The
Reliability of the "Absent Cistern Sign" in Assessing LP Shunt
Function
Abhaya
V. Kulkarni, Paul D. Chumas, James M. Drake and Derek C. Armstrong
Abstract:
Objective: One of the difficulties with lumboperitoneal
(LP) shunts has been non-invasively ascertaining shunt function.
It has been previously reported that in the presence of a
functioning LP shunt the perimesencephalic cisterns become
obliterated &endash; the "absent cistern sign". In order to
more rigorously test this association we performed a retrospective
analysis of LP shunt patients at the Hospital for Sick Children,
Toronto. Methods: The CT scans of all patients
undergoing LP shunting over a 17 year period were reviewed.
The "absent cistern sign" and ventricular size were compared
against the results of either an isotope shunt study or surgical
findings performed within 2 days of the CT. Results:
There were 38 CT scans (27 patients) performed within 2 days
of an isotope shunt study and 15 CT scans (14 patients) performed
within 2 days of a surgical intervention. These results give
the absent cistern sign a sensitivity of 75% and a specificity
of 57% when compared to the shunt isotope findings and a sensitivity
of 100% and a specificity of 50% when compared to the surgical
findings. Over 30% of the CT scans showed ventriculomegaly
in the presence of a functioning shunt and, conversely, nearly
45% of the CT scans had normal or small lateral ventricles
in the presence of a malfunctioning shunt. Conclusions:
The "absent cistern sign" appears to reliably rule out a completely
blocked shunt, but is less reliable in detecting a normal
or partially obstructed shunt. Ventricular size correlates
poorly with LP shunt function.
Résumé:
Fiabilité du signe de la citerne absente dans l'évaluation
du fonctionnement d'une dérivation LP. But:
Une des difficultés associées aux dérivations
lombopéritonéales (LP) est de s'assurer de façon
non invasive que la dérivation fonctionne. On a rapporté
antérieurement que les citernes périmésencéphaliques
sont oblitérées en présence d'une LP
fonctionnelle &endash; le "signe de la citerne absente". Nous
avons effectué une analyse rétrospective des
cas de patients ayant une dérivation LP au Hospital
for Sick Children à Toronto afin d'évaluer de
façon plus rigoureuse cette association. Méthodes:
Nous avons revu les tomodensitométries de tous
les patients qui ont subi une dérivation LP sur une
période de 17 ans. Le "signe de la citerne absente"
et la taille ventriculaire ont été comparés
aux résultats soit d'une étude isotopique de
la dérivation ou aux observations chirurgicales obtenues
dans les 2 jours suivant la tomodensitométrie. Résultats:
38 tomodensitométries ont été effectuées
(27 patients) dans les 2 jours d'une étude isotopique
de la dérivation et 15 tomodensitométries (14
Patients) dans les 2 jours suivant une intervention chirurgicale.
Ces résultats confèrent au signe de la citerne
absente une sensibilité de 75% et une spécificité
de 57%, par rapport aux observations obtenues par l'étude
isotopique et une sensibilité de 100% et une spécificité
de 50% par rapport aux observations obtenues à la chirurgie.
Plus de 30% des tomodensitométries montraient une ventriculomégalie
en présence d'une dérivation fonctionnelle et,
à l'inverse, près de 45% des tomodensitométries
montraient des ventricules latéraux normaux ou petits
en présence d'une dérivation défectueuse.
Conclusions: Le "signe de la citerne absente"
semble éliminer de façon fiable une dérivation
complètement obstruée, mais il est moins fiable
pour mettre en évidence une dérivation normale
ou partiellement obstruée. La taille ventriculaire
est faiblement corrélée au fonctionnement d'une
dérivation LP.
Can.
J. Neurol. Sci. 1999; 26:40-43
Although
lumboperitoneal shunts are being used with decreasing frequency,
one of the difficulties that remains is in non-invasively
assessing shunt function in those who have had prior LP shunt
placement. In 1986, Chuang et al. noted the obliteration of
the basal cisterns on computed tomography (CT) &endash; the
"absent cistern sign" &endash; to be a reliable indicator
of LP shunt function and, conversely, the presence of basal
cisterns to correlate well with shunt malfunction.(1) We,
therefore, decided to confirm this finding by performing a
retrospective review of all the available CT scans on patients
who had undergone insertion of an LP shunt at the Hospital
for Sick Children over a 17 year period. The CTs were compared
to the most accurate assessment of shunt patency currently
available (radionuclide shunt study(2) and operative findings)
in order to truly assess the reliability of the "absent cistern
sign".
