Introduction

Intracranial bypasses are believed to be high-risk operations with poorly studied outcome measures, and they are often considered as the last and final option in the treatment of neurosurgically demanding vascular and oncological lesions. In situ bypasses, such as middle cerebral artery (MCA) to MCA and anterior cerebral artery (ACA) to ACA bypasses, differ from conventional graft-utilizing bypasses in many aspects. For example, in situ bypasses are always occlusive bypasses, contrary to nonocclusive bypasses like ELANA bypasses,^1–3 have only one anastomotic site between two intracranial vessels, and do not utilize extracranial or harvested vessel grafts. Poor collateral network limits the use of in situ bypasses to some extent in anterior circulation and in large proximal vessels, whereas distal segments of the posterior inferior cerebellar artery (PICA) are able to tolerate temporary occlusion for indefinite periods permitting the safe creation of bypasses proximal to their vascular territory. Therefore, a side-to-side in situ PICA-PICA bypass operation can be considered a relatively safe and elegant adjunct to the treatment repertoire of, for example, complex vertebral artery (VA)-PICA vascular lesions, when other treatment options could compromise the blood flow through the anterior medullary segment of the PICA vessel.

Only a few dozen side-to-side in situ PICA-PICA bypass cases have been reported to date.^4–12 The first report of the side-to-side in situ PICA-PICA bypass was published by Takikawa and others in 1991 describing a 42-year-old man with a ruptured right VA-PICA aneurysm treated with a combination of microsurgical aneurysm trapping and the side-to-side in situ PICA-PICA bypass.¹² Due to the lack of technical reports, we will describe this modern bypass technique in the following sections in detail.

Procedure

Decision Process

Although collateral networks in posterior fossa are robust for hemispheric perfusion, sacrifice of the proximal PICA segment can result in catastrophic ischemic injury. This proximal PICA segment maintains the origin of relatively small but extremely critical perforators feeding the medulla oblongata and cerebellum.¹³ To evaluate the need of the PICA revascularization, the PICA should be divided into five segments and two loops as suggested previously^13,15 (Figure 10–1):

1. Anterior medullary segment (extends posteriorly from the origin of the PICA to the inferior olivary prominence and passes near the hypoglossal rootlets)

2. Lateral medullary segment (begins at the site where the PICA passes the most prominent point of the inferior olive and ends at the origins of the 9th, 10th, and 11th cranial nerves)

3. Tonsillomedullary segment (begins at the point where the PICA passes posterior to the 9th, 10th, and 11th cranial nerves, and then extends medially across the posterior aspect of the medulla to the level of the tonsillar midportion, before which the PICA forms a so-called caudal loop at the caudal pole of the tonsils)

4. Telovelotonsillar segment (begins at the middle section of the PICA’s ascent along the medial surface of the tonsils and extends to the suboccipital cortical surface of the cerebellum and includes the so-called cranial loop)

5. Cortical segment (extends to the cerebellar vermis and hemisphere)

Figure 10–1 The PICA segments illustrated.

(Figure redrawn from Ramina R, et al., Distal posterior inferior cerebellar artery aneurysm: case report. Arq Neuro-psiquiatr 2005;63(2a): 335–338, Fig. 1.)

A PICA revascularization procedure should be considered for lesions locating proximal to the telovelotonsillar segment, especially if the treatment of the lesion itself may occlude patent PICA circulation. However, the absence of perforating arteries along the very proximal portion of the anterior medullary segment permits direct clip or coil occlusion of the PICA at its very origin. Since there are no reliable PICA test occlusions, we have to rely on the anatomy-based planning of bypass surgery. In brief, the most important point in treating VA-PICA lesions is to preserve the critical perforating branches of the proximal PICA.

