Spaceflight-Induced Intracranial Hypertension

Abstract

Although once a widely speculated about and largely theoretical topic, spaceflight-induced intracranial hypertension has gained acceptance as a distinct clinical phenomenon, yet the underlying physiological mechanisms are still poorly understood. In the past, many terms were used to describe the symptoms of malaise, nausea, vomiting, and vertigo, though longer duration spaceflights have increased the prevalence of overlapping symptoms of headache and visual disturbance. Spaceflight-induced visual pathology is thought to be a manifestation of increased intracranial pressure (ICP) because of its similar presentation to cases of known intracranial hypertension on Earth as well as the documentation of increased ICP by lumbar puncture in symptomatic astronauts upon return to gravity. The most likely mechanisms of spaceflight-induced increased ICP include a cephalad shift of body fluids, venous outflow obstruction, blood-brain barrier breakdown, and disruption to CSF flow. The relative contribution of increased ICP to the symptoms experienced during spaceflight is currently unknown, though other factors recently posited to contribute include local effects on ocular structures, individual differences in metabolism, and the vasodilator effects of carbon dioxide. This review article attempts to consolidate the literature regarding spaceflight-induced intracranial hypertension and distinguish it from other pathologies with similar symptomatology. It discusses the proposed physiological causes and the pathological manifestations of increased ICP in the spaceflight environment and provides considerations for future long-term space travel. In the future, it will be critical to develop countermeasures so that astronauts can participate at their peak potential and return safely to Earth.

Internal jugular pressure increases during parabolic flight

“…Recent data suggest that ICP during parabolic flight is not different than that measured while supine in 1G (Lawley et al. ), but at this time no IJVP, IOP, and ICP measurements during space flight have been reported (Michael and Marshall‐Bowman ), and thus the relation between these and the development of the VIIP syndrome in long‐duration astronauts is yet to be established. To date, only postspace flight lumbar opening pressures have been reported in six astronauts who have demonstrated VIIP symptoms, and these were only acquired 7–66 day after landing (Mader et al.…”

“…The physiologic adaptations which contribute to the VIIP syndrome have not been clearly elucidated but the cephalad fluid shift that occurs upon entry in weightlessness has been hypothesized to be a significant contributor (Michael and Marshall‐Bowman ); approximately 2 L of fluid shift into the upper body from the legs during weightlessness (Thornton et al. ).…”

“…However, a relationship between changes in IJVP and the development of VIIP symptoms must be confirmed during long-duration exposure to weightlessness. The physiologic adaptations which contribute to the VIIP syndrome have not been clearly elucidated but the cephalad fluid shift that occurs upon entry in weightlessness has been hypothesized to be a significant contributor (Michael and Marshall-Bowman 2015); approximately 2 L of fluid shift into the upper body from the legs during weightlessness (Thornton et al 1987). Although a large portion of the fluid is eliminated from the vascular space, as evidenced by plasma and blood volume loss (Leach et al 1996), some fluid continues to reside in intracellular spaces and in the veins of the upper body.…”

In-flight analysis of intracranial pressure in pilots undergoing variation in Gz

“…Long-term exposure to an increase or absence of gravity has aroused the concern of the aerospace medical community as its effects on the nervous system are unknown. 32 One of the objectives of this experiment was to establish whether there is an increase in blood flow to the brain and as a consequence an increase in ICP during human exposure to an increase in Gz. There is evidence in the results of this work that corroborate with this objective.…”

“…A new emerging science in human factors known as neuroergonomics seeks to understand human performance in complex systems, in relation to underlying brain mechanisms, and to use this knowledge to improve the human in-flight performance system. 32 This research area deals with a major threat for modern fighter jet pilots known as gravity-induced loss of consciousness, which occurs during sudden surges or acrobatics. 7 The G-LOC is caused by a sudden reduction in brain O 2 (rSO 2 ) as a result of the increase in the + Gz force.…”