Analysis of the Posterior Fossa in Children with the Chiari 0 ...

CLINICAL STUDIES
Analysis of the Posterior Fossa in Children with the
Chiari 0 Malformation
R. Shane Tubbs, M.S., P.A.-C., Scott Elton, M.D.,
Paul Grabb, M.D., Stephen E. Dockery, M.D.,
Alfred A. Bartolucci, Ph.D., W. Jerry Oakes, M.D.
Pediatric Neurosurgery (RST, SE, PG, WJO) and Pediatric Imaging (SED), The
Children’s Hospital of Alabama, University of Alabama at Birmingham, and
Departments of Biostatistics (AAB) and of Cell Biology (RST), University of Alabama
at Birmingham, Birmingham, Alabama
OBJECTIVE: We previously reported the resolution of syringohydromyelia without cerebellar tonsillar ectopia in five
patients after posterior fossa decompression of the so-called Chiari 0 malformation. A sixth patient is described.
In this study, the anatomy of the posterior fossa is analyzed using radiological imaging, enabling features of the
posterior fossa in this uncommon subgroup of children to be characterized.
METHODS : Multiple measurements were made on magnetic resonance imaging studies in six children with Chiari
0 malformation to determine the position of the brainstem relative to the foramen magnum. Fifty children with
normal magnetic resonance imaging studies of the brain were used as controls.
RESULTS: All children with a Chiari 0 malformation were found to have the following positive results: obices that
were located more than 2 standard deviations below normal, an increase in the anteroposterior midsagittal
distance of the spinomedullary junction at the level of the foramen magnum, an increase in the angle between
the floor of the fourth ventricle and clivus, and an increase in the anteroposterior midsagittal distance of the
foramen magnum.
CONCLUSION: The findings of this study suggest that the contents of the posterior fossa are indeed compromised
and/or distorted in patients with syringohydromyelia but no tonsillar ectopia. In this group, the brainstem was
caudally displaced more than 3 standard deviations below normal. (Neurosurgery 48:1050–1055, 2001)
Key words: Chiari malformation, Posterior fossa, Syrinx, Tonsillar ectopia
The resolution of syringohydromyelia in children without
cerebellar tonsillar ectopia after posterior fossa decompression—the
so-called Chiari 0 malformation—
was described previously by our group (4, 13, 20). In the
1890s, Hans Chiari described four cerebellar abnormalities
that were later termed Chiari malformations I, II, III, and IV
(2, 3). Many authors have attributed the Chiari I malformation
to overcrowding of the contents of the posterior fossa because
of a small posterior fossa (1, 11, 14, 17). In some patients with
Chiari I malformation, however, the posterior fossa is of normal
dimensions (16, 19). This study analyzes various radiological
measurements of the posterior fossa and craniocervical
junction in 50 children whose brains were deemed normal on
the basis of magnetic resonance imaging (MRI) studies. These
data are compared with measurements found in a population
of six children with Chiari 0 malformation. The goal of this
study was to determine whether the posterior fossa is compromised
and/or distorted, and because of this goal, appro-
1050 Neurosurgery, Vol. 48, No. 5, May 2001
priate drainage of the fourth ventricle was restricted in a
group of children with no tonsillar ectopia but with resolution
of the syrinx after posterior fossa decompression. Also, we
attempted to develop additional radiological criteria to assist
the neurosurgeon in evaluating patients with syringohydromyelia
without herniated tonsils to strengthen the preoperative
assumption that posterior fossa decompression will adequately
resolve the patient’s symptoms.
PATIENTS AND METHODS
This study retrospectively analyzed 50 children (30 boys
and 20 girls) ranging in age from 2 months to 17 years (mean,
6.9 yr; median, 7 yr) with normal brain imaging on 1.5-T MRI
scanning (General Electric Medical Systems, Milwaukee, WI).
