Why does stroke cause weakness on one side?

Messages that tell muscles to move travel from the brain down through the spinal cord and cross over along the route. Damage to a movement area on one side of the brain can therefore reduce strength, control, or coordination on the other side of the face, arm, or leg. Weakness on One Side is a very common presentation of Stroke. The arm may drift down, a smile may become uneven, or walking may suddenly feel difficult.

Call emergency services as soon as sudden one-sided weakness is noticed and record when the person was last known well. Keep the person in a safe position and avoid walking them without support, because a weak leg can lead to a fall. Do not drive them to hospital yourself if emergency transport is available.

New weakness of the face, arm, or leg needs emergency assessment, even when it is mild or short-lived. Act immediately if it occurs with numbness, slurred speech, confusion, vision change, sudden dizziness, or a severe new headache. These signs can start abruptly and should never be managed by waiting at home.

Ask which functions are affected, whether swallowing and fall risk need assessment, and what rehabilitation may be appropriate. During recovery, ask how to move safely, how to care for a weak arm, and when a change in strength should prompt an urgent call.