Structural Brain Changes following Long-Term 6° Head-Down Tilt Bed Rest as an Analog for Spaceflight

Abstract

Background and purpose: Following long-term spaceflight, a subset of the National Aeronautics and Space Administration astronauts present with visual impairment and increased intracranial pressure, known as visual impairment and intracranial pressure syndrome. We investigated structural brain changes following long-term head-down tilt bed rest as a spaceflight analog.

Materials and methods: Volumetric analysis was performed on structural pre- and post-bed rest brain MR images.

Results: Comparing post-bed rest to pre-bed rest images, we found the following: 1) no significant group differences in GM, WM, CSF, or ventricular volumes; 2) shift of the center of mass of the brain upward and posterior rotation of the brain relative to the skull; 3) a significant correlation between posterior brain rotation and changes in ventricular volume; and 4) significant increases in brain tissue density in regions at the vertex, including the frontoparietal lobes, with contraction of adjacent extra-axial CSF spaces, and significant decreases in tissue density in areas along the base of the brain, including the orbitofrontal cortex.

Conclusions: We observed widespread morphologic changes with brain tissue redistribution in response to gravity changes; possible associated functional changes are unknown. The observation that ventricular change is correlated to posterior brain rotation suggests an alteration in CSF homeostasis. Ultimately, to elucidate any structural changes that may play a role in visual impairment and intracranial pressure syndrome, volumetric analysis of pre- and postflight structural scans of astronauts is needed.

Fig 1.

Structural images before and after bed rest. Sagittal images of the brain (subject H in On-line Tables 1–4) at the vertex before (A) and after (B) bed rest show a shift of the brain toward the vertex with contraction of the extra-axial spaces at the vertex and crowding of adjacent structures, including a cortical vein (arrow). Sagittal images of the posterior fossa before (C) and after (D) bed rest (subject A in On-line Tables 1–4). Before bed rest, the occipital lobes (o) lie against the tentorium cerebelli (black arrow). Below the tentorium, there is a thin layer of CSF (red arrow) between the tentorium and the upper aspect of the cerebellum (c). After bed rest, there is an upward shift of the occipital lobes away from the tentorium, now with a thin layer of CSF (red arrow) between the occipital lobes and the tentorium. In the posterior fossa, a layer of CSF is no longer visible between the tentorium and the cerebellum. Instead, the cerebellum now appears compressed against the tentorium. Sagittal images of the frontal lobes before (E) and after (F) bed rest show subtle expansion of the frontal lobe sulci after bed rest (subject A in On-line Tables 1–4).

Fig 2.

Changes in ventricular volume before and after bed rest. Axial images of the brain of the subject with the largest change in ventricular size on the post–bed rest scan (B) compared with the pre–bed rest scan (A). Compared with pre–bed rest, there was a 22.4% reduction in ventricular size post–bed rest in this subject, best appreciated at the level of the atrium of the lateral ventricles (arrow) (subject D in On-line Tables 1–4). Axial images of the brain of the subject with the largest increase in ventricular size on the post–bed rest scan (D) compared with the pre–bed rest scan (C). Compared with pre–bed rest, there was a 10.4% increase in ventricular size post–bed rest in this subject, best appreciated at the level of the frontal horns of the lateral ventricles (arrow) (subject C in On-line Tables 1–4).

Fig 3.

Brain translation and rotation in reference to the skull following bed rest. Parameters (translation and rotation on x, y, z-axes) were estimated on the basis of a rigid-body assumption. The arrow indicates the direction of movement that corresponds to positive values on the graph.

Fig 4.

Correlation between brain rotation and ventricle volume changes. Without the potential outlier, which was the subject that demonstrated the largest ventricles before bed rest, the Spearman correlation is r = 0.893, P = .007. Including the outlier, the Spearman correlation is r = 0.690, P = .058.

Fig 5.

Regions of the brain most significantly affected by bed rest. There is increased brain tissue density (top row of images) at the vertex, particularly affecting the central frontoparietal lobes, with contraction of the adjacent CSF spaces (bottom row of images). There is decreased brain tissue density along the base of the brain, including the orbitofrontal cortex, with expansion of the adjacent CSF spaces.