Cerebrospinal fluid

Cerebrospinal fluid (CSF) is the clear liquid that surrounds and bathes the brain and spinal cord.

Physiology

Production

Cerebrospinal fluid is produced by the epithelium of the choroid plexus within the ventricular system and flows in the direction from the lateral ventricles to the third ventricle, then fourth ventricle and then around the brain and spinal cord. 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. Up to 15% of CSF drains into the lymphatics within the perineural spaces of the cranial and spinal nerves ⁵.

Circulation

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

• 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

Normal volume ranges differ depending on the source but are generally in the 150-250 mL range with approximately 700 mL produced daily, hence the entire CSF volume is replaced 3-4 times per day.

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.

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 lymphatic 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 and does not enhance.