Skull and dural venous sinuses
Skull, Cranial Fossae, Foramina and Dural Venous Sinuses
Skull base anatomy for FCPS Part 1: cranial fossae, foramina and their contents, fracture signs, meningeal haemorrhage, dural venous sinuses and the cavernous sinus. Explains how face, sinus and ear infections reach the cranial cavity.
Cranial fossae and foramina
| Fossa | Opening | Contents |
|---|---|---|
| Anterior | Cribriform plate (ethmoid) | Olfactory nerve filaments |
| Middle | Optic canal (lesser wing of sphenoid) | Optic nerve, ophthalmic artery |
| Superior orbital fissure | III, IV, V1 (lacrimal, frontal, nasociliary), VI, superior ophthalmic vein | |
| Foramen rotundum | V2 (maxillary) | |
| Foramen ovale | V3 (mandibular), lesser petrosal nerve, accessory meningeal artery, emissary veins | |
| Foramen spinosum | Middle meningeal artery and vein, meningeal branch of V3 | |
| Foramen lacerum | Filled with cartilage; internal carotid passes across its upper end; greater petrosal nerve nearby | |
| Carotid canal | Internal carotid artery with sympathetic plexus | |
| Posterior | Internal acoustic meatus | VII, VIII, labyrinthine artery |
| Jugular foramen | IX, X, XI; sigmoid sinus becoming the internal jugular vein; inferior petrosal sinus | |
| Hypoglossal canal | XII | |
| Foramen magnum | Medulla, meninges, vertebral arteries, anterior and posterior spinal arteries, spinal roots of XI |
- Superior orbital fissure, rotundum and ovale carry V1, V2 and V3 in that order.
- Foramen ovale lesion: weak muscles of mastication (jaw deviates to the injured side on opening) and numb lower lip, chin and lower teeth.
- On leaving the jugular foramen, the internal jugular vein lies behind the internal carotid artery, with IX, X, XI and XII passing between or beside them; jugular foramen tumours give IX–XI palsies (Vernet syndrome).
- Small emissary foramina: foramen of Vesalius (cavernous sinus to pterygoid plexus), mastoid and parietal emissary foramina.
Foramina of the facial skeleton
| Foramen | Position | Contents |
|---|---|---|
| Supraorbital notch | Upper orbital margin | Supraorbital nerve and vessels (V1) |
| Infraorbital | Below the orbit | Infraorbital nerve (V2): skin of lower lid, cheek, side of nose, upper lip |
| Mental | Below the second lower premolar, midway up the mandible in adults | Mental nerve (V3): lower lip and chin; site of mental nerve block |
| Incisive (palate) | Behind the central incisors | Nasopalatine nerve and the terminal sphenopalatine (with greater palatine) vessels |
| Greater palatine | Opposite the third molar | Greater palatine nerve and vessels |
Skull fractures and haemorrhage
| Fracture | Signs |
|---|---|
| Pterion (where frontal, parietal, sphenoid and temporal meet; thinnest area) | Tears the middle meningeal artery: extradural haematoma |
| Anterior fossa | Periorbital bruising ("raccoon eyes"), CSF rhinorrhoea, anosmia |
| Middle fossa (petrous temporal) | Battle's sign, haemotympanum, CSF otorrhoea, facial palsy, deafness |
| Type of bleed | Source and shape |
| Extradural | Middle meningeal artery; biconvex; lucid interval; limited by sutures |
| Subdural | Torn bridging (superior cerebral) veins crossing to the superior sagittal sinus; crescent; elderly, alcoholics, shaken infants; crosses sutures |
| Subarachnoid | Ruptured berry aneurysm; thunderclap headache |
- Dura of the anterior and middle cranial fossae is supplied by trigeminal branches (middle fossa mainly V3 via the nervus spinosus, and V2); posterior fossa dura by C2–C3 and the vagus. Dural traction causes headache.
Fetal skull
- Anterior fontanelle closes at about 18 months; posterior at about 2–3 months.
| Presentation | Engaging diameter | Size |
|---|---|---|
| Vertex, well flexed | Suboccipitobregmatic | 9.5 cm |
| Deflexed vertex | Occipitofrontal | 11.5 cm |
| Brow | Mentovertical | 13–13.5 cm (cannot deliver) |
| Face | Submentobregmatic | 9.5 cm |
Dural venous sinuses
- Superior sagittal sinus (upper border of falx) drains to the confluence of sinuses, then transverse, sigmoid and internal jugular vein.
- Inferior sagittal sinus joins the great cerebral vein (of Galen) to form the straight sinus, which joins the confluence.
