Novel pachymetric map analysis detects keratoconus

April 5, 2009

San Francisco-Gaussian fitting on inverse normalized pachymetry maps enables quantification of the depth and width of focal thinning associated with keratoconus and is showing promise as a new method for diagnosing this corneal ectatic disease, said David Huang, MD, PhD, of the University of Southern California, Los Angeles.

San Francisco-Gaussian fitting on inverse normalized pachymetry maps enables quantification of the depth and width of focal thinning associated with keratoconus and is showing promise as a new method for diagnosing this corneal ectatic disease, said David Huang, MD, PhD, of the University of Southern California, Los Angeles.

The technique involves use of corneal pachymetry maps obtained using a Fourier domain-based, optical coherence tomography system (RTVue-CAM, Optovue). The depth and width of focal thinning is quantified by fitting a rotationally symmetric two-dimensional Gaussian waveform to the inverse normalized pachymetry map, which is derived as the average pachymetry map of normal eyes divided by the pachymetry map of the eye under analysis. The Gaussian waveform is centered at the peak and the width adjusted to maximize the cross-correlation product between the waveform and the map.

The technique was investigated in a retrospective study involving 15 normal and 15 keratoconic eyes. Comparisons of the average cone depth and average cone diameter showed statistically significant differences between the two groups. In this small sample, the technique demonstrated 100% sensitivity and 100% specificity.

“We know that topography does not screen out all eyes at risk for postLASIK ectasia, and while corneal thickness offers another clue, thin corneas have been operated on without developing ectasia,” Dr. Huang said. “Looking at the pattern of focal cornea thinning may be a better approach.”

“In a previous study, we developed a system using a general statistical description of a map to detect focal thinning based on a number of parameters and demonstrated it had very good diagnostic accuracy in a small body of test patients,” he concluded. “This new approach to matching the focal thinning with a Gaussian waveform involves fewer parameters. Now its clinical utility should be further evaluated in a larger study that should include eyes with forme fruste keratoconus.”

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