Long‐duration spaceflight alters estimated intracranial pressure and cerebral blood velocity

Abstract

During long-duration spaceflights, some astronauts develop structural ocular changes including optic disc oedema that resemble signs of intracranial hypertension. r In the present study, intracranial pressure was estimated non-invasively (nICP) using a model-based analysis of cerebral blood velocity and arterial blood pressure waveforms in 11 astronauts before and after long-duration spaceflights. r Our results show that group-averaged estimates of nICP decreased significantly in nine astronauts without optic disc oedema, suggesting that the cephalad fluid shift during long-duration spaceflight rarely increased postflight intracranial pressure. r The results of the two astronauts with optic disc oedema suggest that both increases and decreases in nICP are observed post-flight in astronauts with ocular alterations, arguing against a primary causal relationship between elevated ICP and spaceflight associated optical changes. r Cerebral blood velocity increased independently of nICP and spaceflight-associated ocular alterations. This increase may be caused by the reduced haemoglobin concentration after long-duration spaceflight.

Spaceflight not an eye‐popping experience for astronauts

“…Long‐term alterations in hydrostatic gradients change cerebral perfusion pressure (mean arterial pressure at the brain minus ICP) but relatively constant cerebral blood flow or velocity is achieved by autoregulatory adaptation on changing posture or spaceflight. Iwasaki and colleagues (2021) reported higher post‐flight cerebral blood velocity but this finding deserves further investigation. Ocular blood flow is influenced by ocular perfusion pressure (OPP = MAP at the eye minus IOP), but there is evidence that ocular vascular resistance operates over a more limited autoregulatory range to increases in OPP (Schmidl et al .…”

“…In this issue of The Journal of Physiology , Dr Iwasaki and colleagues (2021) make an important contribution to understanding the potential aetiology of SANS in their report of pre‐ and post‐spaceflight non‐invasive estimation of intracranial pressure (nICP). nICP was actually lower or unchanged after spaceflight in 10 of 11 astronauts, a finding counter to the assumption of elevated ICP that prompted the initial syndrome name, VIIP.…”

Effects of −10° and −30° head-down tilt on cerebral blood velocity, dynamic cerebral autoregulation, and noninvasively estimated intracranial pressure

“…Alterations in cerebral circulation and intracranial pressure (ICP) induced by the cephalad fluid shift during spaceflight may be partially related to health problems in astronauts, such as headache ( 1 ) and space-associated neuro-ocular syndrome (SANS) ( 2 ). To better understand these important health issues in the field of space medicine, various studies focusing on cerebral circulation and ICP have been conducted in recent years ( 3 , 4 ). However, the etiology of SANS remains contentious ( 2 ).…”

Cardiovascular autonomic nervous system responses and orthostatic intolerance in astronauts and their relevance in daily medicine

“…If anything, cerebral autoregulation was improved after the mission [94]. During more long-term missions, cerebral blood flow velocity increased in proportion to a reduction in hemoglobin levels [95]. The authors suggested that increased flow may have compensated reductions in blood oxygen carrying capacity.…”