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Editors Selection IGR 18-3

Intraocular pressure: CCT and progression

Comment by Crawford Downs on:

19717 Central corneal thickness as a risk factor for glaucoma, Mehdizadeh A; Hoseinzadeh A; Fazelzadeh A, Medical Hypotheses, 2007; 69: 1205-1207

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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, Mehdizadeh et al. (1342) have proposed that the risk of glaucomatous progression has an inverse relationship to central corneal thickness (CCT). Their hypothesis is based primarily on the weak correlation between CCT and scleral thickness (in one ARVO abstract) and stress in the neural tissues as calculated using Laplace's Law. This paper is important in that it links the eye's biomechanical res-ponse to glaucomatous damage in the neural tissues. Unfortunately, the supporting logic is not well rooted in evidence-based science. Laplace's Law is only valid for calculations of stress in thin-walled, materially isotropic, perfectly spherical pressure vessels of uniform wall thickness, with no geometric discontinuities. As a result, the Law is wholly inadequate to describe the wall stress in any of the ocular coats, which vary in thickness by a factor of four, are made up of layered tissues that exhibit anisotropic, nonlinear responses to load, and have numerous geometric discontinuities (limbus, ONH, pars plana, etc.). The most important limitation of the underlying logic is their implicit assumption that the relationship between IOP-related stress and strain in the neural tissues is straightforward.

The relationship between IOP-related stress and strain in the neural tissues is not straightforward

The authors are correct that it is strain, not stress that damages tissues, and strain is related to stress through the tissues' material properties. The correlation between stress (force/cross-sectional area) and strain (a measure of local deformation) is very complex in the eye for reasons mentioned above. So, while CCT might well be related in some way to glaucomatous susceptibility, it is not likely to be as simplistic a relationship as proposed by the authors. 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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