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Treatment of Normal Pressure Hydrocephalus
Harold L. Rekate, MD
Division of Neurological Surgery, Barrow Neurological Institute,
St. Joseph's Hospital and Medical Center, Phoenix,
Arizona
The treatment of normal pressure hydrocephalus (NPH) is complicated
by the lack of certain diagnostic criteria. The risks associated
with treating NPH are also higher than for other forms of
hydrocephalus. Possible forms of treatment include ventricular
shunts, lumbar shunts, and endoscopic third ventriculostomy. The
lack of well-constructed, randomized, controlled trials on the
treatment of NPH means that all forms of treatment should be
considered "options." Recent technological advances, however, have
improved the management of NPH. The risks of treatment are lessened
by the use of high-pressure valves with devices that retard
siphoning. Better outcomes are associated with lowpressure valves.
When NPH is treated with ventriculoperitoneal shunts, choosing a
programmable valve with a device that retards siphoning maximizes
the efficacy of treatment while minimizing the risks.
Lumboperitoneal shunts are effective in the management of NPH but
have a higher overall failure rate than ventriculoperitoneal
shunts. Furthermore, their continued functioning is difficult to
assess. A few patients with NPH have responded well to endoscopic
third ventriculostomy, and this form of treatment deserves careful
assessment.
Key Words: endoscopic third ventriculostomy,
lumboperitoneal shunt, normal pressure hydrocephalus,
ventriculoperitoneal shunt
The quality of life of carefully selected patients with normal
pressure hydrocephalus (NPH) can be improved significantly by
shunting. Because they are programmable, valves with devices that
retard siphoning, specifically the Codman Hakim Programmable Valve
with Siphonguard, can recreate the normal dynamics of cerebrospinal
fluid (CSF), improve the possibility of good outcomes, and minimize
the risks of mechanical problems with the shunts. Lumboperitoneal
(LP) shunts can manage NPH effectively, but the difficulty of
assessing their function and their lack of long-term reliability
make their use problematic. The use of endoscopic third
ventriculostomy (ETV) has only recently been advocated. No large
assessment studies have been performed, but ETV may hold promise
for the management of this condition without a permanent internal
shunt.
Other articles in this issue consider the pathophysiology of NPH
and patient selection. The starting point of this review is a
patient who has already been evaluated and found to be a potential
candidate for treatment of this condition. What are the treatment
options and how does one select among them to maximize the
risk-benefit ratio associated with treatment?
Methods
Using computerized search engines, the
literature in English was surveyed using the key words, normal
pressure hydrocephalus,with the modifier, treatment. Clinical
information derived from the author's practice and theoretical
considerations based on the concepts of the biophysics of the
condition were also used. Initially, 208 articles met the inclusion
criteria and were scanned by title. Of these, 30 articles were read
entirely and found useful for this review. Alternative forms of
treatment for NPH, including the use of LP shunts and ETV, were
also explored.
Results of
Literature Review
Unfortunately,no randomized controlled trials
have compared patients who have been shunted to matched controls of
patients who may have been candidates but were not shunted.
Furthermore, no randomized control trials have compared the
outcomes of different treatments. Therefore, no Class I data are
available on which to base treatment decisions for NPH.
Three articles retrospectively compared the
treatment outcomes of patients who had undergone two different
types of shunting methods. In two of these studies, low-pressure
valves were superior to medium- or highpressure valves.[3,11] In
one of these studies, however, the complication rate associated
with low-pressure valves was significantly higher than those
associated with the other valves. Therefore,the advantage of the
low-pressure valve was significantly reduced. Nonetheless, the
authors suggested that their data supported the use of low-pressure
valves in patients undergoing shunt treatment for NPH.[3]
One article compared the success of treatment
and complications rates between differential pressure valves and
flow-control valves (Orbis Sigma valve, NMT Corporation, Duluth,
GA),[21] but there were no differences in outcomes. The authors
concluded that flow-control valves and differential pressure valves
are associated with overdrainage, and both can treat the condition
adequately.[21] Of their patients, 90% improved. Those with the
differential pressure valves did slightly better than the other
patients, but the finding was not statistically significant.
Without randomized control trials, the data
are insufficient to recommend any form of treatment of NPH as a
standard. Retrospective studies of a relatively large number of
patients treated with low-pressure valves compared to high- and
medium-pressure valves show an increased likelihood of improvement
with low-pressure valves.[11] The lack of statistical significance
and the higher complication rate associated with low-pressure
valves make it impossible to recommend this form of treatment even
as a guideline. Therefore, all forms of treatment for NPH must be
viewed as options.
