Brain Tissue–Volume Changes in Cosmonauts
Long-duration spaceflight has detrimental effects in several physiological systems. Several studies have shown an upward shift of the cerebral hemispheres, a decrease in frontotemporal volume, and an increase in ventricle size after spaceflight. However, information is limited about the effects of microgravity on brain volume, particularly regarding changes that are evident more than 1 month after spaceflight.
We prospectively studied data from T1-weighted magnetic resonance imaging (MRI) that was performed in 10 male cosmonauts (mean age, 44 years; average space-mission duration, 189 days) at three time points: preflight (in 10 cosmonauts), short-term postflight (average, 9 days postflight; in 10), and long-term postflight follow-up (average, 209 days postflight; in 7). The volumes of gray matter, white matter, and cerebrospinal fluid (CSF) were analyzed with the use of voxel-based morphometry. (The complete methods and additional analyses are provided in the Supplementary Appendix, which is available with the full text of this letter at NEJM.org.) Aging effects that may occur over the interval between preflight and postflight were accounted for by longitudinal data from matched controls.
The gray-matter volume postflight as compared with preflight showed a widespread decrease in the orbitofrontal and temporal cortexes; the maximal decrease was 3.3% in the right middle temporal gyrus. At long-term postflight follow-up, most reductions in gray-matter volume had recovered toward preflight levels (e.g., a 1.2% reduction in gray-matter volume persisted in the right temporal gyrus). The white-matter volume postflight as compared with preflight was reduced along a longitudinal tract of the left temporal lobe, but there was a global reduction of cerebral white-matter volume at long-term follow-up as compared with postflight. The ventral CSF spaces of the cerebral hemispheres and the ventricles had increased in volume postflight as compared with preflight (maximal increase, 12.9% in the third ventricle), while CSF volume below the vertex decreased. At long-term follow-up, the CSF volume in the ventricles had returned toward preflight values, while the CSF volume in the entire subarachnoid space around the brain had increased. Changes in the volumes of gray matter and CSF are shown in Figure 1.
The findings from an average of 7 months after a return to Earth can be summarized as showing that most of the loss in the gray-matter volume that was seen immediately postflight had recovered to preflight levels, while CSF volume continued to increase in the subarachnoid compartment. The expansion of CSF spaces in light of postflight decreases in the gray-matter volume and a reduction in the white-matter volume at follow-up suggests a persistent disturbance in CSF circulation even many months after a return to Earth. These brain-volume changes may relate to clinical findings, such as ocular and visual abnormalities after long-duration spaceflight. Future investigation is required in order to determine the overall clinical significance of the findings and to mitigate risks in long space missions.