Symptoms:

The earliest symptom is subtle blurring of vision that is not correctable with glasses. (Vision is generally correctable to 20/20 with gas-permeable contact lenses.)

Signs and tests:

Keratoconus can usually be diagnosed with slit-lamp examination of the cornea. Early cases may require corneal topography, a test that involves making a stereo image that gives a topographic map of the curvature of the cornea.

When keratoconus is advanced, the cornea may be thinner in areas. This can be measured with a painless test called pachymetry.