How do you assess cranial nerve (CN) XI, the accessory nerve?

Assessment of Cranial Nerve XI (Accessory Nerve)

To assess cranial nerve XI (accessory nerve), test the strength of the sternocleidomastoid and trapezius muscles by having the patient shrug their shoulders against resistance and turn their head against resistance to the contralateral side.

Anatomy and Function of CN XI

The accessory nerve (CN XI) consists of two components:

• Cranial root: Originates from the nucleus ambiguous in the medulla oblongata
• Spinal root: Originates from the ventral horn of the spinal cord between C1 and C5 levels 1

These components join and enter the skull through the jugular foramen 1. The accessory nerve provides motor innervation to:

• Sternocleidomastoid muscle: Responsible for head rotation to the opposite side
• Upper portion of the trapezius muscle: Responsible for shoulder elevation and scapular stabilization

Clinical Assessment Technique

1. Sternocleidomastoid Muscle Testing

• Position: Have the patient seated comfortably
• Technique: Ask the patient to turn their head to the side opposite the muscle being tested
• Resistance: Place your hand on the side of the patient's face/jaw and ask them to turn against your resistance
• Assessment: Observe and palpate the contralateral sternocleidomastoid muscle for contraction
• Comparison: Test both sides and compare strength

2. Trapezius Muscle Testing

• Position: Have the patient seated comfortably
• Technique: Ask the patient to shrug their shoulders
• Resistance: Place your hands on top of the patient's shoulders and ask them to shrug against your resistance
• Assessment: Evaluate the strength of shoulder elevation
• Comparison: Test both sides and compare strength

Interpretation of Findings

Normal Findings

• Equal bilateral strength in the sternocleidomastoid muscles during head turning
• Equal bilateral strength in the trapezius muscles during shoulder shrugging
• No visible atrophy of either muscle group

Abnormal Findings

• Weakness: Indicates possible accessory nerve damage
• Atrophy: Chronic denervation changes in the affected muscles
• Asymmetry: Difference in strength between sides suggests unilateral nerve damage
• Shoulder drooping: May be present with trapezius weakness (shoulder syndrome) 2

Common Causes of Accessory Nerve Dysfunction

1. Iatrogenic injury: Most common cause, particularly during neck surgery or internal jugular vein cannulation 1, 2
2. Trauma: Direct injury to the nerve in the posterior triangle of the neck
3. Neoplasms: Tumors along the nerve pathway
4. Neurological disorders: Such as motor neuron disease

Advanced Assessment

If accessory nerve dysfunction is suspected based on clinical examination, further evaluation may be warranted:

Imaging Studies

• MRI orbits, face, and neck with contrast: Highest rated imaging modality (rating 8/9) for evaluating CN XI 1
• MRI head with contrast: Recommended to assess the intracranial portion of the nerve (rating 8/9) 1
• CT neck with contrast: Complementary to MRI for evaluating the extracranial course (rating 7/9) 1

Electrophysiological Testing

• EMG can help confirm the diagnosis and determine the level of injury 3
• Different segments of the nerve innervate specific muscle portions:
  • C1 rootlets primarily innervate the sternal part of sternocleidomastoid (95.8%)
  • C2 rootlets primarily innervate the clavicular part of sternocleidomastoid (90%)
  • C3 rootlets primarily innervate the superior part of trapezius (66.6%)
  • Lower rootlets innervate the middle part of trapezius 3

Clinical Pitfalls to Avoid

1. Incomplete assessment: Always test both sternocleidomastoid and trapezius muscles
2. Failure to compare sides: Bilateral comparison is essential for detecting subtle weakness
3. Overlooking compensatory mechanisms: Patients may develop compensatory movements to mask weakness
4. Missing associated cranial nerve deficits: CN XI dysfunction may occur alongside other lower cranial nerve palsies 1

Documentation

Document your findings clearly, including:

• Strength grading for each muscle (using standard 0-5 scale)
• Presence or absence of muscle atrophy
• Any associated symptoms (pain, limited range of motion)
• Comparison between sides

Remember that accessory nerve palsy typically manifests as weakness and atrophy of the sternocleidomastoid and trapezius muscles, causing decreased shoulder abduction, shoulder pain, cosmetic disfiguration, and functional disability 1.