Residency · Residency · Neurosurgery

Anatomy of the Sellar and Parasellar Region

Overview

The sellar and parasellar region is one of the most anatomically complex areas in neurosurgery. It houses the pituitary gland, surrounds the cavernous sinuses, and is traversed by critical neurovascular structures. Detailed knowledge of this anatomy is essential for transsphenoidal surgery, skull base approaches, and managing pathology of the sella and cavernous sinus.

Sellar Anatomy

Sella Turcica

The sella turcica is a saddle-shaped depression in the body of the sphenoid bone. The tuberculum sellae forms the anterior wall, with the sulcus chiasmaticus lying just above it. The dorsum sellae forms the posterior wall and is capped by the posterior clinoid processes. The sellar floor is thin bone overlying the sphenoid sinus, and its thickness varies from paper-thin to densely pneumatized. The anterior clinoid processes are lateral projections of the lesser wing of the sphenoid, connected to the optic canal by the optic strut.

Pituitary Gland

The pituitary gland sits within the sella turcica, covered superiorly by the diaphragma sellae. The anterior lobe (adenohypophysis) accounts for approximately 80% of the gland and secretes ACTH, GH, PRL, TSH, FSH, and LH. The posterior lobe (neurohypophysis) stores and releases ADH and oxytocin, which are produced in the hypothalamus. The pituitary stalk (infundibulum) connects the gland to the hypothalamus, passing through an aperture in the diaphragma sellae. The diaphragma sellae itself is a dural fold with a central aperture of variable size; a larger aperture allows suprasellar extension of tumors.

Blood Supply

The superior hypophyseal arteries, arising from the ophthalmic segment of the ICA, supply the stalk, anterior lobe, and optic chiasm. The inferior hypophyseal arteries, arising from the meningohypophyseal trunk of the cavernous ICA, supply the posterior lobe and capsule. The portal venous system connects the median eminence to the anterior pituitary, delivering hypothalamic hormones that regulate anterior pituitary function.

Cavernous Sinus Anatomy

Structure

The cavernous sinuses are paired venous structures lying lateral to the sella turcica. Despite the name, they are not true venous sinuses but rather trabeculated venous plexuses. Each extends from the superior orbital fissure anteriorly to the petrous apex posteriorly.

Contents

Within the sinus proper lie the internal carotid artery (cavernous segment, C4), CN VI (abducens nerve), and the sympathetic plexus. Within the lateral wall, arranged from superior to inferior, are CN III, CN IV, CN V1, and CN V2. The ICA makes an S-shaped curve within the sinus (the carotid siphon) and gives off the meningohypophyseal trunk and inferolateral trunk.

Walls of the Cavernous Sinus

The medial wall is thin, often only a single dural layer, and represents the surgical plane for transsphenoidal surgery. The lateral wall is multilayered dura containing CN III, IV, V1, and V2. The superior wall is formed by the anterior and posterior clinoid processes, the interclinoid ligament, and the oculomotor triangle. The posterior wall consists of dura overlying the posterior fossa and is traversed by CN III, IV, and V as they enter the sinus.

Triangles of the Cavernous Sinus

Defined by Parkinson and refined by Dolenc and Rhoton, these triangles guide surgical access. The clinoidal triangle lies between the optic nerve and CN III and provides access to the clinoid segment of the ICA. The oculomotor triangle is between CN III and V1 in the lateral wall. The supratrochlear triangle lies between CN III and CN IV, and the infratrochlear (Parkinson's) triangle between CN IV and V1 is used for direct access to the cavernous ICA. The anteromedial (Mullan's) triangle between V1 and V2 serves as the entry point for percutaneous trigeminal procedures. The anterolateral (Glasscock's) triangle below V3 contains the middle meningeal artery and greater superficial petrosal nerve. The posterolateral (Kawase's) triangle, medial to V3 and lateral to CN VI, is drilled during Kawase's approach to access the petroclival region.

Suprasellar Region

Optic Chiasm

The optic chiasm is located above the diaphragma sellae, but its relationship to the sella varies. A prefixed chiasm (approximately 15% of individuals) lies over the tuberculum sellae and limits access to the suprasellar space via the transsphenoidal approach. A normal chiasm (approximately 70%) lies directly over the diaphragma sellae. A postfixed chiasm (approximately 15%) lies over the dorsum sellae and allows greater transsphenoidal exposure. Compression of the chiasm by pituitary adenomas classically produces bitemporal hemianopia.

