Cerebral Hemispheres and Vascular Supply

by

Review of Main Functional Areas of Cerebral Cortex

  • Sensory areas for face and hand are on the lateral convexities while the leg areas are in the interhemispheric fissure
  • Broca’s area in inferior frontal gyrus, just anterior to the articulatory areas of the primary motor cortex
  • Wernicke’s area lies in the superior temporal gyrus, adjacent to the primary auditory cortex
  • Optic radiations pass under the parietal and temporal cortex; thus infarcts in these lobes can cause contralateral visual field deficits

General Information Regarding CVAs

  • CVAs can cause seizures due to scar tissue
  • Depression occurs in approximately 30% of CVA patients – such patients have greater functional disability and subsequent mortality.
    • Left-sided CVAs are most closely associated with depression
  • Although function usually improves over time, patients may continue to show worsening deficits or decreased consciousness over the next few days because of cerebral edema (or less often, from extension of the infarct)
    • Severe cerebral edema can cause potentially fatal shift in intracranial structures
  • Locked-in Syndrome Mute and quadriplegic – but intact cognitive capacity
    • Usually result from infarct of inferior portion of the pons or medulla
    • Can respond by opening and closing eyes
    • Alert with normal cognition and affect
  • Anastomes Connections between different cerebral arteries on the cortical surface. Allows perfusion between arterial systems to limit extent of cortical damage in stroke. While this can protect gray matter, it also creates greater vulnerability in the “border zones” or watershed areas between arteries, as they are the most distal portions of the blood supply. There is little collateral circulation in the white matter

Circle of Willis: Anterior and Posterior Circulations

Circle of Willis

  • An anastomotic ring, from which all major cerebral vessels arise
  • Provides abundant opportunities for collateral flow; however a complete ring is present in only approximately 25% of individuals

Main Arteries of the Circle of Willis

  • Cerebral blood supply provided to circle of Willis by the:
    • Internal Carotid Arteries (paired; see text p. 370 for names of artery sections prior to entering Circle of Willis)
      • Supplies most of the diencephalon and cerebral hemispheres
    • Vertebral Arteries (paired) which fuse into the Basilar Artery
  • Cortex and subcortical structures receive blood through three main artery supplies:
    • Anterior Cerebral Artery Arise from internal carotid arteries
    • Middle Cerebral Artery Arise from internal carotid arteries
    • Posterior Cerebral Artery Arise from top of basilar artery
  • Two anastomes in the Circle of Willis
    • Anterior Communicating Artery (anastomes of the anterior cerebral artery)
    • Posterior Communicating Artery (connect internal carotids to the posterior cerebral arteries)

Anatomy and Vascular Territories of the Three Main Cerebral Arteries

  • The 3 main arteries give rise to numerous branches that travel in the subarachnoid space of the surface of the brain
  • Subcortical areas are supplied by penetrating branches that arise from initial segments of the arteries near the Circle of Willis at the base of the brain

Vascular Supplies of the Superficial Cerebral Structures

(see figure 10.5 for visual depiction of information provided below; ACA and MCA are both involved in the ”anterior circulation”)

Anterior Cerebral Artery

  • Supplies most of cortex on the anterior medial surface of the brain, from the frontal to the anterior parietal lobes, usually including the medial sensorimotor cortex (i.e., the sensory cortex for the lower extremity).
  • Travels in the interhemispheric fissure as it sweeps back over the corpus callosum
  • Two branches: Pericallosal Artery and the Callosomarginal Artery

Middle Cerebral Artery

  • Supplies most of the cortex on the dorsolateral convexity of the brain (involving frontal, parietal and temporal lobes).
  • Passes over the insula, around the operculum and then exits the Sylvian fissure onto the lateral convexity
  • Superior Division Supplies cortex above Sylvian fissure and inferior division supplies the region below the Sylvian fissure.

Posterior Cerebral Artery

  • Supplies inferior and medial temporal and occipital cortex (including the visual cortex).

