The spinal accessory nerve (cranial nerve XI) supplies the sternocleidomastoid and trapezius, both of which are tested in the neurological examination of CN XI. Its superficial course through the posterior triangle of the neck makes it vulnerable to injury during posterior triangle lymph node biopsy, neck dissection, and internal jugular vein cannulation, producing the most common iatrogenic peripheral nerve injury in the neck.
| Origin | Spinal accessory nucleus in the anterior horn of the upper cervical cord (C1-C5); rootlets ascend through the foramen magnum (the small cranial root from the nucleus ambiguus joins the vagus) |
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| Course | Rootlets ascend through the foramen magnum and leave the skull through the jugular foramen; the nerve crosses the internal jugular vein, enters and supplies sternocleidomastoid from its deep surface, emerges from the posterior border of the muscle near the junction of its upper and middle thirds, crosses the posterior triangle just under the investing fascia and enters trapezius from its deep surface |
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| Sensory |
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The spinal accessory nerve is the most superficial structure crossing the posterior triangle, lying just under the investing fascia, so it is easily injured in lymph node biopsy, neck dissection and other posterior triangle procedures. Injury paralyses trapezius, causing a dropped shoulder, lateral scapular winging, weak shrug and difficulty raising the arm above 90 degrees, often with persistent shoulder pain. Symptoms may only become obvious weeks after a biopsy as the muscle wastes. Modified and selective neck dissections aim to preserve the nerve, and nerve monitoring can help. Early repair or grafting gives the best results; for permanent palsy a muscle transfer (Eden-Lange procedure, moving levator scapulae and the rhomboids) can restore scapular control.
Trapezius paralysis after posterior triangle surgery or biopsy produces a dropped shoulder, lateral scapular winging and limited arm elevation, ideally treated by early nerve repair or grafting.
When the nerve cannot be repaired, transferring levator scapulae and the rhomboids laterally on the scapula substitutes for trapezius and improves shoulder function and pain.
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