Arteries of the Brain - Posterior Circulation
About, fifteen percent (15%) of the daily cardiac output is utilized by the brain. Owing to the high oxygen and nutrient demand of the organ, it is supplied by two arterial systems. The anterior blood supply arising from the internal carotid arteries is discussed in another article.
This one addresses the posterior arterial blood supply to the brain, which is derived from the vertebrobasilar system. The embryological development and pathway of major branches of the posterior circulation will be discussed. Additionally, the major point of communication between the anterior and posterior circulation (the circle of Willis) will also be addressed. Clinically significant points relevant to this part of the circulation will also be covered.
Embryology of the Posterior Circulation
The posterior circulation is formed in a distal to proximal fashion (i.e. from posterior communicating and posterior cerebral → basilar → vertebral). This process is initiated by the growing brain stem and occipital lobe. Within the 4th gestational week, the superior cerebellar artery perfuses the primitive cerebellum without any assistance. The foetal posterior cerebral artery will become the posterior communicating artery. It fuses with the adult posterior cerebral artery, which is formed from the fusion of nearby adjacent embryonic vessels.
Two parallel neural channels form carotid-vertebrobasilar anastomoses via the hypoglossal, otic, proatlantal, and the trigeminal arteries. In the 5th gestational week, the neural channels fuse to form the proximal portion of basilar artery. The distal part of the basilar artery is formed from the fusion of the posterior communicating and the posterior cerebral arteries. The aforementioned hypoglossal, otic, and trigeminal arteries break down after the posterior communicating artery initiates contact with the distal basilar artery.
During the 5th gestational week, the intersegmental arteries extending from the proatlantal artery to the 6th intersegmental artery fuse to form the vertebral artery. The sixth intersegmental artery merges with the subclavian artery to form the origin of the adult vertebral artery. The proatlantal artery is the most caudal of the pre-segmental arteries mentioned earlier. It persists longer than the others and is later incorporated into the distal parts of the occipital and vertebral arteries.
Posterior Blood Supply
Vertebral Artery & Branches
The subclavian artery is divided into three parts based on anatomical landmarks. The first part extends from its origin to the medial border of scalenus anterior. The vertebral artery originates from this part of the vessel and travels superiorly toward the transverse foramen of the 6th cervical vertebra.
After entering the transverse foramen, it continues superiorly within the five preceding foramina. The vessel has numerous anastomoses with branches of the deep cervical arteries (from the costocervical trunk) and ascending cervical arteries (from the thyrocervical trunk). The vertebral arteries gain access to the cranial vault via the foramen magnum, anterolateral to the brainstem. Each vertebral artery:
- Gives off a posterior inferior cerebellar artery
- Contributes to the formation of the anterior spinal artery via tributaries that converge in the midline anterior to the medulla oblongata
- Contributes meningeal branches near the foramen magnum that supplies the falx cerebelli and the surrounding bone
- May give off the posterior spinal artery; although this vessel usually arises from the posterior inferior cerebellar artery
- Gives off medullary arteries that perfuse the medulla oblongata
Basilar Artery & Branches
There is convergence of the vertebral vessels in the midline at the pontomedullary junction, where they form the basilar artery. This is a relatively large vessel that ascends along the ventral surface of the pons, in its basilar groove, within the pontine cistern. The vessel terminates as it bifurcates into two posterior cerebral arteries.
The basilar artery has the following branches:
- The anterior inferior cerebellar artery arises from the proximal part of the basilar artery. With CN VI, CN VII, and CN VIII travelling dorsal to it, the vessel takes a posterolateral course as it goes to supply the inferior aspect of the cerebellum. It also anastomoses with the posterior inferior cerebellar branch of the vertebral artery.
- Occasionally, a slender internal auditory (labyrinthine) artery arises from the basilar artery. In some instances, this vessel also originates from the superior cerebellar, posterior inferior cerebellar, or anterior inferior cerebellar arteries. Along with CN VII (facial nerve) and CN VIII (vestibulocochlear nerve), the artery traverses the internal acoustic meatus to supply the inner ear .
