Normal Skull Base Venous Variants

Christine M. Glastonbury, MBBS

DIFFERENTIAL DIAGNOSIS

Common

Jugular Foramen Asymmetry
Jugular Bulb Pseudolesion
Pterygoid Venous Plexus Asymmetry
Transmastoid Emissary Vein
Asymmetric Posterior Condylar Canal

Less Common

High Jugular Bulb
Jugular Bulb Diverticulum
Dehiscent Jugular Bulb
Asymmetric Sphenoidal Emissary Vein (of Vesalius)

Rare but Important

Foramen Cecum
Petrosquamosal Emissary Vein

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Intracranial venous flow must exit through skull base foramina
Normal structures may be asymmetric
Normal variants may be uni- or bilateral
Variants may be mistaken for masses, especially jugular foramen pseudolesion
- MR pitfall results from variable signal intensity secondary to venous flow
- Normal CECT ± normal bone CT clarifies
- Asymmetric or enlarged veins may be sign of pathology

Helpful Clues for Common Diagnoses

Jugular Foramen Asymmetry
- Key facts
  - Jugular veins usually asymmetric in size
  - Exaggerated asymmetry may result in larger side mimicking mass
  - More often MR pitfall
- Imaging
  - MR: Unilateral high signal and enhancement
  - Look for ipsilateral enlarged transverse sinus
  - CECT: Normal jugular enhancement
  - Bone CT: Preservation of normal foraminal contours
Jugular Bulb Pseudolesion
- Key facts
  - Turbulent venous flow creates MR pitfall
  - Asymmetric signal mimics pathology
- Imaging
  - MR: Typically high signal intensity and avid enhancement
  - MRV or coronal T1 C+ usually clarify jugular bulb as normal
  - CECT: Normal jugular enhancement
  - Bone CT: Preservation of normal foraminal contours
Pterygoid Venous Plexus Asymmetry
- Key facts
  - Unilateral prominence of deep facial veins that drain from cavernous sinus
  - Often incidental finding
  - Asymmetry may be pathological: Increased venous flow with caroticocavernous fistula
- Imaging
  - CT: Curvilinear enhancement in medial masticator &/or parapharyngeal space
  - MR: Enhancement or flow voids in deep face, but no mass effect
  - Normal signal intensity of adjacent masticator muscles
Transmastoid Emissary Vein
- Key facts
  - Transverse sinus to posterior auricular or occipital veins
  - Large veins may be pathological
  - Skull base dysplasia with small jugular foramina (e.g., achondroplasia)
  - Emissary and suboccipital veins “replace” jugular veins and are surgical hazard
- Imaging
  - CECT: Enhancement of veins traversing mastoid bone
  - Bone CT: Smooth well-corticated channels through bone
Asymmetric Posterior Condylar Canal
- Key facts
  - Synonym: Condyloid canal
  - Posterolateral to hypoglossal canal
  - Important channel for venous flow with atretic jugular veins
  - Contents: Meningeal branch of ascending pharyngeal artery; emissary vein from sigmoid sinus to suboccipital veins
- Imaging
  - Bone CT: Well-corticated venous channel
  - CECT/MR C+: Venous enhancement

Helpful Clues for Less Common Diagnoses

High Jugular Bulb
- Key facts
  - Usually incidental finding
  - May be associated with pulsatile tinnitus
- Imaging
  - Axial bone CT: Bulb at level of cochlea
  - Coronal bone CT: Medial ± inferior to semicircular canals
  - MR: May be mistaken for unilateral mass because of high signal
  - MRV or coronal T1 C+ should clarify
Jugular Bulb Diverticulum
- Key facts
  - Normal variant, incidental finding
  - May be associated with pulsatile tinnitus
- Imaging
  - Bone CT: Best evaluated in coronal plane
  - Small “pouch” projecting from superior aspect of jugular bulb
  - MR: May be mistaken for temporal bone mass because of high signal
  - MRV or coronal T1 C+ should clarify
Dehiscent Jugular Bulb
- Key facts
  - Otoscopy: “Vascular” middle ear mass
  - May be associated with pulsatile tinnitus
  - Absence of bony covering between jugular bulb and middle ear cavity
- Imaging
  - CT: Absence of bony plate over jugular bulb at posterior hypotympanum
Asymmetric Sphenoidal Emissary Vein (of Vesalius)
- Key facts
  - Synonym: Sphenoid emissary foramen
  - Transmits emissary vein from cavernous sinus to pterygoid venous plexus
  - Enlargement is pathological if ↑ venous flow from caroticocavernous fistula or tumor direct invasion
- Imaging
  - May be partially assimilated with ovale or may be duplicated
  - Bone CT: Located medial to anterior aspect foramen ovale; usually < 2 mm

Helpful Clues for Rare Diagnoses

Foramen Cecum
- Key facts
  - Usually closes during embryogenesis
  - Contains vein from sup sagittal sinus
  - Large foramen can be pathological
  - Suspect anterior neuropore anomaly
- Imaging
  - Bone CT: Midline anterior to crista galli
  - Usually < 2 mm
Petrosquamosal Emissary Vein
- Key facts
  - Embryonic venous remnant
  - Connects transverse sinus and retromandibular vein
- Imaging
  - Bone CT: Vertical channel, ≤ 4 mm
  - Posterior to TMJ, anterior to EAC

Image Gallery

Axial CECT shows a normal left jugular foramen, though larger compared to the right. Note the left sigmoid sinus

, jugular foramen

, and internal jugular vein

are all significantly larger than the right.

Coronal CECT in the same patient reveals the larger left jugular foramen

and internal jugular vein

. Such asymmetry is normal and does not imply underlying pathology.

(Left) Axial T2WI FS MR performed during work-up of a possible metastatic disease reveals asymmetric hyperintensity in the right skull base

, which was intermediate intensity on T1 & enhanced with gadolinium (not shown). (Right) Axial CECT in the same patient reveals normal enhancement of the right sigmoid sinus