the contact lens exam
Corneal Topography: More Than Just a Colorful Map
BY DAVID KADING, OD, AND JENNIFER SMYTHE, OD, MS, FAAO
Not all corneal topographers are created equal. Although they map the corneal surface to provide both quantitative and qualitative information, each has its own characteristics. In May 2004, Colleen Riley, OD, described the Orbscan (Bausch & Lomb), Medmont (Art Optical) and Keratron (EyeQuip) topographers. This month, we'll talk about three more that you should give careful consideration to when selecting the right unit for your practice.
The Humphrey Atlas (Carl Zeiss Meditec) uses 22 placido rings and a specialized Cone of Focus, which incorporates the principle of parallax to acquire readings. An Arc Step Algorithm takes into account the asphericity and miniscule elevations in the corneal periphery.
This instrument's Pathfinder Corneal Analysis program identifies irregular corneas by comparing the shape factor, mean toric keratometry value and corneal irregularity measurements. It then compares these values to normal and abnormal corneas to help differentiate corneal distortion, contact lens-induced warpage or keratoconus.
The Humphrey Atlas has two special modules that provide information in addition to the standard axial and tangential maps:
1. The Advanced Refractive Diagnostic module gives the following views: Curvature, power, elevation and irregularity.
2. The Simulated Ablation Difference module displays axial power, simulated ablation and difference topographic maps, which can prove useful for patient education.
Other modules include the Masterfit contact lens fitting module, which empirically selects GP lens parameters based on corneal curvature and shape and the Healing Trends/Stars module, which displays five maps to show progression of corneal healing.
OPD-Scan Wavefront Plus
The OPD (optical path difference)-Scan (Nidek) incorporates a corneal topographer, autorefractor/keratometer and wavefront analyzer into one. The unit uses the principle of skiascopy (similar to retinoscopy), which reflects a noncoherent infrared slit off the fundus. Special photo sensors capture these data points and a time difference measurement calculates the total refraction and the OPD map. The OPD-Scan measures refractive error and corneal topography data points simultaneously while an eye tracker aids reliability.
The OPD-Scan also works as a pupillometer by measuring both mesopic and photopic pupil size. The touch screen and XYZ automatic alignment features make the instrument easy to use. The time it takes to run all three components is similar to that of a standard topographer (two-and-a-half to three minutes).
The EyeSys Vista (Marco) is a convenient, handheld unit that you can attach to a slit lamp mount. The use of a laptop computer to compile the data compliments the portability of the device. The EyeSys Vista takes measurements automatically and monitors them for tilt and alignment. Acquiring proper alignment may seem cumbersome at first, but with practice and patience, the benefits of portability soon become evident.
Accepting Topography's Place
As instruments have become more sophisticated, easier to use and more economical, we should consider corneal topography a standard part of every patient's contact lens examination.
Dr. Kading is currently the Cornea and Contact Lens Resident at Pacific University College of Optometry.
Dr. Smythe is an associate professor of optometry at Pacific University and is in private group practice in Beaverton, Oregon.