A basic understanding of corneal topography is paramount for effective screening and follow-up of refractive surgery patients in order to exclude those who are at risk for postoperative ectasia and to manage those who develop that complication.
New Orleans-Although there are known risk factors for the development of ectasia after refractive surgery, this complication can develop in patients without any predisposing features. In that situation and in situations where the ectasia is not very pronounced, documenting its presence or absence may be a challenge.
Speaking at refractive surgery subspecialty day during the annual meeting of the American Academy of Ophthalmology, Stephen D. Klyce, PhD, discussed criteria for detecting ectasia and considerations for preoperative screening.
"Fortunately, ectasia after refractive surgery is a rare occurrence, and not every steepening of the cornea is necessarily an ectasia. However, patients who present after surgery with progressive loss of correction and reduced [best spectacle-corrected visual acuity (BSCVA)] require careful examination and follow-up so that they can receive an accurate diagnosis and appropriate management," said Dr. Klyce, professor of ophthalmology, Louisiana State University Health Sciences Center, and adjunct professor of biomedical engineering, Tulane University, New Orleans.
Considering that prevention is always the best treatment, it is important to identify pre-existing topographic abnormalities associated with an increased risk of postoperative ectasia so that patients may be appropriately excluded from surgery. This preoperative screening requires good working knowledge of corneal topography and a reliable Placido corneal topographer so that the surgeon can differentiate normal variations in corneal topography from subtle findings associated with forme fruste keratoconus or other topographic abnormalities, Dr. Klyce said.
Availability of interpretive software methods for categorizing topographies using different commercially available hardware platforms can aid surgeons in differentiating between normal and abnormal corneas, he added.
In screening, surgeons need also to consider whether there is a history of contact lens wear, which can lead to corneal warping, as well as the "second surgeon syndrome" scenario, which involves patients who present for re-treatment after having refractive surgery at another location and without complete records available.
A global history
"Never accept patients for second surgery if you are unable to evaluate the full history, because they may have topographic abnormalities that existed but were not detected prior to their first procedure," he said.
In evaluating the corneal topography for changes in shape and power after surgery, it is important to use a fixed scale for interpreting the color-coded contour map of surface powers and a power calculation that makes sense. Dr. Klyce said powers should be presented with the axial power method using 1.5-D power steps.
"Use of instantaneous power can be very confusing as some topographers can make even stable corneas appear to have a peripheral ectasia with a scale that displays the peripheral corneal powers in the red region. Use of the refractive power calculation should also be avoided since plotting of the cornea with this technique may cause the important information to disappear," he said.
Use of difference maps also is essential to identify serial changes that are the basis for documenting ectasia. The changes in the apex power should be plotted with time to validate the trend, Dr. Klyce said.