Table of Contents

cerebellar tonsils - portion of the cerebellum located at the bottom, so named because of their shape

cerebellum - part of the brain located at the bottom of the skull, near the opening to the spinal area; important for muscle control, movement, and balance

cerebrospinal fluid (CSF) - clear liquid in the brain and spinal cord, acts as a shock absorber

Chiari malformation - condition where the cerebellar tonsils are displaced out of the skull area into the spinal area, causing compression of brain tissue and disruption of CSF flow

cine MRI - type of MRI which can measure CSF flow

compliance - a measure of how much a vessel changes in volume due to a change in pressure; dV/dP; the inverse of elastance

control -  in a medical study, a group of subjects who either does not have a disease or does not receive a treatment, which acts as a basis for comparison

cranium - the skull

craniectomy - surgical technique where part of the skull is removed

decompression surgery - general term used for any of several surgical techniques employed to create more space around a Chiari malformation and to relieve compression

dura - tough, outer covering of the brain and spinal cord

duraplasty - surgical technique where the dura is opened and expanded by sewing a patch into it

hydrocephalus - a condition where there is an unusually large amount of CSF in the brain, resulting in swollen ventricles

intradural exploration - general term referred to a surgeon finding and removing any scarring or obstructions to CSF flow that exist underneath the dura

laminectomy - surgical technique where part of a vertebra is removed

magnetic resonance imaging (MRI) - diagnostic device which uses a strong magnetic field to create images of the body's internal parts

posterior fossa - depression on the inside of the back of the skull, near the base, where the cerebellum is normally situated

syringomyelia (SM) - neurological condition where a fluid filled cyst forms in the spinal cord

syrinx - fluid filled cyst in the spinal cord

tonsillar herniation - descent of the cerebellar tonsils into the spinal area; often measure in mm

ventricle - a CSF filled space in the brain

[Ed. Note:  In the spirit of full disclosure, Dr. Terry Lichtor, one of the authors of this study, is a Scientific Advisor to this publication.  In addition, the Editor's family was directly responsible for sponsoring this work.]

