Fig. 1

The CSF model. The model consists of three cerebrospinal fluid (CSF) compartments with their own pressure \(p\), volume \(V\), and compliance \(C\). Changes in volume, from supine equilibrium, are denoted \(\Delta V\). CSF is formed in the cranial CSF compartment (denoted \({Q}_{f}\)) and across the lamina cribrosa (LC) (across \({R}_{LC}\)), and the CSF is absorbed to venous blood (across \({R}_{{out}_{c}}\) and \({R}_{{out}_{s}}\), respectively) and across the optic nerve sheath (ONS) (across \({R}_{ou{t}_{ONS}}\)). The spinal (\({p}_{vs}\)) and cranial (\({p}_{vc}\)) venous pressures as well as the intra-orbital pressure (\({p}_{orb})\) constitute backpressures to CSF absorption and contribute to CSF compliance. Intraocular pressure is denoted as \(IOP\). In between compartments we have the resistances \({R}_{ONSAS}\) and \({R}_{C-S}\) for ONSAS and craniospinal flow, respectively, where the former is dependent on the ONSAS pressure (through MRI—derived ONS distensibility at different sections along the ONSAS [18]). Flow rates \(Q\) and their directions are indicated with arrows. The model allows for postural changes through hydrostatic effects that alter pressures and redistributes CSF volumes. The collapse of the jugular veins is included for controlling postural changes in the cranial venous pressure, and by extension ICP [23, 24, 53]. The reference level for the \({p}_{c}=ICP\) is the level of the auditory meatus, the \({p}_{s}\) is referenced to the venous hydrostatic indifference point (HIP), and \({p}_{ONSAS}\) to the LC