Corneal Sub-basal Nerve Fiber Plexus Tortuosity in Eyes with Ectasia Susceptibility and Keratoconus: A Cross-sectional In Vivo Confocal Microscopy Study

Purpose This study used in vivo confocal microscopy (IVCM) to examine changes in the sub-basal corneal nerve plexus (SBNP) in susceptibility to ectasia and clinical keratoconus (KC). Methods Eighty-one eyes of 81 participants with no previous ocular surgery and no ocular or systemic diseases, with the exception of KC, were divided into three groups: healthy controls (n=22), clinical KC (n=40, further subdivided into mild and moderate cases), and very asymmetric ectasia with normal tomography and tomography (VAE-NTT, n=19), with the other eye showing clinical KC. The eyes were considered susceptible to ectasia. The SBNP images were acquired using IVCM and analyzed automatically and semiautomatically for corneal nerve fiber properties such as length (CNFL) and tortuosity coefficient (TC). In addition, corneal biomechanical properties were asses by Scheimpflug-based air-puff tonometry. Results CNFL, adjusted for age and gender, was significantly lower in KC than in controls (p=0.033) and showed a significant inverse correlation with maximum keratometry values in KC (r=-0.523, p<0.001). However, TC, adjusted for age and gender, was significantly higher in KC and VAE-NTT than in controls (p=0.002 and p=0.047, respectively) and showed no correlation with KC severity. No other SBNP parameter demonstrated significant differences. TC demonstrated a diagnostic ability with an AUC of 0.773 in distinguishing between normal controls and VAE-NTT, however, it was not superior to the Corvis Biomechanical Index. Conclusion Increased corneal nerve fiber tortuosity is associated with susceptibility to ectasia, while reductions in nerve length and area are observed in clinical keratoconus, indicating ongoing structural nerve alterations.