What is it?
Keratoconus is a progressive disorder of the cornea, in which the cornea slowly changes from its normal domed shape, into a cone. This changes the focussing power of the cornea; but because of the abnormal cone shape it is very hard to correct this change with glasses or soft contact lenses
Keratoconus affects both eyes, but usually one eye is less affected than the other. The condition progresses for 15-20 years, and then stabilises.
What causes it?
In most cases the precise cause of keratoconus is not known. Rarely, severe, habitual eye-rubbing may be a factor in some patients.
The layers of collagen fibres which make up the cornea are thought to slide over each other, allowing the cornea to bulge forward.
How is it treated?
Treatment is in two parts: firstly, improving the vision, and secondly treating the condition.
1. Treating the vision
The abnormal corneal shape in keratoconus is impossible to correct fully with glasses, and difficult to correct with soft contact lenses because these simply take up the shape of the cornea itself.
Hard contact lenses (‘rigid gas permeable contact lenses’) do not change shape, and sit on top of the ‘cone’. The natural layer of tears on the surface of the eye fills in the gap between the contact lens and the cornea, effectively creating a new focussing surface and so improving the vision.
The down-side of hard lenses is that it takes time for the eyes to get used to them, and therefore some people who have advanced keratoconus in one eye only decide not to use them, and to manage with one eye.
Wearing contact lenses does not prevent keratoconus progressing.
2. Treating the condition
Until recently there was no treatment for the condition itself. But it is now possible to use ultraviolet light, combined with special eyedrops, to stimulate bonding between the layers of the cornea, locking them in place and preventing progression. This treatment is called Corneal Collagen Cross-Linking (CXL) and was approved by NICE in 2013.
CXL stiffens the cornea and prevents progression of keratoconus. This stabilises the vision and makes correction with contact lenses easier. In some cases vision may even improve after treatment.
What other treatments are available?
If keratoconus is too advanced for CXL, or if the cornea has become very thin and unstable, it may be necessary to replace the cornea with transplanted tissue from a donor eye, with a PK or DALK procedure (please see separate articles for details of these).