3 How Does Trabecular Bypass Affect the Relationship Between Schlemm’s Canal Pressure and Episcleral Venous Pressure? Aqueous humor exiting the anterior chamber through the trabecular (conventional) outflow system must pass through two primary regions of resistance: (1) the trabecular meshwork (TM) and inner wall of Schlemm’s canal; and (2) the distal outflow system consisting of collector channels, aqueous veins, and ultimately the episcleral veins. For the distal outflow system, the pressure difference between Schlemm’s canal and the episcleral veins is the driving force for fluid flow. By reducing TM resistance, trabecular bypass surgery may alter this pressure difference, with long-term consequences on surgical efficacy. A relevant point to consider is how much of the total fluid resistance is proximal to and how much is distal to Schlemm’s canal. In nonhuman primates, direct measurement of Schlemm’s canal pressure and episcleral venous pressure (EVP) suggests that only 10% of resistance is distal.1 In human cadaver eyes, Grant2,3 demonstrated that 75% of outflow resistance was proximal to Schlemm’s canal. However, as reported by Rosenquist et al,4 the perfusion pressures used by Grant in his original experiments were too high to be physiological, given that EVP in cadaver eyes is zero. A perfusion pressure of 25 mm Hg would result in a pressure drop from the anterior chamber to the episcleral veins that is much higher than normal. Assuming an intraocular pressure (IOP) of 15 mm Hg and an EVP of around 7 to 8 mm Hg, the pressure drop should be only 7 to 8 mm Hg instead of 25 mm Hg. When the more physiological perfusion pressure of 7 mm Hg was used, only 50% of total resistance was eliminated by a complete 360-degree trabeculotomy. Therefore, the pressure in Schlemm’s canal could be expected to be approximately equal to the mean of IOP and EVP in normal eyes.
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