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SECTION 12
Dierential Diagnosis
Basilar Invagination 392
Basilar Impression
Cranial Settling
Platybasia
Intrinsic Skull Base Lesion
Foramen Magnum Mass 408
396
400
402
404
Basilar Invagination
DIFFERENTIAL DIAGNOSIS
Common
• Chiari 1 Malformation
• Chiari 2 Malformation
• Klippel-Feil Spectrum
• Atlantooccipital Assimilation
Differential Diagnosis
Less Common
• Basiocciput Hypoplasia
• Occipital Condylar Hypoplasia
• Atlas Hypoplasia
• Achondroplasia
• Down Syndrome
Rare but Important
• Syndromes With Vertebral Anomalies
○ 22q11.2 Deletion Syndrome (DiGeorge Syndrome)
○ Cleidocranial Dysplasia
○ Hajdu-Cheney Syndrome
○ Hemifacial Microsomia (Oculoauriculovertebral
○ CHARGE Syndrome
○ Spondylocarpotarsal Synostosis Syndrome
○ Robinow Syndrome
○ Jarcho-Levin Syndrome
ESSENTIAL INFORMATION
Key Differential Diagnosis Issues
• Basilar invagination is term used for developmental
anomalies of craniovertebral junction where odontoid has
abnormal relationship to foramen magnum (prolapse)
○ Should be distinguished from basilar impression, which
• Basilar invagination variably associated with symptoms,
brainstem compression
• Basilar invagination has been categorized by absence (type
1) or presence (type 2) of Chiari malformation
○ Has also been categorized by presence (type A) or
Helpful Clues for Common Diagnoses
• Chiari 1 Malformation
○ Key facts
○ Imaging
• Chiari 2 Malformation
○ Key facts
○ Imaging
392
Dysplasia)
is characterized by acquired abnormal odontoid/foramen
magnum relationship (rheumatoid arthritis, Paget,
osteogenesis imperfecta)
absence (type B) of clinical/radiographic instability at
craniovertebral junction
– Mismatch between posterior fossa size and cerebellar
tissue
– Low-lying pointed cerebellar tonsils
– Tonsils project ≥ 5 mm below foramen magnum
– Often associated with syrinx, anomalies of 4th
occipital sclerotome, and retroflexed odontoid
– Nearly 100% with neural tube closure defect
– Small bony posterior fossa
– "Notched" clivus
– Large funnel-shaped foramen magnum
– Low-lying tentorium
• Klippel-FeilSpectrum
○ Key facts
– Congenital spinal malformation characterized by
segmentation failure of ≥ 2 cervical vertebrae ±
thoracic, lumbar segmentation failure
– Various syndromes (Turner and Noonan) associated
but often clinically asymptomatic
○ Imaging
– Single- or multiple-level congenital cervical
segmentation and fusion anomalies
– Associated abnormalities include odontoid dysplasia,
basilar impression, C1 assimilation, and
occipitocervical instability
• Atlantooccipital Assimilation
○ Key facts
– Failure of C1 to correctly segment from occipital bone
○ Imaging
– Atlas fusion to occipital bone
– Wide variability ranging from partial to complete, uni-
or bilateral
Helpful Clues for Less Common Diagnoses
• Basiocciput Hypoplasia
○ Key facts
– Basiocciput derives from mesodermal cells of occipital
somites and not from neural crest (as do facial bones)
– Normal lower clivus formed from contributions of 4
occipital sclerotomes with upper portion formed from
basisphenoid
○ Imaging
– Truncated inferior margin with concave superior
margin
• Occipital Condyle Hypoplasia
○ Key facts
– Skull base is flattened with violation of Chamberlain
line
○ Imaging
– Wide variation in severity from minimal with no clinical
impact to severe with basilar invagination
• Atlas Hypoplasia
○ Key facts
– Hypoplasia usually involves posterior arch of C1,
compressing cord and giving high cervical myelopathy
○ Imaging
– Small C1 arch, which may be segmentally hypoplastic
or aplastic, and posterior midline nonunion are
common
– Split atlas with midline anterior and posterior
nonunion may be asymptomatic variant in adult
– Fracture should be excluded
– More aggressive work-up if identified in child with
other associated vertebral anomalies
• Achondroplasia
○ Key facts
– Foramen magnum stenosis is primary clinical concern
with quadriparesis, feeding difficulty, and respiratory
abnormalities
○ Imaging
– Defects in enchondral bone formation with shortened
basiocciput, short clivus
Basilar Invagination
• Down Syndrome
○ Key facts
