Vertebrobasilar System

The vertebrobasilar system is so named because it is formed by the distal segments of the vertebral arteries as they join to form the basilar artery. This portion of the cerebrovascular system is the primary source of blood supply to the brainstem.

Vertebral Artery

The vertebral artery is divided into four segments (V₁ to V₄) on the basis of its anatomic relationships. Segment V₁ is that portion located between its origin from the subclavian artery and the point where this artery enters the transverse foramen of the sixth cervical vertebra. The second segment, V₂, is that portion of the vertebral artery that ascends through the transverse foramina of C₆ to C₂. The third segment, V₃, is located between the exit of the artery from the C₂ foramen and the point where it penetrates the atlantooccipital membrane; this includes the loop that passes through the transverse foramen of C₁ and arches caudally and medially around the lateral mass of the atlas. The V₄ segment passes through the dura, is located within the cranial cavity internal to the atlantooccipital membrane and dura, and will join its counterpart to form the basilar artery.

This circuitous segment of V₄ is vulnerable to injury. For example, hyperextension of the head may compress the vertebral artery between the occipital bone and the posterior arch of the atlas, and extreme rotation of the head may put torsion on this artery and restrict blood flow. The deficits seen in these patients fall under a general classification referred to as vertebrobasilar insufficiency. Once it is inside the subarachnoid space, the vertebral artery is located in the lateral cerebellomedullary cistern.

Branches of the vertebral artery supply the medulla, parts of the cerebellum, and the dura of the posterior fossa. Its first major branch, the posterior inferior cerebellar artery (PICA), arches around the posterolateral medulla and sends branches to this part of the brainstem. Posteriorly the PICA is located in the cisterna magna (dorsal cerebellomedullary cistern). It serves the choroid plexus of the fourth ventricle and then branches over medial parts of the inferior cerebellar surface. In about 75% of brains, the posterior spinal artery is a branch of the PICA; in the other 25%, it arises from the vertebral artery. The posterior spinal artery serves dorsolateral regions of the medulla caudal to the area served by the PICA. The vertebral artery supplies the anterolateral medulla and, just before joining its counterpart on the opposite side, gives rise to the anterior spinal artery. This vessel usually (85% of cases) originates as two small trunks, which join to form a single artery that courses caudally in the ventral median fissure of the medulla and continues into the spinal cord. The anterior spinal artery is found in the premedullary cistern.

Aneurysms of the vertebral artery or its major branches are not common. When present, they are usually found where the PICA branches from the vertebral artery.

Basilar Artery

The basilar artery lies in a shallow depression, the basilar sulcus, on the anterior surface of the pons in the prepontine cistern. Its first large branch, the anterior inferior cerebellar artery (AICA), generally arises from the lower third of the basilar artery and passes through the cerebellopontine cistern as it wraps around the caudal aspect of the middle cerebellar peduncle. The AICA serves ventral and lateral surfaces of the cerebellum, parts of the pons, and the portion of choroid plexus that extends out of the foramen of Luschka into the cerebellopontine angle. The labyrinthine artery is usually a branch of the AICA. It arises close to the origins of the facial and vestibulocochlear nerves and enters the internal acoustic meatus along with these nerves.

The basilar artery gives rise to numerous pontine arteries. These arteries may penetrate the pons immediately as paramedian branches, travel for a short distance around the pons as short circumferential branches, or pass for longer distances as long circumferential branches.

The last major branches of the basilar artery are the superior cerebellar arteries. Just distal to their origin, each superior cerebellar artery divides into medial and lateral branches, which serve their respective regions of the superior surface of the cerebellum and most of the cerebellar nuclei. These vessels pass laterally just caudal to the root of the oculomotor nerve and wrap around the brainstem in the ambient cistern to ultimately serve caudal parts of the midbrain and the entire superior surface of the cerebellum.

Intracranial aneurysms on the vertebrobasilar system are frequently found in relation to the basilar bifurcation and may therefore involve the oculomotor nerve. In similar fashion, an aneurysm of the AICA may produce symptoms of facial or vestibulocochlear nerve involvement because this vessel travels adjacent to these nerves.