17 DURAL VENOUS SINUSES
# 18 Dural Venous Sinuses
Detailed notes
The dural venous sinuses are valveless endothelium-lined channels that sit between the periosteal and meningeal layers of the dura mater. They have no muscular wall and no valves, which means flow is passive and bidirectional under pathological conditions — a fact that explains why infection from the face can travel backwards into the skull.
All cerebral and cerebellar venous blood, together with diploic and emissary vein drainage, ultimately funnels through these sinuses into the internal jugular vein at the jugular foramen.
👩⚕️ Three properties make these sinuses high-yield:
➡ Fixed in position by the dural folds — they tear when those folds tear (vertex trauma → SSS).
➡ Valveless — infection and tumour can spread retrograde into the cranial cavity.
➡ Communicate with extracranial veins via emissary veins — the anatomical basis for cavernous sinus thrombosis.
The master drainage pathway
The single most important diagram to memorise:
Superior sagittal sinus → confluence of sinuses → transverse sinus → sigmoid sinus → internal jugular vein
A parallel deep system feeds in:
Inferior sagittal sinus + great cerebral vein of Galen → straight sinus → confluence
The cavernous sinus drains separately, posteriorly via the superior and inferior petrosal sinuses to the transverse sinus and IJV respectively.
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The dural folds (where the sinuses live)
The sinuses are not free-floating — each occupies a specific edge of a dural reflection:
| Dural fold | Location | Sinuses contained |
|---|---|---|
| Falx cerebri | Vertical, between the cerebral hemispheres | SSS (superior border), ISS (inferior free edge), straight sinus (at junction with tentorium) |
| Tentorium cerebelli | Horizontal, separates cerebrum from cerebellum | Transverse sinus (posterior attached margin), straight sinus (along midline attachment to falx) |
| Falx cerebelli | Vertical, between cerebellar hemispheres | Occipital sinus |
The individual sinuses
Superior sagittal sinus (SSS) — runs in the superior border of the falx cerebri from the crista galli to the confluence. Receives the superior cerebral veins and the arachnoid granulations that reabsorb CSF — which is why SSS thrombosis produces raised ICP and papilloedema. Sits directly under the vertex, so depressed vertex fractures lacerate it.
Inferior sagittal sinus — runs in the free lower edge of the falx. Small and deep; not at risk from external trauma.
Straight sinus — formed by the union of the ISS and the great cerebral vein of Galen at the junction of falx and tentorium. Drains posteriorly to the confluence. A vein of Galen malformation in neonates shunts arterial blood directly into this system and presents with high-output cardiac failure.
Confluence of sinuses (torcular Herophili) — the venous "roundabout" at the internal occipital protuberance. SSS, straight sinus and occipital sinus meet here and redistribute laterally into the transverse sinuses.
Transverse sinus — runs laterally in the attached margin of the tentorium cerebelli, grooving the occipital bone, towards the petrous temporal.
Sigmoid sinus — S-shaped continuation of the transverse sinus that grooves the mastoid part of the temporal bone. Sits immediately deep to the mastoid air cells, which is why mastoiditis can erode into it and cause lateral (sigmoid) sinus thrombosis. It exits the skull through the jugular foramen as the IJV.
Occipital sinus — smallest of the sinuses, in the attached margin of the falx cerebelli; drains to the confluence.
Cavernous sinus — see below; the single highest-yield sinus in MRCS Part A.
The cavernous sinus
Paired venous lakes on either side of the body of the sphenoid, extending from the superior orbital fissure anteriorly to the apex of the petrous temporal posteriorly. The two sides are connected across the midline by intercavernous sinuses, so infection readily becomes bilateral.
Contents — the highest-yield fact on this topic:
| Within the sinus | In the lateral wall |
|---|---|
| Internal carotid artery | CN III (oculomotor) |
| CN VI (abducens) | CN IV (trochlear) |
| V1 (ophthalmic) | |
| V2 (maxillary) |
👩⚕️ Mnemonic — lateral wall, superior to inferior: O TOM CAT — Oculomotor, Trochlear, Ophthalmic (V1), Maxillary (V2). The Carotid Artery and abducens (T for "the sixth") run through the sinus.
