Acute Effects of Intraocular Pressure-Induced Changes in Schlemm's Canal Morphology on Outflow Facility in Healthy Human Eyes

Abstract

Purpose To estimate the outflow facility coefficient (C) as a function of Schlemm's canal cross-sectional area (SCAR) in healthy subjects using noninvasive oculopression tonometry (OPT). Methods In 25 healthy volunteers, intraocular pressure (IOP) decay values were recorded by a ophthalmodynamometer, with a fixed external force (0.15 N) on the inferior-temporal eyelid, every 10 seconds, for four minutes, and again after a 30-minute rest. Schlemm's canal profile images and IOP were obtained pre-procedurally (baseline), immediately (T0), and at 1-minute intervals post-procedurally (T1, T2, T3, and T4). C was calculated for different IOPs. The SCAR, coronal, and the meridional diameter of Schlemm's canal were calculated. Results Mean C 0 for the maximum IOP was 0.020 ± 0.017 µL/min/mm Hg; mean C was 0.018 ± 0.0071 and 0.058 ± 0.0146 µL/min/mm Hg at 40 and 20 mm Hg, respectively. C was nonlinearly dependent on the IOP ( R 2 = 0.945). The SCAR was 5440 ± 3140.82, 3947.6 ± 2246.8, and 5375.7 ± 2662.7 µm 2 at baseline, T0, and T4, respectively. The coronal diameter of SC decreased significantly from the baseline (33.02 ± 11.3 µm) to T0 (26.6 ± 9.37 µm) and recovered at T4 (32.3 ± 9.53 µm). The SCAR and IOP correlated significantly throughout ( R 2 = 0.9944; P < 0.001). C0 significantly correlated with the SCAR at baseline and with changes in the SCAR and IOP from T0 to T4. Conclusions Schlemm's canal dimensions are responsible for the IOP-dependent mechanical forces, and these changes appear to directly affect outflow facility.

Morphological changes in the trabecular meshwork and Schlemm’s canal after treatment with topical intraocular pressure-lowering agents

“…While the SC area remained stable in the placebo group, the SC lumen expanded obviously, and the IOP decreased in the travoprost-treated group; the expansion effect was maintained up to 60 h after administration. However, this expansion was reversed to the baseline from 72 h. In line with this finding, it has been reported that IOP reduction leads to SC area expansion and vice versa 20 , 24 , 25 . Based on microsurgical and perfusion study, Johnstone and Grant 24 reported that the diameter of the SC lumen was IOP-dependent and reached a minimum at high pressures but was enlarged under low pressure.…”

“…Based on microsurgical and perfusion study, Johnstone and Grant 24 reported that the diameter of the SC lumen was IOP-dependent and reached a minimum at high pressures but was enlarged under low pressure. In another study, Chen and colleagues 25 showed the link between the collapse of SC and decrease in outflow facility with acute IOP increase. Further, Hong et al 14 reported that the degree of SC expansion was related to the extent of IOP decrease after trabeculectomy in patients with glaucoma.…”

“…With advances in imaging technology, recent studies have utilized OCT to investigate the association between the microstructure of the outflow pathway and glaucoma status 10 , 22 or IOP level 20 , 24 , 25 . Hong et al 10 showed that the SC area in patients with POAG was significantly smaller than that in normal subjects, and there was a significant relationship between the IOP and SC area.…”

Intraocular pressure variation from ocular compression in low and high myopia

“…During the course of this project, Chen et al 12 reported a decline in IOP during a 4-min ocular compression. The dimensions of the Schlemm's canal were monitored using SD-OCT.…”

“…Older adults, particularly POAG patients, should be recruited in future study. Measurement of the Schlemm's canal was not included and the change of Schlemm's canal dimension and IOP variation was based on the previous study by Chen et al 12 Investigators using ocular compression to study LC shape should measure IOP at both the initial and nal stages of ocular compression.…”