Cavernous Sinus

• Endothelial-lined venous lake located superior to clivus and lateral to sella (hypophyseal fossa); bordered by dura (meningeal and periosteal layers medially but only the meningeal layer laterally)

• Cavernous carotid artery progression: posterior vertical segment → posterior genu → horizontal segment → anterior genu → anterior vertical segment (paraclinoidal segment)

• Two branches of cavernous carotid artery: meningohypophyseal trunk (from posterior aspect of posterior genu) and inferolateral trunk (from middle of horizontal segment)

• It contains cranial nerves III, IV, V1, V2, VI, and the sympathetic plexus.

• The anterior and posterior intercavernous sinuses connect the sinuses across the midline.

Petroclival Region

• The clivus is located anteromedial to the petrous apex; the otic capsule is located posterolateral.

• The petrous ICA runs beneath the petrous apex, then makes a vertical turn anterior to the petrous apex, and past the foramen lacerum to enter the paraclival carotid canal.

Pterygopalatine Fossa

• Boundaries: pterygoid process posteriorly, palatine bone anteromedially, maxilla anterolaterally

• Anterior vascular compartment: contains internal maxillary artery and its branches

• Posterior neural compartment: contains pterygopalatine ganglion, vidian nerve, and greater and lesser palatine nerves

• Communicates with intratemporal fossa through pterygomaxillary fissure

Posterior Cranial Fossa

• Bony structures surround the foramen magnum, and include the basiocciput of the clivus, petrous temporal bone, and the pars lateralis and squama of occipital bone.

• Vertebrobasilar junction (VBJ): located approximately 2 cm above foramen magnum

• Hypoglossal nerve: enters hypoglossal canal 6 mm inferior and 8 mm lateral to VBJ

• Hypoglossal canal: extends from medial surface of occipital condyle to open into the parapharyngeal space on its lateral surface

• Supracondylar groove is an area on the superior occipital condyle formed by muscular and membranous attachments that serves as a reliable landmark for the hypoglossal canal

Abducens Nerve

• Most vulnerable nerve to intradural injury during skull base surgery

• Extracavernous: exits brainstem at pontomedullary junction (level of sphenoid sinus floor), then travels obliquely in the prepontine cistern, continuing between the dural layers to pass under Gruber’s ligament (petrosphenoidal ligament) into Dorello’s canal, and subsequently into the posteroinferior cavernous sinus

• Intracavernous: travels laterally to ICA and medial to V1 into superior orbital fissure

• Carries sympathetic nerve fibers from the ICA into the orbit

Carotid Artery

• Bouthillier classification of ICA segments: cervical, petrous, lacerum, cavernous (paraclival), clinoid, ophthalmic (supraclinoid), communicating (terminal)

• Vidian nerve leads to transition of horizontal/petrous ICA to the vertical/paraclival ICA at the level of the foramen lacerum

• Differential diagnosis:

1. Chondrosarcoma,

2. Ecchordosis physaliphora

3. Chordoid meningioma,

4. Fibrous dysplasia

5. Metastases (especially mucinous adenocarcinoma)

• Symptomatology: most commonly, headache and cranial nerve deficits (abducens nerve palsy most common), depending on structures involved; symptom range from endocrine dysfunction, stroke, epistaxis, ataxia, ptosis, hearing loss, vertigo, etc

• Imaging: high-resolution CT, MRI, CT angiogram

1. CT shows lytic well-circumscribed soft tissue lesion, hyperintense relative to brain, that moderately enhances after postcontrast.

2. MRI shows hyperintense lesion on T2 with heterogeneous enhancement postcontrast in a lobulated honeycomb pattern.

3. CT angiogram is best to delineate the relationship of major vessels within the tumor and surgical corridor.

• Classification:

1. Type I: limited to 1 compartment of the skull base (rare)

2. Type II: involving 2+ contiguous compartments but requiring a single surgery (most common)

3. Type III: involving multiple compartments and requiring 2+ surgeries

• Patterns of involvement:

1. Upper clival (basisphenoidal) lesions often involve pituitary gland, hypothalamus, or CN III, V, or VI.

2. Lower clival (basioccipital) lesions often involve CN IX–XII

• Treatment: ideally includes an expanded endonasal approach (EEA) with gross total resection (GTR) and adjuvant proton-beam or hadron-beam radiotherapy

1. Contraindications to GTR include tumor involving brainstem-perforating arteries, or lateral cavernous sinus.

2. EEA may need to be staged or combined with the open approach if the tumor extends laterally to the ICA, vertebral arteries, or optic nerves; inferiorly to the dens; or into the lateral condyle, mastoid bone, or lateral jugular tubercle.

