Hypohydration Alters Brain Morphology and Function While Impairing Fine Motor Performance
Authors: Wittbrodt Matthew T , Sawka Michael N , Mizelle J.C. , Wheaton Lewis A , Millard‐Stafford Mindy L 
Published: November 2023

The impact of body water deficits on brain morphology and function are poorly defined, while its impact on cognitive‐motor performance is controversial. We hypothesized that body water deficits following exercise‐heat stress would enlarge brain ventricles, reduce volume of periventricular structures, and alter the neural resources required to perform a fine‐motor task in order to maintain performance. Eleven healthy adults (6 males, 5 females) were evaluated for fine motor performance during control (CON) and 45 min after exercise‐heat stress (2.5 h graded treadmill walking at ~3 mph in 45°C, 15% RH) with (Euhydration, EU) and without (Hypohydration, HYPO) fluid replacement. Fine motor (right index finger paced tapping at regular and irregular intervals) performance (reaction time and temporal accuracy) was evaluated in a Magnetic Resonance Imaging scanner while determining changes in brain morphology and Blood Oxygen Level Dependent (BOLD) responses. Body mass loss was −3.0 ± 0.6% (P < 0.01) for HYPO. Plasma osmolality was 286 ± 5 and 281 ± 4 for CON and EU trials but 295 ± 5 mOsm/kg for HYPO (P < 0.01); while plasma glucose was maintained within the normal range (> 4.0 mmol/L). Reaction time was not different between the three trials for either the irregular or regular interval variations. Accuracy was impaired by HYPO (vs. CON) during both regular (−19%, P < 0.01) and irregular interval variations (−18%, P < 0.01) but was not different for EU (vs. CON). Brain morphology (n = 9) changes were evaluated. HYPO (vs. CON) induced lateral ventricle expansion (0.86 ± 0.67 cm3, P < 0.01) while EU (vs. CON) induced lateral ventricle shrinkage (−0.82 ± 0.67 cm3, P < 0.01). Thus, the net difference in lateral ventricle size was 1.68 ± 0.67 cm3 between the EU and HYPO trials. The only periventricular structure that changed was thalamus shrinkage (−0.49 ± 0.41 cm3, P < 0.01) with HYPO (vs CON). BOLD responses (n = 8) were not different between the three trials during the irregular interval variation; however, during regularly‐paced interval variation HYPO (vs. CON) elevated (P < 0.05) BOLD responses in the bilateral thalamus, bilateral basal ganglia (putamen, caudate, pallidum), bilateral hippocampus, and right amygdala. These data demonstrate that hypohydration alters brain morphology and require greater neural resources in areas associated with motor production and emotive responses. Furthermore, the elevated neural resources were insufficient to mitigate impairments in fine motor performance. These findings demonstrate the susceptibility of the cognitive‐motor system and fine motor performance to hypohydration, which has profound implications to occupational/military/athletic populations. In addition, it raises the possibility that hypohydration might exacerbate impairments in cognitive‐fine motor task performance in persons suffering from neurodegenerative disorders (e.g., dementia and traumatic brain injuries).Support or Funding InformationSupported by Center for Advanced Brain Imaging Seed Grant, Neuroengineering Seed Grant, Georgia Institute of Technology, and Gisolfi Research Award, American College of Sports Medicine