Anatomy

Osseous Anatomy

The anterior clinoid process (ACP), formed by the medial extension of the lesser wing of the sphenoid bone, provides a bony roof to the superior orbital fissure (SOF) and the anterior cavernous sinus (Fig. 366-1). The optic strut extends from the inferomedial surface of the ACP to the body of the sphenoid bone, separating the optic canal from the SOF. The ACP and optic strut define and obstruct access to the anterior and lateral borders of the ascending ICA as it exits the cavernous sinus to enter the subarachnoid space.

FIGURE 366-1 Paraclinoid osseous anatomy, dorsal (A) and posterior oblique (B) views. The anterior clinoid process (ACP) is the most medial extension of the lesser wing of the sphenoid bone. The ACP provides a bony boundary to many structures in the region, including a roof to the superior orbital fissure (SOF), an anterolateral border to the clinoidal segment (ClinSeg) of the internal carotid artery (ICA), and an anterolateral margin to the superior aspect of the cavernous sinus. The optic strut (OS) extends from the inferomedial surface of the ACP to the body of the sphenoid body, and separates the optic canal (OpCan) superomedially from the SOF inferolaterally. DS, dorsum sella.

The superomedial portion of the ACP is connected to the planum sphenoidale by the roof of the optic canal, the most posterior portion of which is formed by a dural fold called the falciform ligament. The anterior clinoid process is thus connected to the skull at three main points: the medial aspect of the lesser sphenoid wing, the optic strut, and the roof of the optic canal.

The relevant dural structures in this region represent dural reflections off the ACP, which primarily include the falciform ligament, the (distal) dural ring, and the carotid-oculomotor membrane (COM; Fig. 366-2).⁴ The dural ring represents the circular point of attachment of the dura to the ICA as the vessel penetrates the dura to enter the subarachnoid space. This dural attachment extends radially from the ICA and is continuous with the periosteum covering the superomedial aspect of the anterior clinoid process, the diaphragma sella, and the floor of the optic canal.⁵^–⁹ The plane of the dural ring slopes downward from anterior-to-posterior and lateral-to-medial directions. The subarachnoid space diverticulum medial to the ICA created by this slant is called the carotid cave.⁶^–¹⁰ The COM is formed by dura from the inferior surface of the ACP that extends from the ICA medially to the oculomotor nerve laterally.⁴ This membrane marks the point of exit of the ICA from the cavernous sinus main lumen.

FIGURE 366-2 Paraclinoid dural, vascular, and neural anatomy (schematic): lateral (A), dorsal (B), and anteroposterior (C) views. The cavernous segment (CavSeg), clinoidal segment (ClinSeg), and ophthalmic segment (OphSeg) of the internal carotid artery (ICA) are all in close proximity to the anterior clinoid process (ACP). The CavSeg lies within the venous channels of the cavernous sinus. The ClinSeg has an interdural location between the dural ring (DR) and the carotid-oculomotor membrane (COM). The OphSeg is entirely subarachnoid. Note the paraclinoid branches of the ICA: (1) the ophthalmic artery (OphArt), which arises just past the dural ring and projects beneath the optic nerve (ON) as it travels to the orbit; and (2) the superior hypophyseal artery (SupHypArt), which arises distal to the OphArt along the medial aspect of the OphSeg projecting to the pituitary gland. Note also the location of the oculomotor (III), trochlear (IV), and first division of the trigeminal (V1) nerves within the lateral sinus wall, and the abducens nerve (VI) within the venous lumen. AChorArt, anterior choroidal artery; CavSin, cavernous sinus; PComArt, posterior communicating artery; PetSeg, petrous segment; Pit, pituitary gland.

Neural Structures

The cranial nerves within the cavernous sinus (CN III, IV, V, and VI) are rarely clinically affected by paraclinoid aneurysms because these lesions generally project superiorly or medially away from the superior orbital fissure and lateral cavernous sinus wall (see Fig. 366-2). Nonetheless, their close proximity to the ACP makes knowledge of their anatomy mandatory so as to avoid injury during its removal.