Based on this discussion, the ideal valve for
use with ventriculoperitoneal (VP) shunting is likely to differ
among patients. Such a valve would allow the lowest drainage
pressure and therefore the highest likelihood of decreasing
ventricular size that could be tolerated by a given patient. There
are methods purported to help select such a valve, but none have
been validated.[2,23] Valve selection is discussed further after
the complications associated with treating patients with NPH and
the relationship of shunt biophysics to preventing these
complications are explored.
Complications
Associated with Treatment of NPH
By consensus, complication rates associated
with the treatment of NPH are significantly higher than those
associated with the treatment of other forms of hydrocephalus.
Three types of complications need to be discussed. The first and
most common complication involves the failure of the procedure to
lead to improvement in patients or patients who continue to
deteriorate after a brief period of improvement. There are two
potential explanations for these failures. The first case assumes
that if patients fail to improve, they did not actually have NPH
and were selected for treatment inappropriately. Williams and
colleagues have cast doubt on this explanation and suggest that
many of these patients have nonfunctioning shunts and would benefit
from a shunt revision.[22] The diagnosis of shunt malfunction is
particularly difficult in patients with NPH because marked
decreases in ventricular size are unusual and the signs and
symptoms of increased intracranial pressure (ICP) are lacking.
Before clinicians conclude that patients are not responding to a
shunt,Williams et al.[22] believe that it is essential to assure
that an adequate shunt is in place. Shunt function should be
assessed either clinically or radiographically, or the shunt should
be explored surgically and replaced with a shunt with a lower
opening pressure.
Infectious complications of shunt systems are
a continuous problem in the treatment of hydrocephalus. A recent
multicenter trial in children found a baseline rate of shunt
infection of 8%.[7] The infection rate in adults is likely somewhat
lower, but this issue has not been subjected to multicenter
scrutiny. A recent publication with a relatively large number of
patients with NPH reported an infection rate of 8.5% in adults.[23]
Shunt infection is a serious complication and can be associated
with multiple surgical procedures and weeks in the hospital. It
rarely, however, causes permanent disability. Furthermore, if
patients respond to the treatment, a decision against such
treatment would seldom be made.
Mechanical complications are probably common
in patients who receive VP shunts for the treatment of NPH. The
size of the ventricles seldom decreases. Therefore, obstruction of
the ventricular catheter, which is the most common complication
associated with ventricular shunting in children, is a very rare
complication in adults. Most severe and especially life-threatening
complications are related to overdrainage of the shunt.
Overdrainage leads to the usually benign but troubling complication
of subdural effusions or to the potentially life-threatening
complication of brain collapse and the development of subdural
hematomas. Subdural effusions are usually associated with headaches
that are localized to the side of the effusion. In a Dutch
study,they occurred in 71% of patients with low-pressure
differential valves but only in 34% of patients with
medium-pressure differential valves.[3] Later discussion of this
study postulated that the use of devices that retard siphoning
(DRS) would improve outcomes in these patients.[6]
A recent review of 44 articles documented a
59% rate of improvement and a 38% complication rate in patients
receiving shunts for the treatment of NPH.[9] Of these
complications, 6% were considered severe (i.e., death or permanent
life-changing neurologic injury). Twenty-two percent of the
patients required a second surgical procedure.[9] Some of these
repeat surgeries were needed to treat surgical complications
(misplacement of a catheter) or infection. Understandably, however,
some were related to the valve pressure or configuration. At least
half of these repeat procedures could be avoided with the use of
properly designed programmable valves that contained an effective
DRS. The results reported in a multicenter study likewise showed a
poorer response rate, but the complication rates were almost
identical: 7% of patients either died or suffered permanent
decreased neurologic functioning.[20]
Figure 1. (A) Intracranial pressure (ICP) as a function of time in
the lying and
sitting position in a patient with a Level 1 Delta Valve and in a
(B) normal patient.
