A thorough understanding of the anatomy of the vertebrobasilar system and its variability is paramount before embarking in any type of surgical intervention on the vertebral arteries.

The vertebral arteries are by convention divided into four anatomical segments. The first segment (V1) of the vertebral artery (VA) originates as the first branch of the subclavian artery from its posterosuperior aspect, and typically has no branches. From its origin the VA courses deep over the prevertebral space in a cephalad direction traveling through the sympathetic fibers that connect the intermediate ganglion with the stellate ganglion. The V1 segment ends typically at the inferior border of the transverse foramen of the sixth cervical vertebra. The vertebral vein courses anterior to the artery along the V1 segment. On the left side, the thoracic duct crosses anterior to the proximal aspect of the vertebral artery, and an accessory thoracic duct or ducts may also be found on the right. The inferior thyroid artery crosses anterior to the distal portion of V1 segment, just caudal to its entry into the transverse foramen, where the VA is covered by the longus colli muscle. The origin of the V1 segment is the most common location for occlusive disease in the VA. Fortunately, the length and mobility of the V1 segment makes it amenable to surgical transposition to the nearby carotid or subclavian arteries for correction of proximal occlusive or embolic lesions. The V2 segment travels through the transverse foramina from the sixth vertebra to the upper border of the second vertebrae, and lies anterior to the cervical nerve roots. Through its course along the transverse foramina, the VA gives radiculomedullary and radicular branches that supply the spinal cord, exiting nerve roots, vertebrae, and surrounding muscles. At the level of C3 there is a rather constant branch that provides the medial odontoid and lateral recurrent arteries that supply the spine and musculature. The lateral recurrent artery anastomoses with the ascending cervical and deep cervical arteries and is a major source of collateral flow in cases of proximal VA occlusion. From its entrance into the transverse foramen to the foramen magnum, the VA is surrounded by a venous plexus intimately attached to the adventitia, so a proper vertebral vein does not really exist in this segment. The V2 segment, because of the bony encroachment of the artery, is in general not suitable for arterial reconstruction, but occasionally, vertebral artery compression caused by
osteophytes or other bone anomalies can be surgically relieved by osteotomy at this level. The V3 segment courses from the upper edge of the transverse foramen of the second cervical vertebra to the base of the skull where the artery penetrates the atlanto-occipital membrane and dura, crossing into the cranium through the foramen magnum accompanied by the posterior spinal artery and the first cervical nerve. The vertical portion of the V3 vertebral artery segment, as it courses from the transverse foramen of C2 to the transverse process of the Atlas, is a relatively long and mobile segment and therefore is a suitable target for distal arterial bypasses or arterial transpositions for problems that are not amenable to reconstruction at the V1 level. The VA segment between C1 and C2 is crossed anteriorly by the ventral ramus of the second cervical nerve, and covered anterolaterally by the levator scapula muscle. The distal V3 segment of the VA above the transverse foramen of the Atlas provides in addition the posterior spinal, posterior meningeal and muscular branches. Occasionally, the posteroinferior cerebellar artery (PICA) originates in the extradural segment of the VA. The V3 segment after exiting the C1 transverse foramen continues posterolaterally turning horizontally and from lateral to medial over the upper surface of the posterior arch of the Atlas, finishing at the junction of the atlanto-occipital membrane with the dura. The latter segment of the VA is well protected posteriorly by the muscles that originate in the occipital bone that course caudally. The V4 segment of the VA originates as it crosses the dura through the foramen magnum. From there, it travels intracranially along the anterior surface of the medulla to join the contralateral VA at the pontomedullary junction to form the basilar artery. As the VA crosses the dura into the cranium, it loses its adventitia, most of its medial layers and the external elastic lamina. The PICA most often originates from the proximal intradural vertebral artery, while the anterior spinal artery arises just proximal to the basilar junction from paired medial origins from both vertebral arteries that fuse in the midline, and as a single midline trunk descends along the anterior surface of the medulla and spinal cord.

The anatomical variations of the VAs bear important surgical implications. The left VA tends to be larger in caliber than the right in over 60% of individuals. In about 5% of the population the basilar artery is congenitally supplied only by one VA because the contralateral VA is either atetric or terminates in the PICA.

The VA originates as the first branch of the subclavian artery in over 90% of individuals. However, the VA can occasionally originate distal to the thyrocervical trunk or as a branch of it. In approximately 5% to 8% of the population, the left VA originates directly from the aortic arch between the origins of the left common carotid and left subclavian arteries, and in these cases it will typically enter the transverse foramina at a higher level. The right VA rarely arises from the aortic arch, but it originates at the brachiocephalic trunk bifurcation or from the right common carotid in 4% of the population. The latter origin typically occurs when there is an aberrant retroesophageal right subclavian artery. It is important to remember that some VAs have a double origin, from the arch and subclavian artery or from the subclavian and thyrocervical trunk fusing at the base of the neck to continue their normal course cephalad.

The VA enters the transverse vertebral foramen at C6 level in 90% of the cases, in 5% of individual enters at C5, in 2% at C4, in 1% at C3, and in 2% at C7. Variation of the entry level of the vertebral artery is more common on the left side.