Methods
Patient
Population
Of
143 patients in whom a lumboperitoneal shunt was initially
inserted at the Hospital for Sick Children, Toronto, Canada
between 1974 and 1991 there were 101 in whom follow-up CT
scans were available for review. Out of a total of 309 CT
scans, there were 53 CT's in 36 patients performed within
2 days of a surgical revision or a radionuclide shunt isotope
study and only these were included in the study. Those patients
in whom there had been posterior fossa surgery (8 tumors)
or in whom there was evidence of incidental pathology at the
level of the basal cisterns (1 arachnoid cyst) were excluded
from the neuroimaging analysis.
There
were 23 males and 13 females and the mean age at first insertion
was 3.0 years (range 2 months to 17.7 years). The indications
for shunting were: communicating hydrocephalus (83.3%); pseudotumor
cerebri (2.8%); CSF fistula (2.8%) and miscellaneous pathology
(11.1%). The mean follow-up time period was 9.1 years (range
49 days to 16.9 years).
At
the time of initial shunt placement, 31 patients underwent
insertion of a T-tube shunt via a limited laminotomy and 5
patients had their shunt inserted by the percutaneous technique
using a Touhy needle. All the T-tube shunts were silastic
and at the time of initial insertion incorporated medium pressure
distal slit-valves (apart from 2 patients in whom low pressure
distal slit-valves were used). In the percutaneous group,
1 of the shunts was valveless polyethylene and this was converted
to the silastic type at first malfunction. The remainder were
distal slit-valve unishunts.
CT
Analysis
All
available CT scans were analyzed and the degree of obliteration
of the perimesencephalic cisterns and the size of the lateral
ventricles were noted. The basal cisterns were classified
as open, if a hypodense cerebrospinal fluid space was seen
to delineate the quadrageminal plate region, or closed, if
no such space was seen (Figure
1). The lateral ventricles were classified as small, normal
or enlarged. All the imaging was reviewed by a neuroradiologist
who remained blinded to the clinical state of the patients
or the results of the isotope shunt study or surgical findings.
The
CT scan results were subsequently correlated to either the
operative findings at the time of shunt malfunction or to
the findings from isotope shunt studies. Operative findings
were recorded as demonstrating either drainage or no drainage
of CSF. Isotope shunt studies were recorded as either normally
functioning or malfunctioning (which included obstructed and
partially obstructed shunts, defined as a patent shunt with
slow flow).
Results
There
were 38 CT scans (27 patients) which were performed within
2 days of an isotope shunt study and Table
1 displays the correlation between the perimesencephalic
cisterns, size of lateral ventricles and the shunt-study findings.
Similarly, there were 15 CT scans (14 patients) performed
within 2 days of a surgical intervention and these data are
shown in Table
2.
These
results give the "absent cistern sign" a sensitivity of 75%
and a specificity of 57% when compared to the shunt isotope
findings and a sensitivity of 100% and a specificity of 50%
when compared to the surgical findings. There were 6 false
positives of the "absent cistern sign", i.e., open cisterns
in the presence of a functioning shunt, when compared to isotope
study and 3 when compared to surgical findings. There were
also 6 false negatives, i.e., absent cisterns in the presence
of a malfunctioning shunt, when compared to isotope study
but none relative to surgical findings. It should be noted
that in all the false negative cases the obstruction was only
partial and in no case were absent cisterns recorded in the
presence of a complete shunt blockage.
The
positive predictive value of the "absent cistern sign", i.e.,
the probability of shunt malfunction in the presence of open
cisterns, was 75% when compared to both the shunt study and
surgical findings.
Review
of the size of the lateral ventricles confirms the difficulty
in using this parameter to determine LP shunt function with
36% and 33% of the CT scans showing ventriculomegaly in the
presence of a functioning shunt as determined by isotope study
and surgical findings, respectively. Conversely, 46% and 44%
of the CT scans demonstrated normal or small lateral ventricles
in the presence of a malfunctioning shunt by shunt study and
surgical findings, respectively. In fact, of the 14 CT scans
with an absent cistern sign, ventriculomegaly was found in
7 (50%). Of these, there were 3 patients (all of whom had
studies to confirm the communicating nature of their hydrocephalus)
with CT scans showing the absent cistern sign and ventriculomegaly
in association with a small or non-visible IV ventricle (Figure
2).
