Proximal Portion

This portion runs from the origin of the artery to its point of entry to the cervical spine. The VA usually originates from the posterior surface of the subclavian artery, but it can also originate from the aortic arch and common carotid artery.⁵

The VA runs vertically, slightly medial, and posteriorly lateral to the longus colli and medial to the anterior scalene muscles to reach the transverse foramen of the lower cervical spine, although its exact direction is dependent on its exact point of origin. The artery’s anomalous origin in this region has been suggested as a potential factor increasing the chance of blood flow compromise due to compression by the longus colli or scalene muscles.

In approximately 88% of individuals, the artery enters the transverse foramen of C6, but it has been shown to enter as far superior as the transverse foramen of C4.⁶

Transverse Portion

The second portion of the VA runs from the point of entry at the spinal column to the transverse foramen of C2 (Fig. 24-2). As already described, the origin of this part of the artery is typically at the C6 level, but this may vary between individuals and even from side to side in the same individual.

Throughout this section of the spinal column, the artery travels vertically in a true canal called the transverse canal (TC). The TC is formed by the bony transverse foramina at each spinal level and the overlying anterior and posterior intertransverse muscles, the scaleni, and the longus colli muscle. The dimension of the TC is proportionate to the diameter of the artery which itself is variable. Within the TC, the artery is surrounded by a periosteal sheath that is adherent to the boundaries of the canal and affords protection for the artery. However, the artery is in close proximity to the uncinate processes of each vertebral body on its medial aspect and is prone to compression from osteophyte formation or subluxation from the zygapophyseal joint.^4,7 The VA in the transverse foramen is also adjacent to the anterior spinal roots. Arterial enlargement by an intramural hematoma or a dissecting aneurysm may cause radiculopathy by compressing or stretching the spinal root.⁸

Anomalies in this portion of the artery, which can include looping within the intervertebral foramina, can occur because of an abnormal origin of the artery from the aorta.⁹

The proximal and suboccipital portions of the artery are more elastic and less muscular than the transverse part.¹⁰ This variation is believed to be an adaptation to the greater mobility required in the proximal and transverse portions of the artery.

Suboccipital Portion

This portion of the artery extends from its exit at the axis (C2) to its point of penetration into the spinal canal. This portion can be further subdivided into four parts:

1. Within the transverse foramen of C2 (see Fig. 24-2). This portion lies in a complete bony canal formed by the two curves of the transverse foramen of C2. The inferior curve is almost vertical, whereas the superior curve is more horizontal and orientated laterally.
   
2. Between C2 and C1 (see Fig. 24-2). The second part runs vertically upward to the transverse foramen of C2. Throughout its journey, it is covered by the intertransversarii, the levator scapulae muscle, and the inferior oblique capitis muscle. Compression of the VA may occur in conditions in which these muscles have increased tone or loss of flexibility.¹¹ It is also at this level that the VA is subjected to the most mechanical stress, as more than 50% of cervical spine rotation occurs here.
   
3. In the transverse foramen of C1 (see Fig. 24-2). In its third part, the suboccipital portion of the VA bends posteriorly and medially in the transverse foramen of C1 in which it is completely enclosed.
   
4. Between the posterior arch of the atlas and its entry into the foramen magnum (see Fig. 24-2). On exiting from the transverse foramen of C1, the artery and the nerve of C1 wind behind the mass of the superior articular process of the atlas (C1) to cross the posterior arch of the atlas in a groove in which the artery is held by a restraining ligament. Anomalies in this portion of the artery include ossification of the restraining ligament of the atlantal groove, turning this into a complete bony tunnel. From the medial end of this groove, the artery runs forward, inward, and upward to pierce the posterior atlantooccipital membrane. The artery penetrates the dura mater on the lateral aspect of the foramen magnum about 1.5 cm lateral to the midline of the neck. This upper portion of the extracranial VA is relatively superficial and covered only by the trapezius, semispinalis capitis, and rectus capitis muscles. Having unyielding bone beneath it, and only muscles above, the artery is vulnerable to direct blunt trauma at this part of its course, whereas in the remainder of its course, it is threatened more by penetrating trauma or disease processes such as osteophytosis or atherosclerosis.^12,13

Although the artery is affected by cervical motion in the lower cervical region, it is affected even more between C2 and the occipital bone.¹⁴

In addition, because the transverse foramen of C1 is more lateral than that of C2, the artery must incline laterally between the two vertebrae. At this point, the artery is vulnerable to impingement from the following:

Intracranial Portion

This portion of the VA that runs from its penetration of the dura mater into the arachnoid space at the level of the foramen magnum to the formation of the basilar artery by the midline union of the two arteries at the lower border of the pons (Fig. 24-1).

