20 Cave Carotid Artery Aneurysms
Abstract
The carotid cave is an intradural pouch that extends below the level of the distal dural ring between the wall of the internal carotid artery (ICA) and the dural collar surrounding the ICA. Cave carotid artery aneurysms project medially from the ICA in this location and may be entirely extradural or may have an intradural component. According to their small size and location, most unruptured cave carotid aneurysms will be silent and fortuitously discovered. Their most frequent clinical expression is subarachnoid hemorrhage (SAH) in case of rupture.
For ruptured aneurysms, the treatment is clearly indicated in most cases, except in high-grade SAH and very old patients. For unruptured aneurysms, the threshold for treatment indication is probably 4 to 5 mm, balanced with patient age and remaining life expectancy. For both ruptured and unruptured cave carotid artery aneurysms, the endovascular treatment is the first-line treatment. Several endovascular options are available for the treatment of cave carotid aneurysms including standard coiling, balloon-assisted coiling, stent-assisted coiling, flow diversion, or parent artery occlusion. The surgical treatment of ventral ICA aneurysms (including carotid cave aneurysms) is difficult.
Introduction
Kobayashi et al initially described carotid cave aneurysms in 1989. The carotid cave is an intradural pouch that extends below the level of the distal dural ring between the wall of the internal carotid artery (ICA) and the dural collar surrounding the ICA. It has been reported to be present in 80% of cadaveric specimens. The clinical significance of this region lies in the fact that aneurysms that project medially from the ICA in this location may be entirely extradural or may have an intradural component. These aneurysms are buried in the dural pouch and are often difficult to find, dissect, and clip during microsurgery; therefore, the majority of these aneurysms nowadays are treated endovascularly.
Major controversies in decision making addressed in this chapter include:
How to recognize cave carotid aneurysms?
When is treatment for cave carotid aneurysms indicated?
Open vascular versus endovascular treatment.
Whether to Treat
Carotid cave artery aneurysms have their fundus totally or partially in the carotid cave and can be responsible for subarachnoid hemorrhage (SAH). For ruptured aneurysms, the treatment is clearly indicated in most cases. Only in high SAH grades and in very old patients, the treatment has to be discussed in detail with family as outcome is usually very poor ( 1 in algorithm ).
In case of unruptured aneurysms, a treatment still has to be proposed and several factors are part of the decision making including patient′s age, risk factors (singularly smoking and elevated blood pressure), aneurysm size, multiple aneurysms, and familial aneurysms ( 2 in algorithm ). The fact is that the carotid cave is a small space that limits the growth of aneurysms. In a recent anatomical report, Joo et al indicate that the depth and the length of the cave averages 2.4 and 9.9 mm, respectively. According to Tanaka et al, the mean size of cave carotid artery aneurysms is 4 mm. Therefore, the 7 mm threshold depicted by the International Study of Unruptured Intracranial Aneurysms (ISUIA) probably does not apply really to this subgroup of aneurysms and aneurysms less than 7 mm have to be treated ( 3, 4, 7, 8, 9 in algorithm ). In their large review of the management of unruptured paraclinoid aneurysms, Iihara et al indicate that until 2000 they were treating all cave carotid aneurysms larger than 3 mm in patients younger than 70 years. From 2001, they changed their strategy and decided to treat in principle all aneurysms larger than 5 mm. However, from 2001, still 30% of aneurysms treated were smaller than 5 mm. The threshold for treatment indication is probably 4 to 5 mm, balanced with patient age and remaining life expectancy ( 3, 4, 7, 8, 9 in algorithm ).
Anatomical Considerations
The carotid cave, identified in 68 to 90% of the cadaveric specimens, is a small recess of the dura mater that extends below the level of the distal dural ring on the posteromedial side of the wall of the ICA. On the medial side, this recess is bounded by bone, i.e., the sphenoid body and/or the sella turcica. The carotid cave contains in most cases subarachnoid space, sometimes the arachnoid membrane or the extra-arachnoid spaces. Identification of the distal dural ring and the carotid cave with imaging is not easy. In their detailed anatomical and radiographic evaluation performed in cadaveric specimens, Oikawa et al demonstrated that the plane of the distal dural ring inclines in the posteromedial direction. For them, the marker of the most distal point of the distal dural ring situated to the anterolateral side of the ICA is the superior border of the anterior clinoid process. The marker for the most proximal point of the distal dural ring located posteromedially is the tuberculum sellae. Recently, Watanabe et al reported the usefulness of fusion images with 3D magnetic resonance (MR) cisternography and MR angiography (MRA) to identify these anatomical structures.
