Phase-based OCT

Glaucoma, a leading cause of irreversible blindness, results from an aqueous outflow abnormality. Abnormal intraocular pressure in glaucoma results from changes in the mechanical properties of the trabecular meshwork, a tissue that controls both aqueous outflow and intraocular pressure (IOP). No technology has previously been available to assess the properties of the trabecular tissues that control aqueous outflow.

At University of Washington (UW), Murray Johnstone, a glaucoma specialist and clinical professor with the Eye Institute, took the problem to SPIE Fellow Ruikang Wang, a professor in the Department of Bioengineering. Working with a team of researchers, Johnstone and Wang developed a new conceptual framework based on examining the way the tissue moves, a motion-based optical coherence tomography (OCT) to monitor dynamic tissue movement with sensitivity at the sub-nanometer scale.

As recently reported in the Journal of Biomedical Optics, the group developed a phase-sensitive OCT (PhS-OCT) specifically tailored to measure tiny trabecular meshwork movements in response to pulsatile changes of IOP.

The group demonstrated the feasibility of the PhS-OCT in ex vivo primate eyes subjected to experimentally induced pulsatile pressure-gradient changes (3mm Hg). The pulse amplitudes and frequency mimicked typical in vivo intraocular pressure oscillations resulting from cardiac-pulse-induced changes of intraocular vascular volume.

The UW researchers found that the detected trabecular meshwork movement was highly synchronous with the ocular pulse, experiencing tissue displacement amplitudes of ≤4µm.

Aqueous humor flows through the trabecular meshwork into a venous sinus called Schlemm’s canal. The PhS-OCT was able to measure synchronous pulse-dependent trabecular meshwork velocities, displacement, and strain rate (with ~20 nm sensitivity) while simultaneously using spectral domain OCT to measure Schlemm’s canal volume changes. Analysis of the data indicates that pulse-induced trabecular meshwork excursions into the canal are enough to account for all of aqueous outflow.

Dynamic motion of the trabecular meshwork has previously been unrecognized. This new non-invasive technology provides both quantitative measurements of movement and insights into trabecular tissue biomechanics.

Clinically, this new ability to detect outflow system properties holds the promise of permitting both earlier abnormality detection and recognition of progressive outflow system malfunction that reduces its ability to maintain IOP homeostasis. Such information should permit better-informed and timelier decisions in glaucoma management.

With the technology, Johnstone finds that it is possible to watch the internal motion of the meshwork as the pressure-induced wave propagates outward to the external wall of Schlemm’s canal. Johnstone sees this new technology as a breakthrough in glaucoma management which is currently limited to treating IOP and based on remarkably limited knowledge of the IOP profile over time in individual patients.

“We measure intraocular pressure maybe three to four times a year, each time for about 3 seconds,” he says. “That’s about 12 seconds per year, and we miss out on what’s going on in the other 31 million seconds.”

In addition, Johnstone points out there are issues with how the pressure is measured with the eye looking straight ahead with no movements (the normal frequent eye movements and blinking increase IOP ~30% from baseline).

We also measure IOP during the daytime, whereas pressures often rise 20%-30% at night. Current IOP measurements do not capture these diurnal fluctuations, which are important in determining the real IOP profile and also whether medication is working.

With this new technology, practitioners can measure the mechanical properties of the meshwork and see if movement abnormalities are reducing the trabecular meshwork ability to maintain the IOP profile within a normal range.

“Also, if we see reduced trabecular movement over time, we’re in a position to say this person is getting into further trouble with maintaining pressures within a narrow range. We have a new potentially sensitive predictive tool to help decide if preemptive IOP management is appropriate before the patient goes on to further structural and functional damage to the visual system,” Johnstone says.

In addition to Johnstone and Wang, coauthors of “Phase-sensitive optical coherence tomography characterization of pulse-induced trabecular meshwork displacement in ex vivo nonhuman primate eyes” include SPIE member Zhongwei Zhi, Peng Li, Roberto Reif, Tueng Shen, and Elizabeth Martin.

Source: Journal of Biomedical Optics17(7), 076026 (2012); doi:10.1117/1.JBO.17.7.076026.

– Lihong Wang is editor of the Journal of Biomedical Optics.

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