25th June 2023

CN XI – Spinal accessory nerve

The spinal accessory nerve is formed from a cranial root from the medulla and a spinal root from the superior spinal cord which passes into the cranium via the foramen magnum. The combined accessory nerve leaves the cranium via the jugular foramen. The spinal accessory nerve supplies motor fibres to the larynx, pharynx, and soft palate, and innervates the sternocleidomastoid and the trapezius muscles.

Disorder of CN XI manifests primarily as weakness of the trapezius and sternocleidomastoid with signs of:

• Asymmetric neckline
• Drooping shoulder ipsilaterally
• Winged scapula
• Weakness of elevation of the shoulder ipsilaterally
• Weakness of head turning to the contralateral side – this is because the ipsilateral sternocleidomastoid muscle, which functions to push the head to the contralateral side, is affected

The aetiology is primarily due to trauma and more commonly iatrogenic trauma, e.g. neck surgery, lymph node biopsy.