Keratoconus is a condition that causes the cornea to weaken, get thinner and change shape. It affects about 1 in 2,000 of the population. The exact cause is unknown. It's possibly passed down through the family, or due to an allergy. Keratoconus is one of the most common reasons for corneal transplantation in younger patients.
It doesn’t usually appear until the early teens, but can occur earlier in a few patients. Many cases of keratoconus are mild and can be managed by using contact lenses or glasses. But in some patients it can progress to the point where a corneal transplant is necessary.
Definition from NHS Choices: keratoconus