Large venous space on either side of the body of the sphenoid + sella turcica, in the middle cranial fossa. Interior divided into small spaces (caverns) by trabeculae → hence “cavernous.”

Situation and Extent

Oculomotor + trochlear nerves pierce a triangular depression in the posterior roof to enter the sinus

Relations

Structures in Lateral Wall (Above → Below)

1. Oculomotor nerve (III)
2. Trochlear nerve (IV)
3. Ophthalmic nerve (V1)
4. Maxillary nerve (V2) — leaves via foramen rotundum

Structures Passing Through the Sinus (Medial Aspect)

1. Internal carotid artery — surrounded by venous + sympathetic plexus
2. Abducent nerve (VI) — inferolateral to ICA; enters below petrosphenoid (Gruber’s) ligament

Memory: “O TOM CAT” — Oculomotor, Trochlear, Ophthalmic, Maxillary (lateral wall) + Carotid artery, Abducent nerve (medial/central)

All structures separated from sinus blood by endothelial lining

Tributaries

From orbit: superior ophthalmic vein, inferior ophthalmic vein, central vein of retina (sometimes)

From meninges: sphenoparietal sinus; frontal trunk of middle meningeal vein

From brain: superficial middle cerebral vein; inferior cerebral veins (temporal lobe)

Communications

All communications = valveless → blood flows in either direction → basis for retrograde infection spread

Applied Anatomy

Dangerous Area of Face

• Upper lip, sides of nose, cheeks
• Facial vein + deep connecting veins (deep facial vein → pterygoid venous plexus → emissary veins) = devoid of valves
• Squeezing pustules/pimples here → infection → facial vein → deep facial vein → pterygoid plexus → emissary veins → cavernous sinus → cavernous sinus thrombosis (CST)

Cavernous Sinus Thrombosis (CST)

• Commonest cause: septic emboli via dangerous-area-of-face pathway; also: nasal cavity/paranasal sinus infection
• Signs + symptoms:

Carotid-Cavernous Fistula

• ICA rupture (e.g., base of skull fracture) → abnormal communication between ICA + cavernous sinus
• Arterial blood under high pressure → sinus enlarges → blood into connecting veins
• Signs: pulsating exophthalmos, loud bruit (systolic) over eye, ophthalmoplegia, marked orbital/conjunctival oedema
• Distinguished from CST by: pulsating exophthalmos + audible bruit (direct high-pressure arterial inflow) — absent in simple thrombosis

The cavernous sinus is a large venous space situated on either side of the body of the sphenoid bone and sella turcica, in the middle cranial fossa. Its interior is divided into a number of small spaces (caverns) by trabeculae — hence the name “cavernous” sinus.

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Situation and Extent

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Relations

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Structures in the Lateral Wall

From above downward:

1. Oculomotor nerve (III) — in the anterior part of the sinus, divides into superior and inferior divisions, which leave through the superior orbital fissure
2. Trochlear nerve (IV) — crosses superficial to the oculomotor nerve in the anterior part; enters the orbit through the superior orbital fissure
3. Ophthalmic nerve (V₁) — in the anterior part, divides into lacrimal, frontal, and nasociliary nerves
4. Maxillary nerve (V₂) — leaves the sinus through the foramen rotundum en route to the pterygopalatine fossa

The trigeminal ganglion (within the cavum trigeminale) and its dural cave project into the posterior part of the lateral wall.

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Structures Passing Through the Sinus (Medial Aspect)

1. Internal carotid artery, surrounded by the venous and sympathetic plexus
2. Abducent nerve (VI) — lies inferolateral to the internal carotid artery; enters the sinus by passing below the petrosphenoid (Gruber’s) ligament

The structures in the lateral wall and on the medial aspect are separated from the blood of the sinus by an endothelial lining.

Memory (verify order with source): “O TOM CAT” — Oculomotor, Trochlear, Ophthalmic division, Maxillary division (lateral wall, superior to inferior) + Carotid artery, Abducent nerve (medial/central, surrounded by sinus blood).

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Tributaries

From the Orbit

1. Superior ophthalmic vein
2. Inferior ophthalmic vein (or a branch of it)
3. Central vein of the retina (sometimes — may drain here or into the superior ophthalmic vein)

From the Meninges

1. Sphenoparietal sinus
2. Frontal (anterior) trunk of the middle meningeal vein (may alternatively drain into the pterygoid plexus via the foramen ovale)

From the Brain

1. Superficial middle cerebral vein
2. Inferior cerebral veins (a few, from the temporal lobe)

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Communications

The cavernous sinus communicates with:

All these communications are valveless — blood can flow through them in either direction. This valveless arrangement is the anatomical basis for retrograde spread of infection (see Applied Anatomy).

Factors Aiding Venous Flow Through the Sinus

1. Expansile pulsations of the internal carotid artery within the sinus
2. Gravity
3. Position of the head

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Applied Anatomy

The “Dangerous Area” of the Face

The upper lip, sides of the nose, and (by extension) the cheeks constitute the “dangerous area of the face.” The facial vein and its deep connecting veins (deep facial vein → pterygoid venous plexus → emissary veins) are devoid of valves, providing an uninterrupted retrograde pathway to the cavernous sinus.

Mechanism: Squeezing pustules or pimples in the upper lip, side of the nose, or cheek can introduce infection into the facial vein → deep facial vein → pterygoid venous plexus → emissary veins (through foramen ovale/lacerum/sphenoidal emissary foramen) → cavernous sinus, causing septic thrombosis.

Cavernous Sinus Thrombosis (CST)

Commonest cause: Septic emboli travelling via the dangerous-area-of-face pathway described above (also: infection from nasal cavities and paranasal air sinuses).

Signs and symptoms result from involvement of the structures within and around the sinus:

Carotid-Cavernous Fistula (Arteriovenous Communication)

If the internal carotid artery is ruptured (e.g., in a fracture of the base of the skull), an abnormal communication forms between the artery and the cavernous sinus. Arterial blood under high pressure rushes into the sinus, enlarging it and forcing blood into the connecting veins.

Signs and symptoms:

1. Pulsating exophthalmos — the eyeball protrudes and pulsates with each heartbeat
2. Loud bruit (systolic murmur) audible over the eye
3. Ophthalmoplegia — due to involvement of cranial nerves III, IV, and VI
4. Marked orbital and conjunctival edema — due to raised venous pressure within the cavernous sinus

Distinguishing CST from carotid-cavernous fistula: Both produce ophthalmoplegia and orbital edema/exophthalmos, but the fistula additionally produces a pulsating exophthalmos with an audible bruit, reflecting the direct high-pressure arterial inflow — features absent in simple thrombosis.

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