Congenital Anomalies of The Skull Base

Susan I. Blaser, MD, FRCPC

DIFFERENTIAL DIAGNOSIS

Common

Internal Jugular Vein Asymmetry
Jugular Bulb Diverticulum
Chiari 1
Chiari 2
Neurofibromatosis Type 1

Less Common

Aberrant Internal Carotid Artery
Persistent Stapedial Artery
Carotid Artery, Sphenoid Migration
Agenesis Internal Carotid Artery

Rare but Important

Craniostenoses
4th Occipital Sclerotome Anomalies
Persistent Craniopharyngeal Canal
Medial Basal Canal (Basilaris Medianus)
Chiari 3

ESSENTIAL INFORMATION

Helpful Clues for Common Diagnoses

Internal Jugular Vein Asymmetry
- Key facts: Right internal jugular vein (IJV) dominant in 68-75%
  - Left IJV commonly smaller than right
  - IJV asymmetry < in larger cranial vault
- Imaging
  - Most commonly seen normal asymmetry: Right sigmoid sinus, jugular bulb, and IJV larger than left
  - Increased signal of left IJV due to compression of left brachiocephalic vein during respiratory cycle
Jugular Bulb Diverticulum
- Key facts: Asymptomatic normal variant
- Imaging
  - Coronal best: Bone CT, MRV, or T1 C+
  - “Pouch” projects from jugular bulb
  - High signal on MR may simulate mass
Chiari 1
- Key facts: Mismatch between posterior fossa size and cerebellar tissue
- Imaging
  - Low-lying pointed (not rounded) peg-like cerebellar tonsils
  - Tonsils project below (≥ 5 mm) OR are impacted in foramen magnum
  - 4th occipital sclerotome anomalies in > 50% small occipital enchondral skull
Chiari 2
- Key facts
  - 100% with neural tube closure defect
- Imaging
  - Small bony posterior fossa
  - “Scalloped” petrous pyramid
  - “Notched” clivus
  - Low-lying tentorium/torcular
  - Large funnel-shaped foramen magnum
Neurofibromatosis Type 1
- Key facts: Neurocutaneous disorder
  - Characterized by diffuse neurofibromas, intracranial hamartomas, benign and malignant neoplasms
- Imaging
  - Progressive sphenoid wing dysplasia
  - Enlarged optic foramina and fissures
  - Foramen magnum and skull base defects

Helpful Clues for Less Common Diagnoses

Aberrant Internal Carotid Artery
- Key facts: Displaced ICA courses through middle ear
- Imaging
  - Enters posterior middle ear through enlarged inferior tympanic canaliculus
  - Courses anteriorly across cochlear promontory
  - Joins horizontal carotid canal through dehiscent carotid plate
  - Absent carotid foramen & vertical segment of petrous ICA
  - Look for associated persistent stapedial artery (PSA) in 30%
Persistent Stapedial Artery
- Key facts
  - Embryologic stapedial artery persists
  - PSA becomes middle meningeal artery
- Imaging
  - Enlarged CN7 canal
  - Small canaliculus leaving carotid canal at genu of vertical and horizontal petrous ICA
  - Absent ipsilateral foramen spinosum
Carotid Artery, Sphenoid Migration
- Key facts: Seen in skull base syndromes involving enchondral bone
  - Achondroplasia, branchiootorenal syndrome, bicoronal synostoses (Apert, Pfeiffer, Crouzon)
- Imaging
  - Medial migration of bony carotid artery walls at level of sphenoid
Agenesis Internal Carotid Artery
- Key facts
  - Isolated or syndromic (PHACES, morning glory, Goldenhar, clefting syndromes)
- Imaging
  - Absent or hypoplastic vertical and horizontal petrous portions of ICA

Helpful Clues for Rare Diagnoses

Craniostenoses
- Key facts
  - Syndromic (fibroblastic growth factor, TWIST, and MSX2 mutations) + nonsyndromic premature osseous obliteration of cranial sutures
- Imaging
  - Early fusion of occipital sutures surrounding foramen magnum
  - Enchondral skull base: Achondroplasia, syndromic bicoronal synostoses, kleeblattschädel
4th Occipital Sclerotome Anomalies
- Synonym: Proatlas anomalies
- Hypocentrum of 4th occipital sclerotome (OS) anterior clival tubercle
- Centrum of 4th OS apical cap of dens and apical ligament
- Ventral portion of neural component of 4th OS anterior margin of foramen magnum and occipital condyle
- Caudal portion of neural component of 4th OS lateral atlantal masses and superior posterior arch of atlas
- Imaging: 4th OSA finding
  - Short clivus and atlas assimilation
  - Craniovertebral bony anomalies
Persistent Craniopharyngeal Canal
- Key facts
  - Look for associated canal atresia or stenosis, moyamoya, coloboma
- Imaging
  - Small: Nonpituitary tissue containing remnant channel
  - Large: Contains pituitary gland or frank encephalocele or artery
Medial Basal Canal (Basilaris Medianus)
- Key facts
  - Notochord remnant cephalic terminus
- Imaging: Midline clivus
  - Posterior to sphenooccipital synchondrosis
  - Currarino types A-F denote completeness and location of defect
  - Tortuous canal may indent superior or inferior aspect of clivus
Chiari 3
- Key facts: Intracranial Chiari 2 + meningoencephalocele
  - High cervical meningoencephalocele
- Imaging
  - Occipital squama defect may involve upper cervical vertebrae
  - Bony features of Chiari 2 in addition