Enhanced Screening for Ectasia Susceptibility Among Refractive Candidates: The Role of Corneal Tomography and Biomechanics

Abstract

Progressive ''iatrogenic'' ectasia or keratectasia is a very severe complication of laser vision correction procedures. This is more common after LASIK, in which the lamellar cut promotes a larger biomechanical impact than the excimer laser ablation. However, ectasia has been also reported after surface ablation. Considering the severity of such complication, prevention is the best approach. Preoperative abnormal topography has been classically considered as the most important risk factor for ectasia development. Other risk factors are young age, high myopic corrections, low residual stromal bed and thin cornea. Multiple laser retreatments and thick flaps are additional risk factors, as are postoperative trauma or intense eye rubbing. However, there are mysteries related to the cases that develop ectasia with no identifiable risk factors, and also to the cases of successful LASIK that remain stable despite of multiple risk factors (including abnormal topography). Corneal ectasia may occur due to two distinct mechanisms: 1. preoperative abnormal (weak) corneal stroma; and 2. severe biomechanical impact (weakening) from the procedure. While these mechanisms are distinct, there is an association and overlapping between the level of susceptibility of any cornea and the biomechanical impact of the procedure. Corneal tomography and biomechanical assessment provide an advanced understanding of the cornea that augments the sensitivity to identify a very mild (forme fruste keratoconus) form of ectasia, that may still present with relatively normal front surface topography. Such an enhanced screening approach not only augments the sensitivity to detect susceptible cases, but also provides higher specificity for a cornea with irregular topography, considered as a keratoconus suspect, that may be suitable for laser vision correction.

Astigmatic Vector Analysis of Posterior Corneal Surface: A Comparison Among Healthy, Forme Fruste, and Overt Keratoconic Corneas

“…1 Undiagnosed preoperative mild forms of Kc have been reported to be a principal reason for iatrogenic corneal ectasia following keratorefractive surgery. [7][8][9][10] At present, corneal topography of anterior surface, based on the Placido disk, remains the predominant method for detecting Kc. However, topography screening methods have inherent shortcomings.…”

“…5,16 Newer techniques, such as Scheimpflug-based tomography, have been developed to obtain a complete analysis of corneal geometry, permitting characterization of the anterior and posterior surfaces and pachymetric mapping. 10,13,17 As Kc-associated changes apparently first arise on the posterior corneal surface, it has been suggested that corneal Scheimpflug tomography may readily detect topographically normal Kc cases. 7,10,14 Recent studies, based on Scheimpflug tomography, have found that the shapes of anterior and posterior corneal surfaces correlate in a predictable way in the normal healthy eye, 18,19 although conflicting observations have been reported.…”

“…10,13,17 As Kc-associated changes apparently first arise on the posterior corneal surface, it has been suggested that corneal Scheimpflug tomography may readily detect topographically normal Kc cases. 7,10,14 Recent studies, based on Scheimpflug tomography, have found that the shapes of anterior and posterior corneal surfaces correlate in a predictable way in the normal healthy eye, 18,19 although conflicting observations have been reported. 20 Astigmatism is a vectorial variable: in addition to its magnitude, astigmatism has an orientation defined by its axis.…”

PRK em olhos com topografia atípica e pentacam normal

“…New concepts such as posterior elevation and pachymetric progression can be of great value in identifying individuals who do not have the classic signs of keratoconus but who may be susceptible to the condition after surgery. One study reported changes in posterior elevation and/or pachymetric progression in 88% of contralateral eyes in cases of unilateral keratoconus with normal topography (24) . Apart from changes in corneal anatomy, another factor to be considered is corneal resistance, which plays a key role in the development of ectasia after laser surgery.…”

Application of corneal tomography before keratorefractive procedure for laser vision correction

“…Also, a full pachymetric map can be generated from measurements of the anterior and posterior cornea surfaces on tomography images, but this is not possible in Placido topography. However, the Placido‐based system enables evaluation of tear film, which is also relevant to screening for dry eye after refractive surgery .…”