– Atlantoaxial subluxation with transverse ligament
laxity is key feature
– Multiple other anomalies associated: Atlantooccipital
instability, os odontoideum, odontoid hypoplasia,
basiocciput hypoplasia, poster arch C1 hypoplasia
○ Imaging
– Atlantodental instability in up to 40%, although 1%
are symptomatic
Helpful Clues for Rare Diagnoses
• 22q11.2 Deletion Syndrome (DiGeorge Syndrome)
○ Key facts: OMIM #188400
– Rare congenital disease with symptoms including
recurrent infections, heart defects, and characteristic
facial features
○ Imaging
– Platybasia and upper cervical spine anomalies are
common (dysplastic atlas in 75%)
• Cleidocranial Dysplasia
○ Key facts: OMIM #119600
– Autosomal dominant disorder with clavicular aplasia
or hypoplasia, brachydactyly, dental anomalies, and
vertebral anomalies
○ Imaging
– Nonspecific vertebral and craniovertebral junction
bony anomalies
• Hajdu-Cheney Syndrome
○ Key facts: OMIM #102500
– Rare autosomal dominant disorder with dysmorphic
facies, bowed long bones, and vertebral anomalies
○ Imaging
– Progressive bone destruction with acroosteolysis and
osteoporosis
• Hemifacial Microsomia (Oculoauriculovertebral
Dysplasia)
○ Key facts: OMIM #164210
– Goldenhar syndrome; oculoauriculovertebral
dysplasia
Differential Diagnosis
– Common birth defect involving 1st and 2nd branchial
arch derivatives with highly variable phenotype
○ Imaging
– Nonspecific vertebral and craniovertebral junction
bony anomalies
• CHARGE Syndrome
○ Key facts: OMIM #214800
– Coloboma, heart anomaly, choanal atresia, retarded
growth and development, genital hypoplasia, and ear
anomalies
○ Imaging
– Basiocciput hypoplasia is common and may be severe
• Spondylocarpotarsal Synostosis Syndrome
○ Key facts: OMIM #272460
– Congenital familial syndrome with extensive vertebral
anomalies due to mutation in gene-encoding filamin B
○ Imaging: Platybasia and basilar invagination
• Robinow Syndrome
○ Key facts: OMIM #268310
– Short-limbed dwarfism, dysmorphic facies, vertebral
segmentation abnormalities, and hypoplastic genitalia
due to mutation in ROR2 gene
○ Imaging: Nonspecific segmentation anomalies
• Jarcho-Levin Syndrome
○ Key facts: OMIM #277300
– Rib deformities and widespread vertebral
segmentation anomalies related to mutation in DLL3
gene
○ Imaging
– Wide variability in phenotypes with Jarcho-Levin
– Most severe form with "crab-chest," spondylocostal
dysostosis
SELECTED REFERENCES
1. Wei G et al: Treatment of basilar invagination with Klippel-Feil syndrome:
atlantoaxial joint distraction and fixation with transoral atlantoaxial
reduction plate. Neurosurgery. 78(4):492-8, 2016
2. Dokai T et al: Posterior occipitocervical fixation under skull-femoral traction
for the treatment of basilar impression in a child with Klippel-Feil syndrome.
J Bone Joint Surg Br. 93(11):1571-4, 2011
Chiari 1 Malformation Chiari 1 Malformation
(Left) Sagittal T2WI MR shows
a Chiari 1 malformation st
with the syrinx involving the
upper cervical cord. There is
basilar invagination with mild
mass effect upon the medulla
ſt. (Right) Sagittal midline
bone CT in patient with Chiari
1 shows dens retroflexion. The
anterior C1 ring is large and
abnormally articulates with
the remodeled clivus ſt. The
odontoid tip is prolapsed
cephalad to the foramen
magnum.
393
(Left) Sagittal bone CT shows
a thickened dysplastic
appearance of the dens and
C1 ring ſt with the posterior
C1 ring severely narrowing the
central spinal canal. There is
also narrowing of the C2-C3
Differential Diagnosis
disc space st and fusion of the
C2 and C3 spinous processes
. (Right) Sagittal T2WI MR
shows dysplastic formation of
the dens as well as an
abnormal hypoplastic C1 ring
ſt resulting in severe spinal
canal narrowing and cord
compression with focal syrinx
at the C1 level.
(Left) Sagittal CT shows
typical midline findings of
assimilation of C1 into the
occiput with a high riding C1
arch ſt and upward
translocation of the odontoid
relative to the foramen
magnum. Note the congenital
fusion of C2-C3 st. (Right)
Sagittal T2 TSE MR shows
upward displacement of the
odontoid ſt compressing the
medulla with secondary Chiari
1 malformation due to the
narrowed foramen magnum.
