Trabecular Meshwork Response to Pressure Elevation in the Living Human Eye

Summary

Trabecular meshwork (TM) migration into Schlemm’s canal space can be induced by acute pressure elevation by ophthalmodynamometer, and observed by spectral domain optical coherence tomography. The goal of this method is to quantify the morphometric response of the living outflow tract to acute pressure elevation in living tissues in situ.

Abstract

The mechanical characteristics of the trabecular meshwork (TM) are linked to outflow resistance and intraocular pressure (IOP) regulation. The rationale behind this technique is the direct observation of the mechanical response of the TM to acute IOP elevation. Prior to scanning, IOP is measured at baseline and during IOP elevation. The limbus is scanned by spectral-domain optical coherence tomography at baseline and during IOP elevation (ophthalmodynamometer (ODM) applied at 30 g force). Scans are processed to enhance visualization of the aqueous humor outflow pathway using ImageJ. Vascular landmarks are used to identify corresponding locations in baseline and IOP elevation scan volumes. Schlemm canal (SC) cross-sectional area (SC-CSA) and SC length from anterior to posterior along its long axis are measured manually at 10 locations within a 1 mm segment of SC. Mean inner to outer wall distance (short axis length) is calculated as the area of SC divided by its long axis length. To examine the contribution of adjacent tissues to the effect IOP elevations, measurements are repeated without and with smooth muscle relaxation with instillation of tropicamide. TM migration into SC is resisted by TM stiffness, but is enhanced by the support of its attachment to adjacent smooth muscle within the ciliary body. This technique is the first to measure the living human TM response to pressure elevation in situ under physiological conditions within the human eye.

Introduction

Glaucoma is the world’s second leading cause of irreversible blindness¹. Elevated intraocular pressure (IOP) is a major causal risk factor for the presence and progression of glaucoma^2-7. IOP is regulated by balance between the formation and outflow of aqueous humor⁸. The locations of greatest outflow resistance are the juxtacanicular tissue and the inner wall of Schlemm canal (SC), the interface between SC and the trabecular meshwork (TM) ^9-11. While TM stiffness may contribute to the prevention of SC collapse in the face of IOP elevation, Overby et al. ¹² recent....

Protocol

Ethics Statement: Approval was obtained from the Institutional Review Board of the University of Pittsburgh School of Medicine before subject recruitment began. All subjects provided written informed consent before participation in the study.

1. Data Acquisition

1. Pressure Elevation
   1. Take baseline measurements (IOP and OCT measurements) by instilling one drop of 0.5% proparacaine into the eye. Wait 3 min for efficacy.
   2. Gently apply pressure to the temporal sclera with the ophthalmodynamometer, 30 g in cohort 1, and 5 and then 10 g in cohort 2. Then take the desired (IOP and OCT) measurements as follows
      1. IOP Measuremen....

Results

Using these data acquisition and image analysis techniques, the effects of small and large changes in IOP on outflow tract morphological parameters such as SC cross-sectional area are obtained (Figure 1). We can see that high levels of IOP increase produce an observable collapse of SC, as represented by a large reduction in cross-sectional area. The eye appears to be able to accommodate small increases in IOP, as evidenced by the lack of change in SC-CSA (Figure 1). These results show th.......

Discussion

The present technique leverages non-invasive observation of the mechanical response of soft tissue to quantify SC collapse. Future work using human cadaver eyes is needed to calibrate tissue deflections actual tissue stiffness after dissection. But, such studies will suffer the same limitations of previous outflow models; specifically, that the contributions of living muscle to tissue tension will not be present. Further calibration in a living mammalian eye model may allow calibration of imaging and direct measurem.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Spectral Domain OCT	Zeiss	Cirrus
Imaging Workstation	Apple	iMac
Ophthalmodynamometer	Baillairt Matalene Ophthalmodynamometer, Surgical instruments CO., Inc. New York, NY
Image Processing Program	rsb.info.nih.gov/ij	ImageJ, FIJI