Supraclinoid Internal Carotid Artery Aneurysms
The majority of aneurysms involving the supraclinoid internal carotid artery (ICA) arise at the origins of the posterior communicating or anterior choroidal arteries. We will address aneurysms arising at these locations individually. In addition, over the past decade, increasing recognition has been given to a unique subset of aneurysms involving the ventral supraclinoid ICA not associated with a clear branch point or perforating vessel. These lesions are often dissecting aneurysms. They tend to have a highly malignant natural history and can be extremely unstable.
A pterional craniotomy is utilized for the treatment of aneurysms involving the supraclinoid portion of the ICA. For lesions arising at the origins of the posterior communicating or anterior choroidal arteries, the Sylvian fissure need be opened only to the level of the carotid bifurcation. In most cases, proximal control is readily achieved. We will discuss the management of ventral supraclinoid aneurysms as a separate entity.
Posterior Communicating Artery Aneurysms
Posterior communicating artery (PCommA) aneurysms have traditionally been thought of as “easy” aneurysms from a surgical perspective. In academic training programs, these lesions have often been considered reasonable fodder for mid-level residents yearning to clip their first aneurysms. In our experience, there is great variability in the difficulty associated with these lesions. Although most will be visible immediately upon opening the arachnoid lateral to the ICA just above the level of the optic nerve, these aneurysms can sometimes be obscured from view by a large anterior clinoid process, which can also compromise early establishment of proximal control ( Figs. 4.1, 4.2 ). In addition, when the supraclinoid ICA itself is short or lies in a very horizontal orientation, the aneurysm may point deeply away from the surgeon during a routine pterional exposure. Most often, however, these lesions are well exposed through a standard pterional approach.
Much has been written about proper patient positioning when treating a ruptured PCommA aneurysm based on whether or not the patient presents with a third-nerve palsy. In general, if the patient does not have a third-nerve palsy, then the aneurysm is suspected to be directed laterally, where its dome can adhere to the temporal lobe. In such cases, too much rotation of the head can encourage the surgeon to utilize aggressive temporal lobe retraction, which can cause the aneurysm to rebleed. In our experience, it is best to avoid deep temporal retraction in all PCommA aneurysms, and a standard pterional approach with the head turned no more than 45 degrees works best in such cases. By opening the proximal Sylvian fissure, the surgeon can identify the neck of the aneurysm first, and a small bit of subpial resection can be used to free the aneurysm dome if necessary prior to clip placement. This maneuver avoids any traction being placed on the deep temporal lobe, which can, in theory, result in tearing of the aneurysm.
For all PCommA aneurysms, the proximal Sylvian fissure should be opened sharply, exposing the supraclinoid ICA at the level of the optic nerve . This maneuver will establish proximal control in most cases. Depending on the size of the aneurysm, the surgeon will have to address both the PCommA itself as well as the neighboring anterior choroidal artery (AChA). The PCommA should be identified and its origin preserved ( Fig. 4.3 ). In some cases, the neck of the aneurysm may incorporate the origin of the artery, and a carefully placed clip will enable the reconstruction of the origin of the PCommA. In rare cases, when the PCommA origin is incorporated into the aneurysm neck and there is clear filling of the posterior cerebral artery (PCA) and the PCommA itself in retrograde fashion from the basilar artery, the PCommA origin can be occluded along with the aneurysm neck. In these very rare cases, we always check an intraoperative vertebral arteriogram to be sure the PCA as well as the PCommA both fill promptly. The PCommA typically gives rise to critical anterior thalamoperforating vessels, and these can be injured by overly long clip blades reaching past the aneurysm neck toward the perimesencephalic cistern.
Before operating on a PCommA aneurysm, it is important to understand the anatomy of the ipsilateral PCA based on preoperative angiography ( Fig. 4.4 ). If the PCommA represents a fetal PCA (i.e., the ipsilateral P1 is atretic), then occlusion or stenosis of the PCommA can result in a serious PCA infarction with complete hemianopsia. Ischemic injury due to loss of the anterior thalamoperforators can further complicate the situation. As a result, one should not sacrifice a fetal PCommA.
If the AChA is adherent to or running along the aneurysm dome, it must be thoroughly dissected away, at least from the aneurysm neck, prior to clip placement. AChA occlusion can result in contralateral hemiparesis, hemisensory loss, and hemianopsia, and such occlusion should be avoided in all situations ( Fig. 4.5 ) .
PCommA aneurysms are situated close to the third cranial nerve. An expanding PCommA aneurysm can result in partial or complete third-nerve palsy, a situation that should generally be treated with urgent repair of the aneurysm before a life-threatening subarachnoid hemorrhage (SAH) occurs ( Fig. 4.6 ). There is some controversy as to whether open surgery or endovascular coiling should be performed in this setting. In our experience, the third-nerve palsy tends to improve in the majority of cases treated in either fashion, although there may be a slight advantage to open surgery, which enables immediate deflation of the aneurysm and decompression of the nerve.
When PCommA aneurysms become large, they can become more difficult to treat using simple neck clipping. One important surgical strategy involves the use of fenestrated clips, often with right-angle blades, placed around the ICA to occlude the aneurysm neck ( Fig. 4.7 ). In these cases, one must be particularly careful to avoid compromising the origin of the AChA.
Finally, PCommA aneurysms will occasionally present with a large temporal lobe hematoma or a subdural hematoma, resulting in a life-threatening herniation syndrome. A preoperative CT scan with contrast will often show the underlying aneurysm in such cases, and the patient is brought to the operating room on an emergency basis without a formal preoperative arteriogram. A generous craniotomy is performed to enable decompression, and then an intraoperative angiogram is performed to assess the neurovascular anatomy. In general, one can evacuate some of the hematoma to decompress the brain without rupturing the aneurysm. In these cases, we prefer to open the proximal Sylvian fissure, expose the aneurysm neck, and clip the aneurysm in standard fashion, rather than working through the hematoma cavity to reach the aneurysm. Once the aneurysm has been clipped, remaining hematoma is removed, and a decision can be made whether the bone flap can be replaced safely or whether the degree of brain swelling mandates that the flap be temporarily stored for later re-implantation ( Table 4.1 ).