Localization of Increased Stiffness and Flow Resistance in Glaucomatous Human Eyes
Mark Johnson PhD Northwestern University, Biomedical Engineering, Ophthalmology and Mechanical Engineering
Co-authors: Amir Vahabakashi PhD Ariel Gelman Biqin Dong PhD Ernst R Tamm PhD W Daniel Stamer PhD Cheng Sun PhD Hao Zhang PhD Haiyan Gong PhD
Recent studies have suggested that changes to the stiffness of the inner wall of Schlemm's canal (SC) impact the level and distribution of aqueous humor outflow resistance in glaucoma. We probed the inner wall region of enucleated human eyes using atomic force microscopy (AFM) and perfusion/imaging studies to see where increased stiffness and flow resistance are localized. In fifteen normal and 8 glaucomatous eyes, AFM was used to measure tissue stiffness as a function of distance from the inner wall; outflow resistance and optic nerve axon counts were measured when possible. In 6 normal and 5 glaucomatous eyes, outflow resistance was measured, and then the eyes were fixed at 15 mm Hg, and giant vacuole density and size determined. Giant vacuoles were used as micropressure transducers to estimate transcellular flow resistance across the SC cells.
AFM measurements of SC cells in-situ showed significantly (p>0.003) higher stiffness in glaucomatous eyes (0.74±0.06 kPa) than in normals (0.44±0.06 kPa); deeper measurements of the substrate underlying the SC cells were also higher in glaucomatous eyes (11.7±2.4 vs. 7.6±1.5 kPa, respectively), but the difference was not significant (p>0.15). Outflow resistance correlated closely with SC cell stiffness (p=0.01) but not with deeper measurements of tissue stiffness. We found the density and size of giant vacuoles to be similar comparing normal to glaucomatous eyes (p>0.3). Based on this and flow measurements, transcellular flow resistance across the SC cells was estimated to be at least 5.2 times higher (p<0.02) in the glaucomatous eyes than normals, indicating a large shift in flow resistance distribution in glaucoma. Our data implicate the inner wall endothelium of SC and/or the immediate underlying tissue in generating the elevated flow resistance characteristic of glaucoma.
Acknowledgments: We gratefully acknowledge helpful discussions with Dr. Thomas Read and Prof. C Ross Ethier.
