Transnasal Endoscopic Skull Base Surgery & Image-Guided Navigation

Part I: Transnasal Endoscopic Skull Base Surgery

Overview and Historical Context

• KJL Essential Otolaryngology, p. 627

• Cummings Otolaryngology, p. 3366

Advantages of Endoscopic Skull Base Surgery (ESBS)

• Avoidance of facial scars
• Decreased trauma to normal tissue
• Decreased hospital length of stay
• Improved quality of life
• Diminished morbidity compared to open approaches
• Reduced brain retraction
• Tension pneumocephalus is less common after endoscopic versus open approaches

The Expanded Endonasal Approach (EEA) - Surgical Modules

• Cummings Otolaryngology, p. 3366

Regional Anatomy for Endoscopic Skull Base Surgery

1. The Sellar Region

• Anterior: anterior wall of the sphenoid sinus
• Posterior: middle third of clivus
• Superior: planum sphenoidale
• Inferior: floor of sphenoid sinus
• Lateral: optic nerve, lateral optico-carotid recess (OCR), cavernous sinus, parasellar carotid artery

• The sphenoid sinus has three pneumatization types: sellar (75%), presellar (24%), and concha (1%)
• The concha type has no pneumatization below the sella - a solid block of bone
• The presellar type does not extend beyond the vertical plane of the anterior sellar wall - landmark identification is more challenging
• The sellar type (most common) has pneumatization extending into the body of the sphenoid below the sella and to the clivus
• The pituitary fossa occupies the central part of the body of the sphenoid, bounded anteriorly by the tuberculum sellae and posteriorly by the dorsum sellae
• The chiasmatic groove is bounded by the planum sphenoidale anteriorly and the tuberculum sellae posteriorly
• Onodi cell: a posterior ethmoidal air cell that has pneumatized posteriorly into the sphenoid sinus, defined by a horizontal sphenoid sinus septation above the posterior bony choana - it may contain the optic nerve

• Tuberculum recess - at the junction of the anterior sellar wall and planum sphenoidale
• Medial optico-carotid recess (OCR) - where the lateral tuberculum recess and optic nerve meet (position of middle clinoid process)
• Lateral optico-carotid recess - at the junction of optic nerve and anterior genu of carotid artery (position of optic strut and anterior clinoid process)
• Clival recess - junction of the middle third of clivus and pituitary fossa
• KJL Essential Otolaryngology, p. 627

2. The Anterior Cranial Fossa / Cribriform Region

• Anterior: frontal sinus posterior wall
• Posterior: planum sphenoidale
• Medial: perpendicular plate of the ethmoid (midline)
• Lateral: lamina papyracea

• The ethmoid bone forms the anterior and middle thirds of the anterior cranial fossa, separated by the cribriform plate (with olfactory fila), the perpendicular plate (nasal septum), and two lateral plates (laminae papyraceae)
• The cribriform plate gives rise to the crista galli, onto which the falx cerebri attaches anteriorly
• The roof of the ethmoidal sinuses is the fovea ethmoidalis (part of the frontal bone), attaching the lamina papyracea laterally to the lateral lamella medially
• The anterior ethmoidal artery passes along the fovea ethmoidalis from lateral to medial in an anteromedial direction; it may run on the skull base or in a mesentery (14-43% of cases). It is found 1 or 2 air cells behind the frontal recess in 85% of cases.
• The posterior ethmoidal artery is located at the junction of the planum sphenoidale with the fovea ethmoidalis anteriorly

• KJL Essential Otolaryngology, p. 629

3. The Suprasellar Region

• Anterior: ethmoid roof
• Posterior: third ventricle, basilar apex, mammary body
• Superior: frontal lobe gyri
• Inferior: sella
• Lateral: optic nerves and supracavernous carotid artery

4. The Cavernous Sinus Region

• The intracavernous ICA is divided into five segments: posterior vertical, posterior genu, horizontal, anterior genu, and anterior vertical segment
• Along the lateral wall of the sphenoid, boney projections from superior to inferior: cavernous sinus roof (at the inferior aspect of lateral OCR), trigeminal maxillary impression (V2), and trigeminal mandibular nerve (V3)
• The oculomotor, trochlear, and V1 nerves run in the lateral wall of the cavernous sinus, seen by retracting the anterior genu of the carotid medially
• The abducens nerve runs within the cavernous sinus lumen, hugging the horizontal portion of the intracavernous carotid

• For lesions limited to the medial cavernous sinus: standard sellar approach
• For tumor extending to the lateral and inferior portion: entry into the maxillary sinus required (accessing the lateral recess of the sphenoid sinus)
• Unicinectomy and maxillary antrostomy performed; the sphenopalatine artery is cauterized and divided
• The posterior maxillary sinus wall is exposed and the pterygoid root identified; bone over the carotid protuberance is drilled with a diamond bur

5. The Posterior Cranial Fossa / Clivus

• Anterior: anterior wall of sphenoid sinus, nasopharynx, and oropharynx
• Posterior: pons, basilar artery
• Lateral: vidian nerve, eustachian tube
• Superior: posterior clinoid
• Inferior: foramen magnum

• Upper third: the dorsum sellae
• Middle third: between the floor of the sella and floor of the sphenoid sinus
• Lower third: between the floor of the sphenoid sinus and the foramen magnum

6. Pterygopalatine Fossa

7. Infratemporal Fossa

• Superior: greater wing of the sphenoid bone
• Inferior: medial pterygoid muscles attaching to the mandible
• Medial: lateral pterygoid plate, cartilaginous eustachian tube
• Lateral: temporalis muscle and ramus of the mandible
• Posterior: tympanic bone
• Anterior: posterolateral wall of the maxillary sinus

