From MedlinePlus Genetics
Keratoconus is an eye condition that affects the shape of the cornea, which is the clear outer covering of the eye. In this condition, the cornea thins and bulges outward, eventually resembling a cone shape. These corneal abnormalities, which worsen over time, can lead to nearsightedness (myopia), blurred vision that cannot be improved with corrective lenses (irregular astigmatism), and vision loss.

Other corneal changes typical of keratoconus that can be seen during an eye exam include iron deposits in the cornea that form a yellow-to-brownish ring, called the Fleischer ring, surrounding the colored part of the eye (iris). Affected individuals may also develop Vogt's striae, which are thin, vertical, white lines in the tissue at the back of the cornea.

Keratoconus may affect only one eye at first, but eventually the corneas of both eyes become misshapen, although they might not be affected with the same severity. As keratoconus worsens, people with this condition can develop corneal scarring, often caused by exposure of the abnormally thin cornea to prolonged contact lens use or excessive eye rubbing.

The eye changes characteristic of keratoconus typically begin in adolescence and slowly worsen until mid-adulthood at which point the shape of the cornea remains stable.  https://medlineplus.gov/genetics/condition/keratoconus