6 Pulsatile Aqueous Outflow Observations to Guide Glaucoma Surgery Acqueous outflow is pulsatile, and clinicians can see the pulsations. This simple and well-accepted insight1,2 provides clinicians with a wealth of information to use in surgical management.3–5 This information includes direct surveillance of mechanisms controlling aqueous outflow,6 outflow abnormalities in glaucoma,3,7 surgical technique decisions,8 and the effects of outflow surgery.9 Powerful clinical tools to take advantage of these insights are within immediate reach of every surgeon, and include a slit lamp, careful observation, and patience.4,5 Pulsatile aqueous outflow provides important clues to the functional properties of the outflow system, and its health along the entire outflow pathway from the trabecular meshwork (TM) to the episcleral veins (Fig. 6.1). Pulsatile aqueous outflow synchronous with the ocular pulse10,11 originates in Schlemm’s canal (SC; Fig. 6.1a).5 Such pulsatility has clearly defined requirements12,13: (1) a reservoir or chamber represented by SC; (2) deforming tissue represented by the TM to change reservoir dimensions; and (3) oscillatory compressive forces represented by the ocular pulse (Fig. 6.1a), blinking, and eye movement, each creating a pulse that deforms the trabecular tissue. The clinical signs of pulsatile outflow indicate that the TM has sufficient elasticity and compliance to deform14–16 and to change the SC reservoir dimensions17; the collector channels are able to open18; the intrascleral channels are patent; and both the anterior chamber (AC) and SC pressures are in a delicately poised equilibrium with the episcleral venous pressure (EVP). In glaucoma patients, pulsatile outflow is harder to see, and in advanced glaucoma it may be absent, likely due to loss of trabecular tissue elasticity.7 This loss is associated with pathologically reduced outflow. In this scenario, the intraocular pressure (IOP) becomes more variable as the TM responds less and less effectively to the hemodynamic forces driving pulsatile outflow. Normally, pulsatile outflow is segmental, seen in some regions of the ocular circumference but not in others. The location of segmental pulsatile flow remains unchanged for an individual, probably for a lifetime.3 In eyes with glaucoma, retention of pulsatile features in these regions suggests function at these aqueous vein locations is only partially compromised and it is possible that these pulsatile segments lend themselves to being exploited by minimally invasive trabecular bypass surgery. In contrast, absence of pulsatile flow in these locations suggests minimally invasive glaucoma surgery (MIGS) may be less likely to succeed. The distribution of aqueous veins is highly asymmetric, with 87% seen in the inferior quadrants, of which 58% lie in the inferior nasal quadrant at or below the midline (Fig. 6.2).4,19 Typically only one to three aqueous veins are present in an eye, a finding that should not be surprising because the average aqueous vein has a flow volume of ~ 1 µL/min. The volume of flow per aqueous vein may explain why just two aqueous veins are needed to cope with trabecular outflow.5,20 Aqueous veins at times carry primarily aqueous humor (1–3 in Fig. 6.1b) making them almost transparent; at other times aqueous veins carry primarily blood (4,5 in Fig. 6.1b), making them look red (Fig. 6.3); either condition can make the aqueous veins difficult to recognize.4 A great aid in determining the presence and distribution of aqueous veins is to apply very gentle pressure through the lower lid to transiently and slightly raise the IOP. Often, a bolus of aqueous will then be seen entering a vessel previously containing solely blood (4,5 in Fig. 6.1b and Fig. 6.3). In a quick dynamic response, blood rapidly refills the aqueous vein within seconds. The rapid transition from blood-filled to an aqueous-filled lumen and back again to a blood-filled lumen signals that the vessel is an aqueous vein. In a vessel containing primarily aqueous humor (1–3 in Fig. 6.1b), the previously mentioned exertion of gentle pressure forces a little aqueous humor through the TM, into the venous collector system, and out of the eye, reducing the IOP transiently. When IOP falls below the homeostatic set point,7 aqueous humor no longer flows into the aqueous vein; instead blood fills the previously clear aqueous vein. Within a few seconds, however, the IOP rises again to the homeostatic set point, restoring a pressure gradient and driving aqueous outflow into the aqueous veins. Aqueous humor then enters and displaces blood in the aqueous vein. Once the aqueous veins are identified (Fig. 6.3), the many manifestations of pulsatile aqueous outflow are readily observed, including intermittent boluses of blood from tributary veins, pulsatile laminar flow, and trilaminar flow (Figs. 6.1 and 6.3). Clues from observing outflow clinically may have predictive value that can be used in the operating room to position minimally invasive bypass surgical devices at segments in which the distal outflow system exhibits relatively physiological behavior.8 It is postulated that enhancing outflow in these segments makes trabecular bypass procedures more effective. Observing fluid enter the aqueous veins by pressuring the eye right after inserting a trabecular bypass device ensures that a direct connection with the venous system has been established9; observing late postoperative aqueous outflow into an aqueous vein in the same area provides assurance that the procedure has had an intended and enduring effect.
Pulsatile Flow Origin and Implications
How Do We Find Aqueous Veins?
Surgical Value
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