Cerebrospinal Fluid (CSF)

Cerebrospinal Fluid (CSF), a clear, colorless and water-like fluid secreted by the choroid plexus in the ventricular system (chambers) of the brains. From the ventricles, the CSF normally circulates in the subarachnoid space between the pia and the arachnoid, the two innermost membranes lining the skull and the vertebral canal.

The central nervous system contains approximate 150 cubic centimeters (ccs) of CSF. It is renewed about four times daily. The CSF is absorbed through the meninges by special valve-like structures.

When the lumbar subarachnoid space is punctured in the low back, CSF pressure can be measured by means of a manometer, a transparent calibrated tube calibrated in millimeters of water. The pressure in the CSF drives it to a certain height in the manometer, corresponding to the pressure of CSF. The height is measured in millimeters (mm) and the pressure is expressed in terms of mm of water, or H2O. Although CSF is not physically or chemically identical to H20, it is deemed to be the same for purposes of pressure measurement.

Pressure can also be measured by cannulating the ventricles of the brain. This procedure is more invasive than a lumbar puncture, but often produces more accurate measurements. The ventricular cannula (or needle) is attached to a manometer in the same way as the lumbar puncture needle. Ventricular pressure measured manometrically is also expressed in mm H2O. The advantage of ventricular puncture is that it can be therapeutic as well as diagnostic. Ventricular cannulation permits calibrated external drainage of CSF from the ventricle. External drainage may help reduce elevated ICP.

More recently, pressure sensors have been designed to be implanted intraparenchymally, within the substance of the brain. This simplifies the surgical procedure. Following the convention used in measuring blood pressure, these sensors are calibrated in millimeters of mercury (mm Hg) rather than in mm H2O. The measurements are interchangeable. One mm Hg is equal to 13.6 mm H2O.

Normal CSF pressure is generally cited as approximately 60–250 mm H20 (95% confidence intervals) with a population mean of 180mm in the usual clinical setting. There is considerable variability around this range, however, and a good deal of clinical judgement is be required to assess what value is acceptable for any individual patient.

In most cases, pressure in the ventricles, in the parenchyma and in the lumbar subarachnoid space are almost identical. There are conditions in which the normal circulation of CSF is perturbed, however, in which case the pressure in the various compartments of the central nervous system cannot equilibrate.

Abnormalities of ICP can result from many neurological and neurosurgical conditions, including trauma, tumors, hemorrhage, stroke, hydrocephalus, congenital abnormalities, infection, and other sources of altered CSF or intracranial blood flow dynamics. Elevated ICP (intracranial hypertension) may compromise cerebral perfusion or blood flow when the ICP exceeds mean arterial blood pressure. At that point, the brain may be starved of blood (ischaemia) and oxygen (hypoxia), resulting in neurological dysfunction which can progress to brain damage, visual loss and death. Elevated ICP requires careful monitoring and may require urgent intervention. Low ICP (intracranial hypotension) may also be abnormal and may also require treatment.

Pressure values must be interpreted in light of the clinical setting (Lee SC, Lueck CJ. Cerebrospinal Fluid Pressure in Adults. J Neuroophthalmology 2014. Sep; 34(3):278-83).

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