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MRI-derived diffusion parameters in the human optic nerve and its surrounding sheath during head-down tilt
Darius A Gerlach1, Karina Marshall-Goebel1,2, Khader M Hasan3
1Division of Space Physiology, Institute of Aerospace Medicine, German Aerospace Center (DLR), Cologne, Germany.
NPJ Microgravity
|June 27, 2017
Summary
Head-down tilt, a microgravity analog, increases fluid dynamics in the optic nerve sheath, potentially explaining spaceflight-induced neuro-ophthalmic issues like spaceflight associated neuro-ocular syndrome (SANS).
Area of Science:
- Neuroscience
- Ophthalmology
- Space Medicine
Background:
- Astronauts frequently experience neuro-ophthalmic changes during space missions.
- Microgravity Ocular Syndrome (MOS) is a significant concern, but its cause remains unclear.
- Alterations in cerebrospinal fluid (CSF) dynamics around the optic nerve may contribute to MOS.
Purpose of the Study:
- To investigate changes in optic nerve and sheath diffusivity during head-down tilt (HDT), a ground-based microgravity model.
- To assess the role of cerebrospinal fluid hydrodynamics in the optic nerve sheath during simulated microgravity.
Main Methods:
- Diffusion tensor imaging (DTI) was used to analyze diffusivity in the optic nerve and sheath.
- Nine healthy males underwent various HDT conditions (-6° to -18°, with CO2 and lower body negative pressure).
- Measurements included mean diffusivity, fractional anisotropy, axial diffusivity, and radial diffusivity at baseline and after HDT.
Main Results:
- HDT significantly increased mean, axial, and radial diffusivity in the optic nerve sheath.
- Fractional anisotropy and axial diffusivity increased within the optic nerve itself during HDT.
- These changes suggest increased CSF volume and movement within the optic nerve sheath during HDT.
Conclusions:
- HDT induces measurable changes in optic nerve sheath and optic nerve diffusivity.
- Findings support the hypothesis that altered CSF hydrodynamics play a role in spaceflight-associated neuro-ophthalmic findings.
- This study provides insights into the pathophysiology of MOS and informs future countermeasures.
