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How Shunts Work

The following excerpt is taken from Chapter Four of Hydrocephalus:A Guide for Patients, Families, and Friends by Chuck Toporek & Kellie Robinson, copyright 1999 by O'Reilly & Associates, Inc. For book orders/information, call 1-800-998-9938. Permission is grantedto print and distribute this excerpt for noncommercial use as long as the above source is included. The information in this article is meant to educate and should not be used as an alternative for professional medical care.

When the flow of cerebrospinal fluid (CSF) is normal and unobstructed, new CSF is constantly produced, flowing into the ventricles and out of the brain again. Hydrocephalus can occur for one of two basic reasons: there is an obstruction in one of the CSF pathways, or CSF is not permitted to be reabsorbed. Figure 1, "Before shunt placement," shows a CT image of a brain with CSF build-up in the ventricles (dark area in the middle).

Figure 1. Before shunt placement

When a shunt is implanted in a person with hydrocephalus, the goal is for the shunt system to mimic what would occur in the body naturally. CSF will be drained by the shunt, and the flow will be regulated so that a constant intracranial pressure (ICP) is maintained within the brain. Figure 2, "After shunt placement," shows a post-operation CT image of the same patient's brain. The ventricles have drained and have resumed their normal size. The white area in the middle of the image is the shunt valve.

Figure 2. After shunt placement

CSF enters the shunt system through small holes or slits near the tip of the proximal catheter. As CSF is produced by the choroid plexus, the shunt valve will regulate the amount of ICP by draining fluid from the ventricles. From the proximal catheter, CSF flows through the valve system and into the distal catheter, which drains CSF into another area of the body where it can be reabsorbed (directly or indirectly) by the bloodstream. For instance, in a person with a VP shunt, CSF would flow out of the distal catheter into the peritoneal cavity. This causes the body no harm, because CSF is normally reabsorbed by the superior sagittal sinus; a large venous structure that carries the blood flow away from the brain.

Valve pressure ratings

Most shunt valves are known as differential pressure valves. This means the valve is self-regulating. It is capable of gauging the amount of ICP, and can adjust to differential pressures (DPs) between the ventricles and the distal cavity the shunt drains into. This allows the right amount of CSF to be drained based on ICP.

The most common pressure ratings for differential pressure valves are:

  • Extra-low-pressure: 0-10 mm H2O

  • Low-pressure: 10-50 mm H2O

  • Medium-pressure: 51-100 mm H2O

  • High-pressure: 101-200 mm H2O
The values listed above are a median range, and are based on information supplied from various shunt manufacturers. The amount of fluid that is allowed to flow through the shunt valve depends on the specific design characteristics of the valve, as well as levels that are rated by the manufacturer of the shunt valve. Check with your neurosurgeon to find out the type of shunt you have and its pressure setting. The flow of CSF through the valve can be changed by the pressure of tissue or debris in the shunt system.

Changes in body position

Intracranial pressure is measured according to atmospheric pressure. The atmospheric pressure relates to the location of the ventricular system and the distal end of the shunt catheter. When you are lying down, your ventricles are considered to be level with the distal end of the shunt. In this position, the normal range of ICP can range from anywhere between 50 mm H2O and 200 mm H2O. For infants, the normal ICP is generally less than 60 mm H2O, and less than 40 mm H2O for premature infants (P. H. Chapman et al., "The relationship between ventricular fluid pressure and body position in normal subjects and subjects with shunts: a telemetric study," Neurosurgery 26, no. 2, February 1990: 181-89).

When you sit or stand after lying on your back, the distal end of the shunt system is below the ventricles. This sudden change in body position can cause your ICP to momentarily drop to a level between -50 mm H2O and +50 mm H2O. In order to compensate for this drop in pressure, shunt valves need to be able to accommodate not only a wide range of atmospheric pressures as the patient changes position, but they also need to adapt to the differential pressures of CSF production. Fortunately, most shunt valves today are differential pressure valves. This means the valve has the ability to automatically adjust to a range of pressures to maintain ICP at the correct level for your body.

Without a differential pressure valve (as with some older shunt systems), changes in body position can sometimes cause a siphoning effect in the distal catheter. The siphoning effect occurs when CSF from the distal catheter flows into its drainage cavity (e.g., right atrium of the heart or the peritoneal cavity), causing fluid from the ventricles to drain suddenly. When this happens, you will probably feel a sense of light-headedness. This is normal, and should only last for a few seconds. Your shunt just needs a chance to respond to changes in elevation brought about by a change of body position.

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