Accessory Nerve Muscular Weakness
How to Elicit
- Sternocleidomastoid - ask the patient to turn their head against resistance.A CN XI lesion may cause weakness on head turning away from side of lesion
- Trapezius - ask the patient to shrug their shoulders against resistance.An CN XI lesion may cause weakness in shoulder elevation on ipsilateral side
Causes of Accessory Nerve Muscular Weakness
- Upper motor neuron brainstem or cervical spinal cord
- Lower motor neuron - accessory (XI) weakness
Interpretation
- Isolated Trapezius WeaknessVentral brainstem, lower cervical spinal cord or lower accessory nerve lesion
- Isolated Sternocleidomastoid WeaknessBrainstem or upper cervical lesion
- Weakness of Trapezius and Contralateral SternocleidomastoidUpper motor neurone lesion on the side of the SCM weakness, above the accessory nucleus
- Weakness of Ipsilateral TrapeziusContralateral brainstem, ipsilateral high cervical cord, or accessory nerve lesion
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