CEREBRAL CIRCULATION

FACTS ABOUT BLOOD SUPPLY OF BRAIN Brain – 2% of body weight Receives 17% of cardiac output Consumes 20% of entire Oxygen used by the

body 10 seconds of interruption in blood flow leads

to unconsciousness Most neurologic disorders are due to vascular

lesions

ANTERIOR AND POSTERIOR CIRCULATIONSAnterior – Internal carotid arteries

Posterior – Vertebral arteries

INTERNAL CAROTID ARTERY

Course Entry – carotid canal Middle cranial fossa- beside dorsum sellae Enters cavernous sinus Approaches medial side of anterior clinoid

process

Pierces dura and arachnoid maters Moves lateral to optic chiasma Divides into Anterior and Middle cerebral

arteries

BRANCHES OF INTERNAL CAROTID ARTERY[PRIOR TO BIFURCATION]

Occasionally, Anterior choroidal artery supplies

1. Rostral midbrain2. Ventral thalamus3. Subthalamus

ARTERIAL ‘CIRCLE’/’CIRCLE’ OF WILLIS

“CIRCLE” OF WILLIS

Looks more like a polygon Connects anterior and posterior circulations

COMPONENTS

From anterior1. Anterior communicating artery 2. Anterior cerebral artery3. Posterior communicating artery4. Posterior cerebral artery

Alternate route – inadequate, especially in the elderly [atherosclerosis]

Variations1. In approximately 33% persons Posterior

cerebral artery arises from Internal carotid artery

2. One Anterior cerebral artery may be small- anterior communicating artery is wider

TERRITORIES OF CEREBRAL ARTERIES

ANTERIOR CEREBRAL ARTERY

Midline proximity [longitudinal horizontal fissure]

Joined together by anterior communicating artery

Branches 1. Medial striate/recurrent artery of

Heubner→ventral part of head of caudate nucleus, putamen, anterior limb and genu of internal capsule

AREA SUPPLIED

Medial part of orbital surface of frontal lobe [includes olfactory bulb and tract]

Medial surfaces of frontal and parietal lobes Corpus callosum A strip on lateral surface

FUNCTIONAL AREAS SUPPLIED

Supplementary and cingulate motor areas Dorsal parts of primary motor and

sensorimotor areas

OTHER ASSOCIATED DEFICITS IN ACA BLOCK

Mental confusion and dysphasia[functional loss in prefrontal cortex, cingulate gyrus, supplementary motor area]

EFFECTS OF OCCLUSION

Paralysis and sensory deficits in contralateral leg, perineum

Urinary incontinence [inadequate perineal sensation, defective cortical control of pelvic floor muscles]

If obstruction is proximal to anterior communicating artery [blocked medial striate artery]

UMN weakness of face, tongue and upper limb [lesion in or near genu]

Ipsilateral anosmia [maybe]

POSTERIOR CEREBRAL ARTERY

BRANCHES

Central branches supply1. Midbrain2. Pineal gland3. Lateral geniculate body4. Lentiform nucleus [partly]5. Thalamus [partly]

Temporal branches→ inferolateral and medial surfaces of temporal lobe, much of parahippocampal gyrus

Calcarine and parieto-occipital branches→ peripheral strip on lateral surface

Calcarine branch supplies all of primary visual cortex and some of Visual association cortex

Posterior choroidal artery supplies 1. Choroid plexus of inferior horn of lateral

ventricle2. Choroid plexus of 3rd ventricle3. Thalamus4. Fornix5. Tectum of midbrain anastomoses with Anterior choroidal

artery

EFFECTS OF OCCLUSION

Blindness in contralateral visual fields of both eyes – homonymous hemianopia

Disturbance of memory – transient In case of dominant hemisphere being

affected, the infarct extends into corpus callosum→ disconnection of contralateral visual area from language area of dominant hemisphere→ alexia in addition to homonymous hemianopia

SPECIAL CIRCUMSTANCES

Expanding space-occupying lesion in supratentorial compartment→ herniation of uncus and midbrain→ compression of one or both PCA→ necrosis of areas supplied [even after surgical correction of cause]→ cortical blindness+inability to form new memories

Traumatic Intracranial hemorrhage can lead to the same consequences

MIDDLE CEREBRAL ARTERY

Larger, more direct continuation of ICA Occupies lateral sulcus Central branches →Head of caudate

nucleus,Putamen, Lateral pallidum, Internal capsule [anterior limb, genu, posterior limb], External capsule, Claustrum, Lateral hypothalamus

FUNCTIONAL AREAS SUPPLIED

Most of primary motor and premotor areas Frontal eye field Primary somatosensory area Geniculocalcarine tract

EXCEPT motor and sensory cortex for lower limb and perineumIn most persons, left MCA supplies all cortical areas concerned with language [Wernicke’s,Broca’s]

EFFECTS OF OCCLUSION

Contralateral paralysis of lower face, upper limb

General somatosensory deficits in the same areas

Astereognosis Hemianopia in contralateral visual fields of

both eyes Global aphasia if dominant hemisphere

involved Aprosodia if non-dominant hemisphere

involved

Cortical neglect, if right hemisphere involved

Conjugate eye movements affected[ at rest, eyes turn towards the side of lesion]

Hearing unaffected [bilateral representation] Obstruction to central branches- contralateral

hemiplegia without aphasia