cerebrospinal fluid

cerebrospinal fluid The brain floats on a liquid cushion of cerebrospinal fluid (CSF) within the rigid bony skull. The CSF is contained between layers of the meninges, the membranes that enclose the brain. It fills the subarachnoid space between the delicate arachnoid mater that lines the tough fibrous outer covering, the dura mater, and the pia mater that covers the soft substance of the brain.

Since the brain floats in CSF, the fluid acts in effect to reduce the weight of the brain from some 1000 g to about 50 g, and also protects the brain from knocks on the head. However, since the brain can move within the CS, it can be damaged on the opposite side by a sudden deceleration such as in a car accident (contra coup injury).

The subarachnoid space on the outside of the brain is in continuity with a similar space around the spinal cord and also with the series of interconnected cerebral ventricles within the brain (see figure). Each of the paired lateral ventricles, in the cerebral hemispheres, contains a leaf-like, highly vascular choroid plexus. It is from these structures that the bulk of the CSF is secreted. From the lateral ventricles CSF drains into the central third ventricle, and thence through the aqueduct in the midbrain into the fourth ventricle. Both the third and fourth ventricles contribute to the flow from their own choroidal tissue. From the fourth ventricle, the CSF exits into the subarachnoid space through several openings, and fills the ‘basal cisterns’ beneath the brain. Thence the flow of CSF is mainly up and over the whole brain surface, whilst some flows down around the spinal cord. Completing the circuit back to the bloodstream, the fluid drains via the valve-like arachnoid granulations into the sagittal sinus, the large venous channel lying centrally on the top of the brain; some is also taken up into veins around spinal nerve roots and into the lymphatics of the nose.

The secretion of CSF is an active transport process that moves fluid and solutes from the blood plasma into the ventricles, the choroid plexuses being a specialized part of the blood–brain barrier. CSF secretion involves the pumping of ions and specialized ion channels, with the energy coming from glucose and oxygen in the blood. In the adult human CSF is formed at a rate of about 0.5 ml/min; the total volume is about 200 ml, of which 30 ml is in the ventricles and the remainder in the subarachnoid space. The circulation of CSF leads to the fluid being completely replaced about every 4 hours.

CSF is a weak salt solution with similar inorganic ion concentrations to plasma, but with small and significant differences, whereas the protein content is about 100 times less than that of plasma (0.5 g/litre compared to 50–70 g/litre). Abnormalities of the CSF can be important in diagnosis of some medical conditions; the fluid can be sampled by lumbar puncture from the extension of the subarachnoid space (the lumbar sac) below the lower end of the spinal cord. CSF is normally a clear, amazingly ‘bright’ fluid, and if it is cloudy or contains a raised level of protein or traces of blood this is usually an indication of brain infection, some types of brain or spinal cord tumour, or trauma.

The pressure within the brain, the intracranial pressure (ICP), is transmitted in the CSF around the spinal cord and down into the lumbar sac. With the body horizontal, it is normally low (about 10 cm H₂O); it is markedly affected by posture, and raised by straining or coughing.

Blockage in the drainage pathways for CSF is one of the causes of a raised ICP, since the CSF is actively ‘pumped’ into the ventricular system. In an adult this raised pressure can cause expansion of the ventricles, with loss of neural tissue by compression against the rigid skull. In infants, when CSF drainage pathways have failed to develop normally, the raised ICP causes the head to swell because the junctions between the skull bones are not fused, resulting in hydrocephalus — ‘water on the brain’.

A raised intracranial pressure can often be recognized by looking into the eye with an ophthalmoscope, an instrument which shines a beam of light on to the retina at the back of the eye. The beam is focused onto the ‘optic disc’, where the nerves of the eye converge to pass to the brain. Normally this appears as a clearly-defined, pale concave disc, but if the pressure in the CSF is raised, the disc may bulge forwards into the cavity of the eye. As well as by blockage of CSF circulation, raised pressure can be caused by an expanding tumour or blood clot, or by swelling of a damaged or diseased brain.

The CSF also acts as a drainage route for waste products of brain metabolism, additional to their direct excretion into the capillary blood vessels everywhere in the brain across the blood–brain barrier.

Malcolm Segal

See also blood–brain barrier; hydrocephalus; meninges.