During a radical neck dissection for oral cancer, the surgeon preserves a nerve running along the posterior border of sternocleidomastoid and supplying trapezius. Injury to this nerve would cause:

• A Loss of sensation over the neck and shoulder
• B Loss of taste from the anterior two-thirds of tongue
• C Weakness of shoulder shrugging and drooping of the shoulder ✓
• D Paralysis of the ipsilateral hemidiaphragm

Explanation

The accessory nerve (CN XI) exits the skull via the jugular foramen and crosses the posterior triangle of the neck superficially along the posterior border of sternocleidomastoid before reaching the trapezius. It is purely motor to both muscles. Injury during neck surgery produces dropping of the shoulder, weakness of shoulder shrugging (trapezius function), and difficulty abducting the arm above the horizontal due to poor scapular rotation. The accessory nerve's superficial course in the posterior triangle makes it the most commonly injured cranial nerve in neck dissections.

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

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Written and medically reviewed by the StethoPrep medical team.