Cerebrospinal fluid (CSF) is the clear liquid that surrounds and bathes the brain and spinal cord. It is not only crucial in cushioning the brain and spinal cord and maintaining intracranial pressure but also for the removal of waste products.

The total amount of CSF in the ventricular system and subarachnoid space is approximately 150-160 mL at any one time ⁷.

Physiology

Production

The choroid plexus is believed to produce the majority of CSF. However, there is some evidence that a substantial percentage is also the result of filtration across the parenchymal capillaries and into the extracellular space ⁷.

The process of CSF production is tightly regulated and involves the active transport of ions across the choroid plexus epithelial cells. This results in a net flux of sodium, chloride, and bicarbonate from the blood across the epithelium to the ventricular lumen. This, in turn, generates an osmotic gradient, drawing water through aquaporin-1 molecules, resulting in the production of approximately 500 mL of CSF per day ⁷. 

Glymphatic pathway

Although yet to be fully elucidated, it has been established that a significant proportion of CSF enters the brain parenchyma via perivascular spaces and is involved in the removal of soluble waste products, particularly beta-amyloid ⁷.

For more information, see: glymphatic pathway.

Circulation

The net direction of flow is from the lateral ventricles through the foramen of Monro into the third ventricle and then through the aqueduct into the fourth ventricle. It then leaves the fourth ventricle via the paired foramina Luschka and the midline foramen of Magendie. From there, it flows around the brain and spinal cord, which is somewhat controlled by the presence of arachnoid membranes.

The mechanisms producing the circulation of CSF is complex, and is thought to be due to a combination of ^5,7:

• bulk flow: pressure gradient between choroid plexus and arachnoid granulations

• to-and-fro: due to pulsatile flow in the cerebral arteries

• two-way transport of water and solutes across the blood-brain barrier and also between the CSF and interstitium at cellular membranes

Absorption

Traditional teaching is that the majority of CSF is absorbed into the venous system by the arachnoid villi, which protrude into the dural venous system around the brain, particularly the superior sagittal sinus and transverse sinuses. Further work suggests that this route is not as important as previously thought with a substantial percentage actually draining into the lymphatics within the perineural spaces of the cranial and spinal nerves and around the olfactory bulb ^5,7.

Constituents

The CSF metabolome is a live online database of the chemical content of the cerebrospinal fluid ⁶. 

Most (80%) of the protein in the CSF is derived from the blood, with only 20% from local synthesis within the CNS itself.

Clinically, a fluid or collection can be analyzed for the presence of beta-2 transferrin (asialotransferrin or tau-transferrin) or beta-trace protein to confirm the presence of CSF ^8,9.

Function

There are numerous functions of CSF which include:

• protection of the brain and spinal cord against rapid acceleration and deceleration

• providing buoyancy to the brain

• maintenance of intracranial pressure (see Monro-Kellie doctrine)

• supplies nutrients

• removes metabolites (see glymphatic system)

Radiographic features

CT

CSF has a density close to water, around 0-10 Hounsfield units.

MRI

CSF demonstrates fluid signal on all sequences. Therefore it is of high signal on T2 and ADC sequences and suppresses on FLAIR and DIR sequences. It is of low signal on almost all other sequences (e.g. T1) and does not enhance.