A Primer on Corneal Topography by David A. Wallace MD
 Normal Photokeratoscop © All Rights Reserved. |
 Photokeratoscope Showing Keratoconus © All Rights Reserved. | | Corneal topography has become essential in the modern era of eye care. It is used in evaluation of and planning for all refractive surgery, in planning cataract surgery, and in fitting contact lenses. It can be helpful in troubleshooting visual complaints (ghosting, distortion, etc.) both before and after any contemplated corrective care It is also helpful in monitoring certain medical conditions and diseases of the cornea.
Topography instruments take a video snapshot of the image formed by reflection of the multi-ring light source (called a "Placido disk") off of the cornea. A normal photokeratoscope view is shown in the
upper left image. The smooth, round, concentric rings are characteristic of normal corneas. This simple view can sometimes show information that is not visible on the fancier computer-processed color maps, below. The photokeratoscope view can disclose conditions such as keratoconus , in which there is distortion of surface curvature, and asymmetric steepening causing the rings to appear egg-shaped, as in the lower left image. Keratoconus is a structural disorder of the cornea where abnormal asymmetric thinning of tissue (usually below and nasal to the center), causes bulging or abnormal steepening of curvature over the thin area.
 Axial map of a normal cornea © All Rights Reserved. |
 Axial map of an eye with advanced keratoconus © All Rights Reserved. | | Using computer image analysis, we can display curvature radius over the entire surface. This is helpful in assessing regularity of curvature and contour; and provides a visual display of corneal astigmatism. It can show whether astigmatism is regular or irregular, and whether the astigmatism is symmetric. Shown is an axial map of a normal cornea with a small amount of astigmatism. Note that the curvature range in this normal eye varies from about 40.5 in the periphery to about 45.0 near the center.
Shown is an axial curvature map of an eye with advanced keratoconus. The numeric scale on the left is small, but shows that curvature ranges from 37 diopters on the low end to above 53 on the high end. Note also that the area of steepest curve (most red on the display) is not in the center of the cornea.
Keratoconus is a form of ectasia, or uncontrolled thinning of corneal tissue. In areas where the tissue is thinnest, the biomechanical structural stability is consequently weakened, and the tissue will bend more readily in this area than in an area of normal thickness. For this reason, areas of thinning are associated with topographic steepening of curvature. In keratoconus, the abnormally thin and steep tissue is in the central or paracentral area fo the cornea. Another 'ectasia' condition is Pellucid Marginal Degeneragion, wherein the area of thinning is more peripheral, typically in the lower (or inferior) cornea.
To read more about abnormal topography and asymmetric topography, see Assessment of High Astimgatism and Topographically Asymmetric Astigmatism . |