Why stroke affects the opposite side of the body: crossing pathways and safety

Follow the motor pathway from cortex to limb

Voluntary movement starts in the motor cortex. Its fibres descend through the internal capsule and brainstem as the corticospinal, or pyramidal, tract. At the junction of the medulla and spinal cord, about 85 per cent of these fibres cross to the opposite side in the pyramidal decussation, then continue down the spinal cord to the muscles.

Because of that crossing, the left hemisphere controls movement on the right side of the body and the right hemisphere controls the left. A stroke that damages the motor cortex or internal capsule on one side produces contralateral hemiparesis. Many sensory pathways also cross, so numbness usually appears on the same side as the weakness.

How the side and site of the lesion shape the deficits

Middle cerebral artery strokes typically cause weakness and sensory loss worse in the face and arm than the leg; anterior cerebral artery strokes affect the leg most. A dominant hemisphere stroke, usually the left, often causes aphasia. A nondominant hemisphere stroke can cause apraxia and sensory neglect, where the patient ignores the left side of space and their own body.

Brainstem strokes break the simple rule. Because the lesion lies near cranial nerve nuclei before the motor fibres have crossed, a patient may have facial weakness or numbness on one side with body weakness on the other, called crossed deficits. Posterior cerebral artery strokes can cause loss of the same half of the visual field in both eyes, on the side opposite the lesion.

Expected deficits versus signs of extension

After a stroke, a stable pattern of contralateral weakness, sensory loss, speech difficulty or visual field loss that matches the lesion is expected. Swallowing problems are common and can lead to aspiration, dehydration and pneumonia, so they belong in the expected-risk picture even when the patient seems alert.

Concerning changes are new or spreading weakness, a fall in consciousness, new speech difficulty, severe headache or vomiting, or a change in pupils. These can mean the stroke is extending, bleeding or causing swelling. Sudden numbness or weakness on one side, speech trouble, vision change or loss of balance in anyone not yet diagnosed calls for emergency activation and noting the time of onset.

Turn the mechanism into nursing actions

Use the lesion side to anticipate needs. For right-sided weakness after a left hemisphere stroke, expect communication problems: use short, simple sentences, yes or no questions, gestures and pictures, and give time to respond. For left-sided weakness after a right hemisphere stroke, expect neglect and poor awareness of deficits, which raise fall and injury risk.

Place the call bell and essentials where the patient can see and reach them, and teach scanning toward the affected side as rehabilitation progresses. Protect the weak arm from injury and shoulder strain. Keep the patient nil by mouth until a swallow screen is passed, position upright for meals, and report coughing or a wet voice during eating.

Work through a hypothetical exam-style scenario

Imagine a hypothetical patient with a right middle cerebral artery stroke. He has left arm weakness, insists he is fine and tries to get up alone, and eats only from the right side of his tray. The options are to place him in restraints, to assume the deficit is confusion, or to institute fall precautions and cue him to scan to his left.

Fall precautions with cueing to scan left is the strongest answer. Restraints are not a first-line response and do not address the cause, and calling it confusion misses a predictable deficit. The tested reasoning is that a right hemisphere lesion produces left-sided weakness and neglect, so the patient's lack of awareness, not just weakness, drives the safety risk.

Sources and further reading

MSD Manual Professional: Overview of stroke. Contralateral hemiparesis and sensory loss, MCA and ACA patterns, aphasia with dominant and neglect with nondominant hemisphere strokes, crossed brainstem deficits, hemianopia and swallowing complications.

Europe PMC: Acute onset quadriplegia and stroke, look at the brainstem (review). About 85% of pyramidal tract fibres cross at the pyramidal decussation at the cervicomedullary junction.

CDC: Signs and symptoms of stroke. Sudden one-sided numbness or weakness, speech and vision changes, and calling emergency services while noting the time.