Surgery of Anterior and Posterior Aneurysms

Anatomy and Terminology

The ophthalmic segment (OphSeg) is typically the longest subarachnoid segment of the ICA, beginning at the dural ring where the artery penetrates the dura to enter the subarachnoid space, and ending at the origin of the PCom artery. The OphSeg has two major branches, both of which typically originate just above the dural ring. The first, largest, and most well-known is the ophthalmic artery (OphArt), which provides the majority of blood supply to the ipsilateral optic nerve. It typically arises from the dorsal or dorsomedial surface of the ICA as the vessel exits the cavernous sinus, immediately beneath the lateral aspect of the overlying optic nerve. Several perforating vessels also arise from this segment, the largest of which is called the superior hypophyseal artery (SupHypArt). These perforators supply the dura around the cavernous sinus, the superior aspect of the pituitary gland and stalk, and the posterior portion of the optic nerves and chiasm. They typically arise from the medial or ventromedial surface of the OphSeg, usually along the second, medial-to-lateral bend of the ICA prior to the PCom artery origin.

OphSeg aneurysms are divided herein into three types, depending primarily on an association of the aneurysm neck with the arterial branches of the segment ( Fig. 71.5 ):

• 1.

  OphArt aneurysms arise from the ICA just distal to the origin of the OphArt, and initially project dorsally or dorsomedially toward the optic nerve.

• 2.

  SupHypArt aneurysms have no association with the OphArt and instead incorporate the medial perforating branches in their origins. Small SupHypArt aneurysms usually arise from the inferior medial surface of the ICA and expand toward the sella dura within a small subarachnoid diverticulum called carotid cave. Because this medial space is limited, enlarging lesions will eventually expand superomedially above the diaphragma sella into the suprasellar space.

• 3.

  Dorsal variant aneurysms are much less common and arise from the dorsal aspect of the ICA well distal to and separate from the OphArt origin. Unrelated to any arterial branch point, some are the result of hemodynamic vectors produced by a sharp angulation within the OphSeg. Others initially appear as “blisters” on the dorsal carotid surface that bleed when small and expand quickly into a fusiform-shaped lesion externally, reflecting their dissection origins ( Fig. 71.6 ).

  

Surgical Approaches

With proper exposure and a firm understanding of parasellar and vascular anatomy, most OphSeg lesions are clippable with low risks to the brain or visual apparatus. Carotid ligation should be considered a secondary alternative, as the risks of stroke are high from parent vessel sacrifice, the visual system is not as effectively decompressed, and complete thrombosis of the aneurysm is not assured. Endovascular treatments with coils, stents, and flow-diversion devices are increasingly being applied to these lesions. Although less invasive, these techniques do not effectively decompress the visual system when it is failing and have a substantial recanalization rate compared to clipping.

Excellent visualization of the ICA, its branches, and the dural folds are mandatory for surgically treating lesions in this segment, and cervical carotid exposure for proximal control is often useful. A skull base exposure, including removal of the anterior clinoid process (ACP), is usually necessary for safe and accurate clipping, especially for larger lesions. Because some optic nerve displacement is also usually necessary to visualize the proximal neck, the falciform ligament should be sectioned before the aneurysm is manipulated. Once the ACP is removed down to the optic strut, a gentle curved or side-angled clip, closed down parallel to the course of the ICA, will secure the neck of OphArt aneurysms, followed by lesion collapse to decompress the visual system. SupHypArt aneurysms are usually best obliterated with fenestrated clips whose blades pass over and then run parallel to the ICA, spanning the distance between the posterior communicating artery and the dural ring. The posterior communicating artery, anterior choroidal, and the SupHypArt must be identified and spared from the clip. Dorsal carotid artery “blister” aneurysms are extremely dangerous if clipped while carrying pulsatile flow and are best handled surgically by temporarily trapping the affected segment followed by clip-wrapping to support the diffusely weakened wall (see Fig. 71.6 ).

Anatomy and Terminology

The communicating segment of the ICA begins with the take-off of the posterior communicating (PCom) artery and ends at the origin of the anterior choroidal (AChor) artery. Due to its short length, only one aneurysm develops from this segment and is traditionally termed a PCom aneurysm ( Fig. 71.7 ). The hemodynamic stress from flow through this segment creates a potential aneurysm site that may project posteriorly and laterally below the tentorium toward the 3rd nerve, or posteriorly and laterally above the tentorium toward the uncus.

Surgical Approaches

The operative position and extent of bone removal to expose this lesion is similar to that required for anterior choroidal, ICA bifurcation, and proximal middle cerebral artery (MCA) aneurysms. The sylvian fissure is split from lateral to medial until the entire course of the ICA and M1 segment of the MCA is delineated. Opening of the sylvian fissure must be done with caution in laterally projecting cases, so as to avoid avulsion of the fundus from its temporal lobe attachments. With the posteriorly projecting type, part of the neck may be obscured by the basal dura of the tentorial incisura, especially when the OphSeg is shortened, or when the ACP is elongated. The clinoid may have to be removed to secure adequate exposure and proximal control of the ICA. PCom aneurysms always arise distal to the origin of the PCom artery. With care, it can be gently dissected away from the wall of the aneurysm, and the clip should avoid compromising its patency as well as that of the anterior thalamoperforators, ICA perforators, and anterior choroidal artery.

Anatomy and Terminology

The choroidal segment (ChorSeg) of the ICA begins with the take-off of the anterior choroidal (AChor) artery and ends at the base of the terminal carotid bifurcation into the anterior and middle cerebral arteries. This short segment lies superior and slightly lateral to the communicating segment and has similar hemodynamic vectors. The single named branch from this segment, the AChorArt, typically arises several millimeters distal and lateral to the PComArt. The artery initially swings laterally and then posteriorly following the optic tract, often giving off branches that supply variable portions of the basal ganglia, internal capsule, lateral geniculate body, and brainstem. The main trunk continues posteriorly inferior to the optic tract, then enters the choroidal fissure to supply the choroid plexus. The artery is duplicated in up to 30% of cases.

Surgical Approaches

The surgical approach to AChorArt aneurysms is identical to that of PComArt aneurysms with regard to the temporalis muscle incision, the size of the craniotomy, the degree of basal bone removal, and the method of sylvian fissure splitting. Aneurysms associated with the AChorArt are usually small and thin-walled, accounting for 3%–5% of all intracranial saccular aneurysms. During the initial exposure, the surgeon must be cognizant of the aneurysm’s possible adherence to the temporal lobe and direct the approach more frontally until this relationship can be visually clarified. Once proximal control is obtained, the number and course of the AChorArt(s) must be ascertained and separated from the aneurysm neck. Unlike the PComArt, this artery is vital and often unforgiving, and temporary clipping with flow interruption is not well-tolerated. This artery must be spared from the clip, as its loss can result in a ganglionic, capsular, or brainstem infarct.