Vascular Supplies of the Deep Cerebral Structures

(see figures 10.8 and 10.9 pp 374-375 for visual depiction of information provided below)

Middle Cerebral Artery

  • Lenticulostriate Arteries
  • They arise from the initial portions of the MCA, before it enters the Sylvian Fissure and supply:
    • basal ganglia
    • internal capsule
  • In hypertension, these arteries are prone to narrowing, leads to lacunar infarcts or rupture (hemorrhage).

Anterior Cerebral Artery

  • current Artery of Heubner
  • They supply:
    • head of the caudate
    • anterior putamen
    • globus pallidus
    • internal capsule

Posterior Cerebral Artery

  • Thalamoperforator Arteries
  • Thalamus and sometimes posterior limb of the internal capsule

Vertebral/Basilar System

  • Midbrain, cerebellum, and portions of the temporal and occipital lobes
    • CVAs with total occlusion lead to coma or locked-in syndrome
    • Occlusion of a branch can lead to cranial nerve palsy or contralateral hemiparesis

Internal Carotid Artery

  • Anterior Choroidal Artery
  • They supply:
    • globus pallidus
    • putamen
    • thalamus
    • posterior limb of internal capsule

Clinical Syndromes of the Three Main Cerebral Arteries

(see Table 10.1 pp 376 and 377)

Middle Cerebral Artery (MCA)

  • Occlusion of the proximal part of the artery generally results in contralateral hemiparesis, hemi-sensory loss, homonymous hemianopsia, aphasia (w/left hemisphere lesions) and hemi-inattention, anosognosia, apraxia, neglect, dysprosody, etc. (w/right hemisphere lesions)
    • if anterior – Broca’s aphasia
    • if posterior – fluent aphasia
    • may also see transcortical aphasia or global aphasia (if entire MCA watershed area affected)
    • posterior MCA infarcts may also lead to Angular Gyrus syndrome (fluent aphasia, alexia with agraphia, Gerstmann’s syndrome, constructional problems)
  • Infarcts and ischemic events are more common in the MCA than in the ACA or PCA, partly because of their relatively larger territory. Infarcts occur in 3 general areas:
    • Superior Division Supplies cortex above the Sylvian fissure
    • Inferior Division Supplies cortex of lateral temporal and occipital lobes below Sylvian fissure, and part of lateral parietal cortex
    • Deep Territory Includes internal capsule and much of the basal ganglia
    • Proximal MCA occlusions affecting all 3 regions are called: MCA stem infarcts
  • Large MCA infarcts often have a gaze preference towards side of lesion, especially during acute period
  • Lacunes Small deep infarcts involving penetrating branches of the MCA or other vessels

Anterior Cerebral Artery (ACA)

  • Infarcts are uncommon, but typically produce contralateral lower extremity cortical sensory loss and weakness of the upper motor neuron type
  • Damage to supplementary area can lead to “alien hand syndrome” (semiautomatic movements of contralateral arm that aren’t under voluntary control)
  • Can cause frontal lobe dysfunction typically characterized by:
    • impaired judgment
    • flat affect
    • grasp reflex
    • apraxia
    • abulia
    • incontinence
    • pseudobulbar palsy
  • Bilateral occlusion can cause emotional disturbance with apathy, confusion, and occasional mutism, plus spastic paraparesis

Posterior Cerebral Artery (PCA)

  • Typically cause contralateral homonymous hemianopia
  • If smaller penetrating vessels are involved may lead to infarcts in the thalamus or posterior limb of the internal capsule leading to:
    • Contralateral sensory loss
    • Contralateral homonymous hemianopsia
    • Contralateral hemiparesis
    • Thalamic aphasia (if in dominant hemisphere) – so can mimic MCA infarcts
    • If involve left occipital cortex and splenium of corpus callosum  alexia without agraphia

