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Editors Selection IGR 21-1

Clinical Examination Methods: Exploring ONH deformation with OCT

Comment by Michael Girard on:

81967 In vivo characterization of the deformation of the human optic nerve head using optical coherence tomography and digital volume correlation, Midgett DE; Quigley HA; Nguyen TD, Acta biomaterialia, 2019; 96: 385-399

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Glaucoma has been referred by many as a biomechanical disorder. After all, the optic nerve head (ONH) is exposed to high levels of biomechanical stress arising from various pressures (including IOP) and from optic nerve traction during eye movements. Such stress levels, when they exceed their homeostatic range may be responsible (in part) for the development and progression of glaucoma. Unfortunately, no biomechanical tests for the ONH currently exist clinically, but research is underway.

In this study, Midgett et al. developed OCT-based image processing techniques (digital volume correlation) to map the 3D strains (i.e., deformations) of the ONH following changes in IOP. To achieve this, two OCT scans of the ONH were captured in each subject: before and after a change in IOP. While such techniques have been proposed in the past by other groups, the authors were able to apply them to 3D radial scans of the ONH (instead of raster scans) and used an innovative method to increase IOP by asking three glaucoma- suspect subjects to wear tight-fitting swimming goggles for 15 minutes. In another group of five glaucoma subjects, strains were mapped following IOP lowering surgery. Overall, the authors concluded they could map deformations with a high resolution even with small IOP variations (4 mmHg or less). They also observed a reduction in tissue compression within the lamina cribrosa (LC) following IOP lowering surgery. Finally, the authors were able to establish relationships between LC depth (a parameter that can indicate the movement of the LC with a change in IOP) and LC strains. Such relationships have value because it is currently much simpler to assess LC depth in vivo, and we may be able to use it to predict the underlying tissue compression changes. It is worth mentioning that the authors would benefit from reproducing their results in much larger cohorts, and whether they could confirm that the biomechanical response of the ONH to IOP is indeed associated with visual field loss, as was observed by Tun et al.¹

Moving forward, it is becoming clearer that glaucoma subjects would benefit from a clinical test to assess the robustness of their ONHs

Moving forward, it is becoming clearer that glaucoma subjects would benefit from a clinical test to assess the robustness of their ONHs. Such a test would work by creating a mechanical perturbation (e.g., IOP increase) while observing a response (e.g., tissue displacements with OCT imaging). The method proposed in this publication to raise IOP (tight-fitting swimming goggles) has clear scientific value from a research perspective, but it may not be easily translated clinically owing to patients' discomfort. An alternative could be to map ONH deformations with an existing natural load (e.g., the ocular pulse amplitude), and several research groups are thinking along those lines. In addition, clear biomechanical markers would need to be defined. Strain and stress are extremely useful engineering metrics to understand a biomechanical system, but each is represented by a set of six numbers defined at each single pixel of a ONH, thus causing the problem of 'too much information'. Artificial intelligence may have an opportunity to reduce such complexities into clinically-manageable biomechanical parameters. Mapped strain and stress can also be used to derive ONH biomechanical properties (i.e., stiffness) and this can be done using the virtual fields method or VFM² (not to be confused with visual field). Finally, to realistically compare biomechanical results across patients, one would need to subject them to the exact same loads (i.e., identical IOP increase from an identical baseline IOP), but this remains a clinical challenge.

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