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Editors Selection IGR 8-2

Clinical Forms of Glaucoma: Lamina cribrosa evaluation in pseudoexfoliation syndrome

Comment by Nicholas Strouthidis on:

53623 Evaluation of Lamina Cribrosa in Pseudoexfoliation Syndrome Using Spectral-Domain Optical Coherence Tomography Enhanced Depth Imaging, Kim S; Sung KR; Lee JR et al., Ophthalmology, 2013; 120: 1798-1803

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Pseudoexfoliation (PXF) is a systemic fibrillopathy that predisposes to the development of glaucoma. Recent cadaveric studies have shown that the lamina cribrosa (LC) develops PXF-specific elastin deposition and has reduced 'stiffness' as compared to the lamina of normal eyes. Kim, et al. sought to identify whether there were any detectable differences between laminar thickness and anterior LC depth (ALD) as measured by SDOCT in PXF and POAG eyes with similar levels of IOP and glaucoma damage. Interestingly, they found the LC in PXF eyes was significantly thinner than in POAG eyes. There was no difference in ALD, indicating that prelaminar tissue deformation was probably not directly influencing the laminar thinning observed in the PXF eyes.

There is a risk that the proliferation of articles on this topic may prematurely push laminar thickness measurements into clinical practice

This paper raises a number of interesting 'chicken or egg' questions. Does LC thinning occur as a consequence of PXF and does this then leads to the predisposition to glaucoma damage? Is the LC thinning a consequence of a more 'aggressive' form of glaucoma impacting upon 'less stiff' tissues? Of course, there is no way of extrapolating this information simply on the basis of a single measurement in time. Furthermore, one cannot make assumptions about biomechanical behavior based on this measurement. A piece of balsa wood and a piece of steel of the same thickness will behave differently under the same stress; likewise a simple layered structure will behave differently from a layered structure with interconnected cross-beams. In other words, the way the LC responds to IOP-related stress will depend on the 3D architecture, the load involved, and the tissue properties of the constituent connective tissues, rather than just its 'thickness'. The results of this paper make for a compelling story and, despite the rather low numbers of subject eyes, one suspects that these findings will be found to be consistent in future. One must not, however, underplay the fact that the LC is a complex 3D structure, under-represented by a single thickness measurement. Furthermore, the detection of the posterior surface of the lamina, even with EDI and compensation algorithms, is highly variable. Many experts will struggle to identify the posterior LC surface in the example figure used in this article. There is a risk that the proliferation of articles on this topic may prematurely push laminar thickness measurements into clinical practice. I would favor caution until this measurement can be obtained automatically, consistently and most importantly, when we have a better understanding of how it relates to visual function in glaucoma.

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