A new method for in vivo assessment of corneal transparency using spectral-domain OCT.

Corneal transparency is essential to provide a clear view into and out of the eye, yet clinical means to assess such transparency are extremely limited and usually involve a subjective grading of visible opacities by means of slit-lamp biomicroscopy. Here, we describe an automated algorithm allowing extraction of quantitative corneal transparency parameters with standard clinical spectral-domain optical coherence tomography (SD-OCT). Our algorithm employs a novel pre-processing procedure to standardize SD-OCT image analysis and to numerically correct common instrumental artifacts before extracting mean intensity stromal-depth (z) profiles over a 6-mm-wide corneal area. The z-profiles are analyzed using our previously developed objective method that derives quantitative transparency parameters directly related to the physics of light propagation in tissues. Tissular heterogeneity is quantified by the Birge ratio Br and the photon mean-free path (ls) is determined for homogeneous tissues (i.e., Br~1). SD-OCT images of 83 normal corneas (ages 22-50 years) from a standard SD-OCT device (RTVue-XR Avanti, Optovue Inc.) were processed to establish a normative dataset of transparency values. After confirming stromal homogeneity (Br <10), we measured a median ls of 570 µm (interdecile range: 270-2400 µm). By also considering corneal thicknesses, this may be translated into a median fraction of transmitted (coherent) light Tcoh(stroma) of 51% (interdecile range: 22-83%). Excluding images with central saturation artifact raised our median Tcoh(stroma) to 73% (interdecile range: 34-84%). These transparency values are slightly lower than those previously reported, which we attribute to the detection configuration of SD-OCT with a relatively small and selective acceptance angle. No statistically significant correlation between transparency and age or thickness was found. In conclusion, our algorithm provides robust and quantitative measurements of corneal transparency from standard SD-OCT images with sufficient quality (such as 'Line' and 'CrossLine' B-scan modes without central saturation artifact) and addresses the demand for such an objective means in the clinical setting.

Corneal stromal stem cells restore transparency after N2 injury in mice.

Corneal scarring associated with various corneal conditions is a leading cause of blindness worldwide. The present study aimed to test the hypothesis that corneal stromal stem cells have a therapeutic effect and are able to restore the extracellular matrix organization and corneal transparency in vivo. We first developed a mouse model of corneal stromal scar induced by liquid nitrogen (N2 ) application. We then reversed stromal scarring by injecting mouse or human corneal stromal stem cells in injured cornea. To characterize the mouse model developed in this study and the therapeutic effect of corneal stromal stem cells, we used a combination of in vivo (slit lamp, optical coherence tomography, in vivo confocal microscopy, optical coherence tomography shear wave elastography, and optokinetic tracking response) and ex vivo (full field optical coherence microscopy, flow cytometry, transmission electron microscopy, and histology) techniques. The mouse model obtained features early inflammation, keratocyte apoptosis, keratocyte transformation into myofibroblasts, collagen type III synthesis, impaired stromal ultrastructure, corneal stromal haze formation, increased corneal rigidity, and impaired visual acuity. Injection of stromal stem cells in N2 -injured cornea resulted in improved corneal transparency associated with corneal stromal stem cell migration and growth in the recipient stroma, absence of inflammatory response, recipient corneal epithelial cell growth, decreased collagen type III stromal content, restored stromal ultrastructure, decreased stromal haze, decreased corneal rigidity, and improved vision. Our study demonstrates the ability of corneal stromal stem cells to promote regeneration of transparent stromal tissue after corneal scarring induced by liquid nitrogen.

Quantitative measures of corneal transparency, derived from objective analysis of depth-resolved corneal images, demonstrated with full-field optical coherence tomographic microscopy.

Loss of corneal transparency, as occurs with various pathologies, infections, immune reactions, trauma, aging, and surgery, is a major cause of visual handicap worldwide. However, current means to assess corneal transparency are extremely limited and clinical and eye-bank practice usually involve a subjective and qualitative observation of opacities, sometimes with comparison against an arbitrary grading scale, by means of slit-lamp biomicroscopy. Here, we describe a novel objective optical data analysis-based method that enables quantifiable and standardized characterization of corneal transparency from depth-resolved corneal images, addressing the demand for such a means in both the laboratory and clinical ophthalmology setting. Our approach is based on a mathematical analysis of the acquired optical data with respect to the light attenuation from scattering processes in the corneal stroma. Applicable to any depth-resolved corneal imaging modality, it has been validated by means of full-field optical coherence tomographic microscopy (FF-OCT or FF-OCM). Specifically, our results on ex-vivo corneal specimens illustrate that 1) in homogeneous tissues, characterized by an exponential light attenuation with stromal depth (z), the computation of the scattering mean-free path (ls) from the rate of exponential decay allows quantification of the degree of transparency; 2) in heterogeneous tissues, identified by significant deviations from the normal exponential z -profile, a measure of exponential-decay model inadequacy (e.g., by computation of the Birge ratio) allows the estimation of severity of stromal heterogeneity, and the associated depth-dependent variations around the average ls enables precise localization of the pathology.

Stromal striae: a new insight into corneal physiology and mechanics.

We uncover the significance of a previously unappreciated structural feature in corneal stroma, important to its biomechanics. Vogt striae are a known clinical indicator of keratoconus, and consist of dark, vertical lines crossing the corneal depth. However we detected stromal striae in most corneas, not only keratoconus. We observed striae with multiple imaging modalities in 82% of 118 human corneas, with pathology-specific differences. Striae generally depart from anchor points at Descemet's membrane in the posterior stroma obliquely in a V-shape, whereas in keratoconus, striae depart vertically from posterior toward anterior stroma. Optical coherence tomography shear wave elastography showed discontinuity of rigidity, and second harmonic generation and scanning electron microscopies showed undulation of lamellae at striae locations. Striae visibility decreased beyond physiological pressure and increased beyond physiological hydration. Immunohistology revealed striae to predominantly contain collagen VI, lumican and keratocan. The role of these regions of collagen VI linking sets of lamellae may be to absorb increases in intraocular pressure and external shocks.