Vertebral artery anatomy

The paired vertebral arteries are branches off of the first portion of each subclavian artery. These arteries are generally unequal in size, with the left the larger and dominant of the two. The typical course of the vertebral artery allows for its classic division into 4 segments, V1 though V4. The first segment (V1) starts with the branching of the vertebral artery from the subclavian artery and follows as it travels anterior to the transverse foramen of C7 and into the transverse foramen of C6. The second segment (V2) includes the section of the artery as it passes through the successive vertebral foramina from C6 to C1. V3 comprises the portion from the superior aspect of the arch of the atlas to the foramen magnum; (V4) extends from the foramen magnum to the confluence with the contralateral vertebral artery and together they form the basilar artery ( Fig. 1 ).

Gross dissection of the cervical spine demonstrating typical anatomy of the left vertebral artery, including passage anterior to the transverse foramen at C7. The artery subsequently enters the C6 foramen and passes through each successive foraminae to C1.

Various anatomic relationships throughout the course of the vertebral artery are important to the spine surgeon. In the V2 region, the artery normally remains 1.5 mm or more lateral to the uncovertebral joint. Furthermore, the bony architecture within the region of the V2 segment dictates a mildly convergent course of the arteries through this section; the mean interforaminal distance at C6 is approximately 29 mm compared with 26 mm at C3. Similarly, the mean distance from the medial edge of the longus colli to the medial edge of the vertebral artery decreases from 11.5 mm at C6 to 9 mm at C3. Although the transverse foramina of the subaxial spine are ring-shaped, the transverse foramen of C2 is an angulated canal bordered by the pedicle and lateral mass. Its inferior and lateral openings allow the artery to deviate 45° laterally before continuing its ascent to enter the transverse foramen of C1.

The V3 segment becomes important to the spine surgeon mostly during posterior surgery of the atlantoaxial joint. As the artery exits the foramen of C1, it travels posteriorly and medially inside the vertebral artery groove on the superior aspect of the atlas. At a distance ranging from 8 mm to 18 mm from the midline, the artery abruptly changes course, traveling anteriorly and superiorly toward the foramen magnum.

Anomalous vertebral artery anatomy

Although anatomic anomalies within the V2 segment are rare, their presence can be important, particularly in patients undergoing anterior cervical spine surgery. These anomalies can be divided into 3 major categories: intraforaminal, extraforaminal, and arterial.

Intraforaminal anomalies, also known as vertebral artery tortuosity, can be defined as a vertebral artery which is located medial to, or less than 1.5 mm lateral to, the uncovertebral joint. Generally, this refers to the midline migration of the vertebral artery causing erosion into the vertebral body ( Fig. 2 ). Several hypotheses have been proposed to explain why such tortuosity occurs. These include degenerative changes and posttraumatic changes as well as less common causes, such as infection, tumor, systemic disease, and prior surgical nonunion. Cadaveric studies have shown the incidence of this condition to be 2.7%, with C3 and C4 the most commonly affected levels. More recent MRI-based studies showed a higher incidence, 7.6%, and found that patients with a tortuous vertebral artery tended to be older than patients without this finding ( Fig. 3 ).

Gross specimen demonstrating a tortuous vertebral artery with erosion into the lateral vertebral body wall.

Axial T2-weighted MRI demonstrating tortuous left vertebral artery with erosion into the vertebral body.

Extraforaminal anomalies refer to instances where the vertebral artery runs anterior to the transverse foramen at one or multiple levels between C6 and C1. An analysis of CT angiograms showed that the vertebral artery enters through the C6 transverse foramen 94.9% of the time. Anomalous entry sites at C4, C5, and C7, however, occurred at 1.6%, 3.3%, and 0.3%, respectively. In their MRI-based study, Eskander and colleagues found that only 92% of arteries entered at C6 ( Figs. 4 and 5 ).

Sagittal T2-weighted MRIs of a patient with C5/6 osteomyelitis/diskitis; right parasagittal ( left ), midsagittal ( middle ), and left parasagittal images ( right ) are shown. Note atypical left vertebral artery passage anterior to the C6 foramen and into the C5 foramen.

Axial T2-weighted image of the same patient as in Fig. 4 at the level of the inferior aspect of the C5 vertebral body demonstrating positioning of the left vertebral artery anterior to the vertebral foramina. A typical pattern is seen on the patient’s right side.

Arterial abnormalities are varied but include such findings as dual-lumen and triple-lumen arteries or the presence of a hypoplastic vertebral artery. Although most of these findings have little surgical implication, vertebral artery hypoplasia affects treatment options and potential neurologic sequelae in the case of an inadvertent injury. Hypoplasia occurs in approximately 10% of the population.

At the atlanto-occipital joint, variations occur with greater regularity. Erosion of the C2 transverse foramen has been reported to have an incidence of 33%, occurring more commonly on the left side. Of these anomalies, 20% are severe enough to preclude the safe placement of C2 instrumentation. Similarly, arcuate foramina of C1 have a reported prevalence of 15.5%, with implications on exposure for C1 lateral mass screw placement.

