Before We Start
A cranial nerve that controls the shoulder
CN XI (Spinal Accessory) is unusual among the cranial nerves because its territory extends well outside the head and face — it controls two muscles of the neck and upper back: the sternocleidomastoid (SCM), which turns and tilts the head, and the trapezius, which elevates and stabilizes the shoulder. This makes CN XI directly relevant to a category of patients that other cranial nerves rarely touch: those recovering from neck or head-and-neck surgery.
💡 Why This Nerve Matters So Much After Neck Surgery
CN XI runs a relatively superficial course through the posterior triangle of the neck — a location that puts it directly in the surgical field for several common procedures. This anatomical exposure is exactly why CN XI injury is a well-recognized, specifically watched-for complication after certain neck surgeries.
Assessment
Testing CN XI at the bedside
Sternocleidomastoid Test
Turn head against resistance
Ask the patient to turn their head to one side while the examiner applies resistance against the direction of the turn (pressing on the jaw or cheek). Note the strength and compare side to side. Weakness suggests CN XI dysfunction on the side being tested.
Trapezius Test
Shrug shoulders against downward pressure
Ask the patient to shrug both shoulders upward while the examiner presses down on the shoulders. Compare symmetry and strength between the two sides — asymmetric weakness is the key finding to identify.
💊 Testing both muscles — not just one — matters, because a patient can have isolated weakness in only the trapezius or only the SCM depending on exactly where along its course the nerve was affected.
Common Causes of Damage
Procedures that place CN XI at risk
Radical neck dissection, carotid endarterectomy, and lymph node biopsy in the posterior triangle of the neck are all procedures where CN XI's superficial course puts it at recognized risk of injury.
Signs of Damage
What to look for post-operatively
Shoulder drop on the affected side, difficulty turning the head against resistance, winging of the scapula (the shoulder blade protruding abnormally due to trapezius weakness), and chronic shoulder pain from the resulting muscular imbalance.
🏥 Clinical Scenario — Post-Operative Shoulder Weakness
A patient two days post-carotid endarterectomy reports difficulty raising their arm on the surgical side and the nurse notices the shoulder appears to droop.
Assess
The nurse performs a shoulder shrug test against resistance — the affected side is noticeably weaker than the unaffected side, and mild scapular winging is visible. Given the surgical history and the specific pattern of findings, this presentation is consistent with CN XI injury related to the neck dissection required for the carotid procedure.
Document and Escalate
The nurse documents the specific findings (asymmetric shrug strength, scapular winging) and notifies the surgical team. Precise documentation of the exact deficit helps the surgical team determine whether this reflects nerve stretching (often temporary) versus a more significant nerve injury (potentially requiring further intervention).
Supportive Care
A referral for physical therapy is appropriate to address the shoulder weakness and prevent secondary complications like frozen shoulder from disuse. Depending on severity, a shoulder sling may also provide support while the nerve recovers, if recovery is expected.
📌 NCLEX Application
CN XI questions center on post-surgical recognition:
Recognition: "A patient post-radical neck dissection has difficulty shrugging one shoulder. Which cranial nerve is most likely affected?" → CN XI (Spinal Accessory).
Assessment technique: "How does the nurse assess trapezius strength as part of a CN XI exam?" → Ask the patient to shrug both shoulders against downward resistance and compare symmetry.
⚠️ The Trap — Attributing All Post-Neck-Surgery Shoulder Pain to Normal Healing
Because some degree of discomfort is expected after any neck or shoulder-area surgery, genuine CN XI injury symptoms — a specific pattern of shoulder drop, scapular winging, and shrug weakness — can be dismissed as ordinary post-operative soreness if the nurse isn't specifically screening for the nerve-specific pattern.
The safeguard: After any procedure involving the posterior triangle of the neck, perform a specific, deliberate shrug-strength and head-turn assessment rather than relying on the patient's general pain report alone.
✓ Quick Self-Test
Answer before checking:
1. Which two muscles does CN XI control?
2. What two bedside tests assess CN XI function?
3. Name two surgical procedures that place CN XI at risk of injury.
4. What does "scapular winging" indicate, and which CN XI-controlled muscle is responsible?
Answers:
1. Sternocleidomastoid (SCM) and trapezius.
2. Turning the head against resistance (SCM), and shrugging the shoulders against downward pressure (trapezius).
3. Radical neck dissection and carotid endarterectomy (lymph node biopsy in the posterior triangle is also a recognized risk).
4. Scapular winging reflects trapezius weakness — the shoulder blade protrudes abnormally because the trapezius normally helps stabilize it against the ribcage.
Next Lesson
Tongue Deviation — CN XII Hypoglossal
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