Structural Characteristics of Epithelium and Bowman's Layer in Keratoconic Cornea Evaluated Using Ultrahigh Resolution OCT

Abstract

Purpose: : To investigate optical thickness (i.e. refractive index x physical thickness) and structural nonuniformity of the epithelium (EL) and Bowman's layer (BL) in keratoconic (KC) cornea non-invasively using ultrahigh resolution spectral domain optical coherence tomography (OCT).

Methods: : An ultrahigh resolution OCT with axial resolution of 1.5 µm in corneal tissue was developed and used to image 10 normal and 10 KC eyes in-vivo. Four millimeter-wide vertical OCT scans were centered at the apex of the cornea. EL and BL were segmented manually by the same observer to obtain the optical thickness profile. Manual segmentation was carried out 5 times by the same observer on a normal and KC image to confirm measurement repeatability. Structural nonuniformity of the two layers was quantified based on the standard deviation of backscattered light distribution in each layer.

Results: : The ultrahigh resolution OCT was capable of visualizing EL and BL in both groups. Average optical thickness of EL was 65.5±3.7 µm and 61.1±4.4 µm in normal and KC eyes, respectively. Average optical thickness of BL in the normal and KC eyes was 20.2±3.7 µm and 16.8±2.8 µm, respectively. The decrease in mean average optical thickness of both layers in KC group was statistically significant (p<0.05). Standard deviation of the optical thickness profile across the central 4 mm cornea was on average 1.9±0.6 µm and 6.0±4.5 µm for EL and 2.1±0.5 µm and 2.9±1.0 µm for BL in normal and KC eyes respectively. The increase in optical thickness variability for the KC group was statistically significant (p<0.05) for both layers. The difference in the structural nonuniformity between the two groups was insignificant for both layers. However, significantly increased nonuniformity in the BL was found in KC eyes with a maximum irregular astigmatic keratometry (K) value higher than 52D. One KC subject who had severe keratoconus (K=63D) showed substantially increased amount of nonuniformity in EL.

Conclusions: : Significant changes in the EL and BL were observed for KC cornea. The individual corneal layers in normal and KC subjects can be quantified using an ultrahigh resolution OCT system enhancing our ability to characterize structural changes in keratoconic corneal layers.

Keywords: keratoconus • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • cornea: epithelium