In Keratoconus, the Cornea thins and bulges forward. This can cause irregular astigmatism, which decreases your vision. Most of the time progression starts at age of 16 to 18 years and usually halts at the age of 40. There is several subtypes that can progress after the age of 40. One of the reasons is a genetic predisposition, but mechanical triggers (e.g. eye rubbing, trauma or laser eye surgery) can destabilise the condition. Main risk factors seem to be eye rubbing in patients with itchy eyes in allergies.
If Progression is observed, stabilising the corneal shape with Crosslinking (see Crosslinking) is mandatory. Improvement of vision may need additional measures.

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More information (german only) can be found here