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Valve Selection
for VP Shunts
Recent advances in the design of valves for
shunts have resulted in at least the likelihood of improvement
after treatment for NPH. The first DRS, developed by Portnoy and
reported in 1973, was the anti-siphon device.[15] For the first
time,shunt valves prevented extremely negative ICP when patients
assumed the erect position. However, technical difficulties with
the device and unpredictable failure modes created significant
resistance to its routine use.[10]
In the early 1990s several other devices,
including the Siphon Control Device, Delta Valves (Medtronic, PS
Medical, Santa Barbara, CA), and the Orbis Sigma Valve (NMT,
Duluth, GA), were developed to prevent overdrainage. The use of
these devices in the treatment of NPH has not yet been validated
with randomized controlled trials. Certainly, however, they have
made the treatment of NPH safer and decreased the likelihood that
subdural effusions and hematomas will develop.
As a balance between valves with a low
opening pressure and valves with an effective DRS to prevent
significant negative ICP, I previously recommended using VP shunts
with a Level I Delta valve for the treatment of NPH.[16] Normal ICP
is about 10 mm Hg in the recumbent position and falls slowly to
about –3 mm Hg when the erect position is assumed (Fig.1).
The Level I Delta valve is associated with almost normal ICP in the
recumbent position and also after an erect posture is assumed. It
differs from normal only in that it equilibrates to the nadir of
ICP much more quickly.
Some caveats, however, must be considered
when using Delta valves. First, to function properly, the valve
must be placed at the level of the foramen of Monro. For the
diaphragm mechanism to work properly, the valve also must be placed
under freely moving skin.[8]
The most recent technological advance to
improve the risk-benefit ratio associated with the management of
NPH is the development of programmable valve mechanisms. The first,
the Sophy Valve (Sophysa Corporation), has never been marketed in
the United States. The Medos Valve, now called the Codman Hakim
Programmable Valve (Codman Corporation, Raynam, MA),was the first
programmable valve to be approved by the Food and Drug
Administration. It consists of a dial of differential pressure
valves with opening pressures ranging from 30 mm H2O to 200 mm H2O
in 10-mm steps. The valve pressures are extremely precise and
stable over long periods.[14]
This valve was inappropriate for the
treatment of most cases of NPH because the valve mechanism
equilibrated rapidly and was associated with extremely negative ICP
in the erect position. More recently, the device has been marketed
with the addition of a new DRS called Siphonguard (Codman
Corporation,Raynam,MA). For the first time, it is possible to
recreate normal ICP dynamics in a patient with a VP shunt (Fig. 2).
At present, this valve appears to be ideal for the treatment of
NPH. I now use this valve system for all patients with NPH who
receive a VP shunt.
Figure 2. Graphs generated in the same patient with a Codman Hakim
Programmable valve with Siphonguard at three settings in the lying
and sitting positions compared to a normal patient.
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Figure 3. (A) Computed tomographic (CT) scan of patient with normal
pressure
hydrocephalus whose Codman Hakim programmable valve with
Siphonguard is set at
100 mm H2O. (B) CT of same patient with the valve set at 200 mm
H2O.
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Treatment with
Programmable Valves
Without a compelling reason otherwise, all
patients with NPH are treated with the Codman Hakim Programmable
valve with Siphonguard originally set at 100 mm H2O. Information
from the operating room is rarely available in the hospital or
outpatient setting when a patient is next seen. Therefore, it is
safe to assume that the valve is set at this level unless the
setting has been adjusted in an environment in which the
information is available. In patients with a positive outcome from
the shunt and with no negative symptoms, there is no reason to
change this setting, which is the most likely to recreate the
normal dynamics of ICP. Patients who do not improve after surgery
or who later deteriorate can be treated with a systematic decrease
in the opening pressure of the valve. Patients often respond
initially and later show signs of deterioration. Lowering the
opening pressure of the valve often recaptures the positive effect
of treatment. Williams et al. found this situation to be true of
patients who underwent valve replacements with lower pressure
valves that required surgical intervention.[22]
A small number of patients complain of
postural headache when they assume the erect position. Many have
unilateral headaches and will be found to have subdural effusions
on imaging studies. For example, an 83-year-old man had a
compelling history and imaging studies that led to the diagnosis of
NPH (Fig. 3A). Immediately after surgery, his gait and bladder
control improved markedly. A month later he complained of severe
right-sided headaches, especially when assuming the erect position.
A significant subdural effusion was found. The pressure of his
valve was increased to 200 mm H2O, and the subdural effusion
resolved within 3 weeks (Fig. 3B). Unfortunately, his gait
disturbance returned and he required a walker. The valve pressure
was changed to 170 mm H2O. He again was able to walk without the
walker and continues to improve.
