Abstract
Purpose :
The detection of subclinical keratoconus remains a challenging task. We propose to use an ultrasonic wave-based Optical Coherence Elastography (OCE) system to map the elasticity of healthy and keratoconic corneas. We hypothesize that both advanced and subclinical keratoconus corneas can be located in the cornea and identified as outliers from our baseline biomarkers.
Methods :
An ultrasonic air-coupled transducer co-focused with a swept-source optical coherence tomography system was used to generate quasi-harmonic mechanical perturbation at the corneal apex. Lamb wave speed and thickness maps of the cornea were estimated in a control group (CG) of 30 healthy subjects (n=60 age:20-50 yo; corneal astigmatism<2D) to define baseline metrics of normal biomechanics in two biomarkers: spatial anisotropy of wave speed (SAWS) calculated from the meridionally dependent wave speed map; and the Speed-Thickness Index (STI), defined as the speed deviation of each corneal semi-meridian compared to CG (STI > 0 m/s: normal or stiffer, STI < 0 m/s: abnormal or softer). STI & SAWS were estimated on 15 patients with a clinical diagnosis of keratoconus (KC) in one eye (n=15), and subclinical keratoconus (SK) in the fellow eye (n=15). A 90% confidence level was used as a baseline metric to separate normal (stiffer) from abnormal (softer) corneal elasticity.
Results :
SAWS was statistically significantly higher in KC (0.337, p<0.001) & SK (0.274, p < 0.001) corneas compared to CG (0.207±0.026). Semi-meridional speed and thickness were positively correlated (RMSE=0.738, p<0.001) in the CG corneas in a speed-thickness map (Fig.1a). We found abnormal elasticity (STI < 0) in an average of 10 out of 16 meridians in KC (average STI=-1.22), and in 5 out of 16 meridians in SK corneas (STI=-0.375) compared to CG (average STI=-0.006). STI maps in KC localized abnormally softer regions consistent with cone location (Fig. 1b).
Conclusions :
We have shown marked biomechanical differences in parameters (SAWS & STI) obtained from ultrasound-excited OCE between control, subclinical, and advanced-stage keratoconus, suggesting those as potential biomarkers to identify “at-risk” corneas before changes in topography and pachymetry become evident.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.