Aqueous outflow regulation: Optical coherence tomography implicates pressure-dependent tissue motion

Abstract

Glaucoma is a leading cause of blindness worldwide and results from damage to the optic nerve. Currently, intraocular pressure is the only treatable risk factor. Changes in aqueous outflow regulate pressure; regulation becomes abnormal in glaucoma. From inside the eye aqueous flows out through the trabecular meshwork into a venous sinus called Schlemm's canal, next into collector channels and finally returns to the episcleral vessels of the venous system. The location of aqueous outflow regulation is unknown. Ex vivo and in vivo studies implicate both pressure-dependent trabecular tissue motion and tissues distal to Schlemm's canal in regulation of aqueous outflow. Technologies have not previously been available to study these issues. New ex vivo imaging in human eyes identifies hinged flaps or leaflets at collector channel entrances using a high-resolution spectral domain optical coherence tomography (SD-OCT) platform. The hinged flaps open and close in synchrony with pressure-dependent trabecular meshwork motion. The SD-OCT platform images from the trabecular meshwork surface while experimentally changing transtrabecular pressure gradients. New in vivo imaging in human eyes uses a motion sensitive technology, phase-sensitive OCT to quantitate real-time pulse-dependent trabecular tissue motion as well as absence of such motion when aqueous access to the outflow system is blocked. The recent studies suggest that aqueous outflow regulation results from synchronous pressure-dependent motion involving a network of interconnected tissues including those distal to Schlemm's canal. The new imaging technologies may shed light on glaucoma mechanisms and provide guidance in the management of medical, laser and surgical decisions in glaucoma.

Aqueous outflow - A continuum from trabecular meshwork to episcleral veins

“…The appearance of the outflow system in Fig. 4 & 5 is similar to that observed with SC pressures in the 30–50 mm range during static pressure measurements (Xin et al, 2016b). Such SC pressures result from normal physiologic activities such as body inversions during yoga and gymnastics (Friberg et al, 1987; Friberg and Weinreb, 1985).…”

“…4 and 5). As a result, the motion of the aqueous outflow structures and resultant SC and CCE lumen dimension changes have been recently quantified (Xin et al, 2016b). …”

“…Cannulating SC and then connecting the cannula to reservoirs at controlled heights provides a hydrostatic pressure head that permits systemically monitoring resultant pressure changes within the canal (Xin et al, 2016b). Dilating SC to study TM motion is the same approach used during clinical gonioscopy to directly observe blood filling SC.…”

“…Such movement is particularly likely given recent evidence that the CCE and ISCC geometry and constituent properties permit free movement in response to pressure changes (de Kater et al, 1992; de Kater et al, 1990; Hariri et al, 2014; Xin et al, 2016a; Xin et al, 2016b). …”

Training strategies and outcomes of ab interno trabeculectomy with the trabectome

“…Recent discoveries indicate valve-like structures that appear to guard the collector channel orifices and collapsible aqueous veins 27,28 , calling into question the simplistic view that collector channel openings are round and unobstructed. Structural data indicates that flaps are kept in suspension through string-like attachments.…”