MRI-derived diffusion parameters in the human optic nerve and its surrounding sheath during head-down tilt

Abstract

More than half of astronauts present with significant neuro-ophthalmic findings during 6-month missions onboard the International Space Station. Although the underlying cause of this Microgravity Ocular Syndrome is currently unknown, alterations in cerebrospinal fluid dynamics within the optic nerve sheath may play a role. In the presented study, diffusion tensor imaging was used to assess changes in diffusivity of the optic nerve and its surrounding sheath during head-down tilt, a ground-based model of microgravity. Nine healthy male subjects (mean age ± SD: 25 ± 2.4 years; mean body mass index ± SD: 24.1 ± 2.4 kg/m2) underwent 5 head-down tilt conditions: −6°,−12°, −18°,−12° and 1% CO2, and −12° and lower body negative pressure. Mean diffusivity, fractional anisotropy, axial diffusivity, radial diffusivity were quantified in the left and right optic nerves and surrounding sheaths at supine baseline and after 4.5 h head-down tilt for each condition. In the optic nerve sheath, mean diffusivity was increased with all head-down tilt conditions by (Best Linear Unbiased Predictors) 0.147 (SE: 0.04) × 10−3 mm2/s (P < 0.001), axial diffusivity by 0.188 (SE: 0.064) × 10−3 mm2/s (P < 0.001), and radial diffusivity by 0.126 (SE: 0.04) × 10−3 mm2/s (P = 0.0019). Within the optic nerve itself, fractional anisotropy was increased by 0.133 (SE: 0.047) (P = 0.0051) and axial diffusivity increased by 0.135 (SE: 0.08) × 10−3 mm2/s (P = 0.014) during head-down tilt, whilst mean diffusivity and radial diffusivity were unaffected (P > 0.3). These findings could be due to increased perioptic cerebral spinal fluid hydrodynamics during head-down tilt, as well as increased cerebral spinal fluid volume and movement within the optic nerve sheath.

Lower body negative pressure reduces optic nerve sheath diameter during head-down tilt

“…Written and informed consent was received before subject participation in the study, and all procedures were approved by the ethical board of the regional medical association (Ä rztekammer Nordrhein). The presented results represent a subset of measurements from a larger study including cerebral arterial and venous blood flow, diffusion tensor imaging, intraocular pressure, and cardiovascular and respiratory variables (17,36,37).…”

Focus on the Optic Nerve Head in Spaceflight-Associated Neuro-ocular Syndrome

“…1 Conversely, more definitive evidence collected during brief periods of weightlessness in parabolic flight indicates that ICP in the supine posture is decreased in 0 g compared with 1 g. 5 Proponents of a role for ICP in the development of SANS look to changes in brain structure 6,7 or cerebral spinal fluid dynamics. 8,9 However, the relationship between structural change on brain magnetic resonance imaging (MRI) and optic disc edema (the defining characteristic of SANS) was noted as "inconsistent" likely because brain structural changes were observed in almost all long-duration astronauts, yet only 3 demonstrated optic disc edema. 7 In the single optic disc edema case in which ventricular volume was changed, that astronaut presented with a smaller volume alteration than all but 3 of the astronauts studied.…”

<p>Spaceflight Associated Neuro-Ocular Syndrome (SANS): A Systematic Review and Future Directions</p>

“…During a subsequent 6-month mission, the astronaut developed more widespread choroidal folds and new onset of optic disc edema in the same eye. Suggests there may be a cumulative effect of repeat space travel for ocular structural changes Arbeille 5 et al 2015 10 LDSF astronauts Significant increase in cross sectional area of jugular vein and portal vein during flight, which returned to baseline after return to earth Suggests cephalad pooling of venous blood during LDSF Gerlach 6 et al 2017 9 healthy adult volunteers HDT resulted in increased optic nerve sheath diffusivity, optic anisotropy and axial diffusivity Suggests HDT is associated with increased perioptic and optic nerve sheath CSF volume and movement Lawley 7 et al 2017 8 volunteers who previously had an Ommaya reservoir placed for treatment of hematological malignancy Prolonged periods of simulated microgravity did not cause progressive elevations in ICP Suggests that mechanisms other than raised ICP may contribute to the pathophysiology of SANS Mader 8 et al 2017 1 LDSF astronaut case report Asymmetric structural ocular changes were found inflight and did not resolve immediately upon return to Earth. Choroidal thickening resolved by 30 days postflight; ONH swelling persisted for 180 days, ONH morphologic changes persisted for 630 days, globe flattening persisted for 660 days.…”

“…Wostyn et al 25 reviewed multiple papers regarding the cephalad fluid shift and reported that optic nerve head (ONH) edema may at least in part result from the forcing of perioptic CSF into the ONH along perivascular spaces. Gerlach et al 6 reported that on imaging of the optic nerves of healthy volunteers who underwent head down tilt (HDT) for 4.5 hours, fractional anisotropy and axial diffusivity were significantly increased, suggesting increased CSF volume and movement within the optic nerve sheath.…”