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.