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Borderline Cerebellar Tonsillar Ectopia Vs. Chiari I Malformation

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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.

Report

Description

Faculty

Asim F Choudhri, MD

Chief, Pediatric Neuroradiology

Le Bonheur Children's Hospital

Tags

Spine

Pediatrics

Neuroradiology

Musculoskeletal (MSK)

MRI

Idiopathic

Congenital

Brain

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