The measurements in this group were compared with corresponding
measurements obtained by using the same MRI
scanner preoperatively in pediatric patients with no tonsillar

ectopia and with syringohydromyelia that resolved after decompression
of the posterior fossa. The six patients (two boys
and four girls) ranged in age from 3 to 16 years, with a mean
age of 10.2 years. Postoperative imaging was performed on all
six patients but was primarily limited to the spinal cord for
evaluation of the syrinx; a repeat of the preoperative measurements
was not performed. In three patients, measurement
required the use of the opisthion, which had been removed
during surgery. None of the six children had hydrocephalus.
The 50 children used as controls were arbitrarily chosen because
they had undergone MRI scanning of the head with
normal results and did not have significant medical problems.
In most of the 50 children, imaging had been performed to
evaluate for headaches. Preoperatively, the majority of Chiari
0 patients who underwent computed tomography of the head
did not undergo full brain MRI. Calculation of the volume of
the posterior fossa was not possible, because preoperative
MRI scanning was restricted to the craniocervical junction,
and the view of the posterior fossa was abbreviated just
inferior to the tentorial incisura. We obtained several measurements
with the purpose of determining compression
and/or caudal descent of the brainstem in the posterior fossa
at and around the foramen magnum.
All measurements were performed on sagittal T1-weighted
MRI scans. Distances and angles were measured with calipers
and a goniometer. The first distance measured on the midsagittal
MRI scan was from the basion to the opisthion (Fig. 1). This
measurement was chosen to determine whether the foramen
magnum is smaller in its anteroposterior dimensions in the
group of patients without tonsillar ectopia. We also measured
the distance from the tip of the obex to a midpoint on the
basion-opisthion line, and this relationship was compared with
MRI data reported by Quisling et al. (15) in 150 patients, 50 of
whom were between 1 and 25 years of age (mean, 12.7 yr). This
measurement was chosen to determine whether caudal displacement
of the brainstem was present in any of the six patients. The
FIGURE 1. Drawing of the
midsagittal region of the
craniocervical junction. a,
the distance between the
basion and the opisthion; b,
the anteroposterior distance
of the spinomedullary junction
in the horizontal plane
on a midpoint of a; c, the
relationship of the obex to
the plane of the foramen
magnum (from a midpoint
of a); e, the orthogonal distance
between the fastigium
and the floor of the fourth ventricle; f, the orthogonal distance
between the spheno-occipital synchondrosis and the
basis pontis.
next measurement was the horizontal anteroposterior distance
of the spinomedullary junction at a midpoint of a line connecting
the basion to the opisthion midsagittally (Fig. 1). This distance
was chosen to verify whether the brainstem was displaced inferiorly,
considering that the medulla is larger in diameter than the
spinal cord. We then calculated the angle formed between the
intersecting lines of the clivus and the floor of the fourth ventricle
(Fig. 2). This measurement was evaluated to determine
whether the brainstem was displaced with regard to the base of
the cranium. An orthogonal distance was measured from the
fastigium (the juncture of the superior and inferior medullary
vela) to the floor of the fourth ventricle for the purpose of
evaluating compression of the fourth ventricle, perhaps with the
implication of a compromised posterior fossa (Fig. 1). Finally, the
horizontal distance from the spheno-occipital synchondrosis to
the basis pontis was measured to further evaluate for anterior
and/or inferior displacement of the brainstem (Fig. 1).
RESULTS
Neurosurgery, Vol. 48, No. 5, May 2001
Chiari 0 Analysis
The results of the measurements for the six patients with
Chiari 0 malformation are listed in Table 1. In this group, the
distance from the basion to the opisthion ranged from 30 to 40
mm (mean, 34 mm). The tip of the obex was found to be at the
level of the foramen magnum in two patients, 2.0 mm superior
to the foramen magnum in two patients, 1.0 mm superior to the
foramen magnum in one patient, and 9.0 mm inferior to the foramen
magnum in one patient. The midsagittal horizontal distance
of the spinomedullary junction at a midpoint on a line
connecting the basion to the opisthion ranged from 12 to 15 mm
(mean, 13 mm). The angle created by intersecting lines of the
clivus and floor of the fourth ventricle ranged from 23 to 35
degrees (mean, 28 degrees). At approximately 115 to 118 degrees,
the clival angle was believed to be within normal limits in
all six patients (7). The distance from the fastigium to the floor of
the fourth ventricle ranged from 9.0 to 11.0 mm (mean, 10 mm).
