Interactive Transcript
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This is a brain MRI in a six-year-old
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being performed for evaluation of seizures,
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and the brain parenchyma looks okay.
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I'm not seeing any abnormality that
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we typically look for in a patient
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with seizures.
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However,
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I do see on this axial T2-weighted image,
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the cerebellar tonsils extend to the level of
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the foramen magnum.
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So we need to see,
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is that normal or abnormal?
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A sagittal view is the best one for that.
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And we find the basion,
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the inferior aspect of the clivus,
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in particular,
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the basioccipital portion of the clivus,
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and the opisthion,
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a portion of the occipital bone representing the
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posterior margin of the foramen magnum.
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Drawing a line between the basion and opisthion,
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we can measure perpendicular to that,
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the level of cerebellar tonsillar extent below
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the plane of the foramen magnum.
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And here,
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the cerebellar tonsils extend
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approximately 7 mm just below the plane
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of the foramen magnum.
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Historically, greater than 5 mm below the plane
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of the foramen magnum has been considered
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to be a Chiari Type I malformation.
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That was a somewhat arbitrary cutoff.
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In reality, a Chiari Type I malformation,
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the clinical significance of it relates to
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abnormal CSF flow dynamics and symptomatology.
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We also know in children that five to six,
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even 7 mm, can be a physiologic finding.
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This patient,
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all the CSF spaces around the brainstem
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and the foramen magnum are patent.
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This patient had no symptoms attributable
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to a Chiari malformation.
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So, this is very possibly a normal physiologic
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finding in this age group,
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that even up to 7 mm
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below the plane of the foramen
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magnum could be normal.
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Now, distinguishing between
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normal or borderline cerebellar tonsillar ectopia,
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which is what I would typically call this,
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versus a mild Chiari Type I malformation,
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in some cases is an academic distinction.
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Except
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if a patient is labeled with a definitive
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Chiari Type I malformation.
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There's a risk that they may proceed to surgery
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at even minor symptoms that may or may not be
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attributable to the Chiari malformation.
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A majority of the population at some point
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will have a headache in their life.
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This patient,
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if they carry a diagnosis of a Chiari Type I malformation,
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the first time that they have a headache,
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someone may wonder,
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is it related to the Chiari malformation?
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That's a reasonable thought,
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but we need to be careful
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on imaging to not just
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give measurements and give a diagnosis
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as a black or white answer.
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There are gray areas and this is a gray area.
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So personally,
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I would consider this to be borderline
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cerebellar tonsillar ectopia.
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If you don't feel comfortable with that,
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you can say borderline cerebellar tonsillar
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ectopia versus a mild Chiari Type I malformation.
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And what is the importance
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in distinguishing that?
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What are the other features that
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you need to know?
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Well, one, you need to know what are the clinical
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symptoms of the patient?
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And number two,
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you need to know your local neurosurgeons
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what their philosophy is.
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And this is a topic I've had very detailed
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discussions with the neurosurgeons
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that I work with,
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and they agree.
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They do not want or need their
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clinic being filled up with patients that their
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cerebellar tonsils extend 6 mm below
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the plane of the foramen magnum,
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but are otherwise asymptomatic.
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And so, it can be a tricky distinction.
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But we need to recognize that while historically
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people have been taught that the cerebellar
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tonsils extending 5 mm below
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the plane of the foramen magnum or more,
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represents a Chiari I malformation.
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We need to recognize, number one,
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that a slightly greater caudal extent than that
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can be physiologic in children,
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and number two,
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that the key to determining management of
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a Chiari malformation is not a ruler.
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It is not the measurement.
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It is other features.
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It is CSF flow dynamics.
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It is the presence of syringohydromyelia.
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It is the presence of neurologic symptoms such
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as headache, tinnitus, other things like that.
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So this case is an important one to demonstrate
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that a Chiari Type I malformation is
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more than just a measurement.
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