What is Cerebro-Spinal Fluid?
The Central Nervous System is made of all the nervous tissues which form the encephalon and the spinal cord. It is made of white and grey matter (nerves are excluded).
These noble tissues are essential to the human being functioning, as the encephalon ensures control over the whole body, but they are fragile and must be protected.
The Central Nervous System is protected by three elements. From the outside to the inside:
- the cranium bones (for the encephalon) and the spinal canal (for the spinal cord) act like a “helmet".
- the meninges act like « a protective wrapping »:
- the dura mater, which is the outside layer,
- the arachnoid,
- the pia mater which is directly in contact with the central nervous system.
- the CSF, or Cerebro-Spinal Fluid, which circulates between the arachnoid and the pia mater. It plays the role of a shock absorber.
The brain consists of two cerebral hemispheres (one right and one left), which meet in their lower midline part to form the diencephalon, which extends into the brain stem. The brain stem joins the spinal cord in its lower part.
Each cerebral hemisphere contains a cavity called the lateral ventricle. The two lateral ventricles take the shape of a crescent which is open anteriorly and which consists of three horns (frontal, temporal and occipital). They are connected in the midline to the underlying third ventricle through the two foramina of Monroe. In the middle of these two lateral ventricles, the choroid plexus hold an important role. It takes part in the production of CSF.
The diencephalon is the place where the two cerebral hemispheres meet, in which the 3rd ventricle lies.
The brainstem is located between the brain above and the spinal cord below. It consists of 3 parts, the mesencephalon (or midbrain), the pons and the spinal bulb (or medulla).
The 4th ventricle is delineated anteriorly by the posterior surface of the pons and posteriorly by the cerebellum. It communicates superiorly with the 3rd ventricle through the aqueduct of Sylvius and inferiorly with the ependymal canal and the sub-arachnoid spaces through the two lateral foramina of Luschka and the median foramen of Magendie.
Cerebro-spinal fluid (CSF) is the transparent «gin clear» fluid that fills the Central Nervous System.
CSF composition is similar to serum composition, besides protein, calcium and protein concentrations which are lower.
|Serum normal||CSF normal|
|Na + (mmol/l)||145||150|
|K + (mmol/l)||4||3|
|Ca 2+ (mmol/l)||2.5||1|
|Mg 2+ (mmol/l)||
Any change in the CSF composition is of great significance in the diagnostic of certain brain diseases.
CSF is normally as clear as water and does not contain any blood cell (leukocytes, aka white blood cells and erythrocytes, aka red blood cells).
However, in infections such as meningitis, leukocytes may pass into the CSF and after hemorrhage, red blood cells may be found in CSF.
Protein concentration in the CSF, usually very low, is increased in case of infection and if CSF reabsorption at the level of the arachnoïd villi is impaired.
In a reverse way, glucose concentration is decreased in some pathological conditions (tumor, acute bacterial infection, fungal infections...)
CSF has two important roles:
- A mechanical protective role for the brain and the spinal cord: the CSF bathes the ventricles and the sub-arachnoid spaces. It is a real shock absorber for the brain avoiding the cranium to crush the brain in case of a trauma. It plays also the same role for the spinal cord.
- A metabolic role between the CNS and the rest of the body: the CSF takes part in the metabolic exchanges bringing nutrients and cleaning waste due to cellular metabolism.
CSF is produced in the ventricles by the choroid plexus at a rate of approximately 20 ml/h in adults (8 ml/h in infants). The total volume of CSF in circulation in the CNS is 125 ml to 150 ml. It allows it to play its shock absorber role. Then, the CSF is reabsorbed by the body.
The CSF circulates within the lateral ventricles, the 3rd ventricle and leaves the 4th ventricle through the foramina of Luschka and Magendie, entering the sub-arachnoid spaces surrounding the brain and spinal cord. These sub-arachnoid spaces are fibrous tissues situated between the pia mater and the arachnoid, which behave like a sponge. Ventricles are cavities that have a role of reservoir for the CSF.
The fluid returns to the venous circulation by absorption through small formations, the arachnoid villi (Pacchioni granules), a type of outgrowth from the arachnoid, through the dura mater and spaces in contact with the sagittal sinus at the midline of the brain.
With this cycle of production, circulation and absorption, under normal conditions, a perfect equilibrium exists between secretion and absorption of CSF.
The average CSF pressure in adults when lying down is 80-200 mmH2O (6 to 15 mmHg). The pressure is almost zero or even negative when standing up. The reference measurement of CSF pressure is measured at the level of the cerebral ventricles: this is intraventricular pressure, which is often called intra-cranial pressure (ICP).