The horizontal distance from the spheno-occipital synchondrosis
to the basis pontis ranged from 4.0 to 6.0 mm (mean, 4.8 mm).
The prepontine and the retrocerebellar cisterns were believed to
FIGURE 2. Drawing of the
midsagittal region of the
craniocervical junction
showing the angle between
intersecting lines drawn
between the clivus and the
floor of the fourth ventricle
(d).
1051

1052 Tubbs et al.
TABLE 1. Measurements in the Chiari 0 Group a
Patient
No.
Age (yr)/
Sex
a
(mm)
b
(mm)
c
(mm)
d
(degrees)
e
(mm)
f
(mm)
1 9/F 40.0 15.0 0 24.0 9.0 5.0
2 13/F 31.0 12.0 2.0 sup 24.0 10.0 4.0
3 16/M 30.0 14.0 0 35.0 9.0 4.0
4 10/M 35.0 14.0 2.0 sup 30.0 10.0 6.0
5 3/F 30.0 12.0 9.0 inf 23.0 11.0 5.0
6 13/F 31.0 13.0 1.0 sup 35.0 10.0 5.0
a a, midsagittal distance from basion to opisthion; b, midsagittal
horizontal distance of the spinomedullary junction at the midpoint of
“a”; c, the distance of the obex superior (sup) or inferior (inf) tothe
midpoint of “a”; d, the angle (in degrees) created between intersecting
lines between the clivus and the floor of the fourth ventricle; e, the
orthogonal distance between the fastigium and the floor of the fourth
ventricle; f, the horizontal distance between the spheno-occipital synchondrosis
and the basis pontis.
be within normal limits. None of our patients had basilar invagination.
One patient had midline bone defects of the posterior
arches of C1 and C2, and one had assimilation of the posterior
arch of C1 on the right. No other obvious bony anomalies of
the posterior fossa or upper cervical spine were noted. No statistically
significant differences in sex or age were noted in the
patients in this group (P 0.05).
The results for the 50 pediatric patients with normal MRI
scanning of the brain are listed in Table 2. In this group, the
distance from the basion to the opisthion in the sagittal plane
ranged from 21 to 40 mm (mean, 28.6 mm). The midsagittal
horizontal distance of the spinomedullary junction at a midpoint
on a line connecting the basion to the opisthion ranged from 4.0
to 14 mm (mean, 9.5 mm). The angle created by intersecting lines
of the clivus and floor of the fourth ventricle ranged from 5.0 to
32 degrees (mean, 13.8 degrees). The clival angle was believed to
be within normal limits in all 50 patients. No bony anomalies of
the posterior fossa and craniocervical junction were seen on
imaging. The distance from the fastigium to the floor of the
ventricle ranged from 6.0 to 19 mm (mean, 10.1 mm). Finally, the
distance from the spheno-occipital synchondrosis to the basis
pontis ranged from 4.0 to 12.0 mm (mean, 6.4 mm). Both the
prepontine and retrocerebellar cisterns were found to be within
normal limits in this group. No significant differences were
noted between the sexes among the patients in this group (P
TABLE 2. Measurements of Control Group a
0.05), nor were any significant differences found when groups of
similar age were examined.
DISCUSSION
We previously reported the resolution of syringohydromyelia
in five patients with posterior fossa decompression but
absence of cerebellar tonsillar ectopia (4, 6). In those patients,
clinical and radiological improvement with presumably idiopathic
syringohydromyelia was documented. The patients
continue to have good clinical results after an average
follow-up of approximately 4 years. We have added a patient
with this clinical presentation who has been followed for 1
year. In two of the original five patients, a surgical observation
was made that the posterior fossa appeared to be “crowded”
at the level of the foramen magnum. Of interest, all three
patients described in the original article who underwent both
preoperative and postoperative cine MRI demonstrated decreased
flow preoperatively, and two of the three demonstrated
improved flow postoperatively (6). In a study of syringohydromyelia
conducted before the availability of
computed tomography or MRI, Newton (12) described drainage
anomalies of the fourth ventricle in which some patients
with syringohydromyelia were without hindbrain herniation.
