Keratoconus is a bilateral, noninflammatory corneal ectasia with an incidence of approximately one per 2,000 in the general population, although less obvious cases such as early forme fruste presentations are thought to be much more common. Keratoconus is characterized by a progressive increase in corneal curvature, with apical thinning and irregular corneal astigmatism. Eventually, an obvious cone-shaped protrusion of the corneal surface may develop.

Keratoconus often becomes apparent during the teenage years and classically progresses until the third and fourth decades of life, when many affected individuals experience an arrest of disease progression or at least a reduction in the rate of progression.