Is It Keratoconus or Corneal Warpage?
BY KENNETH A. LEBOW, OD
Certainly there are significant clinical differences between keratoconus (e.g., Vogt's Striae and Fleischer's Ring) and contact lens induced corneal warpage, but in the early stages, there are also many similarities.
Topographically, one of the most common characteristics shared by these two conditions is superior corneal flattening with inferior corneal steepening.
Fitting rigid gas permeable (RGP) or PMMA lenses flatter than the central keratometer reading so that they ride superiorly, under the upper lid, results in molding of the corneal curvature. Before the advent of corneal topography, corneal warpage was generally described as a condition that included distorted keratometer mires with or without irregular astigmatism and reduced vision on post-lens wear refraction. With the increased use of topography, corneal warpage has been re-defined to include central irregular astigmatism with a loss of radial asymmetry (superior flattening and inferior steepening), and a reversal of the normal flattening corneal contour. Early corneal warpage changes due to PMMA and low Dk RGP materials were a function of edema and hypoxia, while similar changes associated with high Dk RGP and soft lens materials are considered mechanical in origin. These changes are associated primarily with lens decentration.
Corneal shape is the most important criteria used to differentiate normal and abnormal corneal flattening. Typically, a normal cornea demonstrates a prolate shape, meaning that the curvature is steeper centrally and flattens toward the periphery. The shape factor is a positive value, generally between 0 and 0.50 in normally shaped corneas. Since RGPs are firmer than the cornea, the corneal curvature loses its elliptical shape and often assumes a less prolate shape or more oblate shape when they are worn. This means that the peripheral curvature is steeper than the central curvature.
From my experience, normal RGP fitting often reduces the shape factor to approximately 0 to 0.15. This does not necessarily represent corneal warpage, but rather a sphericalization of the cornea due to the spherical base curve of the rigid lens.
However, as the shape factor becomes negative, it implies that the corneal surface has assumed an oblate shape and most likely, has become warped. This can be easily differentiated from keratoconus, where typical shape factors are usually high (>0.6), indicating the highly prolate nature of the surface.
The central cornea in Figure 1 is flatter than the peripheral curvature (i.e., radial asymmetry) and shape factor is negative, indicating an oblate shape. This patient has significant corneal warpage, not keratoconus, despite the obvious pattern of superior flattening and inferior steepening.
FIG. 1: Superior flattening and inferior steepening (radial asymmetry) associated with a negative shape factor.
References are available upon request. To receive them via fax, call (800) 239-4684 and request document # 57.
Dr. Lebow is a member of the AOA and a Fellow of the AAO. He is in private practice in Virginia Beach, Virg.