Before MRI, Chiari Malformations were based on a
set of pathological observations made in a limited number of
patients more than 100 years ago. The current understanding of the
Chiari I is a condition that involves herniation of cerebellar
tonsils into the spinal canal. The condition can be associated
with Syringomyelia and malformations at the base of the skull.
Currently it has a narrow definition by radiologist- the
herniation must be at least 3 millimeters beneath the opening of
the skull; otherwise it is not regarded as a Chiari. Radiologists
further disagree whether it should be 3 or 5 millimeters. This
radiographic criteria is considered the beginning and the end of
diagnosis. Since MRI there has been dramatic rise of cases and
with it a great number of unanswered questions.
- What is the cause of the Chiari Malformation?
- What do patients have?
- What does it produce? What is the clinical syndrome?
- What is the risk of inheritance?
To answer these questions, a prospective study was
begun with 364 patients. A database was created with family
history; complete neurological exams and all radiographic findings
were presented. Each patient had MRI's of the head and spine.
Measurements were made of all the structures of
the posterior fossa. A quantitative measurement was done and then
using a sophisticated technique of volume measurement they
measured the volume of the posterior compartment, the volume of
the cerebellar and brainstem tissues in the posterior compartment
and the volume of the cerebrospinal fluid.
The patient study included 126 patients with
Chiari Malformations only, and 238 Chiari patients with a syrinx.
There was a 3 to 1 preference female over male. Many had lifelong
complaints, but the age of onset was defined as the age when the
patient first sought medical attention. The mean age was 25 years.
Diagnosis was made at a mean age of 30 years.
89 patients or of the study population could
identify a precipitating event that brought on symptoms. The most
common happening was an accident of some sort -such as motor
vehicle, sports injury, bump etc. Some experienced Chiari Symptoms
after childbirth or spinal taps. Coughing and sneezing brought on
symptoms in some. The most common complaint of patients with
Chiari with or without Syringomyelia was headache. This was a
specific headache that began in the suboccipital region as
pressure and pain and in some it radiated to the vertex of the
head and behind the eyes, less often it radiated posteriorly down
the shoulders. Often it was accentuated by postural changes or
valsalva maneuvers (coughing, sneezing, straining) and in females
it tended to be worse preceding menses.
Visual phenomena were common such as blurred
vision, seeing stars or wavy lines, uncomfortable feeling in
bright lights; less common was double vision. Females were often
diagnosed with MS based on the ocular symptoms. Otoneurological
symptoms were the leading cause of seeking medical attention. 74%
of the patient population had a Meniere's like syndrome with two
or more of the following complaints: dizziness, disequalibrium,
pressure in the ears, tinnitus (ranging from high pitched sounds
to sounds of oceans flowing), decreased hearing, true vertigo,
images jumping in from of the eyes. Nystagmus were the only major
finding upon clinic examination despite all these symptoms. All
types of nystagmus were found.
66% of patients had two or more cranial nerve,
brainstem or cerebellar disturbances. They included apnea
(episodes of irregular breathing during sleep), hoarseness,
tremors of the hands, poor circulation, pain in throat, pain in
the face, true syncope, shortness of breath, hypertension; 25%
described palpitations. Two people can have the exact Chiari
Malformation and one can be totally asymptomatic. The presence of
a Chiari does not impute a set of symptoms.
We have come to understand the Chiari Malformation
in the following way. It is a volumetrically too small posterior
compartment of the skull. The posterior compartment is formed by
the occipital bone in the back; this particular part of the bone
is called the supraocciput. It's formed by a roof, which is made
up of a membrane called the tentorium. It has a front, which is
the clivus. Then there is a mystical opening at the top of the
tentorium and there's a mystical opening at the bottom known as
the foramen magnum. Now in the Chiari I malformation, the volume
of this compartment is too small. The tentorium is sloped like the
steeple of a church, not the roof of a house, where the normal
tentorium comes out like this, the Chiari tentorium tends to be
high rising. The back bone, as a consequence, tends to be short.
In many patients with Chiari the clivus is short. Some patients
with Chiari have basilar impression where the tip of the second
vertebral body, the odontoid, tilts back. We refer to that as
retroflexion of the odontoid. These make a small posterior
compartment. Oh, and did he leave it out? There can also be
herniation of the cerebellar tonsils through the foramen magnum.
Milhorat has come to understand this as one of the consequences of
a too small posterior fossa- not the actual phenomena. This is not
the whole problem, but just a reflection of the too small
posterior compartment.
In Milhorat's study population, 34 patients or 9%
had typical Chiari symptoms- typical headaches, visual
disturbances, Meniere's like syndrome, lower cranial nerve
disturbances, 2/3 had syrinxes. They had a small posterior fossa,
they had a high steeped tentorium, they had a short supraocciput,
and they had a small clivus. But you know what? Their tonsils were
not down enough to be called a Chiari Malformation so these
patients were kicked out of the group. But if you look here at the
T-2 weighted images where spinal fluid is white behind the
cerebellum you can see this is a compressed posterior fossa with
an absence of CSF posteriorally and lateral to the cerebellum.
