Vertebral Artery Segments
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Direct answer
The vertebral artery is the first branch of the subclavian artery, and its course divides into four segments: V1 (pre-foraminal) from the subclavian to the C6 foramen transversarium, V2 (foraminal) ascending through the foramina transversaria of C6 to C1, V3 (extradural) looping behind the lateral mass of the atlas in the suboccipital triangle to pierce the dura, and V4 (intradural) running up ventrolateral to the medulla to join its fellow at the lower border of the pons, forming the basilar artery. Its branches — posterior inferior cerebellar artery, anterior and posterior spinal arteries and meningeal branches — supply the lateral medulla, the cerebellum and the spinal cord; V4 disease or PICA occlusion produces the lateral medullary (Wallenberg) syndrome.
What you must remember
- V1: arises from the first part of the subclavian, ascends behind the common carotid in the scalenovertebral angle to enter the C6 foramen transversarium — the C7 foramen transversarium exists but transmits only an accessory vertebral vein, an exam favourite.
- V2: climbs through the foramina transversaria of C6 up to C1 accompanied by a venous plexus and sympathetic fibres from the cervicothoracic (stellate) ganglion — the reason cervical spine injury and dissection carry Horner syndrome.
- V3: exits the C1 (atlas) foramen transversarium, turns medially behind the lateral mass in a groove on the posterior arch of the atlas (with the C1 nerve), across the suboccipital triangle, and pierces the dura at the foramen magnum region.
- V4: intradural, ascends along the ventrolateral surface of the medulla in close relation to the hypoglossal rootlets, and joins the opposite vertebral at the lower border of the pons to form the basilar artery.
- Branches: anterior spinal artery (one vessel formed by both vertebrals, supplying the anterior two-thirds of the cord), posterior spinal arteries (often from PICA), PICA — the largest branch, supplying the cerebellum and choroid plexus of the fourth ventricle — plus meningeal and medullary branches.
- Wallenberg syndrome: occlusion of PICA or the vertebral — ipsilateral facial pain and temperature loss (spinal trigeminal nucleus), contralateral body loss (lateral spinothalamic tract), dysphagia and dysarthria (nucleus ambiguus), vertigo and nystagmus (vestibular nuclei), ipsilateral Horner syndrome (descending sympathetic fibres), and ipsilateral limb ataxia (inferior cerebellar peduncle).
- Subclavian steal: stenosis proximal to the vertebral origin (usually left) makes the arm borrow blood retrograde down the vertebral from the basilar — exertional arm fatigue with brainstem symptoms and a blood pressure difference between arms.
- Dissection: neck rotation-manipulation, trauma or spontaneous medial degeneration — sudden neck pain with posterior-circulation symptoms; the V3 atlas loop is the vulnerable segment.
Walking through a lateral medullary stroke
An elderly hypertensive develops sudden vertigo, vomiting, a hoarse voice and a clumsy left side; examination shows loss of pain and temperature on the right body but the left face, left Horner syndrome and left-sided limb ataxia. Every finding belongs to a thumbnail of anatomy at the dorsolateral medulla supplied by PICA or its parent vertebral artery. The crossed sensory pattern is the signature: the spinal trigeminal nucleus (facial sensation) has been hit on the left, and the lateral spinothalamic tract carrying the already-crossed body fibres has been hit on the left, so the body loss appears contralaterally. The nucleus ambiguus injury explains the hoarseness, dysphagia and absent left gag; the vestibular nuclei connection explains the vertigo and nystagmus; the descending sympathetic tract explains the ipsilateral Horner syndrome; and the inferior cerebellar peduncle explains the ipsilateral ataxia. Hiccups (medullary respiratory centres) complete the classical list. Because the pyramid is medial and untouched, strength is preserved — a stroke with ataxia and sensory loss but no weakness, the detail that distinguishes it from a large middle cerebral event at the bedside.
Where students slip
The C6 entry level is the perennial MCQ — the artery enters the sixth foramen transversarium in about nine out of ten people (occasionally C7, C5 or higher), while the C7 foramen carries only the accessory vein. The second slip is the PICA territory description "cerebellum only" — its medullary branches and the lateral medulla are the clinically decisive territory, and the syndrome it produces is named Wallenberg's. Third, remember the two vertebral arteries are usually unequal (dominant, commonly the left), which is why vertebrobasilar symptoms often follow occlusion of a single dominant vessel.
Frequently asked questions
What are the four segments of the vertebral artery and their limits?
V1 from the subclavian origin to the C6 foramen transversarium; V2 within the foramina transversaria of C6 to C1; V3 from the C1 foramen across the suboccipital triangle to the dura; V4 intradural, from the dural pierce to the union with the opposite vertebral at the lower border of the pons.
At which level does the vertebral artery enter the foramen transversarium, and what does C7 transmit?
Usually at C6 — the artery skips the C7 foramen transversarium, which transmits an accessory vertebral vein (and sometimes sympathetic fibres). Variations at C7, C5 or C4 occur.
Which artery supplies the lateral medulla, and what is Wallenberg syndrome?
The posterior inferior cerebellar artery or its parent vertebral artery. Wallenberg (lateral medullary) syndrome comprises ipsilateral facial pain-temperature loss, contralateral body loss, dysphagia and hoarseness, vertigo, ipsilateral Horner syndrome and limb ataxia, with strength spared.
What is subclavian steal syndrome?
Stenosis of the subclavian artery proximal to the vertebral origin, so exercising the arm siphons blood retrograde down the ipsilateral vertebral artery from the basilar. The result is exertional arm fatigue with brainstem symptoms, and the arms often show unequal blood pressures.
How do the anterior and posterior spinal arteries arise?
The single anterior spinal artery forms from the union of a branch from each vertebral artery and supplies the anterior two-thirds of the spinal cord. The paired posterior spinal arteries usually arise from the PICA or vertebral arteries and supply the posterior one-third.