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Spinal Accessory Nerve (CN XI)

nervus accessorius (pars spinalis)

The spinal accessory nerve (SAN) is the most vulnerable nerve in the posterior triangle of the neck, lying superficially between the SCM and trapezius just deep to the investing fascia. Surgical injury during lymph node biopsy, neck dissection, or carotid endarterectomy produces shoulder droop, winging of the scapula, and inability to shrug — trapezius palsy. Its preservation is a specific surgical objective in selective neck dissection for head and neck cancer.

Region: Head & Skull

Anatomical Data

Origin, Insertion & Supply

Origin	Spinal roots C1-C5 (ascending through the foramen magnum to join the cranial accessory root)

Clinical Relevance

Clinical Notes

SAN injury during cervical lymph node biopsy produces the classic delayed presentation of shoulder pain and dropping 6-12 weeks post-biopsy as trapezius wasting develops. The Erb point — where the SAN crosses the posterior triangle between SCM and trapezius at the junction of the upper and middle thirds of the SCM — is the most vulnerable location. SAN exploration and repair or Eden-Lange muscle transfer (levator scapulae, rhomboids) for irreparable palsy.

Pathology

Common Injuries & Conditions

Spinal Accessory Nerve Palsy

SAN injury during neck biopsy producing trapezius palsy and shoulder droop managed with early exploration or muscle transfer for permanent palsy.