Neuroscience Clerkship at UH/VA
 

Large Vessel Stroke in the Posterior Circulation

A stroke may result from occlusion of a vessel in the anterior or posterior circulation of the brain with varying clinical manifestations. This learning objective first defines the arterial vessels that comprise the posterior circulation of the brain. It then reviews the clinical manifestations of stroke resulting from occlusion of each of its vessels. It is important to contrast these clinical manifestations with those involving strokes of the anterior circulation, as the etiology and treatment may differ. For this information, please refer to the previous learning objective, large vessel strokes of the anterior circulation.


Approximately 20% of ischemic strokes occur in the posterior circulation. A large vessel stroke of the posterior circulation occurs when either vertebral artery, the basilar artery, or the posterior cerebral arteries (PCA) are blocked. Less commonly, the smaller branches of the vertebral and basilar artery including the posterior inferior cerebellar (PICA), anterior inferior cerebellar (AICA) and superior cerebellar arteries (SCA) can be infarcted. Most common among them is the PICA which supplies the lateral medulla and inferior cerebellum. The resulting clinical manifestations, depend on the vascular territory supplied by the blocked artery, as well as the degree of collateral vessels helping to perfuse the given area of brain.

The vertebral arteries originate from the subclavian arteries. They ascend through the foramen transversaria in the vertebral bodies. They then wrap C2, then ascend through the foramen magnum. The PICAs are then given off which supply the dorsolateral medulla and inferior cerebellum. The anterior spinal arteries also originate from the vertebral arteries. The two vertebral arteries then join to form the basilar artery at the ponto-medullary junction. The basilar artery gives off numerous small branches that supply the pons and cerebellum. Two large branches, the SCA and AICA originate from the superior and mid-basilar, respectively. The PCAs are formed as a result of the bifurcation of the distal basilar artery and supply the midbrain, thalamus, medial aspect of the temporal lobe, and occipital lobe. Please review the vasculature of the posterior circulation in the diagrams below.


Medial View of the Brain

Lateral View of the Brain


MRA of the Aortic Arch with Contrast Bolus. (1) Aortic arch; (2) Brachiocephalic artery; (3) Right Subclavian artery; (4) Left Subclavian artery; (5) Left Common Carotid artery; (6) Right Common Carotid artery; (7) Left Vertebral artery; (8) Left Common Carotid artery; (9) Left Carotid Bifurcation; (10) Right Vertebral artery; (11) Right Carotid Bifurcation; (12) Right Internal Carotid artery; (13) Left Internal Carotid Artery; (14) Left External Carotid artery; (15) Right External Carotid artery; (16) Basilar artery; (17) Right Internal Carotid artery (intracranial); (18) Left Internal Carotid artery (intracranial); (19) Top of the Basilar artery; (20) Vertebral arteries; (21) Vertebral arteries

MRA of the Neck - Extracranial Large Vessels.  ECA - External Carotid Artery; ICA - Internal Carotid Artery; CCA - Common Carotid Artery; Vert - Vertebral Artery

Magnetic Resonance Angiography (MRA) Intracranial Study. ACA - anterior cerebral artery; MCA - middle cerebral artery; PCA - posterior cerebral artery; SCA - superior cerebellar artery; AICA - anterior inferior cerebellar artery; PICA - posterior inferior cerebellar artery

Anatomy of the Posterior Circulation: PCA - posterior cerebral artery; SCA - superior cerebellar artery; AICA - anterior inferior cerebellar artery; PICA - posterior inferior cerebellar artery: ASA - anterior spinal artery
The next part of this learning objective will review the major clinical manifestations of different types of large vessel anterior circulation including the anterior cerebral, middle cerebral and internal carotid arteries.

Figure above: Vertebral artery angiogram, lateral view. There is a smooth conical tapered occlusion of the distal vertebral artery typical of a dissection.

 

 

 

 

Vertebral Artery

Stroke

 

The vertebral arteries and their distal branches provide the main blood flow to the brainstem, cerebellum and occipital lobes. A stroke involving one vertebral artery may not be symptomatic due to the collateral contribution from the other vertebral, or may be catastrophic if the other vertebral is absent or hypoplastic. The vertebral artery is also important as the PICA, a branch of the vertebral supplies the posterior inferior cerebellum and lateral medulla.

 

Incidence:

Relatively uncommon

Etiologies:

Intrinsic atherosclerosis. The most common locations are at the origin from the subclavian and the intracranial vertebral.

Embolic source from heart or aorta

Dissection 

Major Signs/Symptoms:
 

If other vertebral is patent, may be asymptomatic, or only have symptoms referable to the ipsilateral PICA, a branch of the vertebral artery. This syndrome is known as the Wallenberg or “lateral medullary” syndrome, and may include the following signs and symptoms:

Vertigo, nausea, vomiting

Ipsilateral limb ataxia (dysmetria, dysdiadochokinesia)

Gait ataxia

Nystagmus

Ipsilateral Horner’s syndrome due to disruption of sympathetic tract: ptosis (droopy eyelid), miosis (constricted pupil), hemianhidrosis (loss of sweating)

Loss of pain and temperature in the ipsilateral face and contralateral body (lateral spinothalalmic tract)

Ipsilateral weakness of the palate, pharynx and vocal cords resulting in dysphagia, dysarthria and dysphonia

Loss of gag reflex

If the opposite vertebral is absent and there is absent collateral blood flow to the brainstem or cerebellum, the result may be catastrophic. Please refer to next section on basilar artery stroke for explanation.