Discussion
Although
a relatively simple procedure, the use of lumboperitoneal
shunts has been associated with many worrisome complications
that, in recent years, have limited its indications and use.3-5
However, there are still many patients with LP shunts in place
who present with symptoms of potential shunt malfunction and
the present experience with them may be limited. One of the
difficulties of LP shunts has been accurately assessing shunt
function as alteration in ventricular size on CT appears to
be a rather imprecise and late development. It is with these
factors in mind that the present study was performed.
Gilday
et al. reported on the successful use of intrathecal 111In-DTPA
radionuclide to assess shunt patency.(2) Others have tried
intrathecal contrast.(6) The non-invasive thermosensitive
technique involving the cooling of the skin over the proximal
shunt tubing and measuring the temperature drop distally to
confirm shunt patency has also been described.(7,8) Schutz
et al. reported the use of the lumbar infusion test in which
the CSF pressure change is recorded during the infusion of
artificial CSF into the lumbar subarachnoid space.(9) A blocked
shunt would result in a precipitous pressure rise while this
should not occur with a patent shunt.
In
1986, Chuang et al. described a non-invasive, simple CT sign
&endash; the obliteration of the perimesencephalic cisterns
("absent cistern sign") &endash; to be a reliable indicator
of LP shunt function and, conversely, the presence of an open
cistern to correlate well with shunt malfunction.(1) In that
study 26 of 28 CT scans in asymptomatic patients showed the
absent cistern sign while 18 of 20 CT scans in patients requiring
shunt revision had visible basal cisterns. Their study data
would give the "absent cistern sign" a sensitivity of 90%
and a specificity of 93%.
In
the present study the "absent cistern sign" also appears to
be a very sensitive indicator of shunt malfunction, with values
of 100% and 75% when compared to surgical and shunt study
findings, respectively. However, the relatively low sensitivity
when compared to isotope study was due to 6 false negatives
that were all associated with obstructions that were only
partial in nature. Therefore, the absence of cisterns on CT
very strongly rules out the possibility of complete, but not
partial, shunt obstruction.
Unfortunately,
due to its specificity of 57% when compared to shunt study
findings, the absence of cisterns on CT is less reliable as
an indicator of a normally functioning shunt. While this study
confirms that the absent cistern sign is a useful guide to
LP shunt function, one must consider the significant false
positive rate in normal shunts and the false negative rate
in partially functioning shunts. Sound clinical judgment is
required in interpreting this sign, which should not be used
as the sole criterion for performing a shunt revision.
The
data from the present study reveal the "absent cistern sign"
to have a much lower specificity than had been shown by Chuang
et al.(1) However, in their study a shunt was considered normal
if the patient was asymptomatic and only a minority of these
children had isotope shunt studies to confirm shunt patency.
The present study was more rigorous in that all shunts that
were classified as normal had shunt study confirmation. This
may have altered our normal population since in all cases,
for whatever reason, it was felt that a shunt study was indicated.
As well, there is the question of how accurately isotope shunt
studies assess true shunt function. In the series of Gilday
and Kellam there were 2 false positives in a series of 74
LP shunts tested by this method.(2) Another possible explanation
for the low specificity is the delay of up to 2 days between
the CT scan and the shunt study or surgery. It has been suggested
that intermittent blockage of the shunt tubing may cause transient
visualization of the basal cisterns.(1) Change in shunt function
between the time of the CT and isotope shunt study might result
in a "false positive" finding in this study.
Ventricular
size was found to correlate poorly with shunt function. Over
30% of the CT scans showed ventriculomegaly in the presence
of a functioning shunt and nearly 45% demonstrated normal
or small lateral ventricles in the presence of a malfunctioning
shunt. Of interest is the finding that in 50% of CT scans
with an absent cistern there was ventriculomegaly and in a
small group of patients this ventriculomegaly was seen in
association with a small fourth ventricle (Figure
2). This small group of patients had all had confirmatory
studies establishing the communicating nature of their hydrocephalus
prior to LP shunt insertion. This constellation of findings
(ventricu-lomegaly, absent basal cisterns, and a small fourth
ventricle) may indicate some degree of flow resistance within
the aqueduct. It is possible that this may have occurred secondary
to chronic LP drainage.
Conclusions
The
"absent cistern sign" is reliable in ruling out the presence
of a completely blocked LP shunt, but is less reliable in
detecting shunts that are either functioning normally or are
only partially obstructed. Ventricular size correlates poorly
with LP shunt function. However, CT scan findings still require
significant clinical correlation and, in many cases, can not
replace more invasive, accurate methods of shunt function
assessment.
References