The oculomotor nerve is located within the superior lateral cavernous sinus wall and courses anteriorly just beneath the ACP to enter the orbit through the SOF. The trochlear nerve is also located within the lateral cavernous sinus wall, traveling just beneath and parallel to the oculomotor nerve. The first division of the trigeminal nerve (V1) courses several millimeters below the oculomotor and trochlear nerves within the lateral sinus wall, whereas the abducens nerve courses within the cavernous venous compartment between the cavernous segment and V1. Finally, sympathetic fiber bundles course as a plexus along the cavernous and clinoidal ICA, eventually departing the ICA proximal to the dural ring to project into the orbit. Disruption of these fibers can occur from operative manipulation of the clinoidal segment, causing mild postoperative ptosis or miosis without facial anhidrosis.

In contrast, the optic nerves and chiasm are often directly in line with aneurysm expansion and are commonly distorted by aneurysms arising from this region. The optic nerve courses posteriorly and medially from the back of the globe, runs through the optic canal, and then enters the subarachnoid space superior and medial to the ICA. At the posterior end of the optic canal, the optic nerve is bounded superiorly by the falciform ligament. Most of the floor of the posterior optic canal is created by the optic strut. Both the falciform ligament and the optic strut may play important roles in the production of visual loss, either by the aneurysm itself or during the surgical procedure for aneurysm obliteration. The specific types of visual deficits created by lesions in this region and their relationships to these structures are discussed later in this chapter.

Vascular Structures

Arterial Segments

The paraclinoid ICA is herein composed of two regions—the clinoidal and ophthalmic segments (see Fig. 366-2).¹¹ The clinoidal segment, the distal portion of the anterior ascending vertical segment of the cavernous carotid artery, is located below and medial to the ACP, above the major venous lumen of the cavernous sinus. This region is a transitional ICA segment found between the COM and the dural ring.^4,^9,¹⁰ The clinoidal segment is located neither within the venous channels of the cavernous sinus nor within the subarachnoid space, and is essentially “interdural.”^4,^9,¹⁰ The ophthalmic segment, the distal portion of the paraclinoid ICA region, lies entirely within the subarachnoid space above and medial to the ACP. Beginning at the dural ring, the ophthalmic segment extends to the origin of the posterior communicating artery, and generally represents the longest subarachnoid ICA segment.¹¹

Arterial Bends and Branches

Saccular aneurysms typically form at points of hemodynamic stress where a bend in the vessel and a branch site coincide.¹² Two major bends in the paraclinoid ICA region predispose this region to aneurysm formation. The first bend, seen best on lateral angiogram, is the posteriorly projecting turn that begins at the anterior genu of the cavernous segment and continues as the vessel ascends through the dural ring. This bend creates a strong superior vector on the anterior and dorsal wall of the clinoidal and ophthalmic segments. A less conspicuous second bend, seen best from an anteroposterior or dorsal view, is the gentle lateral-to-medial-to-lateral curve beginning at the anterior genu of the cavernous segment and continuing as the ICA travels toward its terminal bifurcation. This medially projecting hemodynamic vector places significant stress upon the medial aspect of the clinoidal and ophthalmic ICA segments.

The ophthalmic segment harbors several prominent arterial branches that predispose this region to aneurysm formation. The ophthalmic artery is the most prominent and clinically significant branch, and typically arises from the dorsomedial carotid surface just above the dural ring and below the inferolateral portion of the optic nerve.^13,¹⁴ This branch projects anterolaterally to run through the optic canal beneath the optic nerve, and supplies the optic nerve through perforating branches and the retina through the central retinal artery. The superior hypophyseal artery is the second major branch (or series of branches), usually arising from the medial or inferomedial ICA surface just distal to the dural ring, medial and unrelated to the take-off of the ophthalmic artery along the medial-to-lateral ICA bend within the ophthalmic segment.^15,¹⁶ This vessel projects medially to supply the superior aspect of the pituitary stalk and gland, a portion of the cavernous sinus dura, and the posterior optic nerve and chiasm.