We next evaluate the proximity of the left and right PICAs. The distance between parallel PICAs should be less than 4 to 5 mm, if possible, which allows PICA mobilization and the side-to-side anastomosis without excessive manipulation of the vessels and their perforators. Parallel tonsillomedullary and telovelotonsillar segments may have a significant difference in diameter (ratio up to 1:2) without causing technical problems. The suturing of the anastomosis is performed in the cistern below the cerebellar tonsils, a region that is relatively shallow and wide, making the procedure less technically challenging than those in deeper narrow corridors of the anterior or posterior circulation. We believe that neurosurgeons who are familiar with conventional cerebrovascular bypass procedures should be able to master suturing side-to-side, in situ PICA-PICA anastomoses.

Operative Technique

Patients are put on aspirin 325 g/day at 5 days before the operation, and they continue the medication indefinitely. Aspirin sensitivity is assessed with the Accumetrix system. No fractioned or unfractioned intravenous heparin is used during the operation.

Craniotomy

The neurophysiological monitoring of brainstem auditory evoked potentials (BAEPs) as well as cortical and spinal somatosensory evoked potentials (SEPs) is applied and the patient is placed in a prone position in a Sugita head frame. The midline suboccipital and upper cervical regions are scrubbed and draped, and a midline incision cranially to the nuchal line and caudally to the C1 arch is done. Self-retaining retractors are placed in the wound and a burr hole is fashioned in the midline suboccipital region. An oval-shaped or round midline suboccipital craniotomy from foramen magnum to below transverse sinuses is performed, and the dura is opened in a V- or Y-shaped fashion. It is not necessary to visualize the transverse sinuses and the craniotomy can be kept well below them. Usually, good exposure of the tonsillar region of the cerebellum is obtained.

Vessel exposure

The right and/or left cerebellar tonsil is gently retracted with the help of self-retaining retractors. Depending on the exact location of the PICA branches, less or more retraction is required. The tonsillomedullary and telovelotonsillar segments of the PICAs are well visualized at the level of the cerebellomedullary fissure. A number of brainstem perforators at the tonsillomedullary and telovelotonsillar segments may be seen, and these have to be preserved throughout the operation. Following gentle mobilization with microdissection of one or both of the vessel segments and separation of the vessels from pia/arachnoid attachments, they are brought easily to close proximity. PICA flows are then checked using the Transonic Charbel flow probe. Small temporary clips (blade length of 6 mm) are placed cranially and caudally to the anastomosis site as well as on perforating vessels originating from the same PICA segment. Blood pressure is increased during temporary clipping if changes in SEPs or in the EEG are observed, but this is a very uncommon event.

Arteriotomy and suturing

Using, for example, a back-cutting microknife, arteriotomies of approximately 4 to 6 mm (at least double the diameter of the wider PICA) of length are made on the facing surfaces of both PICAs. The lumens of the two vessels are aggressively irrigated with heparinized saline. By using a 9-0 nylon suture, a running anastomosis is made. After approximating the two arteries the first (stay) suture is placed at the apex of the arteriotomy by passing the needle from outside the vessel lumen to inside the same vessel lumen, and then from inside the other vessel lumen to outside the same vessel lumen (Figure 10–2A). The tail of the first suture is left long enough in order to use it when tightening the final suture. After making the stay suture, the needle is taken underneath the knot into the space between the two vessels (Figure 10–2B). A running suture is begun by passing the needle very close to the apical stay suture from outside to inside the vessel lumen, and then from inside to outside the other vessel lumen (Figure 10–2C). After this most crucial step of the anastomosis (Figure 10–2C), the back wall of the bypass can be sutured in a running fashion from the intravascular side (Figure 10–2D). When the other apex of the anastomosis is reached, the needle is passed from inside to outside the apex, and then from outside to inside of the adjacent apex (Figure 10–2D). Then, the anterior wall is sutured from outside the lumen using a continuous suture. (Figure 10–2E). Once the sutures are loosely placed attaching the entire anterior wall, the bypass lumen is inspected using a small nerve hook, and flushed with heparinized saline. If the anastomosis is widely patent, the continuous suture can be tightened utilizing the long end of the very first suture or single interrupted sutures may be used for the final one to two stitches (Figure 10–2F).

Figure 10–2 A-F, The side-to-side in situ PICA-PICA bypass procedure illustrated in detail.