These patients were treated with posterior fossa decompression
and drainage of the syrinx, with good results (12).
The measurements obtained in our study were chosen to
directly and indirectly assess the bony compactness of the posterior
fossa in and around the level of the foramen magnum and
to determine whether the brainstem may have been displaced on
midsagittal MRI scans. Although linear and nonvolumetric,
these measurements on sagittal MRI scans may assist the neurosurgeon
in evaluating patients with syringohydromyelia and
the absence of tonsillar ectopia. First, by measuring the midline
anteroposterior distance of the foramen magnum in the sagittal
plane, a determination can be made regarding whether the outlet
of the posterior fossa was compromised. Next, by assessing
the anteroposterior distance of the spinomedullary junction
at the foramen magnum, the relationship of the amount of soft
tissue to bony encasement at the level of the foramen magnum
can be determined. The normal range of the diameter of the
spinal cord at C1 is 7.0 to 11.0 mm (7). Measurement of the angle
between the clivus and floor of the fourth ventricle, the distance
Group Age (yr) a (mm) b (mm) c (mm) d (degrees) e (mm) f (mm)
1 2–4 22.0–40.0 (28.0) 4.0–10.0 (9.0) 8.5–17.0 (11.0) 5.0–25.0 (14.0) 8.0–15.0 (10.0) 2.0–8.0 (5.4)
2 8–10 21.0–33.0 (29.0) 6.0–14.0 (9.8) 8.5–16.5 (11.0) 5.0–20.0 (12.4) 6.0–12.0 (9.0) 5.0–10.0 (7.8)
3 12–14 30.0–35.0 (33.0) 9.0–12.0 (8.3) 7.5–16.0 (10.5) 6.0–32.0 (15.0) 6.0–12.0 (10.0) 4.0–12.0 (6.0)
4 15–17 28.0–33.0 (30.0) 9.0–12.0 (11.0) 8.0–17.0 (11.5) 12.0–20.0 (16.0) 11.0–19.0 (12.8) 4.0–10.0 (6.5)
a
Values are ranges (means) for associated ages at presentation (1 yr) of the Chiari 0 group. a, distance from basion to opisthion; b, midsagittal
horizontal distance of the spinomedullary junction at the midpoint of “a”; c, the height of the obex superior to the plane “e” of the foramen
magnum; d, the angle (in degrees) created between intersecting lines between the clivus and floor of the fourth ventricle; e, the orthogonal
distance between the fastigium and the floor of the fourth ventricle; f, the horizontal distance between the spheno-occipital synchondrosis and
the basis pontis.
Neurosurgery, Vol. 48, No. 5, May 2001

from the fastigium of the fourth ventricle to the floor of the
fourth ventricle, and the distance from the spheno-occipital synchondrosis
to the basis pontis allowed us to ascertain whether
the fourth ventricle and/or the brainstem were displaced anteroinferiorly,
considering that all clival angles were found to be
within normal limits.
The results of our study suggest that the bony posterior fossa
in patients with a Chiari 0 malformation may be distorted at the
level of the foramen magnum as compared with a control group.
