The spinal accessory nerve is  usually tested by asking the patient to shrug their shoulders against resistance. It supplies the sternocleidomastoid and trapezius muscles. In the unconscious patient, testing the cough reflex - if a cough is elicited - will usually result in a vigorous activity of the trapezius and sternocleidomastoid muscles.

Lesions of the accessory nerve

Obvious features

  • Weakness of the trapesius and sternocleidomastoid muscles

The "central" supranuclear lesions tend to cause mild and transient weakness, because the accessory nerve nuclei receive bilateral cortical input. Hemispheric lesions rarely cause a clinically important CN XI palsy.

Important localising feature:

  • Due to a quirk of anatomy, a lesion at the cervicomedullary junction produces a weakness of the impsilateral sternocleidomastoid, and a weakness of the contralateral trapesius.

Supranuclear accessory nerve lesions

  • Tumour (cerebellopontine angle, acoustic neuroma)
  • Fracture of the petrous temporal bone
  • Vascular disease of the internal auditory artery

Nuclear and peripheral accessory nerve lesions

  • Occlusion of the vertebral or posterior inferior cerebellar artery produces infarction of the medullary tegmentum, with deficits of V, IX, X, and XI (Wallenberg's syndrome)
  • Trouble in the jugular foramen (eg. base of skull fracture)
  • Surgical exploration of the posterior triangle

References

The LITFL summary of cranial nerve lesions is without peer in terms of useful information density.

Walker, H. Kenneth, W. Dallas Hall, and J. Willis Hurst. "Clinical methods." 3rd edition.(1990). Chapter 64. Cranial Nerve XI: The Spinal Accessory Nerve- by Kenneth Walker