Normal brain function relies on a steady supply of oxygen and nutrients through a dense mesh of blood vessels. Blood is supplied to the brain through the common carotid arteries in two divisions: the external, and internal carotid arteries. There exists the anterior cerebral artery, which extends upward and forwards from the internal carotid artery. It supplies the frontal lobes, the parts of the brain that control logical thought, personality, and voluntary movement, especially of the legs. The posterior arteries supply the temporal and occipital lobes of the left cerebral hemisphere and the right hemisphere. Then there is the middle cerebral artery.
The middle cerebral artery is the largest branch of the internal carotid. The artery supplies a portion of the frontal lobe and the lateral surface of the temporal and parietal lobes, including the primary motor and sensory areas of the face, throat, hand, and arm, and in the dominant hemisphere, the areas for speech. The cortical branches of the MCA irrigate the part of the brain in charge of the primary motor and somatosensory cortical areas of the face, trunk, and upper limbs, apart from the insular and auditory cortex.
Physiologic and anatomic variants occur across individuals. Some studies related a higher risk of aneurysm formation in the MCA with certain MCA variants. Bifurcation of the MCA before the genu without a dominating post-division trunk was the most common variant. Other variants include bifurcation and trifurcations before and after the genu. A study suggested that domination of the upper post-division trunk of the MCA has a higher risk of MCA aneurysm formation. Some studies related a higher risk of aneurysm formation in the MCA with certain MCA variants. The anatomic variants can partly explain the aneurysm formation and stroke incidence. A deeper insight into the implication of these variants warrants further studies.
Generally, the middle cerebral artery (MCA) and its branches can be classified into four parts:
- the sphenoidal segment (stem): named due to its origin and loose lateral tracking of the sphenoid bone. Although known also as the horizontal segment, this may be misleading since the segment may descend, remain flat, or extend posteriorly the anterior (dorsad) in different individuals.
- The opercular segment: which extends laterally and outward from the insula towards the cortex. These finer terminals, or cortical segments, irrigate the cortex. They begin at the external margins of the Sylvian fissure and extend distally away on the cortex of the brain.
- terminal branches (upper trunk, lower trunk, and occasionally a middle trunk): bifurcations and trifurcations occur in 50% and 25% of the cases respectively. Other cases include duplication of the MCA at the internal carotid artery (ICA) or an accessory MCA (AccMCA) which arise not from the ICA but as a branch from the anterior cerebral artery.
- The middle trunk that exists in parts of the population, when present provides the pre-Rolandic, Rolandic, anterior parietal, posterior parietal, and the angular artery for irrigation instead of the upper and lower trunks. The branches (ramus) of the MCA can be described by the areas that they irrigate.
What does the middle cerebral artery supply?
The aortic arch gives rise to the brachiocephalic artery., from which subsequently stems the right common carotid artery; the left common carotid artery branches off the aortic arch just downstream the brachiocephalic trunk. The left and right common carotid arteries run parallel to each other and divide near the angle of the mandible to the external and internal carotid arteries. The external carotid supplies the face and neck branching off immediately. The internal carotid arteries do not branch until it meets the origin of the ophthalmic artery bilaterally. The internal carotid arteries bifurcate, or split onto the anterior and middle cerebral arteries, one on each internal carotid artery.
The primary function of the MCA is to supply specific regions of parenchyma, or functional brain tissue, with oxygenated blood. The cortical branches of the MCA irrigate the brain parenchyma of the primary motor and somatosensory cortical areas of the face, trunk, and upper limbs, apart from the insular and auditory cortex. The small central branches split into lenticulostriate vessels, which irrigate the basal ganglia and internal capsule. The superior division irrigates the lateral inferior frontal lobe, which involves the Broca area responsible for speech production, language comprehension, and writing. Broca’s area and other related gray and white matter important for language expression. The inferior division of the MCA irrigates the superior temporal gyrus, which involves Wernicke’s area responsible for speech comprehension and language development. Parts of the posterior parietal lobe are important for 3D perceptions of one’s own body, of the outside world, and for the ability to interpret emotions.
What happens when the middle cerebral artery is blocked?
Blood flow decrease through one of the internal carotid arteries brings about some impairment in the function of the frontal lobes. This impairment may result in numbness, weakness, or paralysis on the side of the body opposite to the obstruction of the artery. Occlusion of one of the vertebral arteries can cause many serious consequences, ranging from blindness to paralysis.
Other common pathologies involving the MCA are described below.
- Embolism: this occurs when a detached mass, typically a dislodged thrombus, gas, or fat, is transported through the blood vessels until it is lodged in the MCA. The arterial occlusion impedes the transmission of oxygenated blood to the brain parenchyma, resulting in an ischemic stroke. Cerebral edema and brain parenchyma tissue necrosis follow.
- Thrombus formation commonly is associated with sites proximal to the MCA, such as internal carotid plaques, common carotid plaques, and atrial fibrillation, resulting in thrombus formation and embolism. Furthermore, it is worth noting that cardiac defects such as atrial septal and ventral septal defects may result in a paradoxical embolism. This subtype is denoted as such because the thrombus being transported through the superior or inferior vena cava bypasses the respiratory circulation through the cardiac defects and potentially result in an MCA embolism.
