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Editors Selection IGR 9-4

Basic research: Topographic change ONH

Comment by Ross Ethier on:

20100 3-D histomorphometry of the normal and early glaucomatous monkey optic nerve head: prelaminar neural tissues and cupping, Yang H; Downs JC; Bellezza A et al., Investigative Ophthalmology and Visual Science, 2007; 48: 5068-5084

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Yang et al. (1243) have used the techniques from their previous paper (1244), and indeed the same eyes ‐ supplemented by a control monkey ‐ to study the factors that determine early topographic change of the optic nerve head in a monkey model of glaucoma. The experimental model is induced ocular hypertension, and the analysis is done early in the disease (see above comment for details). As expected, the authors find that lamina cribrosa posterior deformation increases optic cup volume, but unexpectedly and very counter-intuitively, they also find that pre-laminar tissue volume and thickness increase in early glaucoma. In other words, an increase in the volume and thickness of pre-laminar neural tissue actually acts to reduce cup enlargement, at least in this early glaucoma model in the monkey.

Pre-laminar tissue volume and thickness increase in early glaucoma

The authors suggest that this thickening may be due to blockade of orthograde axonal transport, prelaminar neural edema and/or prelaminar gliosis. Whatever the cause, these results suggest that standard tools for measuring optic nerve topography do not tell us much about what the underlying connective tissues in the nerve are doing, at least early in the disease process. Yang et al. note that this helps motivate the development of new OCT-based imaging technologies that may allow us to see 'deeper' into the nerve. As an aside, it is notable that one of the monkeys in this study never had detectable IOP elevation, despite ONH topographic change and axonal loss. This observation highlights the need for a continuous IOP monitoring system, an important missing piece of the puzzle for understanding the pressure-dependent aspect of the disease process in animal models of glaucoma.

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