According to the previous anatomical description, cave carotid artery aneurysms originate from the most proximal part of the intradural ICA. As they develop in an intradural space, they have a potential of causing SAH. Cave carotid artery aneurysms are in the carotid cave mainly proximal to the origin of the ophthalmic artery. They arise just cranial to the genu of the ICA and the turbulent and high flow caused by the changing course of the ICA will contribute to their growth. However, they are usually small as their growth is limited medially by the sphenoid and laterally by ICA. They grow ventromedially and may extend in the cavernous sinus. In their radiometric analysis of paraclinoid carotid artery aneurysms, Tanaka et al showed that aneurysms located at the supraclinoidal level had a mean size of 7.3 mm, those located at the clinoidal level 5.2 mm, and those located at the infraclinoidal level (most of them cave carotid aneurysms) 4 mm. In some cases, cave carotid aneurysms develop in relation with the superior hypophyseal artery.
Classification
Several classifications of ICA aneurysms located close from the distal dural ring have been proposed. In 1993, al-Rodhan et al suggested the following classification:
Group I: Aneurysms with necks that arose intradurally from the ophthalmic segment of the ICA. This group includes the ventral paraclinoid carotid aneurysms (also called Nutik aneurysms) that originate on the ventral surface of the ICA at or just distal to the ophthalmic artery. These aneurysms originate opposite to the ophthalmic artery origin.
Group II: True ophthalmic artery aneurysms with necks that arise at the junction of the ophthalmic artery and the ICA.
Group III: Cave carotid aneurysms.
Group IV: Transitional aneurysms include cavernous aneurysms in which the neck arises from the cavernous segment of the ICA but the dome projects superiorly into the intradural extracavernous subarachnoid space.
Group V: Intracavernous aneurysms.
In 1996, Kyoshima et al classified these aneurysms in three groups:
Paraclinoid intradural aneurysms arise from the ICA distal to the origin of the ophthalmic artery. They include carotid-ophthalmic and posterior communicating aneurysms.
Carotid cave aneurysms are located in the carotid cave and proximal to the origin of the ophthalmic artery.
Infraclinoid extradural aneurysms.
Workup
Clinical Evaluation
According to their small size and location, most unruptured cave carotid aneurysms will be silent and fortuitously discovered. Their most frequent clinical expression is SAH in case of rupture. As aneurysm fundus can be located in the cavernous sinus, their rupture can potentially induce a carotid-cavernous fistula. It has not been reported in the literature except during the endovascular treatment of a cave carotid aneurysm, probably created iatrogenically.
According to their location, these aneurysms can sometimes induce optic nerve compression, and in patients with unruptured aneurysms, the performance of an ophthalmic evaluation (including evaluation of visual acuity and visual field) is mandatory.
Imaging
Direct visualization of the carotid cave is difficult whatever the imaging modality used, including computed tomography (CT) and MR. In a recent paper, Watanabe et al showed that fusion images with 3D-MR cisternography and MRA were helpful to identify the distal dural ring and to locate the aneurysms in this region. Angiographic identification of cave carotid artery aneurysms is based on criteria proposed by Zhang et al. In the anteroposterior view, carotid cave aneurysms project medially in a semicircular berry shape. In the lateral view, no space is visible between the axilla (area inside the genu of the ICA) and the anterior or anteroinferior view of the aneurysm. On the contrary to cave carotid aneurysms, ventral paraclinoid carotid aneurysms (Nutik aneurysms) are usually superimposed to ICA in anteroposterior view and a space is usually visible between their anterior wall and the axilla.
Treatment
Choice of Treatment
As outlined in several series dealing with paraclinoid aneurysms, the surgical treatment of ventral ICA aneurysms (including carotid cave aneurysms) is difficult. In their series dealing with unruptured paraclinoid aneurysms, Iihara et al reported that most cave carotid aneurysms (36/37, 97.3%) were treated endovascularly and concluded that endovascular treatment was the acceptable first line of therapy (supports steps 3, 6, and 8 in algorithm). Notably, the only aneurysm treated by surgery was not clipped but coated.
Then, for both ruptured and unruptured cave carotid artery aneurysms, the endovascular treatment is the first-line treatment. As several endovascular options are available (see below), most cave carotid aneurysms will be treatable by this approach ( 3, 5, 7, 9 in algorithm ). Surgical approach has to be proposed only in case the endovascular treatment failed ( 4, 8 in algorithm ).
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