Schlemm Canal (SC) Response to Elevated Intraocular Pressure (IOP) Varies with Morphologic Phenotype

Abstract

Purpose : Lower trabecular meshwork (TM) stiffness is associated with increased outflow facility. The TM is softer deep within the angle, near SC at the location of active outflow. We have shown that IOP elevation creates observable compression of the TM into SC, providing a technique to examine TM stiffness within living eyes. Presented in a 2 dimensional projection, two distinct SC (Figure 1) morphometric phenotypes become evident: 1- wide contiguous channels devoid of branches to distal vascular pathways (Figure 2, green arrows), and 2- multi-segmental networks, often continuous with distal vasculature (Figure 2, yellow arrows). We hypothesized that the TM is softer adjacent to high densities of aqueous vessels including and distal to SC, presumably areas of active outflow. The purpose of this study was to compare SC channel and network response to acute IOP elevation.

Methods : The temporal limbus of 27 eyes of 14 healthy subjects (10 male, 4 female, age 36 ± 13 years) was imaged by optical coherence tomography (OCT, Cirrus, Zeiss, Dublin, CA) at baseline and during IOP elevation (ophthalmodynamometer applying 30g force). IOP was measured at baseline during IOP elevation. Corresponding channel and network structures (Figure 2, green and yellow arrows respectively) were identified in 2D vascular en-face projections of the outflow pathway and diameters measured in FIJI. The change in diameter in channels and networks was compared by analysis of variance, and correlations with the change in IOP and between each other quantified by Pearson’s correlation coefficient.

Results : IOP elevation (12.7 ± 2.4 to 36.4 ± 5.3mmHg) statistically significantly reduced the diameter of SC networks more than that of SC channels (-14% ± 11% versus -5% ± 12%; p = 0.0078). There was no significant correlation between change in IOP and change in channel or network diameter, nor was there a correlation between the change in channel and network diameters within eyes.

Conclusions : Larger deformations in regions of dense vascular networks contiguous with the distal outflow system suggest that the TM may be observably softer in regions of greater outflow. This finding in living human eyes is consistent with previous findings in in-vitro models and rat eyes. Acute IOP elevation creates greater deformations of SC in regions of dense vascular networks as compared to wide SC channels spatially isolated from distal outflow pathways.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.