Study identifies parameters that help diagnose ectatic disease

Take-home message: Topometric and tomographic parameters helped researchers detect keratoconus in a study, but the integration of curvature and 3D analysis is needed to identify milder forms of ectatic corneal disease.

By Vanessa Caceres

Ectasia after LASIK is something that refractive surgeons naturally want to avoid. Identifying patients who have keratoconus preoperatively can help pinpoint who is at risk for postoperative ectasia.

Two methods that could help surgeons going forward are topometric and tomographic parameters, said Marcella Q. Salomão, MD, Instituto de Olhos Renato Ambrósio, Rio de Janeiro, Brazil. Topometric parameters derive from the front surface curvature, and tomographic parameters derive from 3D elevation and thickness distribution.

“Corneal tomography provides a three-dimensional reconstruction of the cornea, which allows the characterization of the front and back surfaces of the cornea along with a pachymetric map,” she said. “It allows a comprehensive evaluation of corneal geometry and architecture.”

In a retrospective study led by Dr. Salomão from the Rio de Janeiro Corneal Tomography and Biomechanics Study Group, Pentacam HR (Oculus) indices were used from 282 patients with bilateral keratoconus (one eye was included in the study), 211 with forme fruste keratoconus (FFKC), and 266 normal patients. Forme fruste keratoconus was defined as the eye with a normal front curvature map from patients with keratoconus detected in the fellow eye and the “normal” eyes were the preoperative status of cases that had stable LASIK with more than two years of follow up.

The use of topometric and tomographic indices successfully detected keratoconus in the study with a virtually perfect separation. However, while many parameters had statistically significant differences among normal and FFKC groups, further integration parameters from curvature and 3D analysis were necessary to enhance accuracy to identify more mild ectasia, Dr. Salomão reported.

Parameters that most successfully identified keratoconus were the deviation (D) value on the Belin/Ambrósio Enhanced Ectasia Display, the Ambrósio Relational Thickness (ART) values, and the Index of Height Decentration (IHD).

The integration of age, a surrogate of biomechanical properties of the cornea, along with other parameters, improved the ability to identify ectatic disease.

Clinical implications

Conscious interpretation of corneal shape analysis identifies early ectatic disease, which helps prevent iatrogenic progressive ectasia, Dr. Salomão said.¹

Corneal tomography allows the characterization of the front and back surfaces of the cornea, along with a pachymetric map. (Images courtesy of Marcella Q. Salomão, MD)There is an unquestionable need to improve the Ectasia Risk Score System, which was published in 2008 based on Placido disk-based topographic abnormalities, low residual stromal bed thickness, young age, and higher levels of correction. Although that was a landmark study, the researchers reported up to 8% of false negatives and 6% of false positives, Dr. Salomão said.²

“Interpretation of corneal topography is a paramount factor, but we need to be objective, as we identified in the study from Ramos,” she said.³“In addition, abnormal corneal topography is reported as the most important preoperative risk factor. However, there are reports of post-LASIK ectasia in eyes with apparently normal topographies, which indicate we need to go beyond such classic and still important exam. As we read in the editorial from Randleman and Ambrósio, ⁴we are really seeking to identify very mild or susceptible cases, which may still have normal curvature.” she said.

Ultimately, Dr. Salomão and fellow researchers believe there are two mechanisms responsible for ectasia ‒ preoperative abnormally weak innate biomechanical properties of the patient’s cornea and the amount of biomechanical weakening caused by the procedure.

The challenge is to characterize each case in terms of the amount of biomechanical weakening they could withstand from the refractive procedure, Dr. Salomão said.¹

“Such integration has been developed using artificial intelligence by the BrAIN (Brazilian Study Group of Artificial Intelligence and Corneal Analysis), which aims to enhance safety for our patients,” she said.

References

• Ambrosio R, Jr., Nogueira LP, Caldas DL, et al. Evaluation of corneal shape and biomechanics before LASIK. Int Ophthalmol Clin. 2011;51:11-38.

• Randleman JB, Trattler WB, Stulting RD. Validation of the Ectasia Risk Score System for preoperative laser in situ keratomileusis screening. Am J Ophthalmol. 2008;145:813-8.

• Ramos IC, Correa R, Guerra FP, et al. Variability of subjective classifications of corneal topography maps from LASIK candidates. J Refract Surg. 2013;29:770-5.

• Ambrosio R, Jr., Randleman JB. Screening for ectasia risk: what are we screening for and how should we screen for it? J Refract Surg. 2013;29:230-2.

Marcella Q. Salomão, MD

E: marcella@barravisioncenter.com.br

This article was adapted from Dr. Salomão’s presentation at the American Society of Cataract and Refractive Surgery annual meeting in San Diego earlier this year. The presentation was entitled “Topometric and Tomographic Parameters for Diagnosis of Ectatic Disease.”