This finding held true when comparing data from people of all
ages with our group of six patients and also when comparing the
patients of different ages in our group with children of the same
age in the control group. Our measurements demonstrated that
in all six patients with this entity, the sagittal anteroposterior
distance of the spinomedullary junction is increased, implying
caudal displacement of the more rotund medulla oblongata. This
was statistically different when compared with our control individuals
(P 0.00121). The concept of caudal descent of the
brainstem is further supported by the finding that, in all patients
in this group, the tip of the obex was more than 3 standard
deviations below the normal position; this migration implies a
compromised posterior fossa in these patients (10, 15). The
Chiari I malformation occasionally has been associated with
caudal migration of the brainstem and is sometimes termed the
Chiari 1.5 malformation (5, 13, 14, 20). The anteroposterior distance
of the foramen magnum in our group of patients was
significantly larger (P 0.00458) (7, 15). Of interest, patients with
Chiari I malformation have been described as often having a
smaller than normal foramen magnum (13, 15). The angle between
the clivus and the floor of the fourth ventricle had statistically
significant differences (P 0.00001), with means of 14
degrees for the control group and 28 degrees for the Chiari 0
group. This implies some degree of posterior tilt to the pons and
medulla, because the clival angles of the Chiari 0 group were
believed to be within normal limits. In addition, we found no
instance in which the spinomedullary junction appeared kinked,
nor did we find any retroversion of the odontoid process, both of
which would most likely decrease this angle. The distance between
the spheno-occipital synchondrosis and basis pontis and
the orthogonal distance between the fastigium and floor of the
fourth ventricle were not significantly different between the
Chiari 0 group and the control group (P 0.46126 and P
0.18149, respectively). This suggests that, in patients with Chiari
0 malformation, the contents of the posterior fossa compressed
caudally, and a laterally compromised posterior fossa would
help explain the increase in the basion-to-opisthion distance as
well as the increased angles noted between the clivus and the
floor of the fourth ventricle.
Many authors have discussed the findings that the posterior
fossa volume tends to be decreased and more shallow in the
Chiari I population (1, 8, 9, 11, 17, 18). In this infrequently
encountered group of patients with syringohydromyelia and
without tonsillar ectopia, the measurements demonstrated that
the posterior fossa revealed signs of being compromised. Our
assumption is that, in this rare group of patients, compactness at
the foramen magnum disturbs the normal circulation of cerebrospinal
fluid (CSF), thereby causing syringohydromyelia. This
mechanism of restriction of CSF flow is the same cause of the
syringohydromyelia observed in many patients with a Chiari I
malformation. Of note, the cerebellar tonsils are paramedian
structures and may have been minimally caudally displaced.
This displacement was not appreciated on our midsagittal evaluations.
Also of interest is that the angle between the clivus and
the floor of the fourth ventricle was significantly increased in our
Chiari 0 group. This is a curious observation because all clival
angles were within normal limits, as was the prepontine space in
each patient (6). Although this study did not evaluate posterior
fossa volumes, the radiological measurements imply that, in this
select group, the posterior fossa is smaller than normal. We now
have a protocol for MRI analysis of prospective patients with the
Chiari 0 malformation by which to compare the posterior fossa
volumes and ratios with those of patients with Chiari I
malformation.
CONCLUSION
We have demonstrated that the contents of the posterior fossa
demonstrate signs of being compromised in patients described
as having a Chiari 0 malformation. These findings indicate that
syringohydromyelia may be a result of a smaller than normal
posterior fossa’s compromising normal CSF egress out of the
cranium and that actual cerebellar tonsillar herniation is not
essential to disturb the delicate balance that is involved with CSF
circulation. Although limited by not having volumetric measurements,
abnormal measurements of the variety we obtained on
midsagittal MRI scanning may assist the neurosurgeon to choose
patients more confidently for posterior fossa decompression
when a syrinx is present without cerebellar ectopia. Findings of
particular note include the following: obices that were abnormally
descended when measured from a midpoint on a midsagittal
line connecting the basion to the opisthion, an increase in the
midsagittal horizontal anteroposterior distance of the spinomedullary
junction on the line described above, an increase in the
angle between intersecting lines of the clivus and the floor of the
fourth ventricle, and an increase in anteroposterior dimensions
of the foramen magnum. It is hoped that these findings will add
to current knowledge regarding hindbrain herniation and syringohydromyelia
and perhaps lead to new nomenclature aimed
not at degrees of tonsillar herniation in the currently termed
Chiari I malformation but rather at capaciousness at the foramen
magnum.
ACKNOWLEDGMENT
We thank Bermans J. Iskandar, M.D., for his pioneering findings
and writings on the topic of the Chiari 0 malformation.
Received, August 21, 2000.
Accepted, January 13, 2001.