This is a Chiari Malformation with the tonsils at the level of the
foramen magnum, and not grossly below it. So we are redefining the
Chiari Malformation as not simply how far the tonsils come down,
but it is a diagnosis, which includes 7 specific signs. They
include: the volume of the posterior fossa being small, CSF volume
being reduced, compression owing to smallness of the bones in the
back and most of them have varying degrees of tonsillar herniation.
The failure to have greater than 3 mm to 5 mm herniation does not
exclude the diagnosis of Chiari I in our new definition.
In all 364 patients the most common and reliable
finding was compression of the CSF spaces posterior and lateral
the cerebellum. 332 of these patients had tonsillar herniation
greater than 5 mm but 32 did not and we did not exclude them.
These are patients, who had syrinxes, had headaches, they knew
they had Chiari but they were excluded from having a Chiari and it
was "idiopathic Syringomyelia" because they didn't have
the 5 mm tonsillar descent. These patients are now redefined as
Chiari I Malformations.
In a desire to be more precise about the small
posterior fossa, a control study of 50 normal people and 50
patients with presumptuous diagnosis of Chiari I were age and
sexed matched with the following measured: the length or tentorium,
angle or slope of tentorium, length of supraocciput, length of
clivus, degree of tonsillar herniation.
The Cavalieri Method was used to calculate total
volume of the posterior fossa and total volume of CSF spaces
around the cerebellum. In patients with Chiari I, the size of the
supraocciput was significantly smaller, clivus was significantly
smaller, tentorium angle was increased, and there was an average
of 9.8 mm tonsillar descent in this group. Normal tonsils should
lie 2mm ABOVE the foramen magnum.
Professor Chiari and others throughout time have
regarded the Chiari I Malformation as the least severe form of a
series of hindbrain malformations. But could it be just something
wrong with the bones of the skull and the consequences of
compressing spinal fluid and compressing local structures
contribute to the symptomotology and it has nothing to do directly
with anything being wrong with the central nervous system?
Based on this study there is strong evidence to
suggest it is a defect of mesodermal origin. It is a disorder of
bones, and structures that support bone- muscles and ligaments and
not nervous tissue. None of the 364 patients had evidence of
neural defects such as missing lobes or abnormalities of the
central nervous system. All patients had evidence of skull based
dysplagia. Interestingly, the location of these bony changes- the
back of the skull, the tentorium, the front of the posterior fossa,
the anomalies at the cervical-medullary junction- are consistent
with a defect of a very particular part of mesoderm. All these
structures form together in the embryo and all are linked to one
another in development.
This is powerful evidence that the Chiari I
Malformation, although it shares the name of Chiari, is not a
malformation of the central nervous system, it's not a
developmental anomaly of the hindbrain of the cerebellum and
brainstem. But it is a too small posterior cranial fossa, owing to
a maldevelopment of the skull.
How do symptoms occur? CSF that is compressed has
to go somewhere. In man, under normal conditions, we secrete
spinal fluid. We can increase that secretion but we cannot
decrease it. It happens at .37 cc every minute of our lives. When
we're upright, sleeping, running, resting. We produce 500 cc a
day. It's formed in the ventricles, circulates down through the
3rd ventricle into the aqueduct of Sylvius, into the 4th
ventricle. It exits and then it surrounds all the structures of
the nervous system. There is a watery bath between brain, spinal
cord, cerebellum and the inner table of the skull. In Chiari
patients there is reduced volume. Where does it go? Some of it is
distributed up into the head, squeezed up and some is squeezed
down and this may very well contribute to the formation of SM in
some patients. We believe this displacement of spinal fluid into
the cranial cavity may be a suitable explanation for the typical
Chiari Symptoms. Distribution of CSF out of the posterior fossa
into the semi-circular canals can produce in some a sea-sickness
syndrome- dizziness, disequilibrium, pressure in the ears,
tinnitus, and so forth. It appears to all begin with a too small
posterior fossa that compresses the spinal fluid at the back of
the head.
Direct bony pressure can also produce complaints.
If the compression of the skull is too great, if the retroflexed
odontoid (basilar impression) is too great you can have direct
compression of cranial nerves or even brainstem. That can produce
problems with swallowing, sleep apnea, hoarseness, tremors, and
palpitations. Symptoms of Chiari I appear to be due to two basic
phenomena that share in common a small back of the skull, a bony
dysplagia of the base of the skull and cervical-medullary
junction. These are increase of CSF pressure both in the head and
in the spinal canal and direct compression of nervous structures.
It is not a malformation of the brain itself.
This anomaly can occur sporadically, and probably
occurs most often in this fashion. It can also be associated with
genetic transmission in some families. The precise mode of that
transmission is under intense investigation thanks to a grant from
ASAP to Duke University. 12% of the 364 study population had two
or more affected family members.
In closing, Milhorat emphasized once again that an
MR finding of a Chiari I does not impute a set of symptoms. Nor
does it impute a prognosis except we have learned if you have a
Chiari I Malformation you will do well to wear your seatbelt and
stay out of harms way, and a Chiari I Malformation certainly does
not impute surgical treatment.
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