Changes in brain volume after hypoxic exposure and exercise investigated by magnetic resonance imaging

Abstract

Recent data suggest that hypoxia can increase brain volume by inducing cerebral oedema. Exercise is thought to exacerbate acute mountain sickness (AMS) and possibly to enhance cerebral suboedema. The present study investigated the effect of hypoxic exposure and exercise on cerebral volume and oedema by magnetic resonance imaging. Twelve healthy males performed 3 semi-randomized 11-h sessions with i) an inspiratory O₂ fraction (FiO₂) of 11% and 4-h cycling, ii) FiO₂=21% and 4-h cycling and iii) FiO₂=8.5-11% (adjusted to match arterial O₂ saturation measured during session 1) at rest. Volumetric, apparent diffusion coefficient and arterial spin labelling 3T sequences performed after 30 min and 10 h in each session assessed changes in brain volume, cerebral oedema and cerebral blood flow, respectively. 30 min of hypoxia at rest induced a significant increase in white matter volume (+0.8 ± 1.0% compared to normoxia) that was even larger after 10 h in hypoxia at rest (+1.5 ± 1.1%) or with 4-h cycling (+1.6 ± 1.1%). Total brain parenchyma volume increased significantly after 10 h in hypoxia and 4-h cycling only (+1.3 ± 1.1%). Apparent diffusion coefficient was significantly reduced after 10 h in hypoxia at rest (-2.1 ± 3.5%) or with 4-h cycling (-2.4 ± 1.9%). No significant change in cerebral blood flow was observed. Symptoms of AMS did not correlate with changes in cerebral volume. These results demonstrate early change in white matter volume as soon as after 30 min of hypoxia that worsens after 10 h of exposure, probably due to cytotoxic oedema. Exercise accentuates the effect of hypoxia by increasing total brain volume. These changes may not however underlie AMS symptoms.