Cerebrospinal fluid

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Cerebrospinal fluid (CSF), Liquor cerebrospinalis, is a clear bodily fluid that occupies the subarachnoid space in the brain (the space between the skull and the cerebral cortex—more specifically, between the arachnoid and pia layers of the meninges). It is a very pure saline solution with microglia and acts as a "cushion" or buffer for the cortex.

Physiology

Cerebrospinal fluid (CSF) at glance.

Cerebrospinal fluid also occupies the ventricular system of the brain and the spinal cord. It is a prime example of the separation of brain function from the rest of the body, as all CSF is generated locally in the brain. It is produced by the choroid plexus which is formed by specialized ependymal cells. The choroid plexus enter the lateral ventricles through the choroid fissure, along the line of the fimbria/fornix, and the third and fourth ventricle through their roofs. The CSF formed by the choroid plexuses in the ventricles, circulates through the interventricular foramina (foramen of Monro) into the third ventricle and then via the mesencephalic duct (cerebral aqueduct) into the fourth ventricle, whence it exits through two lateral apertures (foramina of Luschka) and one median aperture (foramen of Magendie). It then flows through the cerebromedullary cistern down the spinal cord and over the cerebral hemispheres.

CSF returns to the vascular system primarily along the olfactory tracts and into the lymphatic channels of the nasal mucosa. CSF also drains along cranial and spinal nerve roots. At high intercranial pressures CSF flows into the venous system by the arachnoid granulations.

The cerebrospinal fluid is produced by the ventricles (mostly the lateral ventricles) at a rate of 500 ml/day. Since the volume that may be contained by the brain is of 150 ml, it is frequently replaced (3-4 times per day turnover), exceeding amounts getting into the blood. This continuous flow through the ventricular system into the subarachnoid space and finally exiting into the venous system provides somewhat of a "sink" that reduces the concentration of larger, lipoinsoluble molecules penetrating into the brain and CSF. (Saunders et al., 1999)

The CSF contains approximately 0.3% plasma proteins, also being 15 to 40 mg/dL, depending on sampling site. (Felgenhauer, 1974)

Pathology

The cerebrospinal fluid has many putative roles including mechanical protection of the brain, distribution of neuroendocrine factors, and facilitation of pulsatile cerebral blood flow. Understanding cardiovascular dynamics is valuable as the flow pattern of arterial blood must be tightly regulated within the brain in order to ensure consistent brain oxygenation. CSF movement allows arterial expansion and contraction by acting like a spring, which prevents wide changes in intracranial blood flow. When disorders of CSF flow occur, they may therefore impact not only CSF movement, but also the intracranial blood flow, with subsequent neuronal and glial vulnerabilities. The venous system is also important in this equation. Infants and patients shunted as small children may have particularly unexpected relationships between pressure and ventricular size, possibly due in part to venous pressure dynamics. This may have significant treatment implications but the underlying pathophysiology needs to be further explored.

CSF connections with the lymphatic system have been demonstrated in several mammalian systems. Preliminary data suggest that these CSF-lymph connections form around the time that the CSF secretory capacity of the choroid plexus is developing (in utero). There may be some relationship between CSF disorders, including hydrocephalus and impaired CSF lymphatic transport.

Diagnosis and therapy

Cerebrospinal fluid can be tested for the diagnosis of a variety of neurological diseases. It is usually obtained by a procedure called lumbar puncture in an attempt to count the cells in the fluid and to detect the levels of protein and glucose. These parameters alone may be extremely beneficial in the diagnosis of subarachnoid hemorrhage and central nervous system infections (such as meningitis). Moreover, a cerebrospinal fluid culture examination may yield the microorganism that has caused the infection. By using more sophisticated methods, such as the detection of the oligoclonal bands, an ongoing inflammatory condition (for example, multiple sclerosis) can be recognized. A beta-2 transferrin assay is highly specific and sensitive for the detection for e.g. cerebrospinal fluid leakage.

Lumbar puncture can also be performed to measure the intracranial pressure, which might be increased in certain types of hydrocephalus.

This fluid has an importance in anethesology. Baricity refers to the density of a substance compared to the density of human cerebral spinal fluid. Baricity is used in anesthesia to determine the manner in which a particular drug will spread in the intrathecal space.

Notes

Meninges of the brain and medulla spinalis

Dura mater - Falx cerebri - Tentorium cerebelli - Falx cerebelli - Arachnoid mater - Subarachnoid space - Cistern - Cisterna magna - Median aperture - Cerebrospinal fluid - Arachnoid granulation - Pia mater