Keratoconus is a condition where the centre of the cornea thins.

The pressure of the aqueous humour behind the cornea causes it to become conical in shape, and to bow forward much more than normal. The result is irregular astigmatism, which initially can be corrected with spectacles.

As the condition progresses, spectacles are inadequate and rigid contact lenses are required. These restore stable visual acuity by replacing the irregular corneal surface with a smooth refractive surface.

As the thinning of the cornea worsens, special management is needed and special keratoconic lenses might be used. Some practitioners also use soft toric lenses in the early stages of the condition, though these need to be made thicker than they would normally.