The visual disturbance caused by keratoconus is mainly due to the irregular shape of the corneal surface. In order to effectively follow the progression of the disease and to fit a keratoconic cornea with contact lenses it is helpful to know the precise shape of the cornea.
Several methods of measuring the corneal shape have been used. The most common clinical method is keratometry. Keratometry has the limitation of measuring the corneal shape from only two points approximately 3 mm apart. The remainder of the cornea is ignored. With keratoconic corneas the area of main concern may not be included in the area measured by the keratometer.
Other instruments using a bull's-eye pattern reflected off the cornea have been used for years to obtain a qualitative evaluation of the corneal shape over a large area. With the introduction of computer assisted topographical analysis the bull's-eye concept can be used to get more quantifiable data and more accurate visualization of the shape of a cornea.
A corneal topographer is a computer linked to a lighted bowl with a pattern of concentric rings inside it. The patient is seated at the bowl with forehead braced against a bar. The technician has only to line the patient up properly and snap an image. The procedure is painless and very fast. The computer then uses the snapped image to produce a printout of the corneal shape using colors to identify different steepnesses, much like a topographic map of the earth describes changes in the land surface.
Below are some maps of keratoconic patients. The red and orange colors show areas of great steepness, and this is where the cone is located. The doctor can use these maps to follow a cone's size, shape, and steepness. The numerical data generated by the computer can also help in contact lens fitting.
|Small Nipple Cone|
|Large Inferior Cone|