The earliest symptom is subtle blurring of vision that cannot be corrected with glasses. (Vision can generally be corrected to 20/20 with rigid, gas-permeable contact lenses.)
Most people who develop keratoconus start out nearsighted. The nearsightedness tends to become worse over time.
Keratoconus is often discovered during adolescence. It can usually be diagnosed with slit-lamp examination of the cornea. The most accurate test is called corneal topography, which creates a map of the curve of the cornea.
When keratoconus is advanced, the cornea will be thinner at the point of the cone. This can be measured with a painless test called pachymetry.
Jain A, Paulus YM, Cockerham GC, Kenyon KR. Keratoconus and other noninflammatory corneal thinning disorders. In: Tasman W, Jaeger EA, eds. Duane's Ophthalmology. 19th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 2009:chap 16C.
Sugar J, Wadia HP. Keratoconus and other ectasias. In: Yanoff M, Duker JS, eds. Ophthalmology. 3rd ed. St. Louis, Mo: Mosby Elsevier; 2008:chap 4.18.
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