Changes in spectral parameters of corneal pulse following canaloplasty

Heartbeat-Induced Corneal Axial Displacement and Strain Measured by High Frequency Ultrasound Elastography in Human Volunteers

Purpose

The purpose of this study was to establish in vivo data acquisition and processing protocols for repeatable measurements of heartbeat-induced corneal displacements and strains in human eyes, using a high-frequency ultrasound elastography method, termed ocular pulse elastography (OPE).

Methods

Twenty-four volunteers with no known ocular diseases were recruited for this study. Intraocular pressure (IOP) and ocular pulse amplitude (OPA) were measured using a PASCAL Dynamic Contour Tonometer (DCT). An in vivo OPE protocol was developed to measure heartbeat-induced corneal displacements. Videos of the central 5.7 mm of the cornea were acquired using a 50-MHz ultrasound probe at 128 frames per second. The radiofrequency data of 1000 frames were analyzed using an ultrasound speckle tracking algorithm to calculate corneal displacements and quantify spectral and temporal characteristics. The intrasession and intersession repeatability of OPE- and DCT-measured parameters were also analyzed.

Results

The in vivo OPE protocol and setup were successful in tracking heartbeat-induced corneal motion using high-frequency ultrasound. Corneal axial displacements showed a strong cardiac rhythm, with good intrasession and intersession repeatability, and high interocular symmetry. Corneal strain was calculated in two eyes of two subjects, showing substantially different responses.

Conclusions

We demonstrated the feasibility of high-frequency ultrasound elastography for noninvasive in vivo measurement of the cornea's biomechanical responses to the intrinsic ocular pulse. The high intrasession and intersession repeatability suggested a robust implementation of this technique to the in vivo setting.

Translational Relevance

OPE may offer a useful tool for clinical biomechanical evaluation of the cornea by quantifying its response to the intrinsic pulsation.

The effect of changes in cardiovascular activity on corneal biomechanics and pulsation in rabbits

The aim was to assess the relationships between cardiovascular activity, corneal pulse characteristics, and corneal biomechanics in rabbits. Seventeen rabbits were randomly assigned to one of two anesthetic regimens to induce differences in arterial blood pressure and heart rate. Experimental protocol included measuring blood flow parameters in the ophthalmic artery by color Doppler imaging, corneal biomechanical parameters using a non-contact tonometer Corvis ST, and the corneal pulse (CP) signal using a non-contact ultrasonic technique. Statistically significantly lower mean values of normalized amplitudes of higher CP harmonics and changes in eight of the twelve corneal biomechanical parameters were observed in the rabbit group with lower arterial blood pressure and higher heart rate, intraocular pressure, and resistive index. The results of partial correlations showed that the CP signal energy and amplitude of its first harmonic correlate with the resistive index, diastolic and mean arterial pressures, whereas no statistically significant correlation was found between any of the CP parameters and intraocular pressure. Our pilot study indicates, for the first time, that non-contact and continuous measuring of corneal pulse allows indirectly assessing changes in cardiovascular activity when the confounding effect of intraocular pressure is eliminated.

Using the entropy of the corneal pulse signal to distinguish healthy eyes from eyes affected by primary open-angle glaucoma

OBJECTIVE

The purpose of this study was to evaluate whether the complexity of the corneal pulse (CP) signal can be used to differentiate patients with primary open-angle glaucoma (POAG) from healthy subjects.

APPROACH

The study sample consisted of 28 patients with POAG and a control, age-matched group of 30 subjects. After standard ophthalmic examination, the CP signal from a randomly selected eye of each participant was measured using non-contact ultrasonic micro-displacement measurement technology. After pre-processing, the complexity of the CP signal was estimated using refined composite multiscale fuzzy entropy (RCMFE) up to scale factor 50. The average RCMFE values were computed from three repeated measurements of the CP signals for each participant and each scale factor.

MAIN RESULTS

The complexity of the CP signal in glaucomatous eyes was higher than that observed in healthy ones. Also, RCMFE of the CP signal was found to differentiate (statistically significantly) between the two groups for scales in the range from 26 to 43. For these scales, the one for which the lowest p-value (t-test, p = 0.017) was obtained when comparing RCMFE between the two groups was selected as the optimal scale. Next, a receiver operating characteristic analysis for the optimal scale showed that the proposed approach of calculating the multiscale entropy of the CP signal has some potential to discriminate between patients with POAG and healthy controls (sensitivity, specificity and accuracy of 0.643, 0.700 and 0.672, respectively).

SIGNIFICANCE

In conclusion, RCMFE, as a complexity measure, may be considered an auxiliary indicator to support glaucoma diagnostics.