Posterior Inferior Cerebellar Artery Aneurysms
Posterior inferior cerebellar artery (PICA) aneurysms are relatively uncommon. Most PICA aneurysms can be treated endovascularly, but we have encountered a high rate of aneurysm recurrence associated with coiling procedures. As a result, surgical consideration should be given to PICA aneurysms, particularly those with wider necks. The classic PICA aneurysm arises at the takeoff of the PICA from the vertebral artery. These aneurysms can be challenging because of the great variability of the local vascular anatomy. At times, the aneurysm can be located along the anterolateral surface of the medulla and even as high as the cerebellopontine angle. We have found it useful to study the preoperative angiogram to determine the location of the aneurysm relative to the skull base when deciding how difficult the surgical exposure will be ( Fig. 8.1 ).
Many PICA aneurysms occur close to the point where the vertebral artery penetrates the dura not far above the craniocervical junction. These cases can be exposed in straightforward fashion, and distal control of the vertebral artery can be achieved without difficulty ( Fig. 8.2 ). In our experience, when the PICA takes origin from the vertebral artery low on its vertical segment, the aneurysm will generally be closer to the foramen magnum. On the other hand, when the PICA arises at the turn of the artery where it becomes horizontal or along its horizontal segment just before the vertebrobasilar junction, the surgeon can anticipate a more challenging exposure ( Fig. 8.3 ).
We use a far lateral suboccipital approach for these lesions ( Figs. 8.4, 8.5 ). The patient can be prone, lateral, or three-quarters prone, depending on the surgeon’s preference. As part of the exposure, we drill down the condyle aggressively to optimize visualization of the critical intradural anatomy. The vertebral artery is readily exposed extradurally along the lateral aspect of the C1 arch, where hemostatic agents may be useful in controlling bothersome venous bleeding from the vertebral venous plexus. Once the dura is opened, the intradural vertebral artery can be traced to the PICA origin, offering early proximal control ( Fig. 8.6 ). Gentle retraction of the cerebellar tonsil may be helpful with exposure. When the distal vertebral artery communicates normally with the vertebrobasilar junction, retrograde bleeding from the opposite vertebral artery can be problematic should the aneurysm rupture intraoperatively. Therefore, in the setting of a recent rupture, we expose the vertebral artery distal to the PICA origin to obtain proximal and distal control prior to dissection of the aneurysm.
While exposing a PICA aneurysm, the surgeon will typically work past the fine and delicate lower cranial nerve rootlets ( Fig. 8.7 ). In the unruptured setting, these can be easily identified and preserved, but in the setting of a subarachnoid hemorrhage (SAH), it can be more difficult to protect these rootlets when they are encased in thick subarachnoid clot. In all instances, the nerves should be carefully protected during surgery, and we generally test patients for ipsilateral swallowing difficulty prior to initiating a regular diet postoperatively.
It should be noted that if formal digital subtraction angiography is planned as an intraoperative measure during the treatment of a PICA aneurysm, the preoperative placement of a long femoral sheath is strongly recommended, as the patient is typically positioned such that it would be awkward or impossible to place the sheath once the procedure is under way ( Table 8.1 ).