8. Orbital Apex

• Superior: lesser wing of the sphenoid bone
• Inferior: optic strut
• Medial: body of sphenoid sinus
• Lateral: anterior clinoid process
• Anterior: lamina papyracea at the foramen of the posterior ethmoidal artery

Reconstruction After Endoscopic Skull Base Surgery

Nasoseptal Flap (Hadad Flap)

• Blood supply: posterior nasal artery (branch of sphenopalatine artery)
• Used for small to large defects, especially transsellar and transcribriform defects

Pericranial Flap

• Indication: large anterior skull base defect
• Traditionally harvested via bicoronal incision; an endoscopic approach has been described
• Endoscopic harvesting uses two endoscopes, similar to an endoscopic brow lift approach
• A horizontal incision is made at the glabella with an osteotomy; the flap is retrieved from the supraorbital region and placed intranasally
• Blood supply: supratrochlear artery
• KJL Essential Otolaryngology, p. 639

Contraindications to Purely Endonasal Approach

• Involvement of the nasal bones
• Anterior table of the frontal sinus
• Skin overlying nasal dorsum or frontal sinus
• Orbital involvement that necessitates exenteration

• Tumor involvement of superficial tissues
• Need for vascular reconstruction
• Expected very long duration of surgery
• Cummings Otolaryngology, p. 3367

Complications of Endoscopic Skull Base Surgery

1. CSF leak - the most common complication; multilayered reconstruction based on defect size and location reduces risk
2. Tension pneumocephalus - less common than after open approaches; presents with rapid neurologic decline, confusion, and loss of consciousness
3. Perioperative bleeding - ranges from venous bleeding to carotid injury
   • Hemostatic techniques: warm irrigation, Gelfoam with thrombin packing, Floseal, cautery, aneurysm clipping
4. Carotid injury (rare but life-threatening):
   • Immediate management: placement of a paddle over the injured vessel with pressure while muscle is harvested from the neck or thigh
   • A piece of crushed muscle is placed over the injured carotid until bleeding ceases
   • The area is covered by a nasoseptal flap and supported by gentle cavity packing
   • After bleeding control: urgent transfer to interventional radiology for angiogram assessment
5. Cerebrovascular accident - due to accidental injury to circulation, venous infarct from CSF pressure changes, or embolic phenomena
6. Infectious complications - rare; typically from postoperative crust formation
7. Minor complications: sinusitis, vestibular burns, anosmia, synechiae

• KJL Essential Otolaryngology, p. 639

Part II: Image-Guided Navigation in Endoscopic Sinus and Skull Base Surgery

Principle and Technology

• Cummings Otolaryngology, p. 878

Accuracy

• Accuracy of within 2 to 3 mm is considered acceptable for image guidance systems
• This makes them most useful for confirming the identity of large compartments (e.g., posterior ethmoid vs. sphenoid sinus) rather than distinguishing millimeter-level increments (e.g., at the skull base itself)
• Navigation systems are aids to the surgeon - they are not a substitute for thorough knowledge of surgical anatomy

Sources of Error

1. Variables introduced during CT scanning (slice thickness, timing relative to surgery)
2. Fiducial (reference device) or headset shift intraoperatively
3. Soft tissue effects during surface registration
4. Operator-dependent errors during registration

Indications for Use (American Academy of Otolaryngology - Head and Neck Surgery Guidelines)

• Revision sinus surgery (altered anatomy from prior surgery)
• Distorted anatomy from previous disease, trauma, or congenital anomalies
• Extensive sinonasal polyposis
• Surgery involving the frontal sinus, especially with frontal recess dissection
• Pathology involving the skull base, orbit, optic nerve, or carotid artery
• CSF rhinorrhea or conditions where skull base/intracranial anatomy is at risk

Advantages of Image-Guided Surgery (IGS)

1. Improved confidence and safety in complex anatomical regions
2. Reduction in complications - particularly orbital and intracranial injuries
3. Particularly valuable in revision cases where normal landmarks are obliterated
4. Real-time 3D orientation within the surgical field
5. Especially useful in endoscopic skull base surgery, where two surgeons operate simultaneously and where surgical corridors are narrow and critical structures nearby
6. Enables surgeons to navigate safely in distorted post-inflammatory or post-traumatic anatomy

Limitations and Practical Considerations

• Navigation systems are not needed for every ESS case
• Come with an associated increase in operative time and cost
• Accuracy is limited at the millimeter level - cannot replace anatomical knowledge
• System calibration must be verified before and confirmed during the case (image guidance calibration is conducted and confirmed accurate as part of the pre-surgical checklist)
• Preoperative CT scans must be protocoled specifically for use in navigation systems (non-contrast, thin-cut protocol)
• Cummings Otolaryngology, pp. 878-879

Surgical Technique: Key Steps in ESS Using Image Guidance

1. Patient positioning (reverse Trendelenburg to reduce bleeding)
2. CT review (correct patient orientation confirmed)
3. Diagnostic nasal endoscopy and topical anesthetic injections
4. Medialization of the middle turbinate
5. Image guidance calibration confirmed accurate
6. Surgery tailored to disease using Messerklinger (anterior-to-posterior) or Wigand (posterior-to-anterior) technique, or a combination
7. Systematic progression through the sinuses with continuous landmark verification using navigation

• Cummings Otolaryngology, p. 881

Summary Table

• K.J. Lee's Essential Otolaryngology, 11th Edition, pp. 627-639 (Endoscopic Skull Base Surgery)
• Cummings Otolaryngology Head and Neck Surgery, 7th Edition, pp. 878-881 (Image-Guided Navigation Systems, FESS technique) and pp. 3366-3367 (Endoscopic Surgery of the Anterior Skull Base)