Recirculation

Temporary clips are first removed distal to the bypass, then proximal to the bypass and finally from the perforators. Upon recirculation, bilateral flow volumes and directions are measured proximal and distal to the bypass and compared to the preoperative flows. Flows in the range of 8 to 12 ml/min are usually measured in both PICAs proximal and distal to the site of the anastomosis. Flow measurements can be repeated before and after an intraoperative indocyanine green (ICG) angiogram, which confirms the patency of the bypass. Flow measurements distal to the bypass can be done following a test occlusion of the proximal PICA. Minimal if any flow decrease in the occluded limb is expected if the bypass is functioning well, and the PICA origin can be sacrificed. The newly made anastomosis then allows the contralateral PICA to supply the anteromedullary segment of the proximally occluded PICA in a retrograde fashion. Should an early bypass occlusion occur, the sutures are opened, the lumens flushed with heparinized saline, and the anastomosis re-sutured.

Closure

A watertight dura closure is of great importance. A dural patch can be used together with tissue sealants. If any doubt of the water tightness of the dura exists, we recommend using a postoperative lumbar drain for a minimum of 2 to 3 days to avoid postoperative CSF leakage problems, which always lengthens the postoperative recovery time. The bone flap is replaced and secured with titanium microplates, a titanium mesh, or some other implant system. The wound is closed using 0-0 absorbable Vicryl sutures for the deep muscle and muscle fascia, 2-0 absorbable Vicryl sutures for the subcutaneous tissue, and an interrupted or continuous 3-0 Ethilon sutures for the skin closure.

Postoperative course

Patients undergo a CT examination and interventional angiography, if needed, on the first postoperative day. At this time, an interventional radiologist can use intravenous heparinization without a significant risk of postoperative bleeding complications. For 3 to 6 months after interventional procedures, patients usually go through first control angiographies.

Discussion

We believe that PICA-PICA in situ revascularization is an essentially better option than occipital artery (OA)-PICA bypass for the following reasons:

Some patients may tolerate PICA sacrifice at its very origin due to compensating retrograde collateral supply, which can reach to the anterior medullary segment of the PICA. However, since there are no definitive ways to reliably assess the collateral flow of the PICA territory, any treatment involving a probable proximal PICA occlusion should be performed only after PICA revascularization. The side-to-side in situ PICA-PICA bypass offers a relatively safe and feasible revascularization option in such cases.

Many neurosurgeons would not consider an in situ bypass as the first-line revascularization option: vessel occlusion or bypass failure during or after in situ bypass operation could compromise both donor and recipient vascular territories with a subsequent risk of bilateral or wide-ranging ischemic events. While this concern is theoretically valid, better revascularization options for the PICA are quite limited. Strong arguments can be made to select the PICA-PICA alternative even when there is an apparently adequate occipital donor vessel. First, there is no evidence that temporary bilateral PICA clamping is more hazardous than unilateral clamping, which is performed during conventional graft-utilizing PICA bypasses. Second, PICA-PICA bypass thrombosis would lead to occlusion of the PICA distal to the telovelomedullary segment, and therefore any resulting cerebellar hemispheric infarction is likely to be clinically mild due to the rich anastomoses through various cerebellar arteries. Third, if the bypass occluded after performing proximal PICA occlusion, resulting thrombosis of brainstem perforators could cause a lateral medullary syndrome, which would be the very same result of occlusion of conventional PICA bypasses. Fourth, not a single ischemic complication or bypass occlusion of side-to-side in situ PICA-PICA bypasses has been reported to date, not even in children;^4,6 however, publishing bias and the small number of conducted in situ bypass procedures certainly explains this extremely low complication rate.

If the treatment plan of ruptured or unruptured VA-PICA aneurysms as well as VA dissections includes occluding the VA and/or PICA at its origin, the side-to-side in situ PICA-PICA bypass can be readily and safely constructed prior to treating the lesion itself. Endovascular treatments may be combined with less aggressive direct surgical procedures, and this combination strategy may reduce the risks of cranial nerve or brainstem compromise.