Course
The vertebral artery is divided into four parts, often labelled V1 to V4, according to its relationship to the cervical spine.
| Part | Extent | Key features |
|---|---|---|
| First (V1, pre-foraminal) | Origin to the transverse foramen of C6 | Ascends between longus colli and scalenus anterior, behind the common carotid artery and vertebral vein |
| Second (V2, foraminal) | Transverse foramina of C6 to C1 | Surrounded by a venous plexus and sympathetic fibres; lies in front of the cervical nerve roots |
| Third (V3, suboccipital) | Transverse foramen of C1 to the dura | Curves back behind the lateral mass of the atlas and lies in a groove on its posterior arch, in the suboccipital triangle |
| Fourth (V4, intracranial) | Dura to the lower border of the pons | Pierces the dura and arachnoid, enters through the foramen magnum and winds forwards round the medulla |
First part
The first part lies in the triangle of the vertebral artery at the root of the neck. The triangle is bounded medially by longus colli, laterally by scalenus anterior, with its apex at the anterior tubercle of the C6 transverse process (the carotid tubercle) and its base formed by the first part of the subclavian artery. The inferior thyroid artery crosses in front of the vertebral artery here, and on the left the thoracic duct also crosses in front.
Second and third parts
The artery usually enters the transverse foramen of C6, not C7; the C7 foramen normally transmits only the vertebral vein. In the axis it turns laterally, then ascends to the wider-set atlas foramen. After leaving the atlas it runs medially across the posterior arch, with the suboccipital nerve (C1 posterior ramus) between the artery and the bone, and pierces the posterior atlanto-occipital membrane.
Fourth part
Inside the skull the artery ascends from the side to the front of the medulla. The two arteries converge and unite at the pontomedullary junction to form the basilar artery, which runs up the front of the pons.
Branches
The vertebral artery has few branches in the neck and its important branches inside the skull.
- Spinal (radicular) branches enter the intervertebral foramina to supply the spinal cord, meninges and vertebrae.
- Muscular branches from the third part supply the deep suboccipital muscles and anastomose with the occipital and deep cervical arteries.
- Meningeal branches supply the dura of the posterior cranial fossa.
- Posterior spinal artery, often arising from PICA instead, descends on the back of the spinal cord.
- Anterior spinal artery: a branch from each side joins its partner to form a single midline artery on the front of the cord.
- Posterior inferior cerebellar artery (PICA), the largest branch, winds round the medulla to supply its lateral part, the inferior surface of the cerebellum and the choroid plexus of the fourth ventricle.
- Medullary branches supply the medulla directly.
The basilar artery continues the supply to the pons, cerebellum, inner ear and midbrain, and ends by dividing into the posterior cerebral arteries, which join the circle of Willis.
Relations
The vertebral artery is well protected by bone for most of its course, but has three vulnerable segments: its origin, its exit from the atlas, and its path across the posterior arch of C1. At the root of the neck it lies deep to the carotid sheath, with the cervicothoracic (stellate) ganglion behind it and the vertebral vein in front.
In the foraminal part, the cervical spinal nerve roots emerge behind the artery. Uncovertebral osteophytes can compress it from the medial side. On the posterior arch of the atlas it lies in the suboccipital triangle, bounded by rectus capitis posterior major and the superior and inferior obliques.
The vertebral vein does not follow the artery into the skull. It begins in the suboccipital region, descends as a plexus in the transverse foramina, and emerges from the C6 or C7 foramen to drain into the brachiocephalic vein.
Clinical relevance
Occlusion or dissection of the vertebral artery causes posterior circulation stroke. Dissection may follow neck trauma, sudden rotation or manipulation, and presents with neck or occipital pain followed by brainstem or cerebellar signs.
Lateral medullary (Wallenberg) syndrome results from occlusion of the vertebral artery or PICA. It produces:
- loss of pain and temperature on the same side of the face and the opposite side of the body;
- ipsilateral Horner syndrome;
- dysphagia and hoarseness from nucleus ambiguus damage;
- vertigo, nystagmus and ipsilateral ataxia.
In subclavian steal syndrome, a stenosis of the subclavian artery proximal to the vertebral origin reverses flow down the vertebral artery when the arm is exercised, taking blood from the posterior circulation and causing dizziness or syncope.
The artery is at risk in upper cervical spine surgery, particularly C1 lateral mass and C1 to C2 screw fixation, and in fractures that cross the transverse foramina. The two arteries are often unequal in size, and the left is more often dominant.