Measurement of corneal parameters using dynamic ultra-high-speed Scheimpflug photography (CorVis ST, Oculus) is an effective method for discriminating between normal and biomechanically compromised or ectatic corneas, including very mild cases of forme fruste keratoconus (FFKC), according to the results of a retrospective study reported by Renato Ambrosio Jr., MD, PhD.
Rio de Janeiro, Brazil-Measurement of corneal parameters using dynamic ultra-high-speed Scheimpflug photography (CorVis ST, Oculus) is an effective method for discriminating between normal and biomechanically compromised or ectatic corneas, including very mild cases of forme fruste keratoconus (FFKC), according to the results of a retrospective study reported by Renato Ambrosio Jr., MD, PhD.
“Theoretically, any patient can develop ectasia after LASIK, depending on how the surgery is performed,” said Dr. Ambrosio, director of cornea and refractive surgery, Instituto de Olhos Renato Ambrosio, Refracta-RIO, Rio de Janeiro, Brazil. “However, the focus of preoperative screening is to identify the level of susceptibility of the examined cornea for developing biomechanical failure and post-LASIK progressive ectasia.
“The results of this retrospective study indicate that the integration of corneal tomography and biomechanical data has the potential to screen best for ectasia risk or susceptibility among LASIK candidates,” he said. “We need to go beyond, but not over, standard screening technologies, such as front-surface curvature topography and central thickness, for enhancing the sensitivity and specificity of ectasia risk detection.”
The Scheimpflug system, which is commercially available outside of the United States but not FDA approved, is a non-contact tonometer that uses an ultra-fast Scheimpflug camera, acquiring 4,330 images/sec, with 8-mm horizontal coverage of the cornea during the deformation induced by an air puff. In addition to determining IOP, its output includes several parameters related to the deformation response, such as corneal velocity, applanation length, and deformation amplitude.
The ability to differentiate between normal and ectatic corneas was investigated in a study that included 292 eyes representing four groups defined by findings from both Placido corneal topography and corneal tomography using a rotating Scheimpflug system (Pentacam, Oculus) and tomographic guidelines published by Dr. Ambrosio and colleagues [Int Ophthalmol Clin. 2011;51:11-38].
There were 177 normal (N) patients and 79 patients with bilateral keratoconus (KC), who had one eye randomly selected for the study. Twenty eyes had FFKC, defined by the contralateral eye with normal front-surface topography patterns from patients with KC diagnosed in the fellow eye. There was a fourth group with 16 eyes from 16 patients that had suspected keratoconus (KCS) based on front-surface curvature (topographic) maps, but had normal biomicroscopy, normal spectacle-corrected visual acuity, normal tomographic patterns, and documented stability over 1 year.
The data from the Corvis ST were previously analyzed by the BrAIN (Brazilian Study Group of Artificial Intelligence and Corneal Analysis) so that a linear model was created for optimizing the separation between normal and KC eyes. The combined parameter (Corvis Factor 1) was calculated for the eyes included on this study. The Kruskall-Wallis Test with Dunn’s post hoc test confirmed the hypothesis of the differences from normal and ectatic corneas. Interestingly, the normal and KCS groups were not statistically different, while both had statistically lower values than the KC and FFKC groups. Also there was no significant difference between KC and FFKC.
“These data validate our initial hypothesis that FFKC corneas have corneal deformation characteristics similar to KC but with milder abnormalities,” Dr. Ambrosio said. “Also, the topographic KCS presentation may occur in a biomechanically normal cornea.”
There is still some overlap between the groups so that the Corvis Factor 1 parameter should not be used alone. Interestingly, the combination of the tomographic data generated by the Pentacam enables the identification of all cases of FFKC.
“We have documented many cases of refractive candidates that have normal front surface topography but have abnormal tomographic patterns on the posterior elevation and/or pachymetric distribution,” Dr. Ambrosio told Ophthalmology Times. “This is typically seen in the FFKC cases, which we consider to be at very high risk for biomechanical failure and ectasia progression if corneal ablation is performed.
“Interestingly, this presentation may occur in both eyes in the same patient, and without tomographic evaluation, the detection of ectasia susceptibility will be compromised,“ he said. “The integration of corneal tomography and biomechanical assessment has the potential to identify such cases and should be considered when screening refractive candidates.”
Dr. Ambrosio is a consultant to Oculus.
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