OCT provides minute details of aqueous outflow pathways, motion

Take-home message: Schlemm's canal and collector channels respond rapidly to changes in pressure gradients.

 Reviewed by Murray A. Johnstone, MD

Seattle-A new optical coherence tomography (OCT) platform goes where others have not gone before, said Murray A. Johnstone, MD.

Analysis of minute tissue structures in the area of Schlemm’s canal and the collector channels shows incredibly rapid responses to changes in pressure gradients, said Dr. Johnstone, clinical professor of ophthalmology, University of Washington, Seattle.

The instrument showed that pressure-dependent opening and closing of collector channels occurs and may be relevant in distal resistance in micro invasive glaucoma surgery.

This became apparent when Murray A. Johnstone, MD, and colleagues evaluated a radial limbal segment facing upward in a Petri dish.

This positioning facilitates the beam having to pass through only the trabecular meshwork and thus effectively image Schlemm’s canal and the collector channels, Dr. Johnstone explained.

“Using radial scans through the limbal region, we can maintain a steady-state gradient by attaching a cannula in Schlemm’s canal to a reservoir and rapidly switch between reservoirs to watch the responses of the tissues to changes in the pressure gradients,” he said.

With OCT scanning through the trabecular meshwork with a cannula in Schlemm’s canal at the distal end, Dr. Johnstone showed that at 10 mm Hg Schlemm’s canal was widely dilated.

“We could actually see the entrances to the collector channels and hinged collagen flaps at each of the entrances to the collector channels,” he said.

To skip ahead to watch video describing how collector channels open and close, click here.

Amira 3D software

Investigators used Amira 3D software (FEI) to look at the X, Y, and Z axes. The software allowed them to scan through the axes and tilt and turn on the axes to look at any plane. By so doing, Dr. Johnstone investigated the details of the various structures and identified the hinged flaps that are free to move because of hinges present on one end.

What was especially noteworthy, he pointed out, was the presence of attachments between the hinged flaps and the trabecular meshwork.

“If the trabecular meshwork moves, these connections cause the hinged flaps at the entrance to the collector channels to also move,” he said. “This was a consistent finding.”

Investigators looked at the behavior of the various tissues at various pressure gradients of 0, 5, 10, 20, 30, and 50 mm Hg. At 0 mm Hg, the region of the collector channels and Schlemm’s canal was only a “potential space,” he said.

With increases in pressure, Dr. Johnstone demonstrated that progressive increases in the dimensions of the tissues became visible. The area of the collector channels and Schlemm’s canal became very large with the higher pressures.

Both OCT and scanning electron microscopy (SEM) clearly showed the series of tissue strands that connect the trabecular meshwork and the hinged flaps. He explained that with high magnification of the tissues, the aqueous pathway at the collector channel entrances is convoluted and constantly changing shape with changes in the pressure gradients.

Marked symmetry

When they quantified the movements of Schlemm’s canal, collector channels, and tissue strands over time, Dr. Johnstone noted there was marked symmetry and synchrony of the various motions. The canal shape changes occurred in 100 ms and those of the collector channels in 300 ms, indicating that these structures are capable of very rapid responses.

Dr. Johnstone explained that the opening and closing of the collector channels may act as flow-controlling resistance sites in normal eyes and sites of abnormal resistance in glaucomatous eyes. These findings may also explain the distal resistance in microinvasive glaucoma surgery.

“Both the OCT and SEM images give me the privilege of stepping into a new world inaccessible to mankind for all these millennia,” Dr. Johnstone said. “The voyage is particularly exciting because somewhere within that wondrous world lies a solution to the pressure problem in glaucoma.”

Murray A. Johnstone, MD

E: johnstone.murray@gmail.com

This article was adapted from Dr. Johnstone’s presentation at the 2015 meeting of the American Academy of Ophthalmology. Dr. Johnstone has no proprietary interest in any aspect of this report.