Overview

The M1 (sphenoidal) segment runs laterally and gives off the lateral lenticulostriate arteries. These thin branches supply the putamen, globus pallidus and internal capsule, and are sometimes called the "arteries of stroke". The M2 (insular) segment usually splits into a superior division, which supplies the frontal lobe, the precentral gyrus and Broca's area, and an inferior division, which supplies the temporal and inferior parietal lobes and Wernicke's area. The M3 (opercular) and M4 (cortical) segments spread over the convexity of the hemisphere.

The MCA continues in a straight line from the internal carotid, so emboli flowing up the carotid tend to lodge in it. This makes the MCA territory the commonest site of ischaemic stroke.

Function

Supplies the lateral frontal, parietal and temporal lobes, including the face and upper-limb areas of the motor and sensory cortex.
Supplies the language areas in the dominant hemisphere and areas for spatial attention in the non-dominant hemisphere.
Supplies the basal ganglia and internal capsule through the lenticulostriate arteries.

Clinical significance

MCA syndrome

An MCA stroke causes weakness and sensory loss on the opposite side, affecting the face and arm more than the leg. There is usually a homonymous hemianopia, and the eyes deviate toward the side of the lesion. Dominant-hemisphere strokes cause aphasia: Broca's (expressive) aphasia with superior division strokes and Wernicke's (receptive) aphasia with inferior division strokes. Non-dominant strokes cause neglect of the opposite side and unawareness of the deficit (anosognosia). Patients with a large-vessel occlusion are treated with thrombolysis and/or mechanical thrombectomy.

Lacunar infarcts and hypertensive haemorrhage

Long-standing hypertension damages the small lenticulostriate arteries. When they become blocked, they cause small deep (lacunar) infarcts, for example a pure motor stroke from an infarct in the internal capsule. When they rupture, typically from tiny Charcot–Bouchard microaneurysms, they cause haemorrhage into the putamen.

Test yourself

Occlusion of the left middle cerebral artery (dominant hemisphere) typically causes:

Right leg weakness more than arm, with urinary incontinence
Right face and arm weakness more than leg, with aphasia
Left homonymous hemianopia with macular sparing only
Quadriplegia with preserved vertical eye movements

Answer: Right face and arm weakness more than leg, with aphasia

The MCA supplies the lateral cortex, where the face and arm are represented, and the language areas in the dominant hemisphere. Leg-predominant weakness suggests the ACA, macular-sparing hemianopia suggests the PCA, and locked-in syndrome suggests the basilar artery.