A diagnostic instrument for contact lens fitting is rapidly gaining popularity: the corneal topographer. This is the machine where you are asked to place your chin in a chinrest, forehead against a headrest, and calmly stare at a red light while a whorl of illuminated concentric rings is brought so close to your eye that you feel as if you are falling into a vortex that will drop you into a Twilight Zone rerun. As bleeping and whirring noises build to a climax, you nervously anticipate a blast of gamma rays or photon particles that will knock you out of your seat. Suddenly, silence. The tech says, “OK, you can relax now.” As you sit back and decide if you’ve been accosted or not, a computer printer comes alive. Seconds later, the printer pops out a colorful and informative map of your cornea.
What this map reveals is the topography, or shape of the corneal surface. A familiar analogy is a topographical land map, which uses a series of wavy lines to show the surface characteristics of a tract of land – the hills, valleys, and plains. The closer the lines are to each other, the more abrupt the change in elevation. Weather maps also use lines, called isobars, to show areas of different barometric pressure. The lines used in corneal topography are the illuminated concentric circles you see in the machine.
When the illuminated rings are brought close to the eye, a reflection is formed in the cornea as if it were a mirror. If the corneal surface is smooth and regular, the reflected lines are circular and equally spaced. When the corneal surface has abrupt changes in elevation and curvature, as in keratoconus, the lines become irregular and distorted. The keratometer, which for years has been used to measure the central corneal curvature, uses the same reflecting principle but has only one illuminated ring. Corneal topographers use numerous rings, supplying information about the corneal surface from center to edge. This expanded information is especially useful in keratoconus, because the cone is often displaced from the center of the eye.
The accuracy of the information from the corneal topographer is heightened as well. Whereas the keratometer estimates corneal curvature by measuring only four points on the cornea, the topographer is linked to a computer that evaluates tens of thousands of points along the reflected lines. The computer then illustrates the information in a color-coded map, similar to the colorful Doppler radar pictures used on weather maps to show differences in rainfall or temperature.
If your contact lens fitter uses corneal maps, ask for a copy of yours. “Hot” colors like red and orange depict the steepest areas, with “cool” colors like blue showing the flattest. Most maps have all the colors from red to blue explained somewhere off to the side, in terms of their corresponding dioptric values (the diopter is the unit of measurement used to express the refractive power of lenses, and eyes). Also represented are simulated K’s, the readings at the center of the cornea that the keratometer might give; rings and/or blocks used to measure out from the center in millimeters; and numbers around the outside to mark the meridians of the circular cornea through 360o. If you have keratoconus, the hottest spot on the map marks your cone. Once located by the color, the dioptric power, distance from center, and meridian are easily determined.
Another handy feature of the corneal topographer allows the operator to view simulated fluorescein patterns. The fit of rigid gas permeable contact lenses is finalized by staining the tears with sodium fluorescein dye (the orange stuff the doctor or contact lens technician puts in your eye, which later comes out your nose). Under a black light, the dye fluoresces a beautiful lime green. Fitters can see how much space is between the contact lens and the eye in a given area by the amount of green that collects in that area. Where the cone is lightly resting against the back of the contact, hardly any green is evident because there is not much space for the tears. Well, the computer software of the corneal topographer can superimpose hypothetical contact lenses over your map, and show the probable fluorescein patterns on the monitor. This feature may save time by reducing the number of lenses that actually need to be tried on the eye and evaluated.
Notice I said may save time. When personal computers were first gaining popularity, I remember hearing how much time they would save. Now I find myself anchored to a keyboard for several hours every day. Despite all their fancy features, there is one thing that corneal topographers cannot replace: the skill of the contact lens fitter. Corneal maps need to be captured properly, interpreted correctly, and filed carefully for future reference. Otherwise, the benefits of the technology are not passed on to patients. And even after investing the time to gather all this advanced information, lenses must still be evaluated on the eye.
Computers can simulate how the back surface of the contact lens might follow the front surface of the cornea, but many more variables affect how a lens actually fits. Patients have different blink rates, lid tension, tear composition, and lens powers. Skilled contact lens technicians know lenses must position properly between blinks, move just the right amount during blinks, provide crisp vision and be comfortable to boot. To achieve success in all these categories, fitters must ultimately watch the lens perform in its dynamic natural environment – on the eye, not on a computer monitor.
Nevertheless, computerized corneal topographers are gaining favor with contact lens fitters across the country. Topographers provide a clearer picture of the surface shape of the cornea, especially in cases where the cornea is diseased or injured. Corneal maps can also detect changes in corneal shape from office visit to office visit, which is extremely helpful in keratoconus. Maps can even be sent over the Internet to contact lens manufacturers who are customizing lenses, or to other doctors for second opinions. So if you haven’t yet seen a map of your cornea, you probably will in the near future. Just keep watching for those illuminated rings spiraling toward your eye.
by Patrick B. Goughary, a Fellow member of the Contact Lens Society of America, teaches contact lens fitting at Camden County College in Blackwood, NJ.