➡ CN VI is the most vulnerable nerve because it sits free in the venous blood, in direct contact with the carotid. It is first to be affected by raised cavernous sinus pressure (thrombosis, fistula, tumour).
➡ V3 does NOT pass through the cavernous sinus — it leaves the middle cranial fossa via foramen ovale. A classic exam distractor.
Tributaries: superior and superior ophthalmic veins (from the face), the sphenoparietal sinus, and the superficial middle cerebral vein.
Drainage: posteriorly via the superior petrosal sinus to the transverse sinus, and via the inferior petrosal sinus to the IJV at the jugular foramen.
Cavernous sinus thrombosis
The exam favourite. Infection reaches the sinus via the valveless ophthalmic veins from the "danger triangle of the face" — the region between the bridge of the nose and the corners of the mouth. Sources include:
- Furuncles or cellulitis around the nose and upper lip
- Sinusitis (ethmoidal, sphenoidal)
- Dental abscesses (upper teeth → pterygoid plexus → cavernous sinus)
- Orbital cellulitis
Clinical features map onto the contents:
- Proptosis and chemosis — venous congestion of the orbit
- Painful ophthalmoplegia — CN III, IV, VI palsies; CN VI first
- V1 / V2 sensory loss — forehead, upper lid, cheek, upper teeth
- Pupil involvement — sympathetic fibres on the ICA may be affected, giving a Horner-like pattern or fixed dilated pupil
- Pulsatile exophthalmos — only if the ICA wall is breached, creating a carotid–cavernous fistula (audible bruit)
- Bilateral signs — because of the intercavernous communications
Treatment is broad-spectrum IV antibiotics covering Staphylococcus aureus (most common organism) plus anticoagulation in many centres. Mortality remains around 20–30%.
Emissary veins and infection spread
Small emissary veins pass through cranial foramina and connect dural sinuses to scalp and facial veins. They are valveless. Clinically they explain:
- Spread of scalp infection into the SSS via parietal emissary veins
- Spread of facial infection into the cavernous sinus via ophthalmic veins
- Spread of pharyngeal infection into the cavernous sinus via the pterygoid venous plexus
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Test yourself
A patient sustains head trauma to the vertex with a depressed skull fracture. Which intracranial structure is most likely to be damaged?
- ((Superior sagittal sinus::☑️ SSS sits in the midline directly beneath the vertex in the falx; vertex fractures lacerate it.))
- ((Inferior sagittal sinus::Lies in the deep free edge of the falx — too deep to be hit by a vault fracture.))
- ((Transverse sinus::Runs laterally along the tentorium — at risk from occipital, not vertex, injury.))
- ((Confluence of sinuses::Sits at the internal occipital protuberance — posterior, not vertex.))
- ((Straight sinus::Lies deep at the falx–tentorium junction — protected from vault injury.))
👩⚕️ The SSS is the most commonly injured dural sinus; clamping risks bilateral venous infarction.
Infection from a mastoid abscess can spread to which of the following structures?
- ((Sigmoid sinus::☑️ Sigmoid grooves the mastoid bone — mastoiditis erodes into it, causing lateral sinus thrombosis.))
- ((Internal carotid artery::Lies in the carotid canal anteromedial to the mastoid — not in direct contact.))
- ((Inner ear::Labyrinth is medial in the petrous bone; labyrinthitis is less classic than sinus thrombosis.))
- ((Occipital sinus::Sits in the falx cerebelli, midline posteriorly — no anatomical relation to the mastoid.))
- ((Cavernous sinus::Anterior beside the sphenoid body — nowhere near the mastoid.))
👩⚕️ Lateral sinus thrombosis presents with picket-fence fevers, headache and papilloedema.
A 45-year-old woman presents with proptosis, chemosis, ophthalmoplegia and pulsatile exophthalmos. A carotid–cavernous fistula is suspected. Which cranial nerve is most likely to be affected first?
