Spinal Accessory Nerve

nervus accessorius spinalis (CN XI)

The spinal accessory nerve (CN XI) has a spinal root arising from C1-C5 anterior horn cells, ascending through the foramen magnum to join the cranial root (from the nucleus ambiguus), then exiting through the jugular foramen. The nerve divides: the cranial root joins the vagus (supplying pharyngeal and laryngeal muscles); the spinal root continues as the true spinal accessory nerve, crossing the posterior triangle to supply the sternocleidomastoid and trapezius.

Region: Neck

Clinical Relevance

Clinical Notes

Spinal accessory nerve injury is the most clinically significant complication of neck dissection (lymph node biopsy at Erb's point, radical neck dissection). Sacrifice produces ipsilateral trapezius paralysis with shoulder droop, scapular winging, and severe shoulder pain from failed supraspinatus abduction initiation. Modified radical neck dissection preserves the spinal accessory nerve in the posterior triangle. Spinal accessory nerve monitoring with EMG during neck surgery reduces inadvertent injury rates.

Pathology

Common Injuries & Conditions

Spinal Accessory Nerve Palsy After Neck Dissection

Sacrifice or traction injury of the spinal accessory nerve during posterior triangle lymph node biopsy or radical neck dissection produces trapezius paralysis with lateral shoulder droop, scapular winging, and inability to abduct the arm above 90 degrees due to loss of scapular rotation; nerve repair or cable grafting and physiotherapy improve outcomes if performed within 6 months.