Note the associated syrinx st.
Basilar Invagination
Klippel-Feil Spectrum Klippel-Feil Spectrum
Atlantooccipital Assimilation Atlantooccipital Assimilation
394
Atlantooccipital Assimilation Atlantooccipital Assimilation
(Left) Sagittal T1WI MR
obtained off midline shows
segmentation failure st of the
occipital condyles, which are
fused to the C1 lateral masses.
(Right) Sagittal CT study
shows assimilation of the
anterior C1 with the occiput
ſt and partial assimilation of
the posterior arch . There is
a widened atlantodental
interval st and upward
translocation of the odontoid
with respect to the foramen
magnum.
Basilar Invagination
Basiocciput Hypoplasia Basiocciput Hypoplasia
Basiocciput Hypoplasia Atlas Hypoplasia
Differential Diagnosis
(Left) Sagittal T1WI MR of
atlantooccipital assimilation,
basiocciput hypoplasia, and
Chiari 1 malformation shows
blunting of the clivus ,
widening of the predental
space ſt, and prolapse of the
odontoid tip into the foramen
magnum. The tonsils are lowlying st, and there is an upper
cervical syrinx. (Right) Sagittal
T1WI MR in atlantooccipital
assimilation shows an
abnormal truncated clivus ſt
and elongated odontoid
process positioned cephalad to
the foramen magnum (basilar
invagination).
(Left) Sagittal T2WI MR shows
a shortened clivus st and an
abnormal C1 arch with the
anterior arch too cephalad
and the posterior arch too
ventral ſt. (Right) Sagittal CT
shows the typical appearance
of a dysplastic (hypoplastic) C1
arch and the abnormal
relationship of the anterior C1
ſt with the foramen magnum.
The posterior C1 arch is not
fused, so it is not visualized on
this midline image.
Atlas Hypoplasia Syndromes With Vertebral Anomalies
(Left) Axial NECT shows an
abnormal left posterior arch
segment ſt that narrows the
spinal canal. (Right) Sagittal
T2WI in a patient with a
hypoplastic atlas and
Marshall-Smith syndrome
(accelerated skeletal
maturation, failure to thrive,
dysmorphic facial features)
shows dorsal compression of
the cervical spinal cord by the
hypoplastic C1 posterior ring
ſt and invagination of the
odontoid process st. The pons
is hypoplastic .
395
Basilar Impression
DIFFERENTIAL DIAGNOSIS
Common
• Rheumatoid Arthritis
• Paget Disease
• Osteomalacia
○ Renal Osteodystrophy
Differential Diagnosis
○ Rickets
• Osteogenesis Imperfecta
Less Common
• Mucopolysaccharidoses
○ Hurler/Hunter
○ Morquio
• Ankylosing Spondylitis
• Osteomyelitis of Skull Base
• Neoplastic Destruction of Skull Base
• Hyperparathyroidism
• Fibrous Dysplasia
Rare but Important
• Syndromes With Metabolic Abnormalities
○ Metaphyseal Chondrodysplasia (Jansen Type)
○ Lowe Oculocerebrorenal Syndrome
• Chondrodysplasias
○ Schwartz-Jampel Syndrome
• SUNCT Syndrome
ESSENTIAL INFORMATION
Key Differential Diagnosis Issues
• Basilar impression is term used for acquired abnormalities
of odontoid relationship with foramen magnum (bone
softening at skull base)
• Basilar invagination is term used for developmental
anomalies of craniovertebral junction where odontoid
process has abnormal relationship to foramen magnum
(prolapse)
Helpful Clues for Common Diagnoses
• Rheumatoid Arthritis
○ Key facts
○ Imaging
• Paget Disease
○ Key facts
○ Imaging
396
– Basilar impression is 1 of 3 directional instabilities that
occur in rheumatoid arthritis (RA)
– Other 2 are atlantoaxial subluxation and subaxial
subluxation
– "Cranial settling" is term applied to basilar impression
in RA
– In cranial settling, skull and C1 ring move as unit with
respect to C2 and rest of spine
– Gives classic upward translocation of odontoid with
low C1 ring due to transverse ligament incompetence
