Our hypothesis was that microgravity, as simulated through acute application of 15° HDT, would result in an increase in cardiac-related pulsatile CSF flow consistent with increased ICP and decreased intracranial compliance. Our results are inconsistent with this hypothesis, instead showing that HDT was significantly associated with a decrease in pulsatile CSF flow at the mid-C2 vertebral level as manifested by reduced systolic peak flow rate and PtPPAcsf. In addition, HDT was significantly associated with decreased cerebral arterial average flow rate, systolic peak flow rate, and PtPPAart. Finally, a significant increase in jugular venous cross-sectional area was also observed.
Prior studies have examined cerebral flow dynamics under HDT. Marshall-Goebel et al., using 9 healthy male volunteers, found a decrease in both arterial and venous flow rate variables as well as an increase in venous cross-sectional area (CSA) from baseline to HDT (62 mm2 to 97 mm2), suggestive of increased venous pressure . Ishida et al., using 15 healthy volunteers, found increases in venous CSA (36 mm2 to 54 mm2), decreases in arterial inflow, increases in venous outflow, and no significant changes in CSF stroke volume or systolic velocity . These observed changes in venous CSA under ground-based HDT are consistent with ultrasound studies performed during spaceflight that demonstrate comparable increases in venous CSA in subjects exposed to microgravity, which lends validity to the results of these HDT protocols [25, 26]. These prior studies collectively corroborate our findings of increased venous CSA and decreased arterial inflow. Finally, we observe a decrease in pulsatile CSF flow not examined in these prior studies.
Our findings of increased jugular venous cross-sectional area suggest increased cerebral venous pressure. Venous pressure has been shown by Holmlund et al. to be predictive of changes in intracranial pressure with increases in venous CSA correlating with increases in intracranial pressure . These findings are, however, tempered by one study that showed that measured ICP and central venous pressure both decreased during acute (<1 minute) episodes of zero G in parabolic flight, which differs from our findings of increased central venous pressure during HDT . This difference may be due to the duration (<1 minute vs. longer duration HDT) or the mode (parabolic flight vs HDT) of gravitational change.
To help interpret our CSF results, it is useful to recall several aspects of CSF physiology under normal gravity conditions, specifically the coupling of CSF and cerebral blood flow dynamics. CSF is produced in the choroid plexus of the brain’s ventricles and slowly circulates into the spinal and cranial subarachnoid spaces . It is then absorbed into the venous system via arachnoid granulations, primarily near the dural venous sinus . An increase in venous pressure of only a few mmHg can alter the pressure gradient across the arachnoid granulations, leading to reduced CSF absorption and therefore CSF stroke volume . However, this process is unlikely to occur in the setting of acute HDT and further, our study investigated CSF flow dynamics at the level of the foramen magnum and not at the superior sagittal sinus where flow across the subarachnoid space would occur. Our study did not directly measure the location of the CSF space center of gravity and therefore cannot conclude that a net CSF shift occurred. However, any shift in CSF center of gravity towards the head would potentially have a downstream impact on CSF flow amplitude by altering intracranial compliance.
As noted above, CSF flow from the brain has significant contribution from cardiac related pulsation, with cerebral arterial flow pulsations having a positive contribution to CSF flow pulsations . SANS may involve perturbations to CSF flow associated with mild increases in intracranial pressure given the pathological findings of optic disc edema, globe flattening, and decreased visual acuity that are also present in Earth-bound conditions of CSF imbalance such as idiopathic intracranial hypertension [2, 30]. CSF dynamics can be altered by changes in production, flow, or reabsorption. However, the former and latter occur slowly, and thus are relatively unimportant in the setting of an acute study, such as we have conducted.
Collectively, these results suggest that changes in cerebral arterial flow are a plausible cause for the observed decrease in CSF pulsatile flow. Because the elevated ICP expected in HDT would be expected to increase CSF flow pulsatility, we further suggest that the observed decrease in arterial pulsatility dominated any effects associated with reduced intracranial compliance [5, 6]. We also note that out observations could also be influenced by spatial heterogeneity in CSF flow, since we measured CSF pulsatile flow at the level of the foramen magnum, which may not be reflective of CSF pulsatile flow in other locations within the intracranial space.
