Corneal Topography: Understanding the Basics
BY KENNETH A. LEBOW, OD, FAAO
Corneal topography (CT) maps are interesting and produce nice slides, but what do they mean to us clinically? What can these maps tell us about corneal shape and how it is influenced by external forces? More importantly, what do the changes in corneal shape signify?
While some practitioners have an excellent grasp of CT maps, most of us, at best, have a limited idea about how to use them in clinical practice. With that premise in mind, I will review different aspects of corneal topography and its clinical applications in optometric practice. It may also be helpful for you to request a copy of one of the many excellent corneal topography primers that are available from your local CT manufacturer.
A Basic Overview
Corneal topography maps, in their simplest form, graphically present a global picture of the corneal curvature. (Bear in mind, topographers present curvature data describing thousands of points on the corneal surface, while keratometers are limited to just four). Most CT maps offer a two-dimensional representation of a three-dimensional shape. Colors are used to represent curvature values, and from these various color-coded curvatures, corneal shapes are categorized. Remember that cool colors (blues) represent flatter curvatures and hot colors (reds) depict steeper curvatures. Since with-the-rule (WTR) corneal astigmatism has its flattest curvature along the horizontal axis, a vertical red bow-tie pattern develops (the steepest axis is vertical). Conversely, against-the-rule (ATR) corneal toricity produces a horizontal red bow-tie pattern (the steepest axis is horizontal). Oblique astigmatism occurs whenever the bow-tie pattern rotates away from the vertical or horizontal meridian.
Corneal toricity may also be symmetrical or asymmetrical. Symmetrical astigmatism is suggested when roughly equal curvature weightings can be seen along either a vertical or horizontal axis (Fig. 1). The upper left image presents marked symmetrical WTR astigmatism (5.00D), while the upper right image represents moderate symmetrical ATR astigmatism (1.75D). The lower left image shows slight asymmetrical WTR astigmatism (0.75D), (notice the heavy weighting of steep curvatures inferiorly), with the lower right image showing asymmetrical ATR astigmatism (1.37D) (notice the heavy weighting of the steep curvatures nasally). Asymmetry in a topography image can be normal, as in the case of a displaced corneal apex or poor patient fixation, or abnormal, as one would anticipate from keratoconus or forme fruste keratoconus or induced from RGP lens wear. I will discuss these differentiations in a future column, after we've established a better understanding of the basics.
Dr. Lebow is a member of the AOA and a Fellow of the AAO. He is in private practice in Virginia Beach, Virg.