cavernous sinus

**Anatomy/location:** Paired, trabeculated dural venous sinuses lateral to the sella turcica, pituitary, and sphenoid sinus; posterior to the superior orbital fissure; superior to the foramen ovale/rotundum region. Left and right sinuses communicate via intercavernous sinuses (anterior and posterior) that encircle the pituitary.

**Contents:** - *In the lateral wall (superior to inferior):* CN III (oculomotor), CN IV (trochlear), CN V1 (ophthalmic), CN V2 (maxillary). - *Within the sinus lumen:* CN VI (abducens) and the internal carotid artery (cavernous segment) surrounded by its sympathetic plexus (postganglionic fibers from the superior cervical ganglion). - CN V3 is NOT in the cavernous sinus (passes through foramen ovale). - Mnemonic: "O TOM" (CN III, IV, V1, V2 in lateral wall: Oculomotor, Trochlear, Ophthalmic, Maxillary); CN VI and ICA are the ones in the sinus itself.

**Venous drainage:** - *Inflow:* superior and inferior ophthalmic veins, superficial middle cerebral vein, sphenoparietal sinus, central retinal vein; also communicates with the facial vein via the ophthalmic veins and with the pterygoid venous plexus via emissary veins. - *Outflow:* superior petrosal sinus → transverse/sigmoid sinus; inferior petrosal sinus → internal jugular vein.

**Clinical significance:** 1. **Cavernous sinus thrombosis (septic):** Usually from spread of infection of the "danger triangle" of the face (nose, upper lip, medial cheek), sphenoid/ethmoid sinusitis, or orbital cellulitis via the valveless facial/ophthalmic veins. Most common organism *Staphylococcus aureus*. Presents with fever, headache, periorbital edema, chemosis, proptosis, ophthalmoplegia (CN VI is usually affected first as it lies free in the sinus), decreased sensation in V1/V2 distribution, and loss of corneal reflex; may spread to the contralateral side via intercavernous sinuses. Dx: MRI/MR venography or CT venogram. Tx: IV broad-spectrum antibiotics (covering MRSA), ± anticoagulation; drain source if needed. 2. **Carotid-cavernous fistula:** Abnormal communication between ICA and sinus (traumatic/basilar skull fracture or ruptured cavernous aneurysm). Causes pulsatile proptosis, orbital bruit, chemosis, and ophthalmoplegia. 3. **Cavernous ICA aneurysm:** Can compress CN III/IV/V1/V2/VI. 4. **Pituitary adenoma:** Lateral extension invades cavernous sinus → ophthalmoplegia (superior extension → bitemporal hemianopia from chiasm compression). 5. **Tolosa-Hunt syndrome:** Idiopathic granulomatous inflammation of the cavernous sinus/superior orbital fissure → painful ophthalmoplegia; responds to steroids. 6. **Mucormycosis:** In diabetic ketoacidosis/immunocompromised patients, rhinocerebral extension can involve the cavernous sinus. 7. **Horner syndrome (partial/postganglionic):** Damage to the sympathetic plexus around the cavernous ICA can cause ptosis and miosis without anhidrosis of the face (sweat fibers travel with the external carotid).

**Key exam pearls:** CN VI is the most vulnerable (lies within the sinus, not the wall); CN V3 and CN II are not in the sinus; infection spreads from the face via valveless veins; infection can cross to the opposite side.