It’s likely that elevated intracranial pressure (ICP) and associated visual disturbances due to microgravity exposure are serious health risks for astronauts. Furthermore, this is an even greater concern for potential longer duration missions such as to the moon or Mars where decreased vision may impact mission performance and may even require early mission termination. Permanent visual losses may result in lifetime disability issues. Almost certainly, during long-term flights, as the hydrostatic pressure gradients due to gravity are abolished with resultant large cephalad fluid shifts, an intracranial adaptive response occurs. Thus direct measurements of cerebral fluid shifts and cerebral hemodynamics as proposed in this study allowed investigators to more fully characterize the intracranial compartment guiding the development of preventive measures, such as specific pharmacological countermeasures, to combat the development and sequelae of increased ICP during future missions.
Idiopathic intracranial hypertension, or pseudotumor cerebri, is a poorly understood clinical condition of unknown etiology. It has a reported incidence of between one and five cases per 100,000 people in the general population, although the true incidence may be larger. There is a strong female predominance. Patients typically present with disabling headaches and vision changes. In more extreme cases, injury to the optic nerve can occur and patients can experience vision loss. Upon testing, these patients have increased intracranial pressure and swelling of the optic nerves. Similar symptoms can occur in astronauts following long-term space flight. This study provided a unique opportunity to investigate the underlying pathophysiology of abnormal intracranial pressure regulation which was applicable to those who suffer from this Earth-based condition. This research also shed light on the pathophysiology of other Earth-based disease processes associated with abnormal cerebral spinal fluid (CSF) flow or venous outflow obstruction such as obstructive hydrocephalus, normal pressure hydrocephalus and multiple sclerosis, and pathologies associated with altered cerebral autoregulation such as traumatic brain injury.
This data served as the basis for the development of fundamental theories describing intracranial adaptation to microgravity. It helped establish a risk stratification strategy based on intracranial anatomy, dural venous sinus and jugular vessel outflow, and cerebral vascular autoregulation. Importantly, it served as a basis for interpretation of in-flight data collected via other techniques such as ultrasound, the primary in-flight imaging modality.
As hypercapnia is a potent cerebrovasodilator, investigators hypothesized that HDT+CO2 would result in an increase in cerebral blood flow. Instead they found the opposite. By bed rest day 7 there was a 25.1% global decrease in cerebral blood flow compared with baseline.
Additionally, there was a significant time-by-SANS interaction indicating that the change in perfusion over time varied as a function SANS status. The non-SANS group significantly decreased perfusion between BDC-13 and between measurements at baseline and HDT29.
The SANS group significantly decreased perfusion between baseline and HDT7. However, they then experienced a significant increase in perfusion at HDT29 compared with HDT7. At HDT7, there was a strong trend for the SANS group to have decreased perfusion values compared with the non-SANS group. At HDT29, the SANS group had significantly greater perfusion than the non-SANS group.
These results represent the first MRI assessment of cerebral perfusion in a space flight analog. In this study, investigators showed that the combination of strict HDT along with chronic 0.5% CO2 results in altered cerebral perfusion that is SANS dependent. These results underscore the need for further studies to determine if the changes in perfusion documented here are clinically relevant, whether or not similar changes occur during space flight, and whether or not altered perfusion contributes to the development of SANS or headaches among astronauts.
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