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Keratoconus Cornea Topography

Eye Anatomy

In keratoconus cornea topography is a particularly important feature that must be mapped and analyzed as part of the process of diagnosis. Most current topography systems scan the surface of the eye at standard points to develop a map that is used to look for corneal problems such as keratoconus. Bausch & Lomb’s Orbscan® IIz provides a powerful diagnostic system that allows much greater understanding of the cornea than mere cornea topography. This is particularly important when diagnosing corneal dystrophies such as keratoconus. The Orbscan II acquires over 9000 data points in 1.5 seconds to make a very detailed map of the entire corneal surface (11 mm), and analyze elevation and curvature measurements on both the anterior and posterior surfaces of the cornea. These cornea topography maps are very important in the diagnosis of various forms of keratoconus. Both anterior and posterior maps of the cornea developed by the Orbscan can aid in the diagnosis of keratoconus. Combined with other clinical observations, the Orbscan is an invaluable aid in the diagnosis and treatment of keratoconus.

Other forms of cornea topography can be used to diagnose keratoconus as well. The clinician will be looking for signs that the cornea is thinning and therefore causing cornea ectasias. Cornea topography will typically show a change in the peak location of the cornea. Generally the point, or cone, is located inferior to the central point of the cornea. In addition there will be a significant difference in corneal curvature across the cornea topography. Keratoconus causes a sag in the cornea which is visible on cornea topography.

Many cornea topography instruments have software included which analyzes the cornea shape and makes a tentative diagnosis. Unfortunately, these topographer rely on statistical averages for normal corneas. A misdiagnosis can often occur under circumstances that are not ideal, such as small pupil apertures, post-refractive surgery, excessive tearing, or dryness. Under these conditions, a cornea topography may be described as irregular and indicative of keratoconus even though in truth keratoconus is not a factor. Post-LASIK patients can often be described as having keratoconus by a device that measures cornea topography. The reason for this is the change in shape caused by the laser can give a false indication that the cornea is irregular in a manner that is representative of keratoconus.

It requires a skilled clinician to make a diagnosis of keratoconus. A cornea topography alone cannot make that decision. It is however very helpful and it is a standard of practice for LASIK providers to take a cornea topography measurement on every patient prior to deciding a patients candidacy for a procedure.