Anomalous vertebral artery anatomy

Although anatomic anomalies within the V2 segment are rare, their presence can be important, particularly in patients undergoing anterior cervical spine surgery. These anomalies can be divided into 3 major categories: intraforaminal, extraforaminal, and arterial.

Intraforaminal anomalies, also known as vertebral artery tortuosity, can be defined as a vertebral artery which is located medial to, or less than 1.5 mm lateral to, the uncovertebral joint. Generally, this refers to the midline migration of the vertebral artery causing erosion into the vertebral body ( Fig. 2 ). Several hypotheses have been proposed to explain why such tortuosity occurs. These include degenerative changes and posttraumatic changes as well as less common causes, such as infection, tumor, systemic disease, and prior surgical nonunion. Cadaveric studies have shown the incidence of this condition to be 2.7%, with C3 and C4 the most commonly affected levels. More recent MRI-based studies showed a higher incidence, 7.6%, and found that patients with a tortuous vertebral artery tended to be older than patients without this finding ( Fig. 3 ).

Gross specimen demonstrating a tortuous vertebral artery with erosion into the lateral vertebral body wall.

Axial T2-weighted MRI demonstrating tortuous left vertebral artery with erosion into the vertebral body.

Extraforaminal anomalies refer to instances where the vertebral artery runs anterior to the transverse foramen at one or multiple levels between C6 and C1. An analysis of CT angiograms showed that the vertebral artery enters through the C6 transverse foramen 94.9% of the time. Anomalous entry sites at C4, C5, and C7, however, occurred at 1.6%, 3.3%, and 0.3%, respectively. In their MRI-based study, Eskander and colleagues found that only 92% of arteries entered at C6 ( Figs. 4 and 5 ).

Sagittal T2-weighted MRIs of a patient with C5/6 osteomyelitis/diskitis; right parasagittal ( left ), midsagittal ( middle ), and left parasagittal images ( right ) are shown. Note atypical left vertebral artery passage anterior to the C6 foramen and into the C5 foramen.

Axial T2-weighted image of the same patient as in Fig. 4 at the level of the inferior aspect of the C5 vertebral body demonstrating positioning of the left vertebral artery anterior to the vertebral foramina. A typical pattern is seen on the patient’s right side.

Arterial abnormalities are varied but include such findings as dual-lumen and triple-lumen arteries or the presence of a hypoplastic vertebral artery. Although most of these findings have little surgical implication, vertebral artery hypoplasia affects treatment options and potential neurologic sequelae in the case of an inadvertent injury. Hypoplasia occurs in approximately 10% of the population.

At the atlanto-occipital joint, variations occur with greater regularity. Erosion of the C2 transverse foramen has been reported to have an incidence of 33%, occurring more commonly on the left side. Of these anomalies, 20% are severe enough to preclude the safe placement of C2 instrumentation. Similarly, arcuate foramina of C1 have a reported prevalence of 15.5%, with implications on exposure for C1 lateral mass screw placement.

Anterior spine surgery

Vertebral artery injury is a rare but profound complication of anterior spinal surgery. Its relative infrequency limits its presence in the literature to case reports or small case series. The largest series of these injuries cite the incidence of injury as approximately 0.3%. This midline migration can cause erosion into the vertebral body. An instance of postoperative presentation with a lateral medullary infarct, however, has also been reported in a patient whose only intraoperative finding was “epidural oozing.”

In patients with normal vertebral artery anatomy, the artery is most susceptible to injury during anterior procedures in its position anterior to the transverse foramen of C7 or during lateral decompressive maneuvers from C3 to C6. Constant orientation to the anatomic midline is paramount in avoiding injury both during exposure and decompression. The midpoint between the longus colli muscles serves as a reliable intraoperative landmark of the midline, and dissection can safely be performed over the uncovertebral joints. From C3 to C6, the artery is protected by the transverse foramen at the level of the uncovertebral joint, allowing for safe exposure of these structures to their lateral extent; however, care should be taken while exposing the C7 uncovertebral joint given the anterior position of this structure at this level. The presence of an aberrant entry level, however, can place the artery at risk anteriorly at other levels if not recognized preoperatively. At the levels of the vertebral bodies, dissection can safely be carried to the downslope of the vertebrae.

During anterior cervical diskectomy, the vertebral artery is at risk during lateral exploration of the neural foramen. By limiting decompression laterally to the bony ridge of the uncovertebral joint, injury can generally be avoided. Removal of more laterally positioned osteophytes, however, can place the artery at risk, as can loss of orientation.

When performing an anterior cervical corpectomy, the recommended width of decompression is approximately 16 mm. Given that the average interforaminal distance varies from 26 mm to 29 mm, this amount of resection should be safe for nearly all patients at all vertebral levels. Excessive vertebral body resection laterally, however, can put the vertebral artery at risk. This can occur with asymmetric burring due to loss of midline orientation or as a result of oblique resection. The body wall opposite the side of surgical exposure is more prone to the latter, with the use of a surgical microscope considered a further risk factor for creating an oblique corpectomy trough. Additionally, the presence of a softened lateral cortex due to tumor or infection has been implicated in vertebral artery injury during corpectomy.

