The posterior triangle of the neck contains the accessory nerve (CN XI) and the roots of the brachial plexus. The accessory nerve crosses the posterior triangle to innervate which muscle, and its injury at this site produces which clinical sign?

• A Sternocleidomastoid muscle; injury causes inability to turn the head away from the side of the lesion
• B Levator scapulae; injury causes inability to elevate the shoulder
• C Trapezius muscle; injury causes drooping and lateral rotation of the scapula, weakness of shoulder shrugging, and winging of the scapula on arm abduction ✓
• D Serratus anterior; injury causes winging of the scapula on arm flexion

Explanation

In the posterior triangle, CN XI travels superficially from its exit point at the posterior border of SCM to enter the trapezius. Iatrogenic injury (cervical lymph node biopsy, neck dissection) damages the trapezius nerve supply, causing: drooping of the shoulder, lateral rotation of the scapula with the inferior angle moving laterally (long thoracic nerve paralysis causes medial rotation), weakness of shoulder abduction above 90 degrees, and winging seen during arm abduction. CN XI also innervates SCM, but in the posterior triangle it has already passed SCM and is purely targeting trapezius. Serratus anterior winging occurs with long thoracic nerve (C5–C7) injury.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.