Right Middle Cerebral Artery (MCA) Territory Infarction: Clinical Localization and Diagnostic Criteria

Anatomical Localization

The right MCA supplies the lateral motor cortex that controls the contralateral (left) face and upper extremity, explaining left‑sided hemiplegia when this territory is infarcted. 1
Infarction of the MCA territory typically produces contralateral hemiparesis that is more pronounced in the face and arm than in the leg, with a parallel pattern of sensory loss. 1
The presence of a left homonymous hemianopia indicates involvement of the optic radiations or visual cortex, structures supplied by the right MCA. 1, 2

Anterior cerebral artery (ACA) infarcts characteristically cause weakness that is most prominent in the contralateral lower extremity, not the face or arm; therefore, a presentation with combined face, arm, and leg weakness points away from ACA involvement. 1
Posterior cerebral artery (PCA) lesions may produce isolated homonymous hemianopia, but the concurrent motor and sensory deficits rule out PCA territory as the primary source. 1
Ophthalmic artery occlusion leads to monocular blindness rather than homonymous hemianopia; thus, homonymous hemianopia signifies retro‑chiasmal pathway damage, not anterior visual‑pathway disease. 1, 2, 3

Ischemia or infarction within the right internal carotid artery or MCA results in a classic triad: left‑sided weakness, left‑sided sensory loss, and left homonymous hemianopsia, often accompanied by left‑side neglect and visuospatial deficits. 1
The motor component of this triad (left hemiplegia) arises from injury to the right motor cortex. 1
The sensory component (left hemisensory loss) stems from damage to the right primary sensory cortex. 1
The visual component (left homonymous hemianopia) is due to disruption of the right optic radiations or visual cortex. 1, 2

Approximately 19 % of patients with cortical stroke may lack overt cortical deficits on initial examination; the detection of a homonymous hemianopia confirms cortical involvement despite an initially bland neurological exam. 1, 3

The MCA supplies the lateral motor cortex, which contains motor representation for the contralateral face and upper extremity, resulting in contralateral hemiparesis affecting the face and arm more than the leg, with sensory deficits following a similar distribution 4
MCA strokes characteristically present with contralateral hemiparesis affecting the face and arm more than the leg, with sensory deficits following a similar distribution 4

The typical MCA stroke presents with weakness that is more prominent in the face and arm compared to the leg 4

ACA territory infarction characteristically causes contralateral weakness that is most prominent in the lower extremity, and the ACA supplies the medial motor cortex containing motor representation for the contralateral lower extremity 4
If the leg were significantly weaker than the arm, you would suspect ACA territory involvement 4

The American Heart Association recommends CT imaging should be completed within 25 minutes of emergency department arrival with interpretation within an additional 20 minutes, and look for hyperdense MCA sign indicating thrombus or embolus in the first portion of the MCA 5
Loss of gray-white differentiation in the cortical ribbon, particularly at the lateral margins of the insula or lentiform nucleus, and sulcal effacement may be detected within 6 hours in up to 82% of patients with MCA territory ischemia 5
Early infarct signs involving more than one-third of the MCA territory are associated with increased risk of hemorrhagic transformation following thrombolytic therapy 5

The American College of Cardiology and the American Heart Association suggest not confusing equal upper and lower extremity weakness with leg-predominant weakness, as the latter would suggest ACA territory or parasagittal lesions, and the question specifically states both extremities are weak, which is the hallmark of MCA territory involvement when affecting the right side from a left hemisphere lesion 4, 6