Neuroradiology #41

Clinical Data

38-year-old female (outpatient setting):

• Appointed for a non-contrast brain MRI

• Requested card information:

• Vertigo

• Unsteady gait

• Headache

• Chronic onset of symptoms that appeared approximately three years ago and got worse in the preceding weeks

T2-weighted axial image:

• The MRI sequences demonstrated no signs of acute or chronic ischaemic lesions, also no mass lesions were detected.

• The signal of the white and grey matter was also otherwise normal.

• Supratentorially, no abnormalities were detected.

T2-weighted axial image:

• Infratentorially, however, the brainstem appeared compressed anteriorly by the dens.

An additional sagittal T2-weighted image was obtained to clarify the anatomy and the cause of obstruction.

• The sagittal slices demonstrated several congenital abnormalities.

Which of the congenital abnormalities contributes the most to the patient’s symptoms?

Which of the congenital abnormalities contributes the most to the patient’s symptoms?

• Basilar invagination (in combination with mild retroflexion of the dens).

• The high riding dens causes compression of the brainstem at the pontomedullary junction.

What other congenital anomaly is basilar invagination commonly associated with?

• Platybasia

Definition of terminology:

Congenital basilar invagination and platybasia are often seen together.

• Basilar invagination:

• A congenital or acquired condition, where the dens protrudes more cranially than it should be, typically above the foramen magnum.

• If the dens causes compression of the brainstem, symptoms might ensue.

• Platybasia:

• A congenital or acquired condition, denoting abnormal flattening of the skull base, formally described as an increased base of skull angle. A simple way to think about it is also a shape of the base of the skull, where the clivus lies more horizontally than it should be (the clivus normally has a downward sloping shape, as clivus is a latin word for slope/hill).

• Translated directly from Greek, platybasia means “flat base” (of skull). And platypus means “flat foot”, just in the case you are wondering.

• Platybasia does not cause symptoms on its own.

The sagittal slices demonstrated several congenital abnormalities:

A CT of the cervical spine and base of skull was recommended to further evaluate the relationship of the bones:

• Basilar impression, platybasia and clivus hypoplasia were redemonstrated.

• CT also showed an additional ossicle/unfused bone immediately superior to the tip of the dens/posterior to the clivus – in the same place as the structure, that appeared to be the sclerotic tip of the dens.

Sagittal and axial reformats:

• The reformats demonstrated a bony arch, which were isolated from the rest of the occipital bone, representing the very rare variant called prebasioccipital arch.

Sagittal and axial reformats, what would we expect in a normal examination from a different patient?

We are still not finished (Part 1):

• Hypoplasia of the occipital condyles, which appear flattened. They are commonly observed together with the aforementioned anomalies.

• Oblique sagittal reformats right and left occipital condyle.

Condylar hypoplasia, oblique coronal reformat.

Oblique right sagittal and coronal reformats, what we would expect (normal examination from a different patient)?

• The normal occipital condyles are convex.

We are still not finished (Part 2):

• Additionally, the angle of the foramen magnum and the plane of the atlas were tilted cranially and posteriorly, resulting in a so-called “lordotic angulation”. The atlas was also noted to be hypoplastic.

Sagittal reformat, what we would expect (normal examination from a different patient)?

Wait a minute! The dens doesn’t actually protrude through the foramen magnum, as the foramen itself is tilted. So…

Are we still dealing with a case of basilar invagination?

Yes, we are.

Although the usual definition of basilar invagination says that the dens should protrude through the foramen magnum, the condition might be defined by several craniometric lineas and measurements. They also take into account conditions, where the foramen magnum itself is abnormally positioned, as in our case.

On sagittal images, they include the McRae, McGregor and Chamberlain lines. On coronal images, the digastric and bimastoid line.

The following Radiopaedia link has them all covered for you: https://radiopaedia.org/cases/basilar-invagination-measurements

In our case, we used the Chamberlain line. It is a line connecting the posterior edge of the bony hard palate to the opisthion (the midline point of the posterior edge of the foramen magnum). If the tip of the dens lies more than 3 mm above this line, we are dealing with basilar invagination.

Chamberlain line; our case and a normal CT of the cervical spine. Basilar invagination is defined, if the tip of the dens lies more than 3 mm above this line (the measurement in our case was 22 mm).

Platybasia measurements

Platybasia is also defined by a craniometric measurement, namely by an abnormally high base of skull angle (>143°).

On CT and MRI, it is defined as angle between the line drawn from the bottom of the anterior cranial fossa (or the nasion) and the tuberculum sellae, and the line drawn from the tuberculum sellae down the posterior margin of the clivus.

Base of skull angle, our case and a normal CT of the cervical spine.

An angle measuring more than 143° defines platybasia. The measurements of the angle were 146° and 67°, respectively.

Conclusion:

When confronted with a congenital anomaly, always be on the lookout, as they are often not isolated.

In the region of the craniocervical junction, congenital basilar invagination and platybasia often occur together.

It is important though to try to put emphasis on the most clinically relevant anomalies. In our case, among the many findings, basilar invagination was the most important one, causing symptoms by exerting pressure on infratentorial structures. Severe cases of basilar invagination can be treated by neurosurgical operative decompression.

Congenital anomalies of the craniocervical junction represent a complex topic. When confronted with difficult or ambiguous cases, looking up and reviewing many craniometric lines and measurements might be helpful.