Internal Carotid Artery

  • Occlusion leads to infarction in the central-lateral portion of the cerebral hemisphere
  • Symptoms are identical to those of the middle cerebral artery occlusion except for occasional ocular symptoms ipsilateral to the affected artery

Hemorrhages

  • Most often result from hypertension
  • Tend to affect basal ganglia, thalamus, pons, and cerebellum
  • Typically occur abruptly and, because of increased ICP produce headache, nausea and vomiting
  • Patients usually lose consciousness and have profound neurologic deficits

Subarachnoid Hemorrhage

  • Nontraumatic cause is usually from a ruptured berry aneurysm; may mimic a migraine or a muscle tension headache (often referred to as “the worst headache in my life”)
  • Also can present with vomiting, diplopia, altered state of consciousness (signs of increased ICP)
  • Since bleeding is around brain rather than into a particular region, typically no sensory, motor or visual signs

Cerebellar Hemorrhage

  • Can be readily evacuated and is diagnosed by occipital headaches, gait ataxia, dysarthria and lethargy

Watershed Infarcts

  • Watershed Zones Regions who lie between adjacent cerebral arteries
  • Bilateral watershed infarcts in the ACA-MCA and in the MCA-PCA zones can occur with severe drops in blood pressure and in patients with carotid stenosis
  • Watershed infarcts produce proximal arm and leg weakness (“man in the barrel syndrome)
  • Can also cause transcortical aphasia syndromes in the dominant hemisphere

Transient Ischemic Attack and Other Transient Neurologic Episodes

  • The most common causes include:
    • TIA
    • Migraine
    • Seizures
    • Other non-neurologic conditions such as cardiac arrhythmia or hypoglycemia

Transient Ischemic Attack

  • Neurologic deficit lasting less than 24 hours, caused by temporary brain ischemia
  • Recent research suggests that typical TIA duration is 10 minutes and that those lasting more than 1 hour are usually small infarcts.
  • BUT, despite the appearance of a small infarct on MRI scan, complete functional recovery can sometimes occur within 1 day.
  • Mechanisms can include an embolism which dissolves, in situ thrombus formation or vasospasm leading to temporary narrowing of the vessel.
  • Are typically indicative of underlying atherosclerotic cerebrovascular disease and increased risk of sustaining stroke.
  • May mimic partial seizures, postictal confusion, migraine and metabolic aberrations.
  • Are slightly more common in the vertebral basilar system (w/c includes the posterior cerebral arteries) than in the carotid system (which includes the middle and anterior cerebral arteries).

Transient Loss of Consciousness Without other Focal Features

  • Cardiogenic syncope most common cause
  • Neurologic causes are responsible for less than 10% of cases

Differential Diagnosis of Transient Neurologic Episodes

  • Vascular (TIA, Migraine, AVM)
  • Seizures
  • CSF flow-related (cyst of 3rd ventricle)
  • Genetic
  • Toxic/metabolic (e.g., medication/toxin related; hypoglycemia)
  • Infectious/inflammatory (encephalitis, MS)
  • Movement disorders (chorea, dystonia, tic disorders)
  • Psychogenic

Stroke: Mechanisms and Treatment

  • Stroke refers to both hemorrhagic events and to ischemic infarcts
  • Stroke is typically produced by one of 3 ways:
    • Sudden reduction in blood pressure and blood perfusion
    • Impaired blood supply as a result of occlusion or stenosis
    • Obstruction of a vessel by embolus
  • In addition to focal neurologic deficits, strokes can be associated with headaches or seizures
    • Headaches
      • Most often unilateral (ipsilateral to infarct)
      • More common for posterior than anterior circulation infarcts (often in carotid or vertebral arteries)
    • Hemorrhagic Conversion When ischemic strokes cause blood vessels to become fragile and rupture

Risk Factors

  • Age (incidence rises exponentially over 65 year)
  • Hypertension
  • Cardiac conditions
  • Diabetes
  • Smoking
  • Possibly oral contraceptives
  • Lack of exercise, type A personality, heavy alcohol use and cholesterol high diets are risk factors for coronary artery disease, but not directly for CVAs