– Often asymptomatic involvement of skull base
– M > F
– Skull base is often only site of involvement
– May be multifocal disease with mixed sclerotic-lytic
pattern
– Expands bone; results in cotton wool appearance
• Osteomalacia
○ Key facts
– Abnormal mineralization in trabecular and cortical
bone
– Most common cause: Renal osteodystrophy
– Other causes: Malabsorption, liver disease, nutritional,
abnormal vitamin D or phosphate metabolism,
anticonvulsants, tumor induced
○ Imaging
– Deformities due to bone softening: Basilar impression,
vertebral endplate compressions, scoliosis
– Long bones: Looser zones (e.g., Milkman fractures,
pseudofractures)
• Osteogenesis Imperfecta
○ Key facts
– Genetic disorder of type I collagen resulting in bone
fragility
– Associated anomalies include blue sclerae, early
hearing loss, brittle teeth, thin fragile skin, and joint
laxity
○ Imaging
– Severe osteopenia, vertebral fractures, kyphoscoliosis
Helpful Clues for Less Common Diagnoses
• Mucopolysaccharidoses
○ Key facts
– Heterogeneous group of inherited lysosomal storage
disorders
○ Imaging
– Craniovertebral junction stenosis, dens hypoplasia,
ligamentous laxity, atlantoaxial instability, thickened
dural ring at foramen magnum
• Ankylosing Spondylitis
○ Key facts
– Early spine involvement
□ Squaring of vertebral bodies → corner erosions →
"shiny corner" (corner sclerosis)
– Late spine involvement
□ Widespread ankylosis ("bamboo spine")
– Craniovertebral junction abnormalities may relate to
accelerated degenerative change due to altered
biomechanics of spine
○ Imaging
– Severe degenerative change at C1-C2 junction due to
abnormal stress from caudal bony fusion
– C0-C1 joint with collapse of C1 lateral mass and
upward translocation of odontoid
• Osteomyelitis of Skull Base
○ Key facts
– Staphylococcus aureus most common in USA
– Mycobacterium tuberculosis most common worldwide
for C1-C2 joint infection
○ Imaging
– Soft tissue mass and bone destruction at C1-C2 level
• Neoplastic Destruction of Skull Base
○ Key facts
– Patient with known malignant neoplasm
○ Imaging
– Lytic destructive lesion of skull base
– Look for associated soft tissue mass
– Check for multiple lesions
• Hyperparathyroidism
○ Key facts
– Primary: Due to parathyroid adenoma, hyperplasia, or
carcinoma
– Secondary: Due to renal failure or, rarely, intestinal
malabsorption
○ Imaging
– Osteopenia
– Resorption of secondary trabeculae (interlinking,
nonweight-bearing trabeculae)
– Rare: Brown tumor (osteoclastoma)
• Fibrous Dysplasia
○ Key facts
– Relatively common lesion of occiput and sphenoid
○ Imaging
– Ground-glass matrix is classic appearance
– Often hypointense on T2WI MR; shows intense
enhancement on T1WI
Helpful Clues for Rare Diagnoses
• Metaphyseal Chondrodysplasia (JansenType)
○ Key facts: OMIM #156400
– Hypercalcemia and hypophosphatemia occur without
parathyroid abnormalities
○ Imaging
– Short stature, short bowed limbs, clinodactyly, small
mandible
• Lowe Oculocerebrorenal Syndrome
○ Key facts: OMIM #309000
– Mutation in OCRL1 gene (phosphatidylinositol 4,5-
biphosphate 5-phosphatase deficiency)
○ Imaging
– Cataracts, mental retardation, vitamin D-resistant
rickets, amino aciduria
• Schwartz-Jampel Syndrome
○ Key facts
– Very rare; also called Stüve-Wiedemann syndrome
– Joint contractures, bone dysplasia, small stature
○ Imaging
– Multiple skeletal abnormalities
Basilar Impression
Differential Diagnosis
• SUNCT Syndrome
○ Key facts
– Short-lasting unilateral neuralgiform headache with
conjunctival injection and tearing
– Short attacks of severe pain with autonomic
symptoms (e.g., tearing, rhinorrhea, conjunctival
injection)