Hypothalamus and Third Ventricle Floor

The floor of the third ventricle lies above the chiasm and pituitary stalk. From anterior to posterior, the structures encountered include the optic chiasm, infundibular recess, tuber cinereum, and mammillary bodies. The lamina terminalis forms the anterior wall of the third ventricle and can be opened for CSF drainage or endoscopic third ventriculostomy.

Circle of Willis Relationship

The AComA complex lies above the chiasm. The ICAs lie lateral to the sella and converge toward the supraclinoid space. The A1 segments cross over the optic nerves and chiasm. The PCAs and basilar tip are positioned posterior to the dorsum sellae.

Endonasal Transsphenoidal Corridor

Sphenoid Sinus

Sphenoid sinus pneumatization is variable. The conchal type (approximately 3%) has minimal pneumatization with dense bone over the sella. The presellar type (approximately 11%) has pneumatization that does not reach the sellar floor. The sellar type (approximately 86%) is well-pneumatized, with a thin sellar floor that is easily accessed. Septations within the sinus are variable and often off-midline; they may attach to the carotid prominences and must never be avulsed. The carotid prominences are bony ridges overlying the cavernous ICAs on the lateral walls. The opticocarotid recess, a depression between the optic canal and carotid prominence, marks the lateral limit of safe bone removal.

Surgical Landmarks within the Sphenoid Sinus

The sellar floor is the central depression between the carotid prominences. The planum sphenoidale is the flat bone above the sella, leading to the tuberculum sellae. The clival recess lies posterior-inferior to the sella, overlying the clivus. The medial and lateral opticocarotid recesses define the boundaries for extended approaches.

<image> Coronal cross-section through the sellar region showing the pituitary gland within the sella turcica, the cavernous sinuses bilaterally with the internal carotid arteries and cranial nerves (CN III, IV, V1, V2 in the lateral wall and CN VI within the sinus). The optic chiasm is shown above the diaphragma sellae with the pituitary stalk passing through. The sphenoid sinus is shown below the sellar floor. Clean medical illustration with labeled structures and color-coded cranial nerves. </image>

<image> Endoscopic endonasal view of the sphenoid sinus showing the sellar floor centrally, the carotid prominences laterally, the opticocarotid recesses, the planum sphenoidale superiorly, and the clival recess inferiorly. Sphenoid septations are shown with one attaching to the right carotid prominence. Key landmarks are labeled for transsphenoidal surgical orientation. Medical illustration simulating the endoscopic perspective. </image>

<image> Lateral schematic view of the cavernous sinus showing the triangles of surgical access: clinoidal triangle, oculomotor triangle, Parkinson's triangle (infratrochlear), Mullan's triangle (anteromedial), and Kawase's triangle (posterolateral). The internal carotid artery course through the sinus is shown with its branches (meningohypophyseal trunk, inferolateral trunk). Cranial nerves are labeled with their positions relative to each triangle. Diagrammatic illustration with color-coded triangles. </image>

Clinical Pearls

The medial wall of the cavernous sinus is the thinnest dural barrier, and pituitary adenomas with lateral extension can easily invade through it. Knosp grading assesses cavernous sinus invasion based on the tumor's relationship to the cavernous ICA. Sphenoid sinus septations can attach to the carotid prominence, so they should never be avulsed but rather cut with instruments to avoid carotid artery injury. A prefixed chiasm limits suprasellar access via the transsphenoidal corridor, making preoperative MRI assessment of chiasm position essential. The opticocarotid recess is the critical lateral safety boundary during transsphenoidal surgery, as bone removal lateral to it risks ICA injury. The inferior hypophyseal arteries supply the posterior pituitary, and their injury during transsphenoidal surgery can cause transient diabetes insipidus. The cavernous ICA can be injured during transsphenoidal or cavernous sinus surgery, with management options including direct packing, muscle patch, or emergent endovascular balloon occlusion. CN VI is the cranial nerve most vulnerable in cavernous sinus surgery because it runs freely within the sinus rather than being protected by the dural lateral wall.

References

• Rhoton AL Jr. "The Sellar Region." Neurosurgery. 2002;51(Suppl 1):S335-S374.
• Rhoton AL Jr. "The Cavernous Sinus, the Cavernous Venous Plexus, and the Carotid Collar." Neurosurgery. 2002;51(Suppl 1):S375-S410.
• Kassam AB, et al. "Expanded Endonasal Approach: The Rostrocaudal Axis." Neurosurgery. 2005;57(ONS Suppl 3):ONS213-ONS228.
• Knosp E, et al. "Pituitary Adenomas with Invasion of the Cavernous Sinus Space." Neurosurgery. 1993;33(4):610-618.

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