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The vertebrobasilar (VB) system, comprised of the vertebral and basilar arteries, serves as a critical arterial supply to the cervical spinal cord brainstem, cerebellum, thalamus, and occipital lobes. Disruptions of VB circulation can have devastating neurologic consequences; thus, a thorough understanding of the anatomy and clinical significance of the VB system is critical for assessing neurologic syndromes and preoperative neurosurgical planning.
Structure and Function of vertebrobasilar (VB) system
The vertebrobasilar system is comprised of bilateral vertebral arteries (VA) and an unpaired basilar artery (BA). The vertebral artery divides into four segments along its course.[rx] The pre-foraminal or V1 segment arises as to the first branch of the subclavian artery, superior to the first rib, and courses posteriorly between the anterior scalene and longus colli muscles. The VA then enters the transverse foramen (also known as the foramina transversarium, TF) of the sixth cervical vertebrae (C6),[rx] alongside the descending vertebral venous plexus and accompanying sympathetic plexus as the foraminal or V2 segment. As the VA ascends through the cervical vertebrae, VA branches at each cervical level supply the surrounding musculature via the anterior spinal arteries. After exiting the TF of the axis, the atlanto or V3 segment of the VA courses laterally to traverse the TF of C1. The VA then travels over the posterior arch of the atlas and through the suboccipital triangle to enter the intracranial space through the foramen magnum as the intracranial, intradural, or V4 segment. Upon entering the cranium, the VA branches off the posterior inferior cerebellar artery before it unites with the contralateral VA at the base of the pons to form the BA.
The basilar artery serves as the primary source of arterial supply to the brainstem and posterior cerebral hemispheres. The BA courses anterosuperiorly within the basilar sulcus of the pons, giving off bilateral anterior inferior cerebellar arteries, multiple paramedian perforating pontine arteries, and the paired superior cerebellar arteries. As the BA approaches the base of the pituitary gland, it divides to give rise to the bilateral posterior cerebral arteries and posterior communicating arteries to complete the circle of Willis. The connection of the BA to the circle of Willis allows for the collateral pathway of the BA to supply anterior brain structures if flow through one of the internal carotid arteries (ICA) is compromised.
Blood Supply and Lymphatics of vertebrobasilar (VB) system
The VB system provides circulation to the cervical spinal cord, cerebellum, brainstem, and posterior cerebrum via its major branches:
- Vertebral arteries
- Anterior spinal arteries supply the cervical spinal cord
- Muscular branches supply the deep cervical musculature
- Posterior inferior cerebellar arteries supply the cerebellum and the choroid plexus of the fourth ventricle
- Basilar artery
- Anterior inferior cerebellar arteries supply the inferolateral pons, anteroinferior cerebellum, and the middle cerebellar peduncle
- Paramedian arteries supply the thalamus and pons
- Superior cerebellar arteries supply the superior cerebellum and parts of the midbrain
Venous drainage from the territory supplied by the VB system is mainly via the vertebral venous plexus surrounding the vertebral arteries within the transverse foramina and the dural venous sinuses, which drain into the internal jugular veins.
The vertebrobasilar system has an intimate relationship with many nerves along its course. The V1 segment of the vertebral artery courses anteriorly to the inferior cervical ganglion and the sympathetic trunk at the level of C7. As V2 ascends within the transverse foramina, it is surrounded by branches of the sympathetic plexus. As V3 crosses the posterior arch of C1, it passes laterally to the suboccipital nerve. After entering the cranium, V4 courses along the medulla oblongata between the anterior nerve root of C1 and the hypoglossal nerve. The basilar artery ascends along the anteroinferior surface of the pons and terminally gives off the superior cerebellar artery and posterior cerebral artery with the sixth cranial nerve traversing between them.