A non-invasive look at trabecular meshwork

Take-Home:

An investigational non-invasive imaging tool, the phase-sensitive OCT can be used to detect and measure movement of the trabecular meshwork in vivo.

Aqueous outflow from Schlemm's canal (SC) is pulsatile, being dependent on trabecular meshwork (TM) motion with optical coherence tomography measurements indicating the TM motion is adequate to account for all of aqueous outflow. (A-P is anterior-posterior SC length. (Figure courtesy of Murray Johnstone, MD)

Seattle-Phase-sensitive OCT (PhS-OCT) for in vivo evaluation is a promising tool for advancing glaucoma research and patient care, according to findings from a human study presented by Murray Johnstone, MD.

The investigational device-developed by Ruikang Wang, PhD, professor of bioengineering, University of Washington, Seattle-has resolution at the nanometer scale and moves OCT technology, which is limited to structural imaging, to a new realm involving characterization of motion and function.

PhS-OCT was highly sensitive for detecting trabecular meshwork motion according to results of an initial laboratory study involving enucleated primate eyes. The study also showed the tissue movement correlated to a simulated cardiac pulse with amplitude trabecular motion sufficient to account for aqueous outflow.

Examining the human study

A second study using PhS-OCT was then conducted in 10 human adults to investigate pulse-induced trabecular meshwork motion, said Dr. Johnstone, clinical professor of ophthalmology, University of Washington.

As in the animal study, PhS-OCT was highly sensitive for detecting trabecular meshwork motion.

Analyses of the trabecular meshwork tissue motion wave, in relation to the digital pulsimetry wave, showed a high correlation between trabecular meshwork wave minima and digital pulse peaks. The research also showed PhS-OCT could be used to measure velocity of the trabecular meshwork motion and the strain rate.

“This technology opens a new window into understanding abnormalities of trabecular meshwork biomechanics leading to glaucoma,” Dr. Johnstone said. “I believe it will eventually revolutionize our approaches to glaucoma management.”

Evaluating need for trabecular meshwork

Dr. Johnstone

Interest in developing technology for evaluating trabecular meshwork motion derives from revised concepts about aqueous outflow and resistance.

Whereas initial work by Morton Grant, MD, posited that the trabecular meshwork acted as a rigid, restrictive filter creating resistance, additional research demonstrated that the tissue was highly compliant. This indicated that movement of the trabecular meshwork becomes appositional with the external wall of Schlemm's canal and represented the most important source of resistance.

Findings from histological studies provided support for this concept, but direct evidence of trabecular meshwork motion in vivo was lacking until the development of PhS-OCT.

“Color velocity maps obtained with this technology show the trabecular meshwork in motion, changing configuration as it moves outward toward the external wall of Schlemm’s canal during systole, then rebounds toward the anterior chamber during diastole,” Dr. Johnstone said.

By providing information about issue motion, he said, PhS-OCT provides an objective measurement of trabecular meshwork tissue elasticity and compliance, and thus its ability to control IOP.

“Just as HbA1c measurements provide a better indication of diabetes control than random blood glucose testing,” he said, “measuring the biomechanical properties of the trabecular meshwork to determine the function of the outflow system may prove to be a better assessment tool than IOP for evaluating eyes with glaucoma and their response to treatment.”

Possible clinical applications

PhS-OCT may also be used to guide medical and surgical treatment decisions, Dr. Johnstone said.

Ocular hypotensive medications that increase pulsatile aqueous outflow (i.e., miotics, adrenergics, prostaglandin analogues) exert their effects on trabecular meshwork motion and occur very quickly, he said. Thus, in-office PhS-OCT might be used to determine if a patient will be a responder following test dose administration.

The ability to characterize trabecular meshwork motion at different sites could also be useful for guiding optimal placement of stent devices used in microinvasive glaucoma surgical procedures.

The technology, Dr. Johnstone said, might be applied as a non-invasive method to determine whether patients are good candidates for canaloplasty.

Canaloplasty pioneer, Robert Stegmann, MD, has shown that response after canaloplasty can be predicted by the ability of intraoperative provocative gonioscopy to cause Schlemm’s canal blood reflux-a surrogate for trabecular meshwork motion, Dr. Johnstone said.

“PhS-OCT might allow screening of canaloplasty candidates without going to the OR, and it might also allow us to better select candidates for laser trabeculoplasty or ab interno trabeculotomy,” he said.

Murray Johnstone, MD

E: johnstone.murray@gmail.com

Dr. Johnstone has financial interests with Alcon Laboratories, Allergan, Cascade Ophthalmics, Healionics, Sensimed, and the University of Washington Center for Commercialization.

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