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SURGICAL TREATMENT OF HYDROCEPHALUS |
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A variety of CSF shunt systems are currently available. No single shunt
has been clearly shown to be superior to the rest, and thus the surgeon’s
familiarity with the shunt equipment is an important factor in its selection.
The consistent use of the same shunt system helps to optimize the surgeon’s
technical proficiency and facilitates the process of shunt revisions.
The equipment for CSF diversionary shunting includes a
proximal shunt catheter,
valve, distal shunt
catheter, and sometimes other components such as a device to prevent
siphoning, an on/off switch or telemetric sensor. Shunt systems may exist as
integral units (no connectors) or as separate parts requiring assembly.
The proximal catheter is the portion of a shunt system which is placed into
the CSF space before the site of obstruction. In ventricular shunt systems, the
proximal catheter is most commonly placed in the lateral ventricle. When the
subarachnoid space is shunted, the proximal catheter is most commonly inserted
into the lumbar thecal sac. Proximal shunt catheters, particularly ventricular
catheters, should incorporate several features in their design to ensure optimal
function. Some ventricular catheters include a subcutaneous reservoir. The
catheters may be straight or right-angled. Right-angled catheters allow easier
fixation to the distal shunt system. The reservoir should be palpable underneath
the scalp and allow percutaneous access to the CSF for sampling, pressure
measurement, and introduction of medication or contrast agents.
Shunt valves come in several varieties. All produce unidirectional flow of
CSF. Most valves are pressure regulated, that is, they respond to a differential
pressure gradient across the valve. A differential pressure is generated either
by an increase in pressure upstream or a decrease in pressure downstream. The
ideal valve would drain only the excess CSF produced by each individual patient
which could not be reabsorbed. Such a valve does not yet exist.
Ventricular shunt valves may be placed proximally or distally along the shunt
system. The majority of shunt valves are proximal valves which are seated just
distal to the ventricular catheter. Distal valves of the slit type are effective
but have two major disadvantages: 1) they are associated with a higher rate of
distal shunt malfunction and 2) they are more difficult to replace at the time
of shunt revision. Examples of proximal differential pressure valves include the
slit valve, ball-in-cone valve, diaphragm valve, and miter valve. |
Shunt Complications
In spite of their dramatic ability to control the symptoms and signs of
hydrocephalus, ventricular shunts are foreign bodies associated with a myriad of
complications. One must be familiar with the full spectrum of complications that
can follow shunt placement because only some occur immediately and many occur
over the long-term. At times the rationale for placement of a shunt presumes
that because the surgery is simple, “a shunt won’t hurt and might help”. While
the latter statement is often correct, the former is a dangerous misconception.
Complications that accompany shunt placement are generally related to
malfunction and infection. Shunt malfunction may result
from either underdrainage or overdrainage of CSF. |
Shunt malfunction from Underdrainage of CSF
Underdrainage of CSF occurs if the shunt system becomes obstructed or
disconnected. Most commonly, this occurs as the result
of an obstruction of the ventricular catheter. This obstruction may
be the result of initial misplacement of the ventricular catheter or a migration
of the catheter into the subependymal tissue or choroid plexus as a result of
collapse of the ventricles, growth or the head, or movement of the entire shunt
system. Even a catheter properly placed within the ventricles may become
occluded by choroid plexus or tissue debris. Occlusion of a catheter as a result
of debris from an immune response has also been described. The shunt valve may
stick or become occluded and malfunction. The distal shunt is also a site which
can become occluded, although this is more apt to occur if a distal slit valve
apparatus is used, and less likely to occur if open ended peritoneal tubing is
used.
Underdrainage
of CSF may also result from a disconnection with the
shunt system. Disconnections tend to occur at connector sites,
particularly when connectors have been placed along the shunt tract to bridge
gaps at the time of previous shunt disconnections. Disconnections may occur
anywhere along the shunt system. When shunts have been in place for long periods
of time the tubing may become brittle and even calcify, making it more prone to
fracture. Disconnections are common during growth spurts and tend to occur at
areas of movement such as the neck or at areas subject to pressure such as the
region overlying the clavicle. Disconnections have become less common with newer
systems that contain fewer connector sites.
