Right-Sided Stroke and Left Shoulder Shrug Weakness
In a right-sided stroke, patients will typically NOT have weakness to shoulder shrug on the left side, because shoulder shrug (trapezius function) is controlled by the spinal accessory nerve (cranial nerve XI), which has bilateral cortical innervation and is therefore relatively spared in unilateral hemispheric strokes.
Understanding the Neuroanatomy
The key to answering this question lies in understanding which muscles are affected by cortical strokes versus those with bilateral innervation:
- Trapezius muscle (responsible for shoulder shrug) receives motor innervation from the spinal accessory nerve (CN XI), which has strong bilateral cortical representation 1
- Unlike muscles controlled purely by the contralateral corticospinal tract, the trapezius typically maintains function even with unilateral hemispheric lesions
- Right-sided (right hemisphere) stroke causes left-sided hemiparesis affecting the face, arm, and leg through damage to the contralateral corticospinal tract 1
Expected Motor Deficits in Right-Sided Stroke
Patients with right hemisphere stroke typically present with:
- Contralateral (left-sided) weakness affecting the face, arm, and/or leg 1
- Preserved shoulder shrug bilaterally due to bilateral cortical innervation of CN XI
- Greater impairment in movement extent (reaching distance) compared to trajectory control 2
- Possible hemihypokinesia (reduced motor activation) on the contralateral side, which may be more pronounced with right hemisphere lesions 3
Clinical Examination Pearls
When examining a patient with suspected right hemisphere stroke:
- Test shoulder shrug specifically - weakness here suggests either bilateral strokes, brainstem involvement, or peripheral nerve pathology rather than a simple hemispheric stroke 4
- Focus on distal arm weakness - this is where contralateral weakness will be most evident 1
- Assess for neglect - right hemisphere strokes commonly cause left-sided spatial neglect, which is distinct from motor weakness 5
- Evaluate scapular positioning - while strength may be preserved, scapular posture and coordination can be altered bilaterally after stroke, with less protraction on the contralesional side 6
Important Caveats
Rare exceptions exist where ipsilateral weakness can occur:
- Previous contralateral stroke with reorganization of motor pathways may result in ipsilateral weakness with a new stroke 4
- Lesions involving the corona radiata, internal capsule, basal ganglia, or thalamus in rare cases of aberrant motor pathway organization 4
- These cases are exceptionally rare and typically involve specific anatomical circumstances or prior neurological injury 4
The presence of shoulder shrug weakness bilaterally or ipsilaterally to a hemispheric stroke should prompt investigation for alternative diagnoses including brainstem involvement, bilateral strokes, or peripheral nerve pathology 4.