Corneal topographer enhances LASIK screening

Lewiston, NY-The Magellan Mapper Corneal Topographer (Nidek) is a valuable new tool for the preoperative evaluation of refractive surgery patients, said Claus M. Fichte, MD.

"Patient-friendly and easy-to-use, this new corneal topography system is an important adjunct for enhancing safety as it can improve the identification of patients who may be at risk for post-LASIK corneal ectasia," said Dr. Fichte, chief of ophthalmology, Mount St. Mary's Hospital, Lewiston, NY, and a private practitioner in Amherst, NY.

The system recognizes various path-ologies and former surgeries and also provides a ranking of the probability (0 to 99%) that the imaged cornea fits within a particular category.

"While there are some corneas that won't fit into the library of diagnostic categories, at least the system alerts you that there may be something unusual that may indicate further evaluation. It also leads me to look at the Orbscan study a little more critically in various patients, and as a result of using these systems together, I find that I am now performing PRK on 15% to 20% of my LASIK candidates compared with about half that number before. Of course there is no way to know whether some of those patients could have undergone uneventful LASIK, but I would rather err on the side of safety," Dr. Fichte said.

An accurate tool He also appreciates the accuracy of the Magellan Mapper. While it projects 30 rings, 60 rings of data are gathered. Consequently, the Magellan Mapper offers more than two-fold higher resolution than other topographers. Therefore, it is less likely to miss fine irregularities.

In addition, using a color-coded system, it alerts users about data that may be less reliable because of examination problems. For example, areas that cannot be adequately imaged with the Magellan Mapper because of a tear film irregularity are highlighted on the display with a gray color.

"Other systems have a data smoothing function for data that is not collected, but there is no information in the display to show there has been interpolation or extrapolation. There is less interpolation with the Magellan Mapper because it uses more rings and it also alerts you to potential imaging problems," Dr. Fichte said.

In addition to the topography feature for refractive surgery, the Magellan Mapper comes equipped with contact lens fitting software. Since fitting contact lenses with complex parameters is not part of his practice, Dr. Fichte has not personally used the information provided by that feature. However, he will image contact lens patients and provide the data from the Magellan Mapper when referring those individuals for fitting outside of his practice.

Presentations about the Magellan Mapper given at the American Society of Cataract and Refractive Surgery (ASCRS) annual meeting stimulated Dr. Fichte's interest in the system. The fact that its software was developed by Stephen Klyce, PhD, and colleagues at the LSU Eye Center as a neural network application based on corneal statistics from a library of hundreds of normal and abnormal corneas was an impressive attribute.

Conventional screening Various reports of ectasia developing post-LASIK in eyes considered normal by conventional topography screening criteria preoperatively highlighted the need for improvements in techniques for screening LASIK candidates. The limitations of existing approaches were reinforced for Dr. Fichte by events at "stump the experts" type program at the ASCRS meeting. During that session in which a panel of skilled Orbscan users was asked to review a series of scans, the participants failed to correctly identify one of the abnormal corneas.

A demonstration of the performance of the Magellan Mapper helped to convince Dr. Fichte that it may be the solution. "A colleague of mine who had an unusual corneal surface after having undergone scleral expansion band surgery volunteered to be imaged. The Magellan Mapper correctly identified that he had corneal surgery whereas other corneal topography systems had failed to do so in the past," Dr. Fichte explained.

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