Reprint requests: W. Jerry Oakes, M.D., Pediatric Neurosurgery, The
Children’s Hospital of Alabama, 1600 7th Avenue South, Acc #400,
Birmingham, AL 35233. Email: joakes@ms.surgery.uab.edu
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Neurosurgery, Vol. 48, No. 5, May 2001
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Neurosurgery, Vol. 48, No. 5, May 2001
COMMENTS
Dr. Oakes and his colleagues previously reported the successful
treatment of syringohydromyelia without Chiari malformation
by posterior fossa decompression (1). Five of the six
children in this study were included in that publication. At
the time of operation, the surgeon had the impression of
“crowding” of posterior fossa structures at the foramen magnum,
although a true Chiari malformation could not be identified.
This article is an attempt to document quantitatively
whether evidence of a small posterior fossa and/or caudal
displacement of the brainstem is present in these patients.
Their findings support that hypothesis. Volumetric analysis of
the posterior fossa and its structures would be a better way to
determine whether the posterior fossa is too small. It is hoped
that this type of analysis can be done in the future.
The authors suggest that such measurements may be helpful
in deciding whether to perform posterior fossa decompression
on a particular patient. This assumes that another group
of children with syringohydromyelia exists without the
changes described. Such patients would not benefit from posterior
fossa decompression. Until this can be determined, I
suspect that the presence of syringohydromyelia per se will
dictate the decision for surgery. In my experience, only a
fraction of the patients examined for evaluation of a Chiari I
malformation have symptoms relevant to that finding. I am a
bit unsettled by the prospect of contending with an even
larger group of patients with nondescript symptoms such as
headache or dizziness whose anthropomorphic measurements
suggest a need for a posterior fossa decompression.
Finally, I agree with the authors’ concluding comment that a
better term is needed to describe this phenomenon. Because of
the euphony of the term and the ease with which it can be
related to a recognized entity, however, I am afraid that the
term will stick.
Paul H. Chapman
Boston, Massachusetts
1. Iskandar BJ, Hedlund GL, Grabb PA, Oakes WJ: The resolution of
syringohydromyelia without hindbrain herniation after posterior
fossa decompression. J Neurosurg 89:212–216, 1998.
It is sometimes difficult not to be a skeptic with regard to
the many draftsperson’s lines sketched through the cranium,
particularly as they apply to structures within the posterior
cranial fossa. How does one reconcile fixed cranial measurements
with those of the contained “pliable” brain tissue, and
at which point, if any, might the respective measurements
negate each other?
On the surface, this article has appeal, to say nothing of the
Chiari 0 phenomenon—its raison d’être. The proposed measurements
are simple and, for the most part, easily obtained.
One wonders, though, about the leap of faith required when
taking the measurement results to the physiological derangements
that create syringohydromyelia. Can one rely upon
linear, nonvolumetric measurements to define a crowded foramen
magnum? The authors believe so, stating that their

intent is to “assist the neurosurgeon” when evaluating patients
who have syringohydromyelia in the absence of cerebellar
tonsillar ectopia.
Following up on this, the posterior fossa is defined as being
compromised or distorted, which brings about tissue shifts
and the caudal migration of the brainstem. This is the essential
message. In the end, it may be that pure geometry is
correct. Certainly, one cannot argue with success (i.e., the
resolution of clinical symptoms after posterior fossa decompression
in patients with the Chiari 0 malformation).
Robin P. Humphreys
Toronto, Ontario, Canada
The authors of this article previously reported five patients
with hydrosyringomyelia but no cerebellar tonsil displacement
beyond the level of the foramen magnum—the so-called Chiari
0 malformation—that resolved after a craniocervical decompression.
The authors have added a sixth patient to this group, all of
whom had multiple measurements obtained from imaging studies
to determine the existence of objective findings that could be
used to guide the decision-making process with regard to
whether surgical decompression would likely benefit similar
patients. From their measurements, the authors determined that
the brainstems of the patients who underwent decompression
were caudally displaced more than 3 standard deviations below
the normal level. Another finding was that the angle between the
clivus and the floor of the fourth ventricle was significantly
increased in the Chiari 0 group as compared with control
individuals.
