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Accessory Nerve (CN XI) and Head Rotation: Anatomy, Testing, and Lesions

The accessory nerve (cranial nerve XI) is the primary motor nerve for head rotation. It has a unique dual origin and exclusive somatic motor function.

Anatomy

  • Cranial root: Nucleus ambiguus (medulla) → joins vagus (CN X) → supplies pharyngeal/laryngeal muscles (not head rotation).
  • Spinal root: Neurons in lateral cervical spinal cord (C1–C5/6) → rootlets ascend through foramen magnum → exit jugular foramen → sternocleidomastoid (SCM) and trapezius.

Head Rotation Mechanics

  • Unilateral SCM contraction → rotates head to opposite side (ipsilateral flexion, contralateral rotation).
  • Bilateral SCM contraction → neck flexion ("chin to chest").
  • Trapezius (upper fibers) → elevates scapula, extends neck, assists contralateral rotation with SCM.

Clinical Testing

  • SCM: Ask patient to rotate head against resistance; palpate/visualize contralateral SCM.
  • Trapezius: Shrug shoulders against resistance; observe bulk/symmetry.

Lesion Localization & Effects

Lesion SiteDeficit
Nuclear (medulla/cervical cord)Ipsilateral SCM/trapezius weakness
Rootlets (foramen magnum/jugular foramen)Same; often with CN IX, X, XII deficits (jugular foramen syndrome)
Peripheral (posterior triangle)Isolated trapezius palsy (shoulder drop, scapular winging)

Common Causes

  • Iatrogenic: Lymph node biopsy, radical neck dissection, internal jugular cannulation.
  • Trauma: Penetrating, stretch (whiplash), carotid endarterectomy.
  • Neoplastic: Metastatic nodes, paraganglioma, schwannoma.
  • Inflammatory: Parsonage-Turner (neuralgic amyotrophy), infections.

Key takeaway: CN XI (spinal root) → SCM → contralateral head rotation. Trapezius → shoulder elevation/neck extension. Injury causes asymmetric shoulder droop and impaired head turning.

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