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Editors Selection IGR 9-4

IOP-Related: Sclera stiffness and IOP

Comment by Crawford Downs on:

22738 Biomechanical effects of intraocular pressure elevation on optic nerve/lamina cribrosa before and after peripapillary scleral collagen cross-linking, Thornton IL; Dupps WJ; Roy AS et al., Investigative Ophthalmology and Visual Science, 2009; 50: 1227-1233

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Ocular biomechanics likely plays an important role in the development and progression of glaucoma, but it is not well understood. In this paper, Thornton et al. (114) have shown that two compounds that engender collagen cross-linking ‐ riboflavin with UV-A light, and glutaraldehyde ‐ significantly stiffen the peripapillary sclera when applied to its outer surface. They also measure scleral and optic nerve head stiffness at both low and high IOP, and report that these tissues are also highly nonlinear, i.e., they stiffen as IOP (and strain) increases. The study is limited by several aspects of the experimental approach, which include insufficient photographic resolution in the circumferential strain measurements and the inability to measure the lamina cribrosa in isolation (it's covered by a layer of retrolaminar nerve of unknown thickness), but the results reported are reasonable and defensible. This paper is important in that it links the ONH's biomechanical response to the level of cross-linking in the collagen of the underlying connective tissues. The authors propose that it might be 'neuroprotective' to pharmacologically stiffen the collagenous tissues of the ONH and therefore reduce IOP-related strain (local tissue stretch, compression, and shear). While this may seem logical, it is unknown whether a stiffer ONH is also a more robust ONH that is less susceptible to IOP-related damage. It has been reported, that ocular connective tissues stiffen with age and glaucoma is principally a disease of the elderly, so is it reasonable to assume that stiffer ONH connective tissues make a person more susceptible to IOP-related glaucomatous damage? Possibly, but glaucoma is a multifactorial disease and it is difficult to separate the effects of pure biomechanics and related blood flow, axoplasmic flow, and cellular reactivity, all of which may also be affected by aging and possibly driven by biomechanics to some degree. Much more work is needed to elucidate the direct and indirect links between IOP, tissue material properties, tissue geometry and glaucomatous damage.

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