Etiologies of Stroke

Most common cause is thrombosis; second causes is emboli originating in a carotid artery and lodging in a cerebral artery

  • Intracerebral hematoma (nontraumatic) – bleeding with blood collection
  • Hypertension – spontaneous bleed secondary to hypertension
  • Arterial Vascular Malformation (AVM) – congenital malformation leading to poor perfusion, weak areas in the vessels that may bleed, or secondary aneurysm at vessel bifurcations or junctures
  • Aneurysm
    • Berry aneurysm, congenital weakness, typically at a bifurcation of blood vessels that balloons over time and can burst resulting in hemorrhagic stroke
    • Ruptured aneurysm can result in severe headache, focal symptoms, possible coma – typically requires emergency surgical clipping of the aneurysm
  • Prematurity can lead to intraventricular hemorrhage (IVH)
  • Neoplasm – tumor growth can disrupt or burst blood vessels

Mechanisms of Ischemic Stroke

Ischemic stroke occurs when there is inadequate blood supply to a region of the brain for enough time to cause infarction (death) of brain tissue

Embolic Infarct

  • A piece of material (usually a blood clot) is formed in one place and then travels through the bloodstream to lodge in and occlude a blood vessel supplying the brain
  • Typically occur more suddenly, with maximal deficits at onset (and are also painful)
  • Treatment involves finding the source so future strokes may be avoided (e.g., heart related disorders or atherosclerotic disease); often involves prescribing anticoagulants (aspirin)
  • Other sources of emboli include (1) air emboli in deep sea divers; (2) septic emboli in bacteria endocartis; (3) fat emboli in trauma to long bones, etc…

Thrombotic Infarcts

  • A blood clot forms locally on the blood vessel wall, usually at the site of an underlying atherosclerotic plaque, causing the vessel to occlude
  • May have more of a stuttering or slow course; also relatively painless
  • Can also lead to the development of emboli
  • May be the result of carotid stenosis (70-99%)

Hypoperfusion with Resultant Ischemia and Hypoxia

  • Leads to borderzone infarcts
  • Certain brain regions are particularly vulnerable to ischemic-anoxic insults including the hippocampus, cerebellum and cerebral cortex
  • The brain regions with the most marginal vascular supply, the border zones are most often affected
  • In large vessel infarcts (involve the surface of the brain) – infarcts most often due to emboli, although thrombosis can occur occasionally, especially in large proximal vessels such as the vertebral, basilar, and carotid arteries

Small Vessel Infarcts

  • Involve vessels that feed the deep structures/subcortical structures and brainstem).
  • Also called lacunar infarcts because they resemble small lakes when brain is examined on pathologic sections.
  • Often associated with small-vessel disease caused by chronic hypertension, and generally affect the deep white matter and nuclei of the cerebral hemispheres and brainstem
  • Common lacunar syndromes for your notes (see page 382, Table 10.3)

Cortical Versus Subcortical Lesions

  • Can sometimes be differentiated based on the presence or absence of cortical signs, such as:
    • Aphasia
    • Neglect
    • Homonymous visual field deficits
    • Cortical sensory loss
  • BUT, each of these can be seen in some cases of subcortical lesions as well

Stroke Risk Factors

  • Hypertension, diabetes, high cholesterol, cigarette smoking, family history and prior history of stroke or other vascular disease.
  • Migraines can also be a risk factor for stroke in young adults, as is sickle-cell disease, cardiac disease and vasculitis

Treatment and Diagnostic Workup of Ischemic Stroke and TIA

  • CT Scans Remember that an infarct will often not be visible on the initial scan, especially if it is done within a few hours – BUT a hemorrhage will almost always be visible
  • Once a hemorrhage has been ruled out by CT, many physicians treat with thrombolytic agents or heparin
  • See page 384 for a more extensive treatment plan