The clinoidal segment is usually devoid of named arterial perforators. On occasion, however, the ophthalmic or superior hypophyseal arteries originate from this segment, typically reaching their end organs through alternate anatomic pathways. The ophthalmic artery, for example, can originate from the distal cavernous segment or clinoidal segment in up to 10% of cadaveric specimens, usually entering the orbit through the SOF or an orifice within the optic strut.^4,^17,¹⁸

Aneurysm Classification

Paraclinoid aneurysms are classified according to the segment of origin of the aneurysm into clinoidal or ophthalmic segment types. Each segment has several subtypes or variants. The classification of aneurysms into their respective subtypes has important implications for the management of and operative approach to the aneurysm.

Clinoidal Segment Aneurysms

Two clinoidal segment aneurysm variants can be differentiated according to their site of origin, direction of projection, and relationships to arterial bends and branches and the adjacent dural and osseous structures within the segment (Fig. 366-3).

FIGURE 366-3 Clinoidal segment (ClinSeg) aneurysm types (schematic): lateral (A), dorsal (B), and anteroposterior (C) views. D, Anterolateral variant of ClinSeg aneurysm: arteriogram (anteroposterior [top left], lateral [top right], and three-dimensional [bottom left] views) and axial magnetic resonance image (bottom right). E, Medial variant ClinSeg aneurysm: arteriogram (anteroposterior [top left], lateral [top right], and three-dimensional bottom left views) and coronal computed tomographic angiography (bottom right). Anterolateral variant ClinSeg aneurysms (hatched area 1) originate from the dorsolateral aspect of the ClinSeg and project toward the anterior clinoid process (ACP), whereas medial variant ClinSeg aneurysms (hatched area 2) originate from the medial aspect of the ClinSeg and project toward the sella. Small ClinSeg aneurysms (<1 cm) typically remain interdural apart from the subarachnoid space, whereas larger lesions (≥1 cm) tend to breach the overlying dura and enter the subarachnoid space. OphArt, ophthalmic artery; SupHypArt, superior hypophyseal artery; DR, dural ring; COM, carotid-oculomotor membrane; ON, optic nerve.

Anterolateral Variant

The anterolateral variant arises from the anterolateral surface of the clinoidal segment as it obliquely ascends toward the dural ring underneath the ACP.¹ The superiorly and slightly medially directed course of the segment promotes an aneurysm that expands lateral and anterior to the ascending ICA, superiorly projecting toward and into the ACP. When small, the anterolateral variant may erode the optic strut and undersurface of the ACP to cause monocular visual loss from ipsilateral optic nerve compression within the optic canal. Larger lesions may secondarily compress the visual system (nerve or chiasm) within the subarachnoid space after extension through the dura medial to the ACP. These aneurysms can sometimes arise in association with a clinoidal segment origin of the ophthalmic artery but are more often associated just with the hemodynamic stress associated with the anterior ascending vertical segment of the cavernous ICA.

Medial Variant

The medial variant extends from the medial surface of the clinoidal segment and enlarges toward the sphenoid sinus and sella as the ICA turns from lateral to medial to lateral during its ascent toward and through the dural ring.¹ Initially, this aneurysm type expands beneath the diaphragma sella into the pituitary fossa. Gradual enlargement may cause hypopituitarism; rarely, aneurysm rupture into the pituitary gland may simulate pituitary apoplexy. Extension and rupture into the sphenoid sinus may cause life-threatening epistaxis. Large or giant lesions may extend through the diaphragma sella to enter the subarachnoid space. Visual loss from this aneurysm type does not occur with small lesions, but field cuts resembling those of pituitary tumors may occur with large or giant lesions that extend into the suprasellar space.

Ophthalmic Segment Aneurysms

Three ophthalmic segment aneurysm variants can be differentiated according to their site of origin, direction of projection, and relationships to arterial bends and branches and the adjacent dural and osseous structures within the segment (Fig. 366-4).