Because this study was retrospective, the authors did not have
adequate information to determine the volume of the posterior
fossa in their patients, although they had evidence that the
volume was reduced. It is hoped that the authors and other
investigators will obtain subsequent volume measurements to
confirm this presumptive finding. It would also be of value for
the patients to undergo cine magnetic resonance imaging studies
to determine whether abnormal cerebrospinal fluid (CSF) flow
patterns are usually or always present preoperatively, as well as
their response to a craniocervical decompression.
Of interest, the authors were successful in resolving the
presence of hydrosyringomyelia in this group of six patients
after a craniocervical decompression procedure. With this rate
of success, why worry about measurements? One wonders
whether this success rate will continue. Given the alternatives
for treating hydrosyringomyelia (i.e., myelotomy, various
forms of CSF diversion), cervicomedullary decompression in
general seems to be the best initial approach because it addresses
the underlying pathophysiology of an alteration in
normal CSF flow patterns.
J. Gordon McComb
Los Angeles, California
This important study is described by a distinguished group
of investigators who previously reported the resolution of
syringohydromyelia after posterior fossa decompression in
five children lacking magnetic resonance imaging (MRI) evi-
Neurosurgery, Vol. 48, No. 5, May 2001
Chiari 0 Analysis 1055
dence of cerebellar tonsillar ectopia. The MRI findings in these
patients seemed to exclude cranial base abnormalities consistent
with Chiari I malformation, although two of five patients
were believed to have a “crowded” foramen magnum at the
time of surgery. To emphasize the possibility that patients
with idiopathic syringohydromyelia might benefit from posterior
fossa decompression, the authors introduced the interesting
and provocative concept of the Chiari 0 malformation.
In this study, the authors reviewed the MRI findings in the
original study population and added a sixth patient. Unfortunately,
none of the patients in the original report had undergone
complete brain MRI before surgery, which made it impossible
for the authors to discuss anatomical features of Chiari I malformation,
such as a reduced volume of the posterior fossa and an
increased slope of the tentorium. To address this issue, the
authors developed a number of radiographic measurements to
assist in establishing the position of the brainstem relative to the
foramen magnum. These measurements are easy to obtain and
seem to provide a reliable index of brainstem displacement. On
the basis of a comparison of findings in six patients with Chiari
0 malformation and 50 children with normal MRI scans, the
authors conclude that children with Chiari 0 malformation have
a “compromised and/or distorted” posterior fossa and caudal
displacement of the brainstem.
Although the term Chiari 0 malformation is certainly attractive,
it is called into question by the demonstration of subtle but
distinct hindbrain and cranial base abnormalities. Perhaps a
more apt description would be “borderline Chiari I malformation
characterized by low-lying cerebellar tonsils, caudal displacement
of the brainstem, and underdevelopment of the posterior
fossa.” The absence of tonsillar ectopia on midsagittal MRI
scans may understate the incidence of minimal tonsillar herniation.
The cerebellar tonsils are paramedian structures, and the
apices of the tonsillar tips are found lateral to the midline. More
work is required to fully understand the pathophysiology and
clinical manifestations of this interesting condition.
Evidence has accumulated in recent years that noncommunicating
syringomyelia is caused by an obstruction of the CSF
circulation at or below the foramen magnum, which increases
the pulsatile systolic pulse wave in the spinal subarachnoid
space and drives CSF into the central canal of the spinal cord.
In patients with idiopathic syringomyelia, it is advisable to
obtain an exhaustive neuroradiological workup that is focused
on the detection of occult CSF blocks. Cine MRI and
computed tomographic myelography are often helpful in this
regard. It is important that pediatric patients undergo MRI
scanning of the thoracolumbar spine to rule out spinal cord
tethering, which is a recognized cause of chronic tonsillar
ectopia and is commonly associated with idiopathic syringomyelia,
presumably on the basis of occult obstruction of CSF
flow at the level of the foramen magnum. The authors have
revised their original conclusions and expanded our understanding
of the Chiari 0 phenomenon.
Thomas H. Milhorat
Brooklyn, New York