Responses of the Human Brain to Mild Dehydration and Rehydration Explored In Vivo by ¹H-MR Imaging and Spectroscopy

Serial data of brain tissue fluid H₂O_brain. Twelve hours of thirsting (dehydration) increased serum osmolality by 0.67% and decreased H₂O_brain by 1.63%. Subsequent oral fluid intake (rehydration) during 1 hour lowered serum osmolality by 0.96% and led to an increase of H₂O_brain by 0.43%. This finding shows that even subtle changes in H₂O_brain on minor osmotic challenges are detectable by ¹H-MR spectroscopy.

Volumetric morphometry at normo-, de-, and rehydration illustrated as percentage volume change normalized to session 2 (dehydration). Volume changes of the entire brain, cerebral cortex, white matter, and hypothalamus/thalamus were significant in repeated measures ANOVA and compatible with cell shrinking during hyperosmolality and cell swelling during hypo-osmolality. Subject-specific pair-wise differences are plotted with respect to dehydration. Post hoc tests revealed hydration states with significant volume changes between each one (indicated by asterisks; for details see On-line Table 3). Box indicates upper and lower quartiles; thick black line, median; whiskers, most extreme values of the interquartile range; crosses, outliers; D, dehydration; N, normohydration; R, rehydration; asterisk, significant difference.

Cortical thickness analysis. Local changes of cortical thickness (A, red-to-yellow: for thinning on dehydration; blue-to-light blue: for thickening on rehydration). Dehydration primarily induces cortical thinning (upper row), which reverses on rehydration (bottom row). Note that these prevailing changes are not uniformly distributed over the cerebral surface. Changes on the mesial surface (not shown) were slightly less pronounced but similar. The corresponding statistical significance (B, red-to-yellow: for thinning on dehydration; blue-to-light blue: for thickening upon rehydration) is expressed by increasingly lower false-positive probabilities across subjects.