DIAGNOSTIC INSTRUMENTS

 Why You Need Corneal Topography,  And How to  
Make Sense of It

Only one in every six O.D.s has a corneal topographer. This clinician makes the case for why it should be in your practice, and what you should expect from the technology.

John Schachet, O.D.
Englewood, Colo.

Corneal topography has been around for about 20 years, and yet so many of our colleagues think it’s a recent technology. The technology isn’t new at all, but the delivery systems—the boxes in which the technology comes—have changed dramatically since the first Reynold’s Keratoscopes came out in 1978.

Despite this long history, optometrists have been slow in embracing corneal topography. Review of Optometry research conducted last summer shows that fewer than 14 percent of all optometrists have topographers in their practices. The two most common reasons I hear from doctors on why they don’t have topographers: It’s too expensive, and they wouldn’t use it that often if they had one. They’re wrong on both counts, for reasons I’ll go into later.

Before I do, consider the quality of information corneal topography gives you. Keratometry gives you the average of four points in the central 3mm of the cornea. The Reynold's Keratoscope measured out to 7-8mm of the cornea. Today’s topography systems can measure out to 10mm and more and project between 6,000 and 11,000 data points on the corneal surface.

In this article, I’ll explain why corneal topography should be part of your everyday practice, and give some insight into what you should look for if you’re shopping for a corneal topographer.

Price and Practicality
The price of topography has declined drastically over the past few years. As recent as five years ago, you could have bought one of those “big-box” topographers for almost $40,000. Today, you can spend a fourth of that on a system that will give you everything you need for evaluating corneal shape in your practice.
Topography has many applications in practice. I use it constantly, day in and day out. It’s invaluable in our contact lens practice, especially when evaluating keratoconus patients. We could not do ortho-keratology without it. We use it for routine contact lens patients, too. With new patients, we just like to get a clear idea of what their corneal shape factors are. With established patients, we like to do annual topography to track corneal changes. If I suspect any kind of a corneal problem, such as an early keratoconus, I need to have topographical maps.

Corneal topography is a must if you’re comanaging refractive surgery. It enables us to rule out any type of corneal problems that would be a contraindication for laser correction. We also follow all corneal changes in the post-op management of LASIK and PRK patients. A disturbing statistic: Review of Optometry research shows that 85 percent of optometrists said in 1997 that they comanaged refractive surgery, yet far fewer than that have corneal topographers. I don’t see how you can comanage laser vision correction without a topographer. It’s like making a recipe and leaving out a major ingredient.

Corneal topography is also invaluable for evaluating corneal pathology, such as chronic dry eye, corneal dystrophies and contact lens-induced corneal warpage.
I started working with corneal topography in 1979. The first systems were rather rudimentary, but they gave more information than keratometers could. And, once you get a feel for working with the cornea as a whole rather than the average of four measured points, you acquire a much greater appreciation for the cornea.

Beyond Bells and Whistles
Over the past few years, corneal topographers have become more user-friendly. Here are a few of the most significant improvements:
• Optometric-driven software. When topographers first became available commercially, manufacturers geared their software toward for ophthalmologists and built nomograms for RK into their software. Over the past few years, topographer makers have developed contact lens-fitting software that’s more oriented to the way optometrists think and work.

• Smaller size. Topographers, of course, are much smaller than they used to be. They used to be big, bulky instruments. Now that they’ve come down to a smaller footprint, it’s easier to find space for them in the practice.

• More accurate contact lens packages. Today’s contact lens-fitting software uses algorithms that use the data from the topographer rather than K readings. The early software incorporated the contact lens manufacturers’ recommendations for fitting lenses, either flatter than, steeper than or on K. Now, this software uses data that comes directly from the topography that you’ve taken.

• Auto-focus and alignment. This has been a selling point for many systems. It’s very difficult to get an accurate reading if the cornea is not aligned or out of focus, and years ago it took a considerable level of user skill to align and focus a topographer. Today’s systems use joysticks to simplify this task, and some automatically focus and align without an operator. Another development centers on the type of device that actually measures the cornea. Most topographers use the placido disk technology that has been around for many decades. There are a couple of new systems out that use non-placido disk technology: the sine wave technology in the Euclid ET-800; and the infrared technology in the reflective scanning system in Orbtek’s Orbscan.

What You Really Need
One of the problems with corneal topography technology today is the lack of standardization across manufacturers’ platforms. What one manufacturer calls a “sagittal” map, another will label an “axial map.” Where one uses “tangential” to describe a map (a term that makes the most sense to those of us who have used topography for a long time, because it’s the term we’ve used in the past), another calls a it “power” map. This makes it difficult for you to compare one topographic system to another.

Determining the corneal shape factor is a critical task topography should help you perform, but this is another area where terms and functions differ among manufacturers. Some define shape factor as eccentricity, but this is a property of corneal shape. How do you tell if the cornea has a positive or negative shape factor? That’s a key piece of information topography must give you if it’s to be useful.

Bells and whistles aside, here’s what a corneal topographers should be able to give you to meet the basics of practice:
• An image that you can either store on a hard drive or print.
• Tangential screen and axial screen view.
• An ability to predict keratoconus.

Some topographers offer an overlay feature so you can compare maps over time. You can go either way on contact lens programs. Most of us aren’t going to use these too often, although for difficult fits some of the newer software is rather impressive. However, it’s not necessary to fit a contact lens like a glove. What I’m concerned about with contact lens wear is corneal change over time. I need a topographer that gives me that information.

Once you understand the basics of corneal topography and what it can deliver for your practice, the technology isn’t all that daunting. Put that together with the ease of use, price and the critical information that it provides, it’s easy to make the case for using a corneal topographer in everyday practice. After all, if you’re practicing full-scope care—contact lenses, corneal pathology, refractive surgery comanagement—you’ll need all the ingredients to make the recipe work.
Dr. Schachet is in private practice and lectures frequently on contact lenses, corneal topography and practice management.

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