the natural lens of the eye during cataract surgery. It is a monofocal lens with astigmatism correction built into it.