The hypothesis of our study was that microgravity, as simulated through acute application of 15° HDT, would increase intracranial pressure and alter cerebral blood and spinal CSF flow by causing: 1) increased venous pressure reflected by increased vein cross-sectional area; and 2) a decrease in pulsatile CSF flow. As hypothesized, HDT did significantly decrease pulsatile CSF flow at the mid-C2 vertebral level as manifested by reduced systolic peak flow rate and PtPPAcsf. A significant increase in venous cross-sectional area was also observed, which is indicative of increased venous pressure. An unexpected finding of our study was that HDT significantly decreased arterial average flow rate, systolic peak flow rate, and PtPPAart.
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 [23, 24]. These prior studies collectively corroborate our findings of increased venous CSA and their significance. Our study also reproduces the decrease in arterial flow variables seen in these studies. Finally, we observe a decrease in pulsatile CSF flow not examined in these prior studies.
Of note, 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 . Moreover, Holmlund et al. also showed increases in venous CSA as well as central venous pressure as tilt angle increased from sitting to supine which is again consistent with our results . 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 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 understand our CSF results, it is useful to recall several aspects of CSF physiology under normal gravity conditions and its coupling with cerebral blood flow. 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 . However, this process is unlikely to occur in the setting of acute HDT. 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.
Beyond production and absorption, CSF homeostasis is directly coupled to venous flow dynamics, which is evident by considering data from respiration studies. During inspiration, the chest wall expands and creates negative intrathoracic pressure, which allows for the lungs to fill and also causes venous outflow from the head [28-30]. CSF flow has been observed to be coupled with these intracranial venous changes, with inspiration yielding net CSF flow towards the head while expiration leads to pronounced downward CSF flow [28-30].
Finally, CSF flow from the brain also has significant contribution from cardiac related pulsation . The relative importance of cardiac versus respiratory gradients for CSF flow remain unclear, but the pressure gradients from these two systems are several orders of magnitude greater than that of static pressure gradients tied to CSF absorption .
SANS may involve perturbations to CSF flow 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 [1, 4]. CSF dynamics can be altered by changes in production, flow, or reabsorption. The former and latter occur slowly, and thus are relatively unimportant in the setting of an acute study, such as we have conducted. This leaves changes in arterial flow and the aforementioned physiological coupling with venous flow as plausible explanations for our observed CSF changes.
As shown in our study, heart rate and blood pressure do not significantly change from baseline to HDT, but there are significant decreases in arterial flow variables. Most notably, a decrease in arterial flow could drive decreased CSF flow pulsation, which was observed. In addition, the cross-sectional area of the veins significantly increased, which indicate increased venous pressure in the neck veins. This increased venous pressure could be responsible for the CSF outflow obstruction that is implied by the observed decrease in CSF flow variables in our study, but as previously discussed, this process is unlikely to occur in an acute setting. This pathophysiological connection is supported by prior HDT MRI studies that consistently demonstrate that simulated microgravity causes increased venous pressure as well as decreased arterial flow, although our study is the first to connect those variables with directly observed changes in CSF flow dynamics.
Our results however do not provide evidence for whether arterial flow or increased venous pressure is the primary driver for our observed CSF changes since we were unable to control each variable independently. One possible interpretation of our results is that the venous changes are likely to have had less of an impact due to the changes we see in CSF pulse amplitude. As is well documented within the literature, decreases in CSF pulse amplitude are considered an indicator of increased intracranial compliance . Since increased venous pressure would be expected to increase intracranial pressure, the observed decreases in CSF pulse amplitude and correlated increases in intracranial compliance are not consistent. Furthermore, the argument for increased venous pressure in our results is moderated by the fact that venous blood flow itself did not significantly change from baseline to HDT.
Moreover, given our limited sample size, we can at best report this as an association and cannot directly linearly tie increased in venous CSA with decreases in pulsatile CSF flow. In support of our findings, however, significant jugular venous blood flow stasis has been reported in International Space Station crew members during long-duration spaceflight missions with significant increases in venous jugular CSA noted when comparing ultrasound-based preflight measurements to measurements taken during spaceflight . Notably, this increase in CSA was associated with stagnant blood flow, and these findings were clinically significant when one crew member was found to have developed an occlusive IJV thrombus . Additionally of note is that the relationship of CSF physiology with arterial and venous flow remains a controversial topic, and any conclusions drawn from the aforementioned relationships must be tempered.
Apart from the pathophysiological alterations suggested by the measurements in our study, these variables are additionally 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 improve and extend these models, and we anticipate that these results will be similarly useful for models seeking to replicate the pathophysiology of SANS .
Other limitations 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. In addition, we observed unexpected changes in arterial blood flow, which should remain relatively constant due to cerebral autoregulation of arterial diameter and flow in response to changes in blood pressure.
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 [9, 34, 35]. 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° [10, 11, 36].
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.
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 simulated microgravity is associated with alterations in intracranial blood flow and spinal CSF flow dynamics. HDT caused a reduction in CSF flow variables measuring peak systolic flow and peak-to-peak pulse amplitude which were coupled with an increase in venous CSA suggesting increased venous pressure with HDT. Decreased arterial inflow seen in previous studies with HDT was confirmed in this study.