September, 2005 -- While the use of cine-MRI to look at CSF flow in Chiari patients is beginning to spread throughout the neurosurgical community, some researchers are beginning to turn their attention beyond this simple measure, to a property known as compliance.  Recently, a new theory on syrinx formation was published with compliance at the center (New Theory Speculates That Compliance Is Key To Syringomyelia And Alzheimer's), and a research team out of Chicago demonstrated that intracranial compliance was linked to surgical success in Chiari (Intracranial Compliance Linked To Surgical Success).
    Now, on the heels of their surgical report, the Chicago based researchers, led by medical physicist Dr. Noam Alperin and neurosurgeon Dr. Terry Lichtor, report in the July, 2005 issue of the Journal of Neurosurgery, that Chiari patients have lower intracranial compliance than healthy people.
     If you have read previous C&S News articles on this topic, you will recall that compliance is a measure of a vessel's, or container's, stiffness.  It is measured as the change in volume of  a vessel in response to a change in pressure.  A highly compliant container, like a balloon, can be expanded by blowing air into it.  A low compliance container, like a glass jar, will not expand much as the pressure inside it is increased (for a further discussion on compliance and pulsatile CSF flow, see New Theory Speculates That Compliance Is Key To Syringomyelia And Alzheimer's).
   You may also recall that with every heartbeat, blood rushes into the brain/cranium via arteries,  blood flows out through veins, and CSF flows from the skull to the spinal area.   Thus, intracranial compliance is a measure of how the cranium/brain area responds to the inrush of blood during a heartbeat.  To measure compliance, the research team used cine-MRI to quantify the total amount of blood and CSF flowing into and out of the skull area during a heartbeat, they quantified the pressure of the CSF, and then mathematically derived a Compliance Index for each subject.
     In this study, the team measured intracranial compliance, plus the maximum cord displacement - meaning how much the spinal cord moves, the velocity of the CSF, the flow rate of CSF, and how much (volume) CSF moves between the cranium and spinal areas during each cardiac cycle.  They scanned 34 Chiari patients and compared the results to 17 healthy controls.
     The Chiari group was comprised of 25 women and 9 men, who on average were 42 years old.  Nine of the Chiari subjects also had syringomyelia, while 3 had hydrocephalus.  The control group was made up of people with no history of neurological problems.
      While many physicians and researchers focus on the velocity, or flow, of CSF in Chiari patients, this study found that there was not a statistically significant difference in these measures between the Chiari group and the healthy group.  However, the results were suggestive of a relationship.  This finding seems to fall in line with previous research, which has shown mixed results when examining CSF velocity.  At this time, the confusing picture on CSF velocity may mean it is not a reliable parameter, or it may mean that the technology and techniques used to measure it are not yet sophisticated and reliable enough to produce consistent results.
     The Chicago team did find, however, that the calculated intracranial compliance for the Chiari group was significantly lower (by 20%) than the healthy control group.  The authors believe their compliance finding can be used to explain two of the hallmark features of Chiari, the cough headache, and the onset of symptoms in adulthood.
     In explaining the cough headache, they point out that coughing results in a rush of blood flowing into the cranial compartment.  With low compliance, a Chiari patient has less ability to buffer this influx, resulting in an increase in the pressure of the CSF in the skull area.  This increase in pressure in turn causes the cough headache.
     One of the big unknowns about Chiari is why, despite being born with the malformation, symptoms often don't appear until a person is an adult.  Addressing this with compliance, Alperin points out that a person's intracranial compliance naturally gets lower as they age.  The dura gets thicker and less resilient, plus scar tissue can form.  It may be that there is a cut-off point, which once a person's compliance gets below, they become symptomatic.  This also could help explain why some people experience long-term recurrence of symptoms, years after what appeared to be a successful surgery.  As they continue to age, their compliance naturally declines and the expansion effect of the surgery is gradually offset.
     Much more research involving the link between Chiari and compliance is needed.  The studies to date have only looked at a few patients, and how compliance is related to clinical symptoms has yet to be explored.  Is there a compliance cut-off point, below which clinical symptoms emerge?
     Even more interesting is considering whether there are non-surgical ways to increase compliance in Chiari patients.  Is there a way to make the dura more pliable and flexible and avoid surgery in borderline cases?
    Hopefully, more researchers will begin to focus on the this promising area of research and begin to answer some of these questions.

--Rick Labuda 

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Key Points

1. Recently, compliance has emerged as a focus area of Chiari research

2. One researcher developed a theory linking compliance and syringomyelia; this research group previously showed that decompression surgery increased compliance in Chiari patients and linked it to surgical success

3. This study looked at MRI derived parameters for 34 Chiari patients versus a group of healthy controls

4. Found that Chiari patients had significantly lower compliance than the healthy group

5. Measures of CSF velocity, flow, and cord displacement, which other researchers have focused on, were not significantly different between the groups

6. Authors believe compliance may be a good measure of the clinical picture of Chiari and explain why symptoms don't appear often until later in life

Table 1
MRI Derived Measurements, Chiari Patients vs Healthy Controls
 

Note:  Sig? refers to whether the result is statistically significant, meaning it is not likely due to chance; S means that while the result did not achieve statistical significance, it is suggestive of a relationship

Source:  Alperin N, Sivaramakrishnan A, Lichtor T. Magnetic resonance imaging-based measurements of cerebrospinal fluid and blood flow as indicators of intracranial compliance in patients with Chiari malformation.
J Neurosurg. 2005 Jul;103(1):46-52.

Related C&S News Articles: 

CSF Flow In Children Before & After Surgery

Studying CSF Flow To Predict Surgical Outcome

Intracranial Compliance Linked To Surgical Success

Decompression Surgery Reduces CSF Velocity

New Theory Speculates That Compliance Is Key To Syringomyelia And Alzheimer's