Quantitation of Stromal Distortion Across the Full Depth of Keratoconic Corneas

This study aimed at gaining new insights into the pathophysiology of keratoconus by providing a comprehensive and quantitative characterization of stromal structural alterations throughout the full thickness of keratoconic corneas and in regions both proximal to and distant from the cone.

Complementary three-dimensional imaging techniques were combined in a pilot exploratory series of four keratoconic and four control human corneas: clinical spectral-domain optical coherence tomography, highly-resolved line-field confocal optical coherence tomography and second harmonic generation (SHG) microcopy. Several structural metrics were computed from SHG images using automatic image analysis pipelines.

Quantitative SHG imaging revealed striking structural shifts in keratoconic corneas compared to healthy controls. Most notably, we observed a significant rise in normalized striation Endpoints (P = 0.008) and End/Branch ratio (P = 0.015), consistent with the presence of wavy lamellae and the loss of the plywood-like structure found in control corneas. Importantly, these structural markers remained consistent in both the cone and outside the cone regions. Quantitative processing of collagen orientation revealed a single widely-spread orientation as opposed to two main orthogonal orientations in control corneas, and slight shifts in orientation between superimposed lamellae rather than the abrupt changes observed in control corneas.

We provide new quantitative data on the structural alterations associated with keratoconus that could serve as useful biomarkers for earlier detection. Our findings are consistent with a decreased stromal stiffness and an increase in the corneal birefringence. They are also in favor of a global stromal disorder rather than a purely localized disorder.