Carotid Stenosis

  • Atherosclerotic disease commonly leads to stenosis of the internal carotid artery just beyond the carotid bifurcation.
  • Thrombi formed can embolize distally giving rise to TIAs or infarcts – most often affecting the MCA, ACA, and ophthalmic artery
  • Thus, is often associated with MCA symptoms such as contralateral face/arm or face/arm/leg weakness, contralateral sensory changes, contralateral visual field defects, aphasia or neglect.
  • ACA territory symptoms include contralateral weakness
  • Carotid occlusions can be asymptomatic if there is adequate collateral flow via the anterior or posterior communicating arteries However emboli may become dislodged and cause TIAs or strokes

“Dissection” of the Carotid or Vertebral Arteries

  • Head or neck trauma, and sometimes even minor events (e.g., sneeze) can cause a small tear to form on the surface of the carotid or vertebrate arteries. This allows blood to burrow in the vessel wall, producing a dissection.
  • Can lead to development of a thrombosis/embolism
  • Some patients report hearing a pop at the onset

Venous Drainage of the Cerebral Hemispheres

(Venous sinuses lie enclosed within folds of the 2 layers of dura; see pp. 386 & 387 for diagrams of venous drainage system and for more specific details regarding the smaller vessels)

Drainage Occurs Two Ways

  • Through Dural Sinuses These are large channels between layers of the dura (between the dura and the arachnoid). They are also where the CSF is reabsorbed into the ventricular system before blood passes out of the brain.
  • Direct Venous Return Only the spinal cord and medulla supply drain directly to the system (the remainder of the brain supply is drained via veins that empty into the dural sinuses).
    • Like the arterial system, venous drainage has superficial and deep territories:
      • Superficial Veins Drain mainly into the Superior Sagittal Sinus and the Cavernous sinus
      • Deep Veins Drain into the Great Vein of Galen
    • Nearly all venous drainage systems merge into the Internal Jugular Veins

Sagittal Sinus Thrombosis

  • Often associated with same syndromes which affect arteries
  • Increased frequency in pregnant women and within the first few weeks postpartum
  • Obstruction of venous drainage usually causes elevated intracranial pressure which can cause:
    • parasagittal hemorrhages
    • also, increased venous pressure can decrease cerebral perfusion leading to infarcts
    • seizures are common
    • headaches and papilledema
    • depressed level of consciousness
  • Thrombosis can also occur less commonly in the deep cerebral veins or in other cortical veins, leading to infarcts or hemorrhage in their territories
  • Empty Delta Sign Defect observed on imaging; the sinus normally fills uniformly with contrast, so a dark region in the middle suggests a filling defect (possibly due to a blood clot).

Notes from the Clinical Cases

  • Sudden onset of severe headache, worse than any experienced should be considered a subarachnoid hemorrhage until prove otherwise.
  • In about 80% of the cases, spontaneous subarachnoid hemorrhage is caused by rupture of an arterial aneurysm in the subarachnoid space.
  • The most common locations for aneurysms are the origin of the ACA, PCA or bifurcation points of the MCA.
  • Subarachnoid hemorrhage can lead to hydrocephalus due to impaired CSF flow in the subarachnoid space. However, in these cases, a lumbar puncture should not be performed as it can occasionally cause aneurysmal rupture by increasing the pressure across the wall of an aneurysm.
  • Pure motor hemiplegia, without sensory abnormalities or cortical signs (e.g., aphasia or neglect) can be localized to the contralateral corticobulbar and corticospinal tracts in the internal capsule or ventral pons. Dysarthria is commonly present, giving rise to the name dysarthria hemiparesis
  • Facial weakness associated with sparing of the forehead is indicative of an upper motor neuron injury
  • Large cortical lesions can be associated with an ipsilateral gaze preference; there is a loss of the ability to drive the eyes toward the side opposite the lesion
  • Balint’s Syndrome Disruption of the “where” system rather than the “what” system. Patients have trouble focusing on an object, but recognize it once they can focus.