Tip of the Month
Publishing date: November 2020
The intraocular pressure lowering effect of procedures that attempt to enhance flow of aqueous into Schlemm’s canal by bypassing or cutting the trabecular meshwork is limited by episcleral venous pressure and the resistance of the distal outflow pathways (e.g. collector channels) (1-3). While this reduces the risk of hypotony, it also limits ability to achieve low target intraocular pressures. Normal episcleral venous pressure is estimated to be between 7 and 11 mmHg (1), however the average IOP achieved with TM bypass procedures is higher, typically in the mid to high teens (4). Patients with glaucoma due to Sturge-Weber syndrome have been reported to have a mean episcleral venous pressure of almost 21 mmHg (5).
Trabecular meshwork bypass procedures are contraindicated in patients with high episcleral venous pressure due to a higher risk of bleeding and low probability of success.
Contributor: Andrew Tatham, Princess Alexandra Eye Pavilion, Edinburgh, UK.
1. Sit AJ, McLaren JW. Measurement of episcleral venous pressure. Exp Eye Res 2011;93(3):291-8.
2. Cameron T, van der Merwe, Fellman RL, Johnstone M, Bhattacharya SK. Aqueous outflow – a continuum from trabecular meshwork to episcleral veins. Prog Retin Eye Res 2017;57:108-133.
3. Lusthaus JA, Meyer PAR, Khatib TZ, Martin KR. The effects of trabecular bypass surgery on conventional aqueous outflow, visualized by hemoglobin video imaging. J Glaucoma 2020;29(8):656-665.
4. Samuelson TW, Sarkisian SR Jr, Lubeck DM, Stiles MC, et al. Prospective, randomized, controlled pivotal trial of an ab interno implanted trabecular micro bypass in primary open angle glaucoma and cataract: two year results. Ophthalmology 2019; 126 (6):811-821.
5. Shiau T, Armogan N, Yan DB, Thomson HG, Levin AV. The role of episcleral venous pressure in glaucoma associated with Sturge-Weber syndrome. J AAPOS 2012;16:61-4.
Tip Reviewer: Roger Hitchings
Tip Editors: Frances Meier-Gibbons, Humma Shahid, Karl Mercieca, Francisco Goni