0:00

This is a brain MRI in a six-year-old

0:03

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

0:14

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

0:24

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.

0:35

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,

0:43

and the opisthion,

0:45

a portion of the occipital bone representing the

0:47

posterior margin of the foramen magnum.

0:50

Drawing a line between the basion and opisthion,

0:53

we can measure perpendicular to that,

0:56

the level of cerebellar tonsillar extent below

0:59

the plane of the foramen magnum.

1:01

And here,

1:01

the cerebellar tonsils extend

1:03

approximately 7 mm just below the plane

1:05

of the foramen magnum.

1:06

Historically, greater than 5 mm below the plane

1:10

of the foramen magnum has been considered

1:12

to be a Chiari Type I malformation.

1:14

That was a somewhat arbitrary cutoff.

1:19

In reality, a Chiari Type I malformation,

1:23

the clinical significance of it relates to

1:26

abnormal CSF flow dynamics and symptomatology.

1:31

We also know in children that five to six,

1:37

even 7 mm, can be a physiologic finding.

1:42

This patient,

1:43

all the CSF spaces around the brainstem

1:47

and the foramen magnum are patent.

1:48

This patient had no symptoms attributable

1:51

to a Chiari malformation.

1:53

So, this is very possibly a normal physiologic

1:59

finding in this age group,

2:01

that even up to 7 mm

2:02

below the plane of the foramen

2:04

magnum could be normal.

2:07

Now, distinguishing between

2:10

normal or borderline cerebellar tonsillar ectopia,

2:14

which is what I would typically call this,

2:16

versus a mild Chiari Type I malformation,

2:19

in some cases is an academic distinction.

2:23

Except

2:24

if a patient is labeled with a definitive

2:28

Chiari Type I malformation.

2:30

There's a risk that they may proceed to surgery

2:33

at even minor symptoms that may or may not be

2:37

attributable to the Chiari malformation.

2:39

A majority of the population at some point

2:42

will have a headache in their life.

2:44

This patient,

2:45

if they carry a diagnosis of a Chiari Type I malformation,

2:48

the first time that they have a headache,

2:50

someone may wonder,

2:51

is it related to the Chiari malformation?

2:53

That's a reasonable thought,

2:55

but we need to be careful

2:57

on imaging to not just

2:59

give measurements and give a diagnosis

3:02

as a black or white answer.

3:05

There are gray areas and this is a gray area.

3:07

So personally,

3:08

I would consider this to be borderline

3:10

cerebellar tonsillar ectopia.

3:13

If you don't feel comfortable with that,

3:16

you can say borderline cerebellar tonsillar

3:18

ectopia versus a mild Chiari Type I malformation.

3:22

And what is the importance

3:24

in distinguishing that?

3:25

What are the other features that

3:27

you need to know?

3:28

Well, one, you need to know what are the clinical

3:31

symptoms of the patient?

3:32

And number two,

3:34

you need to know your local neurosurgeons

3:36

what their philosophy is.

3:38

And this is a topic I've had very detailed

3:42

discussions with the neurosurgeons

3:43

that I work with,

3:44

and they agree.

3:45

They do not want or need their

3:47

clinic being filled up with patients that their

3:51

cerebellar tonsils extend 6 mm below

3:53

the plane of the foramen magnum,

3:55

but are otherwise asymptomatic.

3:57

And so, it can be a tricky distinction.

4:01

But we need to recognize that while historically

4:05

people have been taught that the cerebellar

4:08

tonsils extending 5 mm below

4:10

the plane of the foramen magnum or more,

4:12

represents a Chiari I malformation.

4:14

We need to recognize, number one,

4:16

that a slightly greater caudal extent than that

4:20

can be physiologic in children,

4:23

and number two,

4:24

that the key to determining management of

4:28

a Chiari malformation is not a ruler.

4:30

It is not the measurement.

4:32

It is other features.

4:33

It is CSF flow dynamics.

4:35

It is the presence of syringohydromyelia.

4:37

It is the presence of neurologic symptoms such

4:40

as headache, tinnitus, other things like that.

4:43

So this case is an important one to demonstrate

4:47

that a Chiari Type I malformation is

4:49

more than just a measurement.