- Middle artery syndrome: a stroke of the MCA. Symptoms presented involve contralateral sensory loss of the legs, arms, and lower two-thirds of the face due to tissue necrosis of the primary somatosensory cortex. Clinically, it can also be diagnosed as muscle weakness, spasticity, hyperreflexia, and resistance to movement (upper motor neuron signs). Ipsilateral eye deviation may be observed due to frontal cortex Brodmann area 8 becoming ischemic, impairing planning of eye movement, symptoms that are exacerbated by contralateral homonymous hemianopsia. A dominant, most commonly left-sided, hemisphere stroke results in Broca aphasia if the superior division of the MCA is affected.
- Wernicke’s or conduction aphasia: the inability to repeat words. May be seen if the inferior division of the MCA is affected. A non-dominant, most commonly right-sided, hemisphere stroke.
- Lenticulostriate Infarct: Lenticulostriate vessels tend to result in lacunar infarcts, which may be differentiated from those above due to lack of cortical involvement. The most common clinical presentation of lacunar infarcts of the lenticulostriate branches is one of pure motor involvement. This presentation is due to the posterior limb of the internal capsule commonly being affected. It is stipulated that lipohyalinosis may have a large role in the development of lenticulostriate infarcts; however, additional evidence to support this claim is needed.
- Charcot-Bouchard Microaneurysms: potentially caused by chronic hypertension in small terminal lenticulostriate vessels. The rupture of the Charcot-Bouchard aneurysms is associated with intracerebral hemorrhages, which tend to cause tissue necrosis due to lack of anastomosing blood vessels. Common sites affected by Charcot-Bouchard microaneurysms include the thalami, basal ganglia, cerebellum, and pons.
- Arteriovenous Malformations Arteriovenous malformations (AVMs): congenital arteriovenous connections which can be found anywhere in the brain. Clinically, the most commonly are seen in young patients ranging from 20 to 40 years of age and carry a yearly hemorrhage risk between 1% to 4%. Conventional surgery, radiosurgery, and endovascular embolization are commonly used to treat AVMs.
- Saccular Aneurysms (Berry aneurysms): the most common subtype of an aneurysm. Most of the MCA aneurysms are located at the MCA bifurcation often projecting laterally in the plane of the M1 segment, followed by the proximal M1, then distal MCA. These usually present in areas of bifurcation/trifurcation due to blood vessel weakness and outpouching. Saccular aneurysms present about a third of the time in the MCA. They are associated with risk factors such as autosomal dominant polycystic kidney disease (ADKPKD), Ehlers-Danlos syndrome, cigarette smoking, hypertension, and age.
The middle cerebral artery is the most common, pathologically affected blood vessel overall, often through strokes. It is among the more common locations for cerebral aneurysms with a reported incidence between 14.4% and 43% of all diagnosed aneurysms.
Brain cells that do not receive a constant supply of oxygenated blood can die, which can cause permanent damage to the brain. Brain tissue death is called necrosis.
A stroke is usually named by the injured part of the brain or by the blocked blood vessel. There are two main types of stroke: ischaemic strokes and hemorrhagic. Many people have misconceptions about how someone will be affected by a stroke. You might automatically envision mobility difficulties and hemiplegia, or perhaps swallowing problems, or maybe even being unable to speak and communicate. How a stroke manifests will depend on many factors. When a stroke happens, the area of the brain deprived of oxygen supply corresponds with subsequent symptoms presented in a patient.
An MCA stroke is an interruption of blood flow to the areas of the brain that receive blood through the middle cerebral artery. These regions include the frontal, parietal, and temporal lobes as well as the internal capsule. Symptoms of MCA stroke are consistent with the symptoms people usually associate with strokes, such as weakness and/or numbness on one side, facial droop, and difficulties with speaking. A middle cerebral artery stroke (MCA) stroke may cause language deficits, as well as weakness, sensory deficits, and visual defects on the opposite side of the body.
Alternatively, if only a small branch of the middle cerebral artery is blocked, then a small-vessel stroke results. A small section of the middle cerebral artery territory is impacted and is usually less serious. These symptoms are similarly covered in different pathologies as listed above.
Although it is among the most easily recognized types of stroke, you may need to undergo imaging tests to confirm the diagnosis. Because an MCA stroke may be considered a large stroke, the short-term situation is handled with the utmost care. Some people who experience an MCA stroke are candidates for urgent treatment with tissue plasminogen activator (TPA) or blood thinners, while others may need careful fluid management and close observation. Symptoms of MCA stroke are consistent with the symptoms people usually associate with strokes, such as weakness and/or numbness on one side, facial droop, and difficulties with speaking. If only a small branch of the middle cerebral artery is blocked, then a small-vessel stroke results, impacting a small section of the middle cerebral artery territory. This is often less serious.
The middle cerebral artery is an area that controls a multitude of human functions depending on which part of the brain it supplies oxygen to, like breathing and facial muscles, but also higher functions like speech, spatial recognition, and recognition of emotion. Blood flow needs to be constant in order to maintain brain function. The Blockages associated with this part of the brain manifest themselves in a variety of symptoms, often associated with traditional thoughts of symptoms of a stroke. However, that is not always the case. The severity of the stroke in the MCA can also vary depending on where it occurs.
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