– May be caused by variety of intracerebral tumors and
posterior fossa deformities
○ Imaging
– Necessary to exclude posterior fossa pathology
SELECTED REFERENCES
1. Botelho RV et al: Angular craniometry in craniocervical junction
malformation. Neurosurg Rev. 36(4):603-10; discussion 610, 2013
2. Brockmeyer DL: The complex Chiari: issues and management strategies.
Neurol Sci. 32 Suppl 3:S345-7, 2011
3. Krauss WE et al: Rheumatoid arthritis of the craniovertebral junction.
Neurosurgery. 66(3 Suppl):83-95, 2010
4. Smoker WR et al: Imaging the craniocervical junction. Childs Nerv Syst.
24(10):1123-45, 2008
5. Riew KD et al: Diagnosing basilar invagination in the rheumatoid patient. The
reliability of radiographic criteria. J Bone Joint Surg Am. 83-A(2):194-200,
2001
6. Nanduri VR et al: Basilar invagination as a sequela of multisystem
Langerhans' cell histiocytosis. J Pediatr. 136(1):114-8, 2000
7. Smoker WR: MR imaging of the craniovertebral junction. Magn Reson
Imaging Clin N Am. 8(3):635-50, 2000
8. Crockard HA: Transoral surgery: some lessons learned. Br J Neurosurg.
9(3):283-93, 1995
9. Zeidman SM et al: Rheumatoid arthritis. Neuroanatomy, compression, and
grading of deficits. Spine (Phila Pa 1976). 19(20):2259-66, 1994
10. Rajshekhar V et al: Haemangioma of the skull base producing basilar
impression. Br J Neurosurg. 3(2):229-33, 1989
11. Sherk HH: Atlantoaxial instability and acquired basilar invagination in
rheumatoid arthritis. Orthop Clin North Am. 9(4):1053-63, 1978
Rheumatoid Arthritis Rheumatoid Arthritis
(Left) Sagittal bone CT shows
an odontoid tip projecting
through the foramen magnum
st (cranial settling). The
anterior C1-odontoid distance
ſt is increased (normal
distance < 2 mm at inferior
aspect of C1 arch). (Right)
Sagittal NECT shows cranial
settling with upward
translocation of the odontoid
process relative to the
foramen magnum ſt. There
are also dense erosions st and
increased atlantodental
interval .
397
(Left) Sagittal T1WI C+ MR
shows the typical appearance
of a severe basilar impression
with flattening of the anterior
skull base (platybasia) ſt and
upward displacement of the
odontoid process and
Differential Diagnosis
posterior skull base st. (Right)
Coronal T1 C+ MR shows a
basilar impression due to bony
softening of the skull base
with upward displacement of
the skull base and mild
effacement of the temporal
lobes ſt.
(Left) Sagittal T1WI MR shows
heterogeneous increased
signal from an expanded clivus
ſt due to Paget disease. There
is upward displacement of the
odontoid process st relative
to the Chamberlain line as
well as Chiari 1 malformation
and cervical syrinx. (Right)
Sagittal NECT reconstruction
shows a horizontal orientation
of the clivus (platybasia) st
and protrusion of the dens
into the foramen magnum
(basilar impression) ſt.
Basilar Impression
Paget Disease Paget Disease
Paget Disease Osteogenesis Imperfecta
398
Osteogenesis Imperfecta Osteomyelitis of Skull Base
(Left) Sagittal T1WI MR shows
horizontal clivus orientation
ſt and upward protrusion of
the dens into the foramen
magnum . Note the
associated angular deformity
of the brainstem . A large,
heterogeneous extraaxial
subdural hematoma is present
st. (Right) Axial T1WI MR
shows a soft tissue component
of the infection involving the
basion ſt and anterior arch of
C1 with a phlegmon adjacent
to the cervicomedullary
junction .
Basilar Impression
Osteomyelitis of Skull Base Osteomyelitis of Skull Base
Osteomyelitis of Skull Base Neoplastic Destruction of Skull Base
Differential Diagnosis
(Left) Sagittal T1WI C+ MR
shows diffuse enhancement of
the phlegmon and destruction
of the C2 body with soft tissue
extension. The subluxation
causes anterior cord
compression from the C2 body
ſt and posterior cord
compression due to posterior
arch C1 compression st.
(Right) Sagittal T1WI MR in
partially treated C1-C2
Staphylococcus aureus
osteomyelitis shows abnormal
low signal involving a partially
collapsed odontoid process
that has migrated superiorly
to compress the ventral
medulla ſt.
(Left) Axial T1WI MR in this
case of partially treated C1-C2
joint osteomyelitis shows the
abnormal odontoid process
with adjacent phlegmon and a
compression on the
cervicomedullary junction ſt.
(Right) Axial NECT shows a
destructive mass ſt involving
the skull base, C1, and
adjacent nasopharynx in a
patient with multiple
myeloma and plasmacytoma.
There is upward translocation
of the odontoid process
with basilar impression.
Neoplastic Destruction of Skull Base Fibrous Dysplasia
(Left) Sagittal T1W MR shows
a very large mass ſt
expanding the clivus in a
patient with plasmacytoma.
The plasmacytoma is nearly
isointense to the brain on T1
images. There is basilar
impression due to the
generalized softening of the
skull base. (Right) Sagittal FS
T2WI MR shows an abnormally
sloped and expanded clivus
with mild platybasia due to
fibrous dysplasia . Also note
the anomalous spur of dense
cortical bone that projects
dorsally, indenting the dura at
the cervicomedullary junction
ſt.
399