Contact Lens Case Reports
Elevating Your Knowledge of Corneal Topography
By Patrick J. Caroline, FAAO, & Mark P. André, FAAO
The color-coded axial display maps produced by corneal topography do an excellent job of describing the dioptric power of the cornea and illustrating the shape and location of corneal astigmatism. It is the elevation maps, however, that provide the most information about the true shape of the cornea, which is important when fitting GP lenses.
Using the Elevation Map
Figure 1 shows a cornea with keratometric readings of 43.00 @ 007/45.50 @ 097 with 2.50D of with-the-rule corneal astigmatism. The corneal topographer selects a theoretical spherical surface (the best-fit sphere) and runs it through the cornea out to a chord of 8.0mm (the yellow line). Along the flat, horizontal meridian, the theoretical spherical surface penetrates the cornea nasally and temporally to a depth of 22 microns. Along the steeper, vertical meridian, the theoretical sphere lifts away from the cornea 46 microns superiorly and inferiorly. As shown on the elevation map, the highest areas of the cornea are red, while the lowest areas are blue.
Figure 1. This patient has 2.50D of with-the-rule corneal astigmatism.
When a spherical GP contact lens is placed over the elevation map, you can see that the lens touches first along the flattest meridian because it is the highest (red) area of the cornea. The lens lifts away along the steep meridian superiorly and inferiorly because these are the lowest (blue) areas of the cornea.
Many contact lens practitioners prefer to describe this cornea not as one with 2.50D of astigmatism, but as a cornea in which there is a 68-micron difference in elevation (at a chord of 8.0mm) between the horizontal and vertical meridians. An elevation difference of 45 microns or greater indicates that a toric GP lens design may be required.
The differences between axial and elevation maps is clearly illustrated in the following example.
Our patient is a 43-year-old woman with a history of herpes simplex virus. The axial display map (Figure 2) shows the shape and location of the corneal astigmatism. The keratoscopy view graphically illustrates the resulting irregular astigmatism. Finally, the elevation map (Figure 3) shows exactly where the corneal surface is highest (red) and where it is the lowest (blue). It is easy to see the strong correlation between the elevation map and the on-eye fluorescein pattern. CLS
Figure 2. Axial and keratoscopy maps of the patient in our case example.
Figure 3. Elevation map and actual fluorescein pattern of the patient in our case example.
|Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant to CooperVision.|