Process of aqueous outflow uses unifying concept

Orlando, FL-A new paradigm of aqueous outflow positing that it is a dynamic, pump-like process regulated through pulsatile mechanisms provides a conceptual framework unifying structure and function throughout the entire trabecular meshwork in both health and disease, said Murray A. Johnstone, MD, in his delivery of the Robert N. Shaffer Lecture at the annual meeting of the American Academy of Ophthalmology.

He also told attendees that research undertaken to explore this new model of aqueous outflow has led to the identification of optical coherence tomography (OCT) imaging as a potential clinical tool for evaluating trabecular meshwork function and a guide to glaucoma management.

"Historically, aqueous outflow has been thought to be a passive phenomenon based on a traditional model suggesting resistance is in the extracellular matrix (ECM) of the juxtacanalicular space, pressure dissipates in the ECM material, and flow is through low-resistance pores in the endothelium in Schlemm's canal," said Dr. Johnstone, clinical professor of ophthalmology, University of Washington, Seattle.

Dynamic model

"We were surprised by these findings and the exquisite sensitivity of the trabecular meshwork tissues to moderate changes in IOP," Dr. Johnstone said. "The compliance demonstrated in these first experiments was confirmed in a number of subsequent studies, and has never been challenged or refuted.

" In vivo studies in living primates also yielded surprising results. When IOP was decreased below a physiologic level, tubes extending across Schlemm's canal became apparent. The tubes collapsed when IOP was below episcleral venous pressure, and appeared to act as functional valves.

"The combination of these findings demonstrating the compliance of the trabecular tissueand the identification of valve-like structures within Schlemm's canal led me to think about a pump-like mechanism of aqueous outflow, and I have pursued that concept since," Dr.Johnstone said.

Dr. Johnstone explained that when the left ventricle contracts and drives blood into the choroid, IOP increases in response to the change in choroidal volume and the pressure waves drive the trabecular meshwork outward. During diastole, IOP and choroidal pressure drop, the pressure gradient drops, and the meshwork, if it has sufficient elasticity, recoils and aqueous can enter Schlemm's canal.

With the next ventricular contraction, aqueous is driven out of Schlemm's canal into the episcleral veins. Pressure in the aqueous veins remains in a delicate equilibrium with pressure in the episcleral veins, causing aqueous discharge to the episcleral veins during each systolic wave followed by lack of discharge during the diastolic wave.