Surgical Management of Midbasilar and Lower Basilar Aneurysms

Background

Aneurysms of the midbasilar and lower basilar artery (BA) are located below the level of the superior cerebellar artery and involve the anterior inferior cerebellar artery (AICA), the inferior basilar trunk, and the vertebrobasilar junction (VBJ). Aneurysms of the midbasilar and lower BA represent less than 5% of intracranial aneurysms. In addition to presenting with subarachnoid hemorrhage as with other types of aneurysms, patients with aneurysms of this region may present with cranial nerve neuropathy or brainstem compression. These BA lesions have a poor natural history, especially if ruptured. Although there has been a shift toward using endovascular therapy to treat aneurysms, perhaps most pronounced with lesions in the posterior circulation, certain aneurysms still require microsurgical treatment. ^, Herein we discuss various surgical approaches ( Table 51.1 ; Fig. 51.1 ) for clip reconstruction of midbasilar and lower BA aneurysms. The treatment of giant, fusiform, dolichoectatic basilar trunk aneurysms deserves an entire chapter in itself, given its complicated management paradigms, usually requiring bypass. ^,

Principles of Management

The overall principles of midbasilar and lower BA aneurysm and subarachnoid hemorrhage management are the same as for aneurysms elsewhere in the brain, with the following nuances. Aneurysms of the midbasilar artery and inferior BA are located in a small, restricted area encased within thick, dense bone, which is situated within a limited subarachnoid space and filled with the densest collection of vital cranial nerve and vascular structures in the nervous system. Therefore, skull base approaches with extensive bone removal are necessary to obtain adequate exposure of the aneurysm with minimal brain retraction.

During aneurysm dissection and clipping, preservation of the small perforating arteries from the BA to the brainstem is of particular importance, as injury or occlusion can lead to a devastating stroke. In certain cases, in which the size of the aneurysm precludes adequate visualization of the parent vessel and perforators, additional exposure can be obtained by using hypothermic cardiac arrest with barbiturate cerebral protection. An alternative to complete cardiac standstill is rapid ventricular pacing, which is a method that dramatically decreases blood pressure for a short period of time.

Extended Retrosigmoid Approach

The extended retrosigmoid approach is excellent for many small- and medium-sized VBJ and BA–AICA aneurysms. The approach provides a wider surgical view compared to the traditional retrosigmoid approach ( Fig. 51.2 ). The approach may be combined with the far-lateral approach to reach aneurysms with a significant vertebral artery (VA) component and to achieve better proximal control of the bilateral VAs. The extended retrosigmoid approach has the advantage of providing excellent proximal control (albeit with some limitations of achieving distal control). It allows for more working room in comparison with a Kawase approach to the VBJ. It permits better exposure of the VBJ than is possible with a pterional approach, which is only suitable for midbasilar trunk or aneurysms that are more distal. Disadvantages include the risks to the cranial nerves that are placed directly between the surgical entry point and the aneurysm ( Table 51.2 ).

Surgical Technique

The skin incision involves a C-shaped incision behind the ear starting just superior to the pinna of the ear, arcing behind the ear and continuing to the mastoid tip ( Fig. 51.3 ). The craniotomy/craniectomy may be performed in a number of different fashions, but all should result with a 4 to 5 cm window posterior to the sigmoid sinus and inferior to the transverse sinus. In addition, the sigmoid sinus should be skeletonized from the transverse sigmoid junction cranially to the point where it curves posteriorly toward the midline at the origin of the jugular bulb caudally. Importantly, remaining occipital bone at the inferior edge of the craniotomy should be drilled away to allow for more immediate access to the foramen magnum and cisterna magna for CSF release, eliminating the need for lumbar drainage and its inherent risks.

Intradural Exposure

Immediately after opening the dura, the cisterna magna should be opened sharply to allow CSF release and brain relaxation. After CSF has been released, the dissection of cerebellopontine angle (CPA) arachnoid begins. The arachnoid overlying cranial nerves VII and VIII are subsequently released with care to preserve any small arterial branches entering the porous area of the internal auditory canal (IAC). It is also sometimes beneficial to separate the arachnoid layers between the flocculus and the cerebellar hemisphere, to allow for increased visualization medially with retraction of the cerebellum laterally. At this point, cerebellar branch arteries are identified and traced back to the vertebrobasilar vessels. Mobilization of these vessels also allows them to be dissected free more easily at later portions of the surgical procedure, when preparing for aneurysm clipping, which allows easier inspection and visualization when performing indocyanine green angiography.

The VA proximal to the aneurysm is identified to assure proximal control. Moving distally along the VA allows identification of the VBJ region aneurysm. The aneurysm dome is carefully dissected away from the overlying arachnoid of the CPA. In cases of subarachnoid hemorrhage, this dissection should be performed with a temporary clip in place on the ipsilateral VA (proximal V4 segment) found at the inferior extent of the dural opening. The neck of the aneurysm must be carefully identified and cleavage between the neck and the normal vessel created to allow for clip application on either side of the aneurysm neck. With VBJ region aneurysms involving the AICA or posterior inferior cerebellar artery, the cerebellar artery adjacent to the aneurysm must be identified and dissected free from the aneurysm as well. The aneurysm clip is usually placed with the tips pointing superiorly, in parallel with the course of the parent VA ( Fig. 51.4 ). As with any aneurysm, following clip placement, inspection should be performed to identify residual aneurysm or vessel compromise, and this should be supplemented with indocyanine green angiography and Doppler flow probe.

Transpetrosal Approaches

The transpetrosal approaches are more aggressive and may be used for large or giant true midbasilar aneurysms. The anterior brainstem and clival regions can be reached by removing portions of the petrosal bone with almost no brain or brainstem retraction. The transpetrosal approach has three variations ( Fig. 51.5 ). The retrolabyrinthine technique involves petrous bone resection and preserves hearing. The translabyrinthine technique increases the amount of petrous bone resected and sacrifices hearing. Finally, the transcochlear technique involves maximal petrous bone resection, sacrifices hearing, and requires transposition of the facial nerve. Moving through these three variations represents a gradual increase in the amount of petrous bone resected and in the exposure of the brainstem and clivus (Video 51.1).

Video 51.1 Tranpetrous clip occlusion of a giant basilar trunk aneurysm using hypothermic cardiac arrest. Used with permission from Barrow Neurological Institute, Phoenix, Arizona.