the contact lens exam

Topography Maximizes Results
BY JEFF SCHAFER, OD, MS

While fitting specialty contact lenses without a topographer is possible, it's much simpler when you're armed with knowing the detailed shape of the patient's cornea. In particular, before starting a new fit of a keratoconus patient, take time to obtain a topography map. If your office doesn't have a topographer, consider referring out for this procedure before beginning the fitting process.

If you refer more than a few patients for topography, then consider buying or leasing your own topographer. A new CPT code, 92025, is now available for topography that you can bill directly to the patient or their insurance when medically justified.

If you've already started fitting an irregular cornea patient without topography and it's not going well, think about putting it on hold while you obtain a map. The unique shape of each patient's cornea can have drastic implications on the strategy of fitting a custom-designed GP lens.

If you're just starting with topography or find yourself intimidated by the amount of data, start with axial, tangential and elevation maps. Axial maps display the curvature of the cornea in diopters and are subject to an overall smoothing effect of the data as they represent a running average of curvatures across the cornea. Axial maps provide more accurate central detail, but often show flatter steep curvatures and steeper flat curvatures.

Tangential maps are calculated using the fact that all light rays aren't refracted perfectly along the optical axis, which allows these maps to be more sensitive to abrupt curvature changes. They display more accurate corneal shape with more detailed patterns.

Elevation maps display the difference in cornea height (in microns) relative to a best-fit reference sphere. Elevation maps are useful in fitting contact lenses. Higher areas (shown in warmer colors) will correspond with areas of touch, while lower areas (cooler colors) will represent areas that will pool fluorescein.

What Topography Reveals

I get many referrals for fitting irregular cornea patients. I first obtain one or several topography readings to see what I'm dealing with. I commonly see new patients diagnosed with keratoconus who've been unsuccessful with GPs. A surprisingly high percentage of the time, topography reveals either a drastically decentered cone apex or a classic pellucid marginal degeneration map (Figure 1). Shape factor can also be useful differentiating keratoconus from PMD. Keratoconic eyes are generally more prolate (SF>0.6), while PMD topographies generally have low prolate or oblate shapes.

When fitting a patient who has PMD, my fitting philosophy varies greatly from typical keratoconus. PMD patients may have been fit in steep, small diameter keratoconus design in the past, when in fact a much larger diameter lens is needed to center the optics over the visual axis.

A smaller lens always centers over the cornea's steepest part (in these cases inferiorly). A larger lens (10.5mm to 12.0mm) allows the lens to center. Carefully control the optic zone size and base curve to allow for minimal touch over the steep inferior cornea and to prevent bubble formation in the flatter superior midperiphery. Large-diameter GP lenses also come in handy with decentered cones or larger globus cones.

Dr. Schafer is a clinical assistant professor and chief of the contact lens service at The Ohio State University College of Optometry.