LP Shunting
LP shunting is a well-established form of
treatment for NPH, but little has been published on the use of this
technique.[19] High-volume withdrawal of CSF and the use of chronic
controlled lumbar drainage in the diagnosis of this condition
suggest that the technique would be an excellent option for
treating NPH (Marmarou A, personal communication, 2002). Unless
hydrocephalus is caused by aqueductal stenosis, LP shunting is an
option in the treatment of any form of hydrocephalus. In infants LP
shunting can lead to hindbrain herniation that could become
symptomatic after a number of years.[4,5] The use of valves in an
LP shunt can prevent this complication.[18] No studies have
implicated LP shunts as causing hindbrain herniation in patients
with NPH or other causes of hydrocephalus in adults.
Compared to VP shunting in patients with
NPH,LP shunting has several advantages. That the cranium is not
entered is an advantage because intraoperative bleeding is
troublesome. However, subdural hematomas can still develop as a
result of negative ICP. Published infection rates for LP shunts are
significantly lower than those for VP shunts, possibly 10-fold
lower.[7,19]
For two interrelated reasons, LP shunts are
not ideal forms of treatment for NPH. The size of the ventricles
does not decrease routinely after shunting, even when the outcome
is excellent. Therefore, the function of the shunt cannot be
assessed by a cranial imaging study. ICP does not increase markedly
when the shunt has failed, nor are there signs of increased
intracranial hypertension. It therefore is difficult to determine
whether the shunt is working. If the patient does not respond, it
may be impossible to discern whether the patient is a nonresponder
or whether the shunt is not working properly.
The second problem is related to the overall
lack of confidence in LP shunts for prolonged function. This lack
of confidence stems from their use to manage pseudotumor cerebri.
The failure rate of LP shunts is quite high, and their long-term
function is the exception. Because the risk associated with this
procedure is relatively low,LP shunting might be seen as an
effective diagnostic test in patients who may be less likely to
respond to shunting.
ETV
Most cases of NPH are classified as
communicating hydrocephalus. Such patients are not candidates for
ETV. The ideal candidate for ETV has aqueductal stenosis.
Information based on modeling NPH reflects the likelihood that
many, if not most, cases of NPH are caused by an obstruction to CSF
flow between the spinal subarachnoid space and the cortical
subarachnoid spaces.[17] Patients with this form of communicating
hydrocephalus are indeed candidates for ETV.[1]
In two small series, NPH in nine patients was
treated successfully with ETV.[12,13] Meier and colleagues selected
patients by performing both ventricular and lumbar infusion tests.
Patients who showed a high resistance to CSF outflow in the
ventricle and low resistance in the lumbar theca were offered ETV.
In the selected patients, the floor of the third ventricle was
bowed into the interpeduncular cistern, suggesting CSF flow had
failed between the third ventricle and cistern (Meier U,personal
communication, 2002).
An alternative method for selecting patients
for ETV that might allow a greater number of patients to be
considered as candidates for the procedure is suggested by the
results of isotope cisternography. This technique has been used to
select patients with NPH for shunting. Patients were selected in
whom the radioactive tracer linked to albumin was injected into the
lumbar theca. In a positive response, the dye entered the ventricle
quickly but stayed there for prolonged periods. The dye flowed into
the ventricle, but its entrance into the cortical subarachnoid
space was restricted as suggested by the mathematical model.[17]
The blockage is at the skull base between the spinal and cortical
subarachnoid spaces; therefore, ETV will be an effective internal
bypass. I have performed the procedure in one patient who had an
excellent outcome. However, the use of ETV must be evaluated in a
larger series before the procedure can be recommended.
Conclusion
NPH is one of the few surgically treatable
forms of dementia. In truth, it is one of the few dementing,
degenerative conditions for which any hope exists at all. It is
frustrating that the diagnosis cannot be made with certainty and
that all tests are associated with both false-negative and
false-positive results. What choices do these patients have? They
can accept their progressive deterioration, or they can take the
courageous course to undergo a relatively risky operation that
might reverse or stabilize the progress of the disease. It is the
responsibility of clinicians to give patients and families the
information they need to decide whether the risks are worth taking.
The availability of programmable shunts, advances in diagnostic
acumen, and the potential of alternative forms of treatment are
improving the quality of life for a growing number of older
patients.
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