Basilar Artery Stroke

The clinical manifestations of basilar artery stroke vary depending on the degree of occlusion of the vessel, the location of occlusion and the extent of collateral flow. It is not infrequent to have prodromal TIA symptoms, as the mechanism of basilar artery stroke is frequently intrinsic atherosclerotic disease. For a list of TIA symptoms of the vertebrobasilar circulation including basilar artery, please click on the link for learning objective “Significance of a TIA and Differential of a Brief Neurological Spell.”

Incidence:

     Uncommon

Etiologies:

Intrinsic atherosclerotic disease with thrombosis (more likely if proximal or mid-basilar)

Embolic source (more likely if distal basilar)

Extension of vertebral dissection

Major Signs/Symptoms:
 

If a basilar artery stroke occurs, the symptoms may range from mild to catastrophic. In the basis of the pons is affected leaving the tegmentum intact, the “locked-in” syndrome may occur with the following signs:

Tetraplegia (all four limbs, face and bulbar muscles)

Intact level of consciousness and sleep-wake cycles

Impairment of horizontal eye movements

Vertical eye movements and blinking spared
 
Bilateral babinski signs

Thus, the patient has relatively intact perception of the environment, but is unable to communicate except with blinking or vertical eye movements. These deficits occur due to massive infarction of the brainstem including basis pontis (ventral pons) which results in disruption of the corticobulbar and corticospinal tract fibers. The wakefulness of the patient are intact due to sparing of the reticular activating system in the tegmentum. The vertical eye movement and blinking are intact due to sparing of the dorsal midbrain supranuclear ocular pathways.

 

In the case the tegmentum is also affected, the patient will be in coma [from involvement of the ascending reticular activating system (ARAS)]

 

 

 

There may also be signs and symptoms referable to the distal branches of the basilar artery resulting in lateral midbrain, thalamic, occipital and medial temporal lobe infarction:

Memory impairment (medial temporal lobe)

Loss of sensation (thalamus)

Visual loss (occipital lobe)


 

 

 

Posterior Cerebral

Artery

Patients with a stroke of the PCA usually present with visual complaints, however depending on whether the PCA is occluded proximally or distally, and whether occlusion is unilateral or bilateral will determine the clinical manifestations. 
 

Incidence:

Common

Etiologies:

Embolic source (proximal vertebral, aorta or heart)

Rarely, intracranial artherosclerosis


Major Signs/Symptoms:

Single PCA infarction:

Homonymous visual field defect

Scintillations at the edge of hemianopic field

Visual perseverations

Absence of visual neglect (e.g., if a patient is given something to read they will usually read the entire sentence, whereas with right parietal stroke they will ignore the contralateral side
 

      Less Common:

Memory disturbance

Peduncular hallucinosis (visual hallucinations of brightly colored scenes and objects).

Hemiballismus, with subthalamic nucleus involvement

Hemiplegia, with cerebral peduncle involvement

3rd nerve palsy, with midbrain involvement

Alexia without agraphia (dominant lesion)

Transcortical sensory aphasia

Gerstmann's syndrome (R/L disorientation, finger anomia, acalculia, agraphia, constructional apraxia)

 

Bilateral PCA Infarction:

Anton’s syndrome (cortical blindness with denial of symptoms)

The patient may confabulate visual perception

Balint’s syndrome [asimultagnosia (inability to see the whole of an object); optic ataxia (inability to visually guide a hand to its target); gaze apraxia (inability to focus on a desired target)


Proximal PCA Infarction:

If the very proximal PCA is infarcted, the small perforators to the thalamus and cerebral peduncle may be affected, resulting in hemi- or quadriplegia, and a depressed state of consciousness


 

 

 

Posterior Inferior

Cerebellar Artery

 

(PICA)

The PICA is the largest branch originating from the vertebral artery. It wraps around the medulla, supplying the lateral medulla before supplying the inferior cerebellum. Infarction the lateral medulla results in the Wallenberg's syndrome, a classic neurologic condition (scan above).

 

Incidence:

Relatively uncommon

Etiologies:

Same as the vertebral artery:

Intrinsic atherosclerosis. The most common locations are at the origin from the subclavian and the intracranial vertebral.

Embolic source from heart or aorta

Dissection
 

Major Signs/Symptoms:

Vertigo, nausea, vomiting

Ipsilateral limb ataxia (dysmetria, dysdiadochokinesia)

Gait ataxia

Nystagmus

Ipsilateral Horner’s syndrome due to disruption of sympathetic tract: ptosis (droopy eyelid), miosis (constricted pupil), hemianhidrosis (loss of sweating)

Loss of pain and temperature in the ipsilateral face and contralateral body (lateral spinothalalmic tract)

Ipsilateral weakness of the palate, pharynx and vocal cords resulting in dysphagia, dysarthria and dysphonia

Loss of gag reflex


Note: Infarctions of the PICA are sometimes confused clinically with gastroenteritis, because of the prominent nausea and vomiting, especially if a neurologic exam in not performed. If the entire territory of the PICA is affected, infarction of the inferior cerebellum may result in swelling and secondary compression of nearby structures, including the fourth ventricles and brainstem (see below).


If the swelling obstructs the 4th ventricle, acute hydrocephalus can result. If not treated, it can lead to transtentorial herniation and death (see scan below)