Contact lenses for keratoconus
Keratoconus is an eye condition in which the cornea (the transparent front part of the eye) becomes cone-shaped rather than round. The estimated prevalence for keratoconus in the general population is 54 per 100,0001. It usually occurs in both eyes, although one eye is normally affected before the other, and the cause is unknown.
Contact lenses play an important part in treating keratoconus. The first line of treatment is usually to correct the irregular cornea with rigid gas permeable (RGP) contact lenses, although some people with early keratoconus may be able to wear glasses or soft contact lenses. Contact lenses do not slow down the rate of progression of the cone, but they do give good vision, which could not otherwise be achieved.
RGP lenses work by trapping a 'liquid lens' between the cornea and the lens that effectively neutralises the irregular front surface of the eye. There may also be a contact zone, which compresses slightly on the cornea, so that it takes up the regular shape of the back surface of the lens.
Some hydrogel lenses have been designed especially for keratoconus in recent years. These are thicker than regular soft lenses so they retain a rigid shape. Another alternative is combination rigid/soft lenses: either a 'piggyback' fitting with an RGP lens fitted on top of a soft lens; or a 'hybrid' design which has an RGP centre with a soft surround.
The other option is scleral lenses, which as the name suggests, fit onto the sclera or white of the eye. Scleral lenses were the first contact lenses ever developed in the 19th century and were the only lenses available until the 1950s. They are larger than corneal lenses, up to 23-25mm in diameter, and are made of gas permeable materials. These lenses are sometimes used for other medical indications, such as severe dry eye.
More recent developments include the use of semi-scleral RGP lenses, which are between 13-16mm in diameter. They still require specialist fitting and are more readily available than full scleral contact lenses.
Our photograph shows a comparison of a scleral lens for keratoconus (left) and a regular RGP lens (courtesy of Ken Pullum and the Keratoconus Group).
If you have keratoconus, your eyecare practitioner will advise you on the best form of correction for your eyes or refer you for specialist advice.
1. Romero-Jimenez, M., Santodomingo-Rubido, J. and Wolffsohn, J. S. (2010) Keratoconus: a review. Contact Lens and Anterior Eye 33 (2010): 157-166.