title: Characterization of the mechanical behavior of the optic nerve sheath and its role in spaceflight-induced ophthalmic changes.
abstract: Visual impairment and intracranial pressure (VIIP) syndrome is characterized by a number of permanent ophthalmic changes, including loss of visual function. It occurs in some astronauts during long-duration spaceflight missions. Thus, understanding the pathophysiology of VIIP is currently a major priority in space medicine research. It is hypothesized that maladaptive remodeling of the optic nerve sheath (ONS), in response to microgravity-induced elevations in intracranial pressure (ICP), contributes to VIIP. However, little is known about ONS biomechanics. In this study, we developed a custom mechanical testing system that allowed for unconfined lengthening, twisting, and circumferential distension of the porcine ONS during inflation and axial loading. Data were fit to a four-fiber family constitutive equation to extract material and structural parameters. Inflation testing showed a characteristic "cross-over point" in the pressure-diameter curves under different axial loads in all samples that were tested; the cross-over pressure was [Formula: see text] mmHg ([Formula: see text]). Large sample-to-sample variations were observed in the circumferential strain, while only modest variations were observed in the circumferential stress. Multiphoton microscopy revealed that the collagen fibers of the ONS were primarily oriented axially when the tissue was loaded. The existence of this cross-over behavior is expected to be neuroprotective, as it would avoid optic nerve compression during routine changes in gaze angle, so long as ICP was within the normal range. Including these observations into computational models of VIIP will help provide insight into the pathophysiology of VIIP and could help identify risk factors and potential interventions.
authors.affiliation: Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, 315 Ferst Dr. NW, Atlanta, GA, 30332, USA.
authors.forename: Julia
authors.initials: J
authors.lastname: Raykin
authors.affiliation: Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, 315 Ferst Dr. NW, Atlanta, GA, 30332, USA.
authors.forename: Taylor E
authors.initials: TE
authors.lastname: Forte
authors.affiliation: Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, 315 Ferst Dr. NW, Atlanta, GA, 30332, USA.
authors.forename: Roy
authors.initials: R
authors.lastname: Wang
authors.affiliation: Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, 315 Ferst Dr. NW, Atlanta, GA, 30332, USA.
authors.forename: Andrew
authors.initials: A
authors.lastname: Feola
authors.affiliation: Department of Ophthalmology, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: Brian C
authors.initials: BC
authors.lastname: Samuels
authors.affiliation: NASA Glenn Research Center, Cleveland, OH, USA.
authors.forename: Jerry G
authors.initials: JG
authors.lastname: Myers
authors.affiliation: Universities Space Research Association, Houston, TX, USA.
authors.forename: Lealem
authors.initials: L
authors.lastname: Mulugeta
authors.affiliation: NASA Glenn Research Center, Cleveland, OH, USA.
authors.forename: Emily S
authors.initials: ES
authors.lastname: Nelson
authors.affiliation: Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, 315 Ferst Dr. NW, Atlanta, GA, 30332, USA.|George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, USA.
authors.forename: Rudy L
authors.initials: RL
authors.lastname: Gleason
authors.affiliation: Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, 315 Ferst Dr. NW, Atlanta, GA, 30332, USA. ross.ethier@bme.gatech.edu.|George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, USA. ross.ethier@bme.gatech.edu.
authors.forename: C Ross
authors.initials: CR
authors.lastname: Ethier
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D006801:Humans; D007427:Intracranial Pressure / Q000502:physiology*; D008954:Models, Biological; D009900:Optic Nerve / Q000502:physiology*; D013026:Space Flight; D013552:Swine
keywords: Biosolid mechanics; Constitutive model; Intracranial pressure; Optic nerve sheath; Spaceflight physiology; VIIP