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

Preoperative Evaluation

A clear understanding and visualization of the PICA anatomy is of utmost importance. The PICA is a rather complex, tortuous, and variable artery,¹³ which originates from the intracranial portion of the VA in 80% to 95% of cases (on average 8.6 mm above the foramen magnum and approximately 1 cm proximal to the vertebrobasilar junction).^13,¹⁴ The gold standard for visualizing the PICAs is a vertebrobasilar angiography for both vertebral arteries; 1.5-T MRA images and CT angiographies can often provide supplementary information for in situ PICA-PICA bypass planning. It is of essence to understand PICA-related anatomical structures, as it will help to clarify the approach for the revascularization 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.