Stiffness of the Trabecular Meshwork In Living Eyes

Abstract

 

Purpose

 

Trabecular meshwork (TM) stiffness has been shown to be elevated in cadaveric glaucomatous eyes (mean 80.8, range 0.5-565 kPa) compared with normal eyes (4.0, 0.5-10kPa) [Last et al, IOVS 2011(52)5:2147-2152]. However; those measurements were made in isolated tissues by atomic force microscopy. In-vivo, the TM is suspended in tension between the posterior cornea and ciliary body. Ciliary muscle activity may increase the effective stiffness of the TM. We examined this hypothesis in living eyes, comparing the effect of IOP on outflow pathway morphology with previous measurements made in cadaveric eyes.

 

Methods

 

The temporal limbus of 33 eyes of 19 healthy subjects (12 male, 7 female, age 40 ± 15 years) was imaged by spectral-domain optical coherence tomography (Cirrus HD-OCT, Zeiss, USA) at baseline and during IOP elevation (ophthalmodynamometer applied at 30 Grms force). IOP was measured at baseline and during IOP elevation by Goldmann applanation tonometry. Vascular landmarks were used to identify corresponding locations in baseline and IOP elevation scan volumes. Schlemm's canal cross-sectional area (SC-CSA), mean inner to outer wall distance (IOD) and mean TM thickness (TMt), the distance from the anterior chamber to SC inner wall, were measured at 5 locations within a 1 mm length of SC using ImageJ as described previously [IOVS 2010; 51(8): 4054-4059]. Data were compared with measurements by VanBuskirk et al in cadaveric eyes (IOVS, 1982;22(5):625-32, Table).

 

Results

 

A mean increase in IOP of 23 mmHg (Table) led to a decrease of SC-CSA (39%) and IOD (18%) while TMt was not unchanged (Table). In cadaveric eyes, the decreases in SC-CSA were greater than in live eyes (Table), but the TMt was also insensitive to IOP elevation.

 

Conclusions

 

The collapse of Schlemm's canal with increasing IOP while TM thickness changed little suggests that the TM acts as a membrane under tension. This was true both in live and cadaveric eyes. However, the 39% decrease in SC-CSA and IOD with increasing IOP in living eyes, was smaller than the decrease observed in cadaveric eyes. This suggests that the tension and thus, the effective stiffness of this membrane may be greater in live eyes, presumably due to ciliary muscle contraction. The contribution of the TM in preventing collapse of the SC may differ in living versus cadaveric eyes.

 

 

Keywords: 735 trabecular meshwork • 550 imaging/image analysis: clinical