Recommended strategies for avoidance of these complications include the use of multiple anatomic landmarks before and during decompression to assure safe resection. Prior to dissecting the longus colli, the midline can be marked using either a marking pen or electrocautery. Ensuring adequate visualization of the uncovertebral joints and planning a resection based on the use of a measuring standard of known width are important steps before beginning a corpectomy. Once the decompression is started, further anatomic clues, such as the lateral curvature of the vertebral body, the location of epidural veins and fat, pedicle palpation, and visualization of the nerve roots, can all serve as verification of orientation.

Lastly, the presence of a tortuous vertebral artery with erosion into the vertebral body can place the artery at risk despite strict adherence to the aforementioned principles. Routine cervical MRI has been shown a reliable imaging modality for evaluation of this condition. Studies have shown, however, that radiology reports of cervical spine MRIs often fail to comment on these and other vertebral artery anomalies and, therefore, all images should be scrutinized by an operating surgeon before any planned corpectomy or diskectomy.

Should vertebral artery injury occur, options for management include tamponade, ligation, embolization, and repair. The therapeutic goals in treatment are 3-fold and progressive: (1) obtaining control of local hemorrhage, (2) prevention of immediate vertebrobasilar ischemia, and (3) prevention of cerebrovascular complications.

Initial tamponade should include the use of large pieces of hemostatic agents combined with pressure from surgical patties; this maneuver should be able to provide temporary hemostatis and can allow for anesthesia staff to obtain further vascular access. The use of particulate materials, such as bone wax, has been discouraged due to the theoretic risk of embolization. Because of the risk of postoperative hemorrhage, delayed embolic complications, and fistula or pseudoaneurysm formation, tamponade alone has largely been abandoned as definitive treatment. Additionally, arterial ligature without prior visualization is not recommended due to the risk of nerve root damage.

Once tamponade has provided some degree of hemostasis, resuscitation by the anesthesia staff should be performed before exposure of the vertebral artery for repair or ligation. Blood loss before obtaining temporary control is likely considerable, with reports ranging from 2300 mL to 4500 mL. Exposure of the artery for repair or ligation is obtained by carrying dissection of the longus colli out further laterally over the transverse processes cephalad and caudad to the site of injury. If the injury occurs ipsilateral to the side of exposure, this can be facilitated by various maneuvers. These include partial or complete transection of the sternocleidomastoid at the level of arterial injury, distal release of the sternocleidomastoid from its insertion site, and mobilization and retraction of the carotid sheath.

Once exposed, transverse foramen can then be opened anteriorly by using either a high-speed burr or Kerrison rongeur. Additionally, the intertransversarii muscles covering the artery between the bones should be resected for improved exposure. Clamps can be applied to the artery at this point for temporary control. Patency of the circle of Willis and adequate collateral circulation can be verified by noting continued bleeding through the site of arterial injury with maintenance of proximal clamping and simultaneous release of the distal clamp. Surgical repair with the use of a 7-0 or 8-0 polypropylene suture has been recommended as the treatment of choice if possible, particularly when collateral circulation is not patent. Ligation, however, remains an option in patients with adequate collaterals.

The decision to ligate an injured vertebral artery is not without consequence. Although the vast majority of patients can tolerate unilateral vertebral artery ligation, in others it can lead to cerebellar or brainstem infarction. Patients with absence of a contralateral vertebral artery, a stenotic or hypoplastic contralateral vertebral artery, or inadequate collateralization at the circle of Willis are at risk of grave neurologic compromise with vertebral artery ligation. The reported incidence of left vertebral artery hypoplasia is 5.7% and the reported incidence of total absence of the left vertebral artery is 1.8%; these rates are 8.8% and 3.1% on the right. In patients without these anomalies, collateral flow can be compromised by atherosclerotic disease. Overall mortality with unilateral vertebral artery ligation has been reported as high as 12%. Other neurologic complications, such as Wallenberg syndrome, cerebellar infarction, isolated cranial nerve paresis, quadriparesis, and hemiplegia, have also been reported.

For these reasons, as well as the technical difficulty associated with open repair or ligation, angiography and coiling have been proposed as other treatment options, both acutely at the time of injury or with manifestation of late complications, such as pseudoaneurysm. With angiography, the patency of collateral circulation can be definitively confirmed before embolization. This treatment option is dependent on the skill and availability of interventional providers at the time of injury, however, and only remains viable if patent collaterals exist.

In the few reported cases of vertebral artery injury during anterior spine surgery, a wide variety of outcomes exist. These vary from no significant neurologic or non-neurologic complications to cerebellar infarction to intraoperative exsanguination and death. In cases where a successful arterial repair was performed, none reports any long term neurologic or non-neurologic complications, making this the treatment of choice should injury occur.