- At the distal aspect of the basilar artery (prior to its bifurcation), the superior cerebellar artery branches off in a lateral direction, caudal to CN III. It courses around the pedunculi cerebri (cerebral peduncles). It passes below CN IV to access the superior aspect of the cerebellum, which it supplies, along with the tela choroidea of the 3rd ventricle, the pineal body, pons, and superior medullary velum. It also forms an anastomosis with derivatives of the inferior cerebellar arteries.
- The basilar artery also gives off numerous pontine arteries from its lateral surface (bilaterally) as well as posteromedial (paramedian) arteries from the distal bifurcation.
The basilar artery bifurcates to form the paired posterior cerebral arteries. It travels superior to the smaller superior cerebellar artery, and is separated from the same by CN III. The artery continues in a course lateral to the midbrain (adjacent to CN IV). Along its course, it joins with the posterior communicating artery to complete the circle of Willis.
Subsequently, it courses around the pedunculi cerebri toward the tentorial aspect of the cerebrum. Here, it supplies the occipital and temporal lobes. Like the middle and anterior cerebral arteries, the posterior cerebral artery has cortical and central branches. Posteromedial central arteries arising from the proximal posterior cerebral artery and the posterior communicating artery enter the substance of the brain via the posterior perforated substance. They go on to perfuse the lateral wall of the third ventricle, globus pallidus, along with the anterior thalamus and its associated subthalamus. The lateral geniculate body and the posterior portion of the inferior horn of the lateral ventricle are supplied by the posterior choroidal segments of the posterior cerebral artery. The fornix, along with the choroid plexus of the lateral and third ventricles, is also supplied indirectly by the posterior communicating artery.
The temporal cortical artery supplies the parahippocampal, uncus, along with the medial and lateral occipitotemporal gyri. The occipital cortical arteries perfuse the posterolateral region of the occipital lobe along with the cuneus and lingual gyrus. Finally, the parieto-occipital branches bring oxygenated blood to the precuneus and cuneus. Collectively, the artery supplies the visual pathway.
It should be noted that an alphanumeric nomenclature exists for the posterior cerebral artery. From the origin of the vessel at the basilar bifurcation to the junction with the posterior communicating artery is known as P1. The P2 segment extends from the junction with the posterior communicating artery to the segment in the perimesencephalic cistern. The P3 segment continues in the calcarine fissure.
Circle of Willis
There is a major communication between the dual supply to the brain. The anterior communicating artery provides communication between the contralateral anterior cerebral arteries. The posterior communicating artery bridges each ipsilateral internal carotid artery with the posterior cerebral artery. The completed structure is known as the circle of Willis.
It surrounds the optic chiasm and infundibulum, as it rests within the interpeduncular cistern. The circular anastomosis was initially believed to provide alternative route for blood flow in the event of vascular occlusion. While that theory still stands, there are additional concepts that suggest the vascular bed may also serve as a pressure relief system to accommodate increased blood flow in instances of raised intracranial pressure.
- The brain is supplied anteriorly by the internal carotid artery and posteriorly by the vertebrobasilar system.
- Formation of the arterial supply to the brain spans the 3rd to 7th gestational weeks and involves both vasculogenic and angiogenic processes.
- The circle of Willis is believed to function as a pressure relief system that can accommodate additional blood flow should cerebral perfusion be increased.
- The posterior cerebral arteries have been divided into three segments for clinical purposes.
Like in the anterior circulation, the vascular network of the posterior circulation is also susceptible to ischaemic and haemorrhagic insults that can result in a cerebrovascular accident (stroke). Posterior circulation infarcts are characterized clinically by:
- bilateral motor and sensory decline
- cerebellar dysfunction
- ipsilateral cranial nerve palsy
- disordered conjugate gaze
It is more difficult to isolate the specific vascular lesion associated with the posterior circulation because of the wide anatomical variety.