topography topics

Clinical Applications of Axial and Tangential Maps

BY KENNETH A. LEBOW, OD, FAAO
OCTOBER 1999

Comparing axial and tangential maps is especially important when examining keratoconic patients. Because these patients represent the extremes of the curvature range, differences between the two maps are more visible.

The two images in Figure 1 represent axial and tangential maps of keratoconus. The axial map (left) shows a large area of diffuse mid-peripheral and peripheral steepening, while the tangential map (right) is smaller and slightly more centrally located. The central apical curvature in the tangential map is steeper (49.7D) than it is in the axial map (47.5D) because the tangential map demonstrates local curvature more precisely.

The axial map is less sensitive to changes in curvature. In either situation, the actual cornea is the same, it is simply being viewed with different algorithms, much like a radiologist would evaluate a tumor with different views of an MRI.

Finding the Cone

The apex location of the cone is an important landmark in keratoconus.The cone apex may represent the area of steepest curvature. It will show a bearing pattern when a spherical base curve contact lens is applied to the ocular surface. It is smaller with steeper curvatures in the tangential view, making its exact location easier to find.

When the average maximum tangential and axial curvatures of 50 keratoconic eyes were compared (personal data), the maximum tangential value was 1.07D steeper than the corresponding maximum axial value. When 50 normal eyes were measured, not only were the curvature values lower, but the difference between the maximum axial and tangential curvatures was only 0.14D.

The differences in axial and tangential map readings may become clinically significant at the extreme ranges of the corneal curvature.

Tips to Remember

So although differences between axial and tangential measurements are typically present for the average cornea, they are clinically small and only represent a difference in visual presentation rather than a statistically significant difference in shape.

Remember, each corneal topography map represents a different picture of the same corneal shape. It's important to view both tangential maps and axial maps to obtain a better understanding of the actual corneal shape.

Dr. Lebow is a member of the AOA and a Fellow of the AAO. He is in private practice in Virginia Beach, Virg.