FIGURE 24.

Extracellular fluid efflux and intracranial fluid egress. To distinguish between outflow of fluid from the brain parenchyma and outflow from the intracranial and spinal compartments to the peripheral circulatory systems, we have defined the following terms that we shall use categorically to distinguish between these two serially connected fluid outflow systems. Efflux: glymphatic efflux, also known as brain clearance, is the process by which extracellular fluid and its solutes in intra-axial compartments of the brain parenchyma transfers to cerebrospinal fluid (CSF) compartments or directly to egress sites. For example, glymphatic efflux includes the outflow of waste proteins in the brain extracellular space along major cerebral draining veins, possibly connecting with dural lymphatics and dural parasagittal spaces. Egress: this refers to the transport of CSF, extracellular fluid, solutes and cells from the central nervous system compartment to the periphery, e.g., the transport of CSF tracers from the subarachnoid space through dural lymphatics to the cervical lymphatic system.