Appreciate the Topography in Keratoconus

Photokeratoscopy and videokeratography enable practitioners to appreciate the diverse corneal shapes in keratoconus -- nipple, oval, temporal and globus. These shapes are most likely the result of where the apex is positioned on the cornea and the mid-peripheral tissue response to the steepening.

The rarest of the corneal shapes is temporal keratoconus. In 1990, Rodger Kame, O.D., first described this condition which is hallmarked by temporal steepening at or near the 180° meridian which results in significant nasal flattening along the same meridian. This form of keratoconus varies significantly from the more common oval shape, in which the corneal apex is displaced inferiorly below the corneal midline. In classic keratoconus, the inferior ectasia creates an island of normal or flatter-than-normal cornea superiorly 180° from the apex.

SHIFTING TOPOGRAPHY CAUSES LENS INTOLERANCE

This month's case history involves a 52-year-old woman with a 20-year history of bilateral keratoconus. Over the past two years, she experienced a gradual decrease in lens tolerance OU. Her wearing time was six to eight hours a day. Her habitual lenses were OD
-1.50D power (48.00D), 9.0mm diameter, 7.6mm optic zone; OS
-5.25D power (46.00D), 9.0mm diameter, 7.6mm optic zone. With fluorescein, both lenses showed heavy mid-peripheral nasal bearing corresponding to the flattest portion of the cornea.

    

FIG. 1: CORNEAL MAP OF RIGHT EYE.  FIG. 2: CORNEAL MAP OF LEFT EYE.

    

FIG. 3: FLUORESCEIN PATTERN OF ROSE K OD.  FIG. 4: FLUORESCEIN PATTERN OF ROSE K OS.

In refitting the patient, we knew it was important to appreciate the diverse topographies along the 180° meridian. The right eye revealed a temporal apical radius of 55.75D, which dramatically flattened nasally to 41.37D. The left eye had a temporal apical radius of 57.75D, which flattened 4.0mm nasally to 40.75D. In this situation, videokeratography (Figs. 1 & 2) clearly identified where the cornea was mid-peripherally steepest (where the contact lens would be loosest) and flattest (where the lens would be tightest).

To decrease the nasal impingement, we fitted diagnostic Rose K lenses. We chose this design because of its smaller posterior optic zone diameter and wider, flatter periphery (Figs. 3 & 4). The final lenses were OD 7.32mm base curve (46.00D), -1.75D power, 8.7mm diameter, 6.2mm optic zone; OS 7.50mm base curve (45.00D), -3.00D power, 8.7mm diameter, 6.4mm optic zone. Visual acuities with the lenses were OD 20/25 and OS 20/30, and wearing time dramatically increased to all-day wear.

This case clearly illustrates one of the many topographical manifestations of keratoconus. In this situation, videokeratography enabled us to identify the steepest and flattest portions of the cornea. This information helped to clarify the peculiar fluorescein patterns noted on the patient's initial visit. The topographical information also identified the flatter nasal quadrants as the areas of greatest fitting concern. CLS

Dr. Campbell is medical director of the Park Nicollet Contact Lens Clinic & Research Center, Minnetonka, Minn. Patrick Caroline is an assistant professor of optometry at Pacific University, Forest Grove, Ore., and director of contact lens research at Oregon Health Sciences University in Portland.