Keratoconus is classified as a noninflammatory ectatic disease of the cornea. Though no specific inheritance pattern is known, keratoconus has been linked to chromosome 21 and can run in families. Keratoconus is also frequently noted in patients with Down syndrome, atopic disease, and eye rubbing. With keratoconus, the central cornea thins and takes on a conical shape. This process tends to begin in patients in the first few decades of life. Symptoms tend to progress for several years and then slow down in the fourth to fifth decade. The change in shape of the cornea causes progressive myopia and irregular astigmatism. Patients may first complain of frequent changes in glasses or soft contact lenses. Eventually, glasses will no longer be able to fully correct vision and patients will require soft toric or gas permeable contact lenses that help correct for irregular astigmatism. As the disease progresses, the central corneal can develop scar tissue, further reducing best corrected vision, and some patients will require corneal transplantation. As the cornea thins, breaks or tears can develop in Descemet's membrane. These breaks allow aqueous fluid to rush into the corneal stroma, causing rapid and severe corneal edema and reduced vision. This condition is called "corneal hydrops." Though the presentation can be quite alarming, it is usually self-limited and will resolve as the break in Descemet's membrane heals.