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
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.
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
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
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.