Chapter 34: Endoscopic Skull Base Surgery

Anatomy

Endoscopic Skull Base Surgery

• Improved understanding of endoscopic endonasal anatomy has allowed surgeons improved familiarity with endoscopic techniques.

• Advances in endoscopic technology and instrumentation over the last several decades have led to popularization of the endonasal route to the skull base. Reduced surgical morbidity, improved recovery time, greater surgical exposure, and the avoidance of external scars have allowed endoscopic skull base procedures to become the gold standard technique to the sella and beyond.

The Sellar Region

• Predominantly made up of by the body of the sphenoid bone

Boundaries

• 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, cavernous sinus, parasellar carotid artery

Key Points

1. The sphenoid sinus is subject to variation in size and shape and variation in the degree of pneumatization. Main development occurs after puberty. There are three types of pneumatization: sellar (75%), presellar (24%), and concha (1%).

2. The concha type is where the area below the sella is a solid block of bone without pneumatization.

3. The presellar type has pneumatization not extending beyond the vertical plane parallel to the anterior sellar wall. This makes identification of landmarks more challenging during dissection.

4. The sellar type is most common, with pneumatization extending into body of sphenoid below sellar and to the clivus.

5. 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.

6. The chiasmatic groove (a shallow depression between the optic foramina) is bounded by the planum sphenoidale anteriorly and the tuberculum sellae posteriorly.

7. The roof the sphenoid sinus is referred to as the planum sphenoidale. It articulates with the roof of the ethmoid sinuses anteriorly and the sella posteriorly.

8. Pneumatization of the sphenoid sinus creates variably present recesses.

   • Tuberculum recess is at the junction of the anterior wall of the sella and the planum sphenoidale.

   • Medial optico-carotid recess is located where the lateral tuberculum recess and optic nerve meet (position of middle clinoid process).

   • Lateral optico-carotid recess is located at junction between optic nerve and anterior genu of carotid artery (position of optic strut and anterior clinoid process).

   • The clival recess is the junction of the middle third of the clivus (posterior wall of sphenoid sinus) and the pituitary fossa.

9. The optic strut is a strut of bone that separates the optic foramina from the superior orbital fissure. Hence, the oculomotor nerve can be seen close to the lateral side of the lateral optico-carotid recess.

10. The septae within the sphenoid sinus vary greatly, particularly in reference to their size, shape, thickness, and location. The cavities within the sinus are rarely symmetrical.

11. An Onodi cell is a posterior ethmoidal air cell that has pneumatized posteriorly into the sphenoid sinus, ...