From the Guidelines
A left middle cerebral artery (MCA) stroke typically causes weakness on the right side of the body, affecting the right arm and right leg. This right-sided hemiparesis occurs because the left MCA supplies blood to the motor cortex in the left hemisphere of the brain, which controls movement on the opposite (contralateral) side of the body, as assessed using the National Institutes of Health Stroke Scale (NIHSS) 1. The NIHSS is an 11-part scale that measures the neurologic examination in a codified manner, including assessments of motor function such as the Motor Arm and Motor Leg components, which evaluate the patient's ability to move their arms and legs against gravity 1. The weakness is often more pronounced in the face and upper extremity compared to the lower extremity, as the MCA primarily supplies the lateral cerebral cortex. This pattern follows the principle of contralateral innervation, where each hemisphere of the brain controls motor function on the opposite side of the body due to the crossing of motor pathways in the medulla oblongata of the brainstem. Key components of the NIHSS relevant to assessing limb weakness include:
- Motor Arm: arms outstretched 90 degrees (patient sitting) or 45 degrees (patient supine) for 10 seconds, with scores ranging from 0 (no drift) to 4 (no movement) 1
- Motor Leg: Raise leg to 30 degrees and hold for 5 seconds; test both sides, with scores ranging from 0 (no drift) to 4 (no movement) 1 The severity of weakness can vary from mild paresis to complete paralysis depending on the extent of the stroke and the specific branches of the MCA affected.
From the Research
Limb Weakness in LEFT MCA Stroke
- The left Middle Cerebral Artery (MCA) stroke typically affects the right side of the body, leading to weakness or paralysis in the right limbs 2.
- The pattern of weakness can vary depending on the specific location and extent of the lesion in the MCA territory.
- In general, a left MCA stroke can cause:
- Right arm weakness or paralysis
- Right leg weakness or paralysis
- Facial weakness or paralysis on the right side
- The severity and distribution of weakness can depend on the individual's specific anatomy and the location of the lesion within the MCA territory.
- Lesions restricted to the rear portion of the medial part of the precentral gyrus can cause a contralateral predominantly distal leg weakness, while lesions involving the medial part of the premotor cortex, the supplementary motor area (SMA), and the rear portion of the medial part of the precentral gyrus can cause a contralateral, severe leg-predominant hemiplegia 2.