- ((Abducens nerve (CN VI)::☑️ Runs free within the sinus next to the ICA — first nerve compressed by any cavernous lesion.))
- ((Oculomotor nerve (CN III)::Sits in the lateral wall — protected, affected later.))
- ((Trochlear nerve (CN IV)::Lateral wall, below CN III — similarly protected.))
- ((Ophthalmic nerve (V1)::Lateral wall — produces forehead sensory loss, not the first sign.))
- ((Maxillary nerve (V2)::Lowest in the lateral wall — last to be involved.))
👩⚕️ "Six runs through the sinus" — CN VI inside, all others in the wall.
The great cerebral vein of Galen drains into which dural venous sinus?
- ((Straight sinus::☑️ Vein of Galen + inferior sagittal sinus unite at the falx–tentorium junction to form it.))
- ((Superior sagittal sinus::Receives superior cerebral veins and arachnoid granulations, not Galen.))
- ((Transverse sinus::Receives the straight sinus indirectly via the confluence.))
- ((Cavernous sinus::Receives ophthalmic and middle cerebral veins — unrelated to deep drainage.))
- ((Inferior petrosal sinus::Drains the cavernous sinus to the IJV — nothing to do with Galen.))
👩⚕️ Vein of Galen malformations cause neonatal high-output cardiac failure.
A patient develops papilloedema, headache and visual disturbance. MR venography shows thrombosis at the internal occipital protuberance. Which structure is thrombosed?
- ((Confluence of sinuses::☑️ Torcular Herophili lies at the internal occipital protuberance — junction of SSS, straight and occipital sinuses.))
- ((Superior sagittal sinus::Runs along the calvarial midline; terminates at, but is not located at, the protuberance.))
- ((Sigmoid sinus::Grooves the mastoid laterally — far from the midline protuberance.))
- ((Occipital sinus::Small sinus in the falx cerebelli; isolated thrombosis rarely raises ICP.))
- ((Basilar venous plexus::Lies on the clivus anteriorly — wrong location entirely.))
👩⚕️ Confluence thrombosis blocks outflow from both SSS and straight sinus, hence the raised ICP.
A patient with septic cavernous sinus thrombosis develops numbness over the forehead and upper eyelid. Which structure is responsible?
- ((Ophthalmic nerve (V1)::☑️ V1 in the lateral wall supplies forehead, upper lid and nose.))
- ((Maxillary nerve (V2)::Supplies cheek, upper lip and upper teeth — wrong territory.))
- ((Mandibular nerve (V3)::Does not pass through the cavernous sinus; exits via foramen ovale.))
- ((Abducens nerve (CN VI)::Pure motor to lateral rectus — causes diplopia, not numbness.))
- ((Facial nerve (CN VII)::Never enters the cavernous sinus; exits via stylomastoid foramen.))
👩⚕️ Infection reaches the sinus from the facial "danger triangle" via the valveless ophthalmic veins.
Revision summary
➡ Drainage path: SSS → confluence → transverse → sigmoid → IJV. Deep system: ISS + vein of Galen → straight sinus → confluence.
➡ Sinuses are valveless — explains retrograde infection spread and emissary vein connections.
➡ SSS sits under the vertex (trauma risk) and reabsorbs CSF via arachnoid granulations (thrombosis → raised ICP).
➡ Confluence (torcular Herophili) at the internal occipital protuberance.
➡ Sigmoid sinus grooves the mastoid — mastoiditis causes lateral sinus thrombosis.
➡ Cavernous sinus contents: ICA + CN VI inside; CN III, IV, V1, V2 in the lateral wall ("O TOM CAT"). V3 NOT included.
➡ CN VI is first affected in any cavernous sinus pathology.
➡ Cavernous sinus thrombosis: from facial "danger triangle", dental or sinus infection via valveless ophthalmic veins. Proptosis, chemosis, painful ophthalmoplegia, V1/V2 sensory loss, often bilateral. Pulsatile exophthalmos = carotid–cavernous fistula.
➡ Dural folds: falx cerebri (SSS, ISS), tentorium cerebelli (transverse, straight), falx cerebelli (occipital sinus).