Although quite rare, a persistent embryologic proatlantal artery may be encountered. The proatlantal artery communicates the distal internal or external carotid arteries with the VA at the suboccipital level, and is typically associated with hypoplasia or absence of the more proximal VA. This artery may provide essential collateral flow to the basilar artery and PICA in cases of proximal and contralateral vertebral occlusion. Persistent hypoglossal and trigeminal arteries occur and communicate the internal carotid with the ipsilateral intracranial segment of the vertebral arteries. The hypoglossal artery arises extracranially and travels with the hypoglossal nerve across the foramen lacerum into the cranium to join the intracranial VA. The persistent trigeminal artery originates from the intracranial internal carotid and anastomoses with the VA intracranially. Persistence of these embryonic arteries typically is associated with agenesia or hypoplasia of the more proximal VA segment.

Reconstruction of the extracranial VA is indicated in patients presenting with signs or symptoms of vertebrobasilar ischemia secondary to occlusive or embolic lesions located in the extracranial segments of the VA. Vertebrobasilar ischemia secondary to low-flow phenomena can be produced by a fixed stenosis of the VA, generally from atherosclerotic origin, from a fixed external compression by bone or muscle, or from transient external compression of one or both vertebral arteries occurring during neck rotation or extension. These patients typically present with repetitive symptoms related to neck motion or postural changes that produce a transient decrease in flow through the VA. Hemodynamic compromise may also follow VA dissection, fibromuscular dysplasia, or kinking from arterial elongation. In general, surgical reconstruction is indicated in symptomatic patients with significant stenosis of a single or dominant VA, or when both VAs are significantly stenotic.

One-third of the cases of posterior cerebral ischemia are attributable to embolic phenomena. Surgical intervention is indicated in the latter patients when the embolic source is located in the VAs and can be eliminated by surgical bypass or transposition. These patients typically present with posterior circulation stroke, and often have evidence of ischemic infarctions in the basilar territory in cross-sectional imaging studies. Occasionally, the embolic source can be located in the basilar artery itself in which case vertebral reconstruction is unwarranted.

It is important to remember that an isolated unilateral VA lesion, regardless of its severity, has no hemodynamic significance as long as the contralateral VA can be considered normal. However, most patients with hemodynamic vertebrobasilar ischemia have significant bilateral VA stenosis, or have a hypoplastic, absent or completely occluded VA on one side, and a severely diseased contralateral one.

Less commonly, the presence of arteriovenous fistulae or aneurysms may constitute an indication for surgery in the vertebral territory. In some of these cases a combined approach with catheter-directed embolic techniques combined with the reconstructive techniques described in this chapter may be indicated.

High-quality imaging studies are of paramount importance in the surgical planning of VA reconstruction. Computerized tomographic angiography (CTA) is extremely useful in the localization of fixed arterial stenosis or potentially embolic plaque. In addition, it provides excellent detail on the origin of the artery and the relationship between the VA and the surrounding osseous structure. Conventional digital angiography is still favored to assess the quality of the arterial segments that will serve as surgical targets. When the symptoms of posterior cerebral ischemia are secondary to positional neck changes, it is mandatory to obtain a dynamic arteriogram that can precisely locate the site of arterial compression during positional changes of the neck. This could be due to kinking in one of the free segments of the VA, or compression of the VA by vertebral bony structures or by the neck muscles. During preoperative planning, it is also important to evaluate the status of the carotid and subclavian vessels as they will most likely serve as the inflow source for a bypass or transposition of the VA. The adequacy of collateral flow through the posterior communicating arteries to the posterior circulation has to be established to assess the risk of posterior cerebral ischemia during VA clamping. In cases where the VA subject of surgery is the only inflow source to the basilar system, maintenance of a systolic blood pressure over 150 mm Hg during clamping is advisable. In addition, superficial hypothermia and steroids are used to improve cerebral tolerance to ischemia. For this purpose, we use a cooling blanket over the body and cold bags placed in the axillae and groins. Careful monitoring of core temperature is extremely important, allowing a maximum temperature drop to 33.5 degrees centigrade. Experience with superficial cooling is necessary, as rewarming should be initiated around 34.5 degrees
centigrade to avoid a dangerous overshooting in temperature drop below 33°C. The adequacy of an autogenous saphenous vein should be assessed preoperatively, and it should be included in the surgical field for possible harvesting.

The use of a mechanical retractor is very helpful to maintain a steady surgical field throughout the case. The VA, and more so in its V3 segment, is extremely delicate and will dissect and tear very easily upon handling. Therefore, fine surgical instruments and appropriate technique should be employed. More importantly, clamping of the V3 vertebral segment requires very small and delicate partial occlusion clamps that are not normally commercially available and may need to be specially ordered. To avoid arterial or neurologic injuries, bipolar electrocautery is mandatory during VA surgery at any level, and surgical magnification loops should be used for VA surgery.

Two types of techniques are available for extracranial VA reconstruction; arterial bypass with autogenous vein or arterioarterial transpositions. These techniques can be done at three different levels with increasing degree of technical complexity. While proximal VA surgery at the V1 segment is fairly straightforward in experienced hands, reconstruction of the VA involving the V3 segment between the C1 and C2 vertebrae, or at the suboccipital level are demanding operations that require a high level of expertise in the anatomy and surgery of this region.

We will describe the exposures and more common techniques that can be applied at three different VA segments from proximal to distal.