Keratoconus (KC) is a disorder of the eye that results in progressive thinning of the cornea. This may result in blurry vision, double vision, nearsightedness, irregular astigmatism, and light sensitivity leading to poor quality-of-life. Usually both eyes are affected. In more severe cases a scarring or a circle may be seen within the cornea. While the cause is unknown, it is believed to occur due to a combination of genetic, environmental, and hormonal factors. Patients with a parent, sibling, or child who has keratoconus have 15 to 67 times higher risk in developing corneal ectasia compared to patients with no affected relatives. Proposed environmental factors include rubbing the eyes and allergies. The underlying mechanism involves changes of the cornea to a cone shape. Diagnosis is most often by topography. Topography measures the curvature of the cornea and creates a colored "map" of the cornea. Keratoconus causes very distinctive changes in the appearance of these maps, which allows doctors to make the diagnosis. Initially the condition can typically be corrected with glasses or soft contact lenses. As the disease progresses, special contact lenses (such as scleral contact lenses) may be required. In most people the disease stabilizes after a few years without severe vision problems. In 2016, the FDA approved corneal collagen cross-linking to halt the progression of keratoconus. In some cases, when the cornea becomes dangerously thin or when sufficient vision can no longer be achieved by contact lenses due to steepening of the cornea, scarring or lens intolerance, corneal cross-linking is not an option and a corneal transplant may be required. Keratoconus affects about 1 in 2,000 people. However, some estimates suggest that the incidence may be as high as 1 in 400 individuals. It occurs most commonly in late childhood to early adulthood. While it occurs in all populations it may be more frequent in certain ethnic groups such as those of Asian descent. The word is from the Greek kéras meaning cornea and the Latin cōnus meaning cone. Signs and symptoms People with early keratoconus often notice a minor blurring or distortion of their vision, as well as an increased sensitivity to light, and visit their clinician seeking corrective lenses for reading or driving. At early stages, the symptoms of keratoconus may be no different from those of any other refractive defect of the eye. As the disease progresses, vision deteriorates, sometimes rapidly due to irregular astigmatism.