Base of Skull Foramina
Overview
The skull base is a perforated bony barrier transmitting all twelve cranial nerves and the principal cranial vessels. Radiological mastery of base of skull foramina is essential for staging head and neck malignancies, identifying perineural tumour spread, and localising cranial neuropathies.
Anterior Cranial Fossa
- Cribriform Plate (Ethmoid): CN I (olfactory nerve fibres). Perforated by ~20 small foramina. Fracture here causes CSF rhinorrhoea and anosmia.
- Optic Canal (Sphenoid lesser wing): CN II + Ophthalmic Artery. Best seen on MRI thin axial/coronal views.
Middle Cranial Fossa (Sphenoid)
- Superior Orbital Fissure: CN III, IV, V1 (ophthalmic), VI + Superior Ophthalmic Vein.
- Foramen Rotundum: CN V2 (Maxillary division) → pterygopalatine fossa.
- Foramen Ovale: CN V3 (Mandibular division) → masticator space. Most easily seen on axial CT.
- Foramen Spinosum: Middle Meningeal Artery → epidural space. Lateral to foramen ovale.
- Foramen Lacerum: ICA passes OVER this foramen (not through it), sealed by fibrocartilage.
Posterior Cranial Fossa
- Internal Acoustic Meatus (IAM): CN VII (facial) + CN VIII (vestibulocochlear).
- Jugular Foramen (Pars Nervosa): CN IX, Inferior Petrosal Sinus.
- Jugular Foramen (Pars Vascularis): CN X (Vagus), CN XI (Accessory) + Internal Jugular Vein.
- Hypoglossal Canal: CN XII only.
- Foramen Magnum: Brainstem (pons-medulla junction), vertebral arteries, anterior and posterior spinal arteries, CN XI roots emerge here.
WarningPerineural Spread in Malignancy
Head and neck malignancies (especially adenoid cystic carcinoma, SCC) can spread along cranial nerve perineural sheaths retrograde into the skull base. Key signs on MRI: nerve enlargement, enhancement, and obliteration of the normal fat pad in the foramen. Foramen ovale is the most common site (V3 involvement).
High Yield Facts
LightbulbFRCR / MD Prep Pearl
Mnemonic for Middle Fossa exits: 'Standing Room Only' — SOF (V1), Rotundum (V2), Ovale (V3). SOF syndrome = combined CN III, IV, V1, VI palsy from a lesion in/near the superior orbital fissure. Cavernous sinus pathology can mimic SOF syndrome — it additionally affects the ICA and sympathetic fibres.