Abstract
Background
Meta-analyses show a variable relationship between optic nerve sheath diameter (ONSD) and the presence of raised intracranial pressure (ICP). Because optic nerve sheath (ONS) tissue can be deformed, it is possible that ONSD reflects not only the current ICP but also prior deforming biomechanical exposures. In this post hoc analysis of two published data sets, we characterize ONS Young’s modulus (E, mechanical stress per unit of strain) and calculate threshold pressure for plastic deformation.
Methods
The authors of two previously published articles contributed primary data for these unique post hoc analyses. Human cadaveric ex vivo measurements of ONSD (n = 10) and luminal distending pressure (range 5 to 65 mm Hg) were used to calculate E and the threshold pressure for plastic deformation. Clinical in vivo measurements of ONSD and ICP during endotracheal tube suction from patients with traumatic brain injury (n = 15) were used to validate the ex vivo cadaveric findings.
Results
Ex vivo ONS estimate of E was 140 ± 1.3 mm Hg (mean ± standard error), with evidence of plastic deformation occurring with distending pressure at 45 mm Hg. Similar E (71 ± 10 mm Hg) was estimated in vivo with an average ICP of 34 ± 2 mm Hg.
Conclusions
Ex vivo, ONS plastic deformation occurs at levels of pressure commonly seen in patients with raised ICP, leading to distortion of the ICP–ONSD relationship. This evidence of plastic deformation may illustrate why meta-analyses fail to identify a single threshold in ONSD associated with the presence of raised ICP. Future studies characterizing time-dependent viscous characteristics of the ONS will help determine the time course of ONS tissue biomechanical behavior.
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Data Availability
The data sets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
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RCT conceived the study. KH, HCH, IMM, and RJS were authors of the previously published data. ML, AAA, and RCT performed the statistical analysis of the data set and wrote the original manuscript. All authors read and approved the final manuscript.
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Post hoc analysis of previously published data. RCT contacted the authors of the two primary reports and gained approval for the analyses as well as co-opting the main authors as collaborators in this article.
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Luchette, M., Helmke, K., Maissan, I.M. et al. Optic Nerve Sheath Viscoelastic Properties: Re-Examination of Biomechanical Behavior and Clinical Implications. Neurocrit Care 37, 184–189 (2022). https://doi.org/10.1007/s12028-022-01462-x
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DOI: https://doi.org/10.1007/s12028-022-01462-x