Postural influence on intracranial fluid dynamics: an overview

Abstract

This review focuses on the effects of different body positions on intracranial fluid dynamics, including cerebral arterial and venous flow, cerebrospinal fluid (CSF) hydrodynamics, and intracranial pressure (ICP). It also discusses research methods used to quantify these effects. Specifically, the implications of three types of body positions (orthostatic, supine, and antiorthostatic) on cerebral blood flow, venous outflow, and CSF circulation are explored, with a particular emphasis on cerebrovascular autoregulation during microgravity and head-down tilt (HDT), as well as posture-dependent changes in cerebral venous and CSF flow, ICP, and intracranial compliance (ICC). The review aims to provide a comprehensive analysis of intracranial fluid dynamics during different body positions, with the potential to enhance our understanding of intracranial and craniospinal physiology.

Keywords: Brain blood flow; Cerebral autoregulation; Cerebrospinal fluid; Craniospinal system; Head-down tilt; Head-up tilt; Intracranial compliance; Intracranial pressure; Postural changes.

Fig. 1

Two graph variants of the volume-pressure curve that depict ICC. A The classical volume-pressure curve formed by the ratio of changes in intracranial volume (∆V) to changes in ICP (∆ICP). B The graph with absolute ICP values on the ordinate axis instead of differential ICP values. In this graph, the volume-pressure curve and its possible continuation in a dashed line are roughly shaped according to the first graph and the sources [58, 59, 61, 62]

Fig. 2

Proportional changes in volumes of intracranial compartments and ICC in spatial compensation and decompensation phases. Under normal physiological and compensatory conditions, ICC is high, indicating that there is spare space in the cranium and ICP remains relatively stable. However, in the decompensation phase, due to abnormal increases in overall intracranial volume, ICC becomes critically low, resulting in high ICP values. In this phase, any additional volume added will cause ICP to increase exponentially

Fig. 3

Concept diagram of potential volume interrelationships of intracranial compartments and compliance. Since brain volume does not significantly change under normal physiological conditions, it is considered constant and is not included in the diagram. The up and down arrows over the compartments indicate an increase or decrease in the respective fluid flow. The dashed arrows and lines represent hypothetical changes due to limited evidence or controversial data. On the right side, the literature studies supporting the depicted volume distribution and ICC changes are presented. The ICP values in the antiorthostatic position are assumed to increase to an upper normal limit of 20 mmHg in healthy individuals