Underdrainage may also be the result of loculation
within the ventricular system. Loculated ventricles may occur
following hemorrhage or infection. Occlusion of the foramen of Monro can create
a trapped lateral ventricle. Multiple septations may occur within the
ventricles, and a single shunt may be ineffective in draining these isolated
fluid collections. Ventriculoscopic techniques can be used to fenestrate the
septum pellucidum as well as the walls of loculated cysts. This allows
simplification of shunt systems and in some cases the need for a shunt may be
eliminated (e.g., after septostomy to bypass a blocked foramen of Monro, a
trapped lateral ventricle may drain through normal pathways on the other side).
Ventriculoatrial shunts have a high rate of malfunction. These shunts require
frequent revision because the distal end migrates out of the cardiac atrium
during growth of the child. Ventriculoatrial shunts are also subject to problems
of vascular or cardiac perforation, embolism, and an immune-mediated
glomerulonephritis. |
Shunt Malfunction from Overdrainage of CSF
All extracranial CSF shunts (ventriculo-peritoneal, -pleural and -atrial) are
subject to malfunction from overdrainage. In each of these shunts there may be
markedly negative pressures generated by the hydrostatic column of fluid in the
tubing distal to the shunt valve when the patient assumes the upright position.
The problem of overdrainage of CSF is common to the differential pressure
valves, as the negative hydrostatic pressure generated in the upright position
can overcome the effect of even the “high pressure” valves.
The most common symptom from overdrainage is
headache. This low pressure headache must be distinguished from the
high pressure headache due to CSF underdrainage. Overdrainage headache is worse
in the upright position and improved when the patient is recumbent. It is often
transient and will abate if the patient is allowed to adjust to the upright
position slowly. Occasionally, upgrading the valve or use of a device to prevent
siphoning is necessary.
When headaches are persistent one must consider the
possibility of a more dangerous condition such as a subdural hematoma or hygroma.
This can be a very difficulty problem to treat, particularly in patients who are
shunt dependent. Burr hole drainage of the subdural collection, or subdural to
peritoneal shunting with a valveless system can be utilized.
The
slit-ventricle syndrome (figure to the
right) is a condition characterized by intermittent headache and symptoms
suggestive of shunt malfunction from underdrainage of CSF that occurs in
children with small slit-like ventricles. When these children are symptomatic,
the intracranial pressure is elevated. These children have usually been shunted
as infants and often have small heads. The shunt valve refills slowly. There may
be papilledema or cranial nerve abnormalities and, occasionally, hypertension
and bradycardia. Many authors regard the symptoms and
signs of the slit-ventricle syndrome to be manifestations of intermittent shunt
obstruction due to small ventricular size occurring in a brain that
lacks compliance, perhaps due to a diminished craniocephalic ratio. Although
many children with indwelling ventricular shunts have small or slit-like
ventricles, the slit-ventricle syndrome is relatively rare, occurring in 1-5% of
infants shunted for hydrocephalus. |
Shunt Infection
Shunt infections make up a substantial percentage of the complications that
accompany shunt placement. Despite a markedly improved trend in infection rate
over the past two decades, shunt infections are still
reported in 2-10% of cases. The presence of ventriculitis has been
shown to adversely affect intelligence. In one study, the average IQ of shunted
patients having had ventriculitis was 72 as compared to 95 in shunted patients
not having had ventriculitis.
Staphylococcus epidermidis is the most common organism to infect ventricular
shunts. The majority of shunt infections occur within two months of shunt
insertion. For this reason it is presumed that the infection is the result of
intraoperative contamination. It is interesting that cultures of the skin of
shunt recipients do not consistently match subsequent cultures of infected
shunts. This suggests either that the source of seeding can be from an area
other than the recipient’s skin, such as the nasopharynx, or from the skin of
operating room personnel. |
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