Role of the spinal canal compliance in regulating posture-related cerebrospinal fluid hydrodynamics in humans.

Mechanical compliance of a compartment is defined by the change in its volume with respect to a change in the inside pressure. The compliance of the spinal canal regulates the intracranial pressure (ICP) under postural changes. Understanding how gravity affects ICP is beneficial for poorly understood cerebrospinal fluid (CSF)-related disorders. The aim of this study was to evaluate postural effects on cranial hemo- and hydrodynamics. This was a prospective study, which included 10 healthy volunteers (three males, seven females, mean ± standard deviation age: 29 ± 7 years). Cine gradient-echo phase-contrast sequence acquired at 0.5 T, "GE double-doughnut" scanner was used. Spinal contribution to overall craniospinal compliance (CSC), craniospinal CSF stroke volume (SV), magnetic resonance (MR)-derived ICP (MR-ICP), and total cerebral blood flow (TCBF) were measured in supine and upright postures using automated blood and CSF flows quantification. Statistical tests performed were two-sided Student's t-test, Cohen's d, and Pearson correlation coefficient. MR-ICP and the craniospinal CSF SV were significantly correlated with the spinal contribution to the overall CSC (r = 0.83, p < 0.05) and (r = 0.62, p < 0.05), respectively. Cranial contribution to CSC increased from 44.5% ± 16% in supine to 74.9% ± 8.4% in upright posture. The average MR-ICP dropped from 9.9 ± 3.4 mmHg in supine to -3.5 ± 1.5 mmHg. The CSF SV was over 2.5 times higher in the supine position (0.55 ± 0.14 ml) than in the upright position (0.21 ± 0.13 ml). In contrast, TCBF was slightly higher in the supine posture (822 ± 152 ml/min) than in the upright posture (761 ± 139 ml/min), although not statistically significant (p = 0.16). The spinal-canal compliance contribution to CSC is larger than the cranial contribution in the supine posture and smaller in the upright posture. Thereby, the spinal canal plays a role in modulating ICP upon postural changes. The lower pressure craniospinal CSF system was more affected by postural changes than the higher-pressure cerebral vascular system. Craniospinal hydrodynamics is affected by gravity and is likely to be altered by its absence in space. LEVEL OF EVIDENCE: 4 TECHNICAL EFFICACY STAGE: 2.