- Arachnoid granulations project into the superior sagittal sinus and return CSF to the blood.
- Sinuses and many connecting veins have no valves, so infection can travel either way.
Cavernous sinus
- On each side of the sella turcica and sphenoid body. Receives superior and inferior ophthalmic veins, superficial middle cerebral vein, sphenoparietal sinus; drains to superior and inferior petrosal sinuses and, through emissary veins, the pterygoid plexus.
- Lateral wall (above down): III, IV, V1, V2.
- Within the sinus: internal carotid artery with its sympathetic plexus and VI beside it, so VI is often affected first.
- Thrombosis: proptosis, chemosis, painful ophthalmoplegia, V1/V2 sensory loss, often bilateral through intercavernous sinuses.
- Carotid–cavernous fistula: pulsating proptosis with an orbital bruit.
Routes of intracranial spread
| Source | Route | Result |
|---|---|---|
| Boil of nasal vestibule or upper lip ("danger triangle") | Facial vein, angular vein, ophthalmic veins (valveless); or deep facial vein to the pterygoid plexus | Cavernous sinus thrombosis |
| Ethmoid sinusitis (children) | Through the thin lamina papyracea of the medial orbital wall | Orbital cellulitis; then ophthalmic veins to cavernous sinus |
| Chronic otitis media | Through the thin tegmen tympani (roof of the middle ear) | Extradural abscess, meningitis, temporal lobe abscess |
| Mastoiditis | Through the mastoid wall to the sigmoid sinus | Sigmoid sinus thrombosis, cerebellar abscess |
| Scalp infection | Emissary veins in the loose areolar layer | Sinus thrombosis, meningitis |
Remember Ovale for V3 and the lesser petrosal; spinosum for the middle meningeal; jugular foramen for IX, X, XI.
Exam trap VI runs through the cavernous sinus beside the internal carotid; III, IV, V1 and V2 lie in its wall.
Exam trap Extradural haematoma is arterial and stops at sutures; subdural haematoma is venous and crosses them.
How FCPS asks about this
- Artery through the foramen spinosum: middle meningeal; injured at the pterion causing extradural haematoma.
- Cause of subdural haemorrhage: torn bridging veins.
- Structures through foramen ovale and rotundum: V3 (with lesser petrosal) and V2.
- Nasal vestibule boil to the dural sinuses: facial and ophthalmic veins to the cavernous sinus.
- Temporal lobe abscess from otitis media: spread through the tegmen tympani.
- Mental foramen position: below the second lower premolar.
- Nerve lying inside the cavernous sinus: abducens (VI).
Test yourself on this topic
5 of the 57 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A child with chronic suppurative otitis media develops fever, headache and confusion. CT shows a temporal lobe abscess. Through which part of the middle ear did infection most likely spread?
Answer: Tegmen tympani (roof)
Walls of the middle ear: roof—tegmen tympani, a thin bony plate separating it from the middle cranial fossa and temporal lobe (spread causes extradural abscess, meningitis or temporal lobe abscess); floor—jugular bulb (lateral sinus thrombosis); anterior wall—carotid canal and Eustachian tube opening; posterior wall—aditus to the mastoid antrum (mastoiditis, then cerebellar abscess via the sigmoid sinus plate); medial wall—promontory (basal turn of cochlea), oval and round windows, and the facial nerve canal (facial palsy); lateral wall—tympanic membrane.
Middle ear walls
- Roof (tegmen): temporal lobe, meningitis
- Posterior: mastoid → cerebellar abscess
- Medial: promontory, facial nerve canal
- Floor: jugular bulb
Remember Thin roof, brain above.
Why the other options are wrong
- Floor over the jugular bulb Leads to jugular bulb or sinus thrombosis.
- Anterior wall, carotid canal Carotid canal; rarely involved.
- Lateral wall (drum) The drum; spread outward.
- Eustachian tube Connects to nasopharynx.
A 50-year-old woman has a meningioma at the anterior margin of the foramen magnum. Besides long-tract signs, she has weakness of the left sternocleidomastoid and trapezius. Which structure that ascends through the foramen magnum is compressed?
Answer: Spinal root of accessory nerve
The spinal root of the accessory nerve arises from the upper five cervical segments, ascends alongside the spinal cord and enters the skull through the foramen magnum. It then turns to leave through the jugular foramen and supplies sternocleidomastoid and trapezius. A foramen magnum tumour can therefore weaken these muscles together with long-tract signs. The foramen magnum also transmits the medulla, vertebral arteries and the spinal arteries.
