Skull Base Foraminal or Fissural Variants

Christine M. Glastonbury, MBBS

DIFFERENTIAL DIAGNOSIS

Common

Jugular Foramen Asymmetry
Glossopharyngeal Canal

Less Common

Petromastoid Canal
Asymmetric Sphenoidal Emissary Vein (of Vesalius)
Persistent Craniopharyngeal Canal
Medial Basal Canal (Basilaris Medianus)
Internal Auditory Canal Hypoplasia
Enlarged Emissary Vein, Transmastoid
Asymmetric Posterior Condylar Vein

Rare but Important

Enlarged Inferior Tympanic Canaliculus
Absent Foramen Spinosum
Innominate Canal
Accessory Foramen Ovale

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Normal variants may be uni- or bilateral
When normal structures are asymmetric, this may be normal variation or sign of pathology
Many are incidental findings
Imaging strategy
- CT: Best defines or clarifies variants
- MR: Often source of pitfalls

Helpful Clues for Common Diagnoses

Jugular Foramen Asymmetry
- Key facts
  - Common developmental asymmetry of jugular foramen
- Imaging
  - CT: Smooth cortical bone surrounds enlarged jugular foramen
  - MR: Various flow phenomena may result in misinterpretation of mass lesion
Glossopharyngeal Canal
- Key facts
  - 30% have partial or complete canal
  - Glossopharyngeal nerve (CN9) enters canal prior to jugular foramen
- Imaging
  - CT: Small funnel-shaped canal leading to anterior portion of jugular foramen (pars nervosa)
  - MR: High-resolution T2 may show CN9

Helpful Clues for Less Common Diagnoses

Petromastoid Canal
- Key facts
  - Synonyms: Subarcuate canaliculus
  - From posterior CPA to mastoid air cells
  - Canal passes between and below crura of superior semicircular canal
  - Vestige of neonatal subarcuate fossa
  - Contents: Subarcuate artery and vein; branch of AICA or labyrinthine artery
  - In infant, subarcuate artery pseudolesion may confound radiologist
- Imaging
  - If seen in adult: ≤ 1 mm curvilinear canal below superior semicircular canal
  - May be ≥ 2 mm in young children; subarachnoid space association may mimic lesion
Asymmetric Sphenoidal Emissary Vein (of Vesalius)
- Key facts
  - Transmits emissary vein from cavernous sinus to pterygoid venous plexus
  - May enlarge with ↑ venous flow from caroticocavernous fistula
- Imaging
  - Located medial to anterior aspect of foramen ovale; usually < 2 mm
  - May be partially assimilated with ovale or may be duplicated
Persistent Craniopharyngeal Canal
- Key facts
  - Synonym: Trans-sphenoidal canal; persistent hypophyseal canal
  - Considered embryologic remnant of vascular channel
  - Site of trans-sphenoidal cephalocele
- Imaging
  - Bone CT: Midline sphenoid, < 1.5 mm
  - Anterior to sphenooccipital synchondrosis
Medial Basal Canal (Basilaris Medianus)
- Key facts
  - Considered remnant of cephalic end of notochord
  - Rarely enlarged to form basal cephalocele
- Imaging
  - Midline sphenoid; < 1.5 mm
  - Posterior to sphenooccipital synchondrosis
Internal Auditory Canal Hypoplasia
- Key facts
  - Contents: CN7 and CN8
  - Hypoplasia from congenital absence or deficiency of CN8
  - Associated with inner ear malformations
  - Normal canal diameter: 4-8 mm
- Imaging
  - CT: Small caliber IAC, < 4 mm
  - MR: High-resolution imaging may show deficient CN8
Enlarged Emissary Vein, Transmastoid
- Key facts
  - Connects transverse sinus to posterior auricular or occipital veins
  - Enlargement may be associated with small jugular foramen (JF)
- Imaging
  - Horizontal canal through posteromedial mastoid bone adjacent to occipital suture
Asymmetric Posterior Condylar Vein
- Key facts
  - Contents: Emissary vein from sigmoid sinus to suboccipital veins; meningeal branch of ascending pharyngeal artery
  - Enlargement associated with small JF
- Imaging
  - Well-corticated curvilinear channel

Helpful Clues for Rare Diagnoses

Enlarged Inferior Tympanic Canaliculus
- Key facts
  - Normally transmits Jacobsen nerve (inferior tympanic branch of CN9)
  - Aberrant internal carotid artery (ICA) enters middle ear through this
- Imaging
  - Aberrant ICA widens canaliculus to reach cochlear promontory
  - CTA or MRA to confirm aberrant ICA
Absent Foramen Spinosum
- Key facts
  - Normally transmits middle meningeal artery (MMA)
  - May be absent if MMA arises from ophthalmic artery or replaced by persistent stapedial artery (PSA)
  - Foramen rarely duplicated when MMA has anterior and posterior branches
- Imaging
  - Foramen normally < 3 mm
  - Posterolateral to foramen ovale
  - Look for PSA and aberrant ICA
Innominate Canal
- Key facts
  - Synonym: Canal of Arnold
  - Contains lesser petrosal nerve
- Imaging
  - Between foramen ovale and spinosum
  - Usually ≤ 2 mm
Accessory Foramen Ovale
- Key facts
  - Contains accessory meningeal artery
- Imaging
  - Lateral to foramen ovale; < 2 mm
  - May be partially assimilated with ovale resulting in posterolateral groove