Acute mountain sickness (AMS) and high-altitude cerebral edema share common clinical characteristics, suggesting cerebral swelling may be an important factor in the pathophysiology of AMS. Hypoxia and hypocapnia associated with high altitude are known to exert strong effects on the control of the cerebral circulation, yet how these effects interact during acute hypoxia, and whether AMS-susceptible subjects may have a unique response, is still unclear. To test if self-identified AMS-susceptible individuals show altered brain swelling in response to acute hypoxia, we used quantitative arterial spin-labeling and volumetric MRI to measure cerebral blood flow and cerebrospinal fluid (CSF) volume changes during 40 min of acute hypoxia. We estimated changes in cerebral blood volume (CBV) (from changes in cerebral blood flow) and brain parenchyma swelling (from changes in CBV and CSF). Subjects with extensive high-altitude experience in two groups participated: self-identified AMS-susceptible (n = 6), who invariably experienced AMS at altitude, and self-identified AMS-resistant (n = 6), who almost never experienced symptoms. During 40-min hypoxia, intracranial CSF volume decreased significantly [-10.5 ml (SD 6.9), P < 0.001]. There were significant increases in CBV [+2.3 ml (SD 2.5), P < 0.005] and brain parenchyma volume [+8.2 ml (SD 6.4), P < 0.001]. However, there was no significant difference between self-identified AMS-susceptible and AMS-resistant groups for these acute-phase changes. In acute hypoxia, brain swelling occurs earlier than previously described, with significant shifts in intracranial CSF occurring as early as 40 min after exposure. These acute-phase changes are present in all individuals, irrespective of susceptibility to AMS.
Individuals susceptible to high altitude pulmonary edema show altered pulmonary vascular responses within minutes of exposure to hypoxia. We hypothesized that a similar acute-phase vulnerability to hypoxia may exist in the brain of individuals susceptible to acute mountain sickness (AMS). In established AMS and high-altitude cerebral edema, there is a propensity for vasogenic white matter edema. We therefore hypothesized that increased cerebral blood flow (CBF) during acute hypoxia would also be disproportionately greater in white matter (WM) than grey matter (GM) in AMSsusceptible subjects.We quantified regional CBF using arterial spin labeling MRI during 30-minutes hypoxia
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