Apart from the pathophysiological alterations suggested by the measurements in our study, these data are useful inputs for models seeking to simulate volume and pressure alterations in the head and eye in microgravitational settings. Prior studies have relied on blood and aqueous humor dynamics as inputs to models, and we anticipate that these results will be similarly useful for models seeking to replicate the pathophysiology of SANS .
The limitations of our study included the fact that our recorded venous outflow did not account for the entirety of arterial inflow, indicating missing venous collateral changes in flow. Jugular venous flow, as opposed to venous flow through other vessels such as the vertebral venous system, increases from an erect to supine position . As a result, changes in body position from upright to supine to HDT could have effects on collateral venous flow that went unmeasured in our study. In addition, observed changes of flow in the jugular veins from supine to HDT in our study could plausibly be a result of the increased jugular recruitment seen in changes in body position from upright to supine . We focused on the jugular veins in our study since they were the only veins that could be consistently identified in the images, but not being able to account for changes in collateral venous flow and their significance is a limitation.
A primary limitation of our study was the use of HDT as a proxy for microgravity. Although HDT has been utilized in many previous studies to simulate microgravity, it cannot reproduce other spaceflight-related nongravitational factors such as alterations to fluid and electrolyte balance, cardiovascular and pulmonary function, and metabolism, and therefore the generalizability of any findings to spaceflight has these inherent limitations [10, 32, 33]. An additional limitation includes the duration of the study. As previously mentioned, ocular symptoms in astronauts increase in prevalence with duration of spaceflight, and a 30-minute HDT analog captures only acute changes. Moreover, an ideal angle for HDT has not been established, with a range of angles from 6° to 15° being used. The 15° angle used in our study may have been too steep and we cannot ensure that our CSF findings would have been replicated with a lower degree of HDT. A study using a 15° HDT protocol to assess venous jugular blood flow before, during, and after spaceflight on International Space Station crew members demonstrated that venous CSA increased from sitting to supine to HDT with a similar magnitude of change between those positions preflight and postflight, although HDT in both of those settings overestimated increases in venous CSA found in-flight . However, our venous and arterial changes from baseline to 15° HDT are corroborated by prior studies at these lower degrees of HDT, which lends support to the validity of our CSF findings at 15° [11, 12, 24].
Our scan protocol did not specify a defined stabilization period prior to the initial supine MRI scan. However, each volunteer experienced lay supine for at least 20 minutes before the scan used for evaluating flow. An additional limitation was that, due to time constraints, we did not repeat the scan protocol afterwards again in the supine position to assess for return to baseline. We also were not able to monitor CO2 during our scan protocol and so cannot correlate our findings with any such changes in a patient’s breathing while undergoing these positional changes. Finally, the radiofrequency coil required that the head be at a 0° orientation above the neck. Excessive neck flexion has been associated with increased intracranial pressure and therefore could be a potential confounder of our findings .
In terms of limitations in statistical analysis, we chose not to apply a Bonferonni correction for multiple comparisons (due to the multiple variables in our study) so as to not make a type II error more likely. However, a lack of correction for multiple comparisons does increase the likelihood of a type I error. As a result, chance associations may exist, and our positive findings should to be replicated. A final limitation includes the lack of well-defined minimal clinically important differences (MCIDs) in our flow variables with respect to SANS.
Future studies should compare arterial, venous, and CSF flow variables in astronauts pre- and post-flight, particularly in those experiencing SANS symptoms in order to generate MCIDs. Non-invasive measurements of these flow variables in astronauts during spaceflight would also be ideal if permitted by technology. In addition, comparisons of HDT variables with imaging from astronauts would help validate HDT as a tool for further investigation of physiological changes to the head in microgravity, which would be critical to justify future studies examining specific populations of interest such as women, minorities, various age groups, or even children to create more individualized risk profiles and predictive models.
In conclusion, we have demonstrated that acute application of 15° HDT to simulate microgravity was associated with alterations in intracranial blood flow and spinal CSF flow dynamics, specifically a reduction in CSF pulsatile flow variables. When combined with our observed decreases in cerebral arterial flow variables as well as a significant increase in jugular venous cross-sectional area, these findings collectively suggest that decreases in cerebral arterial flow were the principal drivers of our observed decreases in CSF pulsatile flow.