3. These naturally radioresistant tumors require high doses of radiotherapy (standard regimen is 75 Gy in 1.8 Gy per day fractions).

1. Classic subtype (80% to 85%): physaliphorous cells with lobular pattern of islands of eosinophilic vacuolated cells in basophilic mucoid stroma

2. Chondroid (5% to 15%): low-grade tumor, mostly composed of chondrocytes and hyaline cartilage

3. Dedifferentiated (1% to 8%): mesenchymal component with sarcomatoid appearance

• Immunohistochemistry: stains + for S-100, vimentin, cytokeratin, epithelial membrane antigen, CEA, MUC1, brachyury

1. Duplications in brachyury (transcription factor T gene) are unique to chordomas; when immunohistochemistry for brachyury is combined with cytokeratin, the sensitivity and specificity of diagnosis is 98% and 100%, respectively.

2. Absent or reduced expression of fragile histidine triad (FHIT) is found in two-thirds of chordomas.

• Outcomes: high local recurrence rate, overall median survival of 6.29 years; 5-year overall survival of 60% to 80%; 10-year overall survival of 32% to 50%

1. Chondroid subtype: best prognosis with median survival of 15.8 years

2. Classic subtype: median survival of 4.1 years

Chondrosarcoma

• Etiology: from degenerated chondroid cells in the petroclival synchondrosis

• Epidemiology: rare tumor accounting for 0.15% of intracranial tumors and 6% of skull base tumors; mean age of presentation, 40 years

• Natural course: slow-growing, locally invasive tumor that invades adjacent regions (cavernous sinus, jugular tubercle, cerebellopontine angle, jugular foramen, middle cranial fossa, hypoglossal canal, brainstem, cerebellum) but rarely transgresses dura

• Symptomatology: headache, cranial nerve deficits (VI and IX–XII)

• Imaging: high-resolution CT, MRI, CT angiogram

1. MRI shows low-to-intermediate signal on T1, and high signal on T2 and FLAIR.

2. CT shows stippled pattern of calcification; 50% of tumors show bone destruction and calcification.

3. CT angiogram is best to delineate the relationship of major vessels within the tumor and surgical corridor.

• Subtypes:

1. Conventional (most common)

2. Mesenchymal

3. Clear cell

4. Dedifferentiated

• Myxoid chondrosarcoma may appear similar to chordomas on light microscopy.

• Immunohistochemistry: stains + for S-100 and vimentin but negative for cytokeratin, epithelial membrane antigen, CEA, MUC1, and brachyury

• Outcomes: better prognosis than for chordoma; 5-year overall survival of 82%, 10-year overall survival of 50%, survival higher with surgery + radiation than with surgery alone

1. Mesenchymal subtype associated with poorer prognosis

Plasmacytoma

• Monoclonal plasma cell proliferation resulting in local destruction of bone

• May be solitary plasmacytoma, extramedullary plasmacytoma, or associated with multiple myeloma

• Imaging: may suggest the diagnosis, although not specific

1. CT shows lytic lesion with narrow zone of transition and invasive outline.

2. MRI shows isointense lesion on T1 and hyperintense lesion on T2 with homogeneous contrast enhancement.

• Treatment: radiation for solitary lesions; consider endoscopic resection if gross total resection feasible

Lesions Invading Clivus

• Metastases:

1. Malignancies that commonly spread to bone including lung, breast, prostate, renal, thyroid, and lymphoma

2. Most commonly present with abducens nerve palsy and headache

• Local invasion: pituitary macroadenoma, meningioma (clival, petroclival, chordoid), jugular foramen tumors, cerebellopontine angle tumors, craniopharyngioma, nasopharyngeal carcinoma, nasopharyngeal rhabdomyosarcoma, trigeminal schwannomas, cholesteatoma

Congenital, Inflammatory, and Traumatic Disease

Ecchordosis Physaliphora

• Etiology: ectopic notochord remnant located in the midline of the craniospinal axis occurring anywhere from the dorsum sellae to the coccyx

• Epidemiology: found in 2% of autopsies

• Symptomatology: often asymptomatic but may become symptomatic secondary to brainstem or cranial nerve compression

• Classic appearance: midline cystic lesion close to dorsum sella and protruding intradurally into prepontine cistern with a stalk-like median bony protuberance connecting to clivus

• Imaging: may suggest the diagnosis but is not specific

1. CT may show lytic lesion undistinguishable from chordoma.

2. MRI shows hyperintense T2 lesion without postcontrast enhancement (chordomas enhance with contrast).

• Diagnosis usually requires histological analysis.

• Can predispose to intracranial hemorrhage or CSF leak.

Fibrous Dysplasia

• Etiology: developmental anomaly characterized by replacement of mature bone by weak immature woven bone and fibrous tissue