Keratoconus
Keratoconus and Irregular Cornea
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The term “keratoconus” comes from the Greek (kerato -horn, cornea and konus – cone) and literally means “conical cornea”.Generally, the definition of keratoconus suggests it is a bulging of the cornea due to thinning which implies the cornea actually stretches. However, research over the last few years has shown that in keratoconus, the cornea does not increase in volume or surface area. This means that wherever the cornea is steeper and thinner, it is offset by areas that are flatter and thicker. This implies that keratoconus is more of an extreme type of warping than actual stretching. This can be understood better by looking at topography – which is a form of mapping of the cornea rather like an Ordnance Survey map – showing the highs and lows. Steep areas are represented by redder colours while flatter areas are shown by greens and blues. |
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The topography to the left shows a typical case of keratoconus with a steep inferior cone surrounded by flatter cornea. A “normal” cornea would be uniformly green. This sort of mapping is used both to diagnose keratoconus and to assist in the fitting of contact lenses. The area surrounding the cone tends to be flatter than a normal cornea and it is these differences in corneal shape that make vision in glasses so poor. Spectacle lenses are designed to work for normal eyes where the cornea is a regular, almost hemispherical shape. The centre of the cornea, the centre of the pupil and the macula at the back of the eye (the small area you see best with) are all lined up on the “visual axis”. In keratoconus, often the centre of the cornea is displaced – usually downwards – and this causes a distortion in vision that no spectacle lens can correct. | |
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A 3D representation of a keratoconic cornea is shown to the left. The balance of steep and flat is clearly shown and it can be appreciated how this irregular surface makes it difficult to produce a regular image. The resultant distortion can be represented by what are called “Higher Order Aberrations” – these cause images to become doubled or even multiples and produces the severe “ghosting effects” typical of the condition. In order for the keratoconic to see clearly again, this irregular surface has to be “smoothed out” in some fashion. The first line of treatment is the fitting of contact lenses which serve to create a “new” surface to the cornea which is regular and eliminates the distortions. |