Foramen magnum contents
- Medulla and meninges
- Vertebral arteries, spinal arteries
- Spinal roots of accessory nerve (ascending)
- Tectorial membrane, apical ligament
Remember Accessory goes up through the magnum, out through the jugular.
Exam trap The accessory nerve passes through two skull openings.
Why the other options are wrong
- Hypoglossal nerve rootlets Leaves through the hypoglossal canal; does not pass the foramen magnum.
- Vagal rootlets of the medulla Leaves through the jugular foramen; does not ascend through the magnum.
- Glossopharyngeal nerve rootlets Leaves through the jugular foramen.
- Anterior root of first cervical Does not supply these muscles.
A right occipital condyle metastasis leaves the tongue wasted on the right and deviating right on protrusion. Which of the following openings is affected?
Answer: Hypoglossal canal
The hypoglossal canal passes through the occipital bone just above the occipital condyle, transmitting the hypoglossal nerve (XII). A tumour in the condyle compresses the nerve here, causing wasting and fasciculation of the same side of the tongue, which deviates towards the weak side on protrusion because the healthy genioglossus pushes it across. Occipital condyle syndrome often also causes occipital pain. The jugular foramen transmits IX, X and XI.
Hypoglossal canal
- Above the occipital condyle
- Transmits XII
- Lesion: tongue wasted, deviates to same side
- Condyle metastasis: occipital pain
Remember Tongue points to the side of the lesion.
Exam trap In an upper motor neurone lesion the tongue deviates away from the lesion side and is not wasted.
Why the other options are wrong
- Jugular foramen Transmits IX, X and XI; tongue spared.
- Foramen magnum Transmits medulla and accessory spinal roots.
- Stylomastoid foramen Transmits the facial nerve.
- Foramen lacerum Filled by cartilage; nothing important passes through.
A young man is knocked out by a cricket ball to the side of the head, recovers and talks normally, then becomes drowsy with a dilating ipsilateral pupil. CT shows a biconvex hyperdense collection. Which vessel is most likely torn?
Answer: Middle meningeal artery
Extradural haematoma: blow to the temple fractures the thin squamous temporal bone near the pterion and tears the middle meningeal artery (branch of the maxillary artery, entering via foramen spinosum). Blood strips the dura from the skull, forming a biconvex (lens-shaped) collection that does not cross suture lines. Classic course: brief loss of consciousness, a lucid interval, then rapid deterioration with ipsilateral pupil dilatation (oculomotor compression from uncal herniation) and contralateral weakness. Urgent craniotomy and evacuation.
Extradural haematoma
- Middle meningeal artery (pterion)
- Biconvex; does not cross sutures
- Lucid interval then rapid decline
- Ipsilateral fixed pupil (uncal herniation)
Remember Pterion hit, lucid interval, lens-shaped bleed.
Why the other options are wrong
- Bridging cerebral veins Cause subdural haematoma (crescent-shaped).
- Middle cerebral artery Causes intracerebral haemorrhage or stroke.
- Superior sagittal sinus Causes venous extradural or sinus thrombosis, less typical.
- Basilar artery Posterior circulation, not this picture.
In transsphenoidal pituitary surgery, after crossing the sphenoid sinus, the surgeon opens the thin bony floor of the sella. This floor is formed by:
Answer: Body of the sphenoid
The sella turcica is a depression on the upper surface of the body of the sphenoid bone, and the sphenoid sinus lies within that body, directly below and in front of the sella. In transsphenoidal surgery the surgeon enters through the nose, opens the anterior wall of the sphenoid sinus, crosses the sinus and opens its thin roof, which is the floor of the sella. The cavernous sinuses with the internal carotid arteries lie on each side, so dissection strays laterally at great risk.
Sella turcica
- On the body of sphenoid
- Sphenoid sinus below and in front
- Lateral: cavernous sinus and ICA
- Above: diaphragma sellae, optic chiasm
Remember Through the nose and the sphenoid sinus to the pituitary.
Exam trap Straying laterally from the sella risks the internal carotid artery in the cavernous sinus.
Why the other options are wrong
- Greater wing of sphenoid Forms the floor of the middle cranial fossa laterally.
- Lesser wing of sphenoid Forms the back of the anterior fossa and the anterior clinoids.
- Basilar part of occipital Forms the clivus behind the dorsum sellae.
- Cribriform plate of ethmoid Lies in the anterior cranial fossa.
Where this sits in the CPSP syllabus
- Paper 1: Head, neck and back, Skull, cranial fossae, foramina and dural venous sinuses
- Surgery & Allied, Paper 2: Cranium and brain, Skull, cranial fossae, foramina and dural venous sinuses