Surgical Treatment of Paraclinoid Aneurysms

Incidence

The reported incidence for paraclinoid aneurysms varies from 5% to 9% of anterior circulation aneurysms. They are more frequent in females and tend to occur in association with multiple aneurysms in more than 20% of the cases, including the so-called mirror aneurysms.

Anatomic Aspects

In 1938, Fisher published an anatomic nomenclature for the ICA based on the angiographic course of the artery, describing five segments designated C1 through C5. However, these segments were numbered opposite to the direction of blood flow and the extracranial ICA was excluded ( Fig. 46.1A ). Recently, other classifications have been published that include the extracranial and intracranial segments, and the carotid segments have been numbered according to the direction of the blood flow. Therefore the paraclinoid segment of the ICA comprises the C2 and C3 segments of the original Fisher classification (1938), the distal C3 and proximal C4 segment of Gibo et al. (1981), and the C5 and C6 segment of the Bouthillier classification (1996) (see Fig. 46.1 ). Because of the close topographical vicinity of these aneurysms to osseous, fibrous, nervous, and vascular structures of the skull base, they may present with clinical symptoms due to compression of the optic nerve or other surrounding structures instead of the classic subarachnoid hemorrhage frequently seen in aneurysms in other locations. ^, The anatomic structures of the paraclinoid area offer a limited space for expanding vascular lesions and for microsurgical treatment. Therefore proximal vascular control and minimal manipulation of nerve structures around these lesions are of utmost importance for achieving optimal outcome. ^{, , ,}

Classification

Given the variability in these aneurysms, several classifications have been reported based on the neck location or the origin site of the aneurysm, the projection of the dome, and the aneurysm relationship with adjacent branches arising from the ICA. ^{, , , , , , ,} The vast majority of saccular aneurysms arise within the angle formed by the parent artery and a significant arterial branch. Therefore aneurysms related to these arteries are named accordingly (e.g., ophthalmic and superior hypophyseal artery aneurysms). ^, In addition, aneurysms unrelated to branches occur in this segment (distal ophthalmic aneurysms). Ventral paraclinoid carotid aneurysms originate from the ventral surface of the ICA with the proximal aneurysm neck approximately at the level of the ophthalmic artery and the distal aspect of the neck proximal to the PComA ( Fig. 46.2 ). These aneurysms also project caudally or slightly medial and caudal. Dorsal paraclinoid aneurysms project from the dorsal aspect of the ICA. Carotid cave aneurysms are another distinct type that originate at a non-branching site of this segment and arise from the medial wall of the proximal intradural ICA and grow within this small dural recess, with the apex of the sac directed toward the cavernous sinus ( Fig. 46.3 ). This denomination by location and projection is rather complex, since aneurysms are not always related to a branching artery and may point in any direction: laterally, medially, ventrally, or dorsally (see Fig. 46.2 ).

In order to simplify the classification of these aneurysms, we prefer to name them according to their site of origin in relation to the circumference of the ICA and some branching artery (if any), because this is relevant to microsurgical approaches and clipping. Thus the paraclinoid aneurysms can be classified as follows: (1) dorsal type aneurysms, (2) ventral type aneurysms, (3) carotid cave aneurysms, and (4) global type aneurysms ( Figs. 46.3 to 46.7 ).

Dorsal Type Aneurysms

These aneurysms include the proximal and distal dorsal type aneurysms with the proximal type corresponding to the carotid ophthalmic aneurysms arising from the ophthalmic ICA segment in close relationship with the ophthalmic artery. ^{, , , ,} On lateral angiograms, the neck of the aneurysm is located just distal to the origin of the ophthalmic artery ( Figs. 46.2A , 46.4 , and 46.8 ). These aneurysms grow rostrally and can exercise pressure on the optic nerve and patients can present with visual deficits. The distal dorsal type aneurysms (also known as dorsal wall aneurysms) originate at the dorsal surface of the ICA distal to the ophthalmic artery and do not seem to originate from any ICA branch. ^, It is unknown whether they arise at bifurcations of vestigial arteries or because of hemodynamic stress at the curvature of the carotid siphon. The dorsal surface of the ICA is also a common site of blister–like aneurysms. These are dangerous small lesions with fragile walls consisting of normal adventitia or fibrin nets. Primary treatment in the acute stage is challenging due to the substantial risk of intraoperative bleeding, resulting in the formation of a large defect in the ICA. To treat these aneurysms, sometimes it is necessary to use specially designed encircling clips or some wrapping procedure; a bypass procedure with trapping of the aneurysm can be used as an alternative. Despite the complexity of blister-like aneurysms, they can be treated with good results (59% with modified Rankin Scale scores of 1 or 2) . ^,

Carotid Cave Aneurysms

This type of aneurysm originates between the proximal and distal carotid rings. They grow ventromedial proximal to the ophthalmic artery and are visible mainly on the anterior or oblique angiographic views. On the lateral view, they are seen as a double density over the ICA ( Figs. 46.3 and 46.10 ). Carotid cave aneurysms are transitional in type between paraclinoid intradural and cavernous sinus aneurysms. They may grow out of the cave into the intradural subarachnoid space. During surgery, they project ventrally at the level of the carotid genu ( Fig. 46.11 ).

Surgical Technique

Although microsurgical treatment of paraclinoid aneurysms is challenging, clipping requires essentially the same microsurgical principles, including (1) proximal control of the ICA, (2) optimal splitting of the sylvian fissure, (3) extensive microsurgical dissection of the subarachnoid cisterns, and (4) adequate exposure of the neck (Video 46.1). Although the size and location of the aneurysm can have significant implications on the exposure, complete removal of the ACP, unroofing the optic canal, and complete opening of the proximal dural ring represent the basic microsurgical tenants for adequate exposure. Opening the infraclinoid carotid groove sinus for exposure of the surgical genu and axilla of the ICA can be achieved by packing along the proximal walls of the ICA or injecting fibrin glue in the cavernous sinus to control venous bleeding. The great improvement in surgical results compared with the early efforts to treat these aneurysms is mainly related to the incorporation of these techniques and new tools along with a systematic approach that includes the following steps as discussed in subsequent sections.