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Editors Selection IGR 14-3

Optical Coherence Tomography: Biomechanics and SD-OCT

Comment by Crawford Downs on:

27930 Laminar and prelaminar tissue displacement during intraocular pressure elevation in glaucoma patients and healthy controls, Agoumi Y; Sharpe GP; Hutchison DM et al., Ophthalmology, 2011; 118: 52-59

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Optic nerve head (ONH) biomechanics likely plays an important role in the development and progression of glaucoma, but it is not well understood. IOP-induced deformation of the ONH surface measured in pre - vious studies has been presumed to reflect deformations of the underlying lamina cribrosa. Agoumi et al. (226) performed in an in vivo imaging study of human eyes characterized as openangle glaucoma, healthy controls agematched to the glaucoma group, and young healthy controls (n = 12 each). Each patient's ONH was SD-OCT imaged at baseline IOP, and after an IOP elevation of ~12 mmHg; the positions of the ONH surface and anterior lamina were measured. The study has two very important findings that hold across all groups. First, there was no association between IOP-induced ONH surface displacement and laminar displacement, and second, although the ONH surface displaced significantly with acute IOP elevation, the lamina did not displace at all. The ONH surface displaced most in young healthy patients (15% of the thickness of the prelaminar tissues) somewhat less in the old healthy group (8%) and least in the glaucoma group (3.5%). The authors conclude that "the lamina is relatively rigid", which is not likely to be true. Recent work in ocular biomechanics suggests that the observed lack of posterior laminar displacement upon IOP elevation results from the simultaneous acute expansion of the scleral canal, which stretches the lamina taut. Hence, while the lamina cribrosa is significantly stretched, it does not displace posteriorly even though it is much more compliant than the sclera. The prelaminar tissues are also stretched across the expanding scleral canal as IOP is elevated, which could explain the reported thinning of those tissues. This phenomenon could also account for the different prelaminar tissue thinning seen between groups, as scleral canal expansion should be greatest in young healthy eyes with compliant sclera, somewhat less in older healthy eyes due to age-related stiffening of their connective tissues, and the least in older glaucomatous eyes whose connective tissues are stiffest due to imaging at higher baseline IOP, age-related collagen cross-linking, and chronic IOP-induced remodeling.

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