Functional assessment of the aqueous humour distal outflow pathways in bovine eyes using time-of-flight magnetic resonance tomography.

The major part of the aqueous humor leaves the eye through the "conventional outflow pathway", consisting of the trabecular meshwork, Schlemm's canal, collector channels, an intrasceral plexus and the episcleral veins. While the trabecular meshwork is well characterized, little is known about anatomical and functional features of the peripheral outflow tract beyond Schlemm's canal. The emergence of minimally-invasive glaucoma surgery directly targeting the outflow resistance in the trabecular meshwork has elicited growing interest in these structures. We used time-of-flight magnetic resonance imaging in ex vivo bovine eyes to map fluid flow under physiological conditions. We were able to identify the peripheral outflow vessels solely by the signal detected from the fluid flow inside their lumina. A question of clinical relevance is, whether localized opening of the trabecular meshwork leads to only localized or to a 360° increase in intrascleral flow. To address this, a goniotomy ab interno was performed in 3 eyes and the flow signal intensity was quantified sectorially. A significant increase in fluid flow was observed in the sector distal to the goniotomy (p = 0.0005) but not in the other sectors (p = 0.1). As a proof of concept we demonstrated that TOF-MRI based detection of flow in the peripheral aqueous outflow tract is feasible. The functional link observed between trabecular meshwork sectors and their distal outflow tract sectors may be relevant for minimally-invasive glaucoma surgery in humans.