After its penetration of the cranium, the VA inclines medially toward the medulla oblongata. It courses up the front of the medulla to reach the lower border of the pons, where the artery from each side unites with its partner to form the basilar artery. A major change in the structure of the VA occurs as it becomes intracranial.⁴ The tunica adventitia and tunica media become thinner, and there is a gross reduction in the number of elastic fibers.¹⁰ This decrease in elasticity can result in the distortion of the VA during cervical extension and rotation.^14,16,19

Branches

Intracranially, the VA first generates small meningeal branches that supply the bone and dura mater of the cerebellar fossa. It is possible that ischemia of these tissues, which occurs with VA occlusion, could be responsible for the suboccipital pain that often accompanies damage to the artery.¹⁵

Spinal Branches⁸

Each of the vertebral arteries gives off single branches (Fig. 24-1), which fuse to form the anterior spinal artery (ASA) that descends in the median anterior fissure and is additionally supplied by anterior radicular arteries (typically two to four). While the ASA is bilaterally derived, the anterior radicular arteries may arise exclusively or predominately from one VA. This helps to explain how a unilateral insufficiency of the VA may cause bilateral spinal cord infarction.

The paired posterior spinal arteries (PSAs) originate superiorly from the vertebral arteries or posterior inferior cerebellar arteries (PICAs). The PSAs are also supplied by posterior radicular arteries (usually two or three), which themselves arise exclusively or predominately from one VA, again demonstrating how a unilateral VA dissection can lead to bilateral spinal cord infarction. The PSAs supply the posterior one-fifth to one-quarter of the spinal cord including the posterior columns, the posterior (dorsal) horns, and parts of the corticospinal and spinothalamic tracts. The ASA is the exclusive arterial supply to the gray matter of the cord. The watershed area between the PSA and ASA encompasses the anterior (ventral) horns and part of the corticospinal and spinothalamic tracts.

Muscular Branches

The muscular branches arise from the suboccipital part of the artery as it winds around the superior articular process of the atlas. They supply the deep suboccipital muscles and anastomose with the occipital and cervical arteries.

Posterior Inferior Cerebellar Artery

The PICA (see Fig. 24-1) is the largest branch of the VA, usually being formed opposite to the medulla oblongata about 1 cm below the formation of the basilar artery. It supplies, either directly or indirectly, the medulla and the cerebellum and, via the PSAs, the posterior (dorsal) portion of the spinal cord.

Basilar Artery

The formation of the basilar artery at the lower border of the pons marks the termination of the VA. The basilar artery runs in a fissure on the anterior surface of the pons (see Fig. 24-1). Directly and indirectly, the basilar artery supplies the pons, the visual area of the occipital lobe, the membranous labyrinth, the medulla, the temporal lobe, the posterior thalamus, and the cerebellum.

Collateral Circulation

Fortunately, there is an inherent redundancy in the vascular supply to the brain. The posterior and anterior vessels, and the vessels stemming from the internal carotid arteries form an anastomotic network via the circle of Willis. Thus, occlusion of the left VA may be compensated for by perfusion of the right VA, the occipital artery, the ascending and deep cervical arteries, and the internal carotid arteries.^21,22 In fact, there is evidence that suggests that blood flow velocity and blood flow volume can increase in the arteries to compensate for occlusion in another vessel.²³ It is worth considering that palpation of the carotid pulse would give the clinician information regarding the ability of the carotid to perform its normal function, and if the pulse appears excessively strong, this could be an indication that the carotid artery is compensating for loss of VA flow.^24,25