FIGURE 366-4 Ophthalmic segment (OphSeg) aneurysm types (schematic): lateral (A), dorsal (B), and anteroposterior (C) views. D, Ophthalmic artery (OphArt) aneurysm: arteriogram (anteroposterior [top left], lateral [top right], and three-dimensional [bottom left], views) and axial magnetic resonance image [bottom right]. E, Superior hypophyseal artery (SupHypArt) aneurysm: arteriogram (anteroposterior [top left], lateral [top right], and three-dimensional [bottom left] views) and axial magnetic resonance image [bottom right]. OphArt origin (black arrow) is proximal to aneurysm, and posterior communicating artery (white arrow) is distal to aneurysm. F, Dorsal variant internal carotid artery (ICA) aneurysm: growing blister aneurysm, arteriogram (anteroposterior view on post–subarachnoid hemorrhage [SAH] days 0 [top left], 13 [top right], and 17 [bottom left]; lateral view on post-SAH day 17 [bottom right]). Note the distance between OphArt origin (black arrow) and the aneurysm. OphArt aneurysms (hatched area 1) arise from the dorsomedial ICA surface just distal to the OphArt origin, whereas SupHypArt aneurysms (hatched area 2) arise from the inferomedial ICA surface independent of the OphArt origin, in close association with the perforators that supply the optic chiasm and parasellar dura. Dorsal OphSeg aneurysms (hatched area 3) are rare and arise from the dorsal ICA surface well distal to the OphArt origin. ACP, anterior clinoid process; COM, carotid-oculomotor membrane; DR, dural ring; ON, optic nerve; Pit, pituitary gland.

Ophthalmic Artery Aneurysms

Ophthalmic artery aneurysms arise immediately distal to and in relation to the origin of the ophthalmic artery.^1,^2,¹⁹^–²² These lesions typically arise along the posterior bend of the ICA just distal to the ophthalmic artery and dural ring, project dorsally or dorsomedially, and elevate the lateral edge of the nerve against the falciform ligament. The pressure from the falciform ligament against the superior surface of the nerve results in an inferior nasal field defect. A monocular inferior nasal field defect is initially produced; as the lesion enlarges, however, the entire nasal field can become affected, followed by a superior temporal field loss in the contralateral eye. If the clinical course is protracted, ipsilateral blindness with marked contralateral deficits can occur. Larger ophthalmic artery aneurysms tend to thicken or even calcify along their anterior portion, a factor that must be accommodated during clip placement.

Superior Hypophyseal Artery Aneurysms

Superior hypophyseal artery aneurysms arise in association with the superior hypophyseal artery along the medial surface of the ICA, usually just distal to the dural ring.^2,²⁰ There are two main variants. The parasellar variant projects inferiorly toward the sella into the carotid cave, whereas the suprasellar variant projects superiorly into the suprasellar space. Parasellar superior hypophysal artery aneurysms are usually asymptomatic. Because they project into the dura of the sella or cavernous sinus, purely parasellar aneurysms are unlikely to rupture.¹ Their medial projection and proximity to ICA perforators also makes them more difficult to treat. Suprasellar or large lesions have a much higher tendency to rupture. When large or giant, these aneurysms tend to elevate the optic chiasm and produce visual changes and relationships to the visual system similar to those seen with pituitary tumors. Larger superior hypophyseal artery aneurysms tend to develop thickenings or calcifications along the anterior-medial aspect, near their origin from the ICA, similar to ophthalmic artery aneurysms. Because these lesions extend medially underneath the optic chiasm, they can result in bilateral visual field defects.

Many small lesions burrow inferiorly and medially toward and beneath the diaphragma sella, expanding the carotid cave. These lesions, termed parasellar variant superior hypophyseal artery aneurysms, have also been called carotid cave aneurysms.^1,^2,^7,²⁰ When small, the fundus of this variant is invested by adjacent lateral parasellar dura, and their risk for subarachnoid hemorrhage is quite low. With growth, however, these lesions expand superomedially above the diaphragma sella into the suprasellar space, where the hemorrhage risk becomes greater. A second type, the suprasellar variant superior hypophyseal artery aneurysm, projects medially above a shallow carotid cave and expands into the medial suprasellar space above the diaphragm, without investment by the parasellar dura. Superior hypophyseal artery lesions cause their visual compression by expansion into the suprasellar space and elevation of the optic chiasm, producing superior bitemporal or other patterns of visual loss more suggestive of pituitary tumors.

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