How volume changes in the epidural space drives respiratory cerebrospinal fluid flow

How cerebrospinal fluid (CSF) circulates around the brain and spine is important to understand solute transport and the mechanisms of CSF flow disorders. It has recently been shown that respiratory-associated spinal CSF flows are influenced by intrathoracic and abdominal pressures, as well as by cranial blood volume. The mechanism of this remains unclear, and we hypothesise that differences in thoracic and lumbar pressures during respiration drive spinal epidural blood volume changes, which in turn drive CSF movement. We tested this hypothesis using a simple model of the whole spinal subarachnoid space (SSAS) and deformed the boundaries of the SSAS to simulate the effect of changes in epidural venous blood volumes. The model showed that the direction of cervical CSF flow depended on the relative difference in the volumes of the thoracic and lumbar SSAS. When the volume increase of the thoracic SSAS was the same or larger than the reduction of the lumbar SSAS, cervical CSF was drawn caudally, but when the change in thoracic SSAS was smaller, cervical CSF was displaced cranially. These models showed that the direction of cervical CSF flow was sensitive to small differences in the thoracic and lumbar SSAS. Since the SSAS volume change depends on the intrathoracic and abdominal pressures that drive venous blood through the epidural veins, these models suggest that respiratory manoeuvres that produce a large pressure gradient across the diaphragm are more likely to draw CSF caudally from the cranium into the SSAS.