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Merck Manual

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Melvin I. Roat

, MD, FACS, Sidney Kimmel Medical College at Thomas Jefferson University

Last full review/revision May 2020| Content last modified May 2020
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Keratoconus is an eye disorder that involves a gradual change in the shape of the cornea (the clear layer in front of the iris and pupil), causing it to become irregular and cone-shaped, which worsens vision.

An Inside Look at the Eye

An Inside Look at the Eye

The condition usually begins between the ages of 10 and 25. Both eyes are always affected, causing major changes in vision and requiring frequent changes in prescription for eyeglasses or contact lenses in many people. The cause is unknown, but people are more likely to develop keratoconus if they have any of the following:


  • Contact lenses

  • Ultraviolet light treatments

  • Corneal ring segments

  • Corneal transplantation

Contact lenses often correct the vision problems better than eyeglasses. There are many designs of contact lenses (for example, rigid gas permeable, hybrid, scleral) that may be well tolerated and give good vision. But, sometimes the change in corneal shape is so severe that any contact lenses either cannot be worn or cannot correct vision.

Ultraviolet light treatments that stiffen the cornea (called collagen cross-linking) are useful in early keratoconus to prevent further progression.

The insertion of corneal ring segments (objects that change the shape of the cornea to help correct refraction) seems to improve vision by allowing people to better tolerate wearing contact lenses. Corneal ring segments prevent certain people from needing corneal transplantation.

In severe cases, corneal transplantation may be needed to restore vision.

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