What is the step-wise method for examining the eleventh (XI) cranial nerve, also known as the accessory nerve?

Examination of the Eleventh Cranial Nerve (Accessory Nerve)

Overview

The examination of the accessory nerve (CN XI) involves a systematic assessment of the sternocleidomastoid (SCM) and trapezius muscles through inspection, palpation, and strength testing against resistance. 1, 2

Step-by-Step Examination Method

Step 1: Inspection

• Observe both shoulders for symmetry and positioning while the patient is seated or standing with arms relaxed at their sides 2
• Look for visible atrophy or asymmetry of the trapezius muscles, particularly noting any drooping of the shoulder on the affected side 3
• Inspect the neck contour for SCM muscle bulk, comparing both sides for any obvious wasting 1

Step 2: Palpate the Muscles

• Palpate the SCM muscle belly bilaterally to assess for muscle tone and bulk 1
• Palpate the upper trapezius muscles along the shoulder girdle to detect atrophy or fasciculations 3

Step 3: Test Sternocleidomastoid Function

• Ask the patient to turn their head to one side against your hand resistance placed on the jaw 1
• The SCM on the opposite side (contralateral) contracts to turn the head - for example, the left SCM turns the head to the right 1
• Test both sides and compare strength 4
• Note: The sternal and clavicular portions of the SCM have distinct innervation patterns, with upper cervical rootlets (C1) primarily innervating the sternal portion 1

Step 4: Test Trapezius Function

• Ask the patient to shrug both shoulders upward against downward pressure from your hands placed on top of the shoulders 3, 4
• Assess the strength and symmetry of the shoulder elevation 2
• The superior portion of the trapezius is primarily responsible for shoulder elevation, innervated by cervical rootlets at the C3 level and below 1

Step 5: Additional Trapezius Assessment

• Have the patient abduct the arm beyond 90 degrees - weakness in this movement suggests trapezius dysfunction since the trapezius stabilizes the scapula for full arm abduction 2
• Observe for scapular winging when the patient pushes against a wall with extended arms 4

Anatomical Considerations

Functional Organization

• The spinal root of CN XI has a cranio-caudal motor organization: C1 rootlets innervate the sternal SCM, C2 rootlets innervate the clavicular SCM, C3 rootlets innervate the superior trapezius, and lower rootlets innervate the middle trapezius 1
• The cranial root of CN XI contributes to vocal cord innervation, not to SCM or trapezius function 1

Clinical Pitfalls

• Do not confuse CN XI palsy with cervical radiculopathy - CN XI dysfunction causes isolated SCM and trapezius weakness without sensory loss 4
• The accessory nerve has significant anatomical variations (documented in 15% of cases), including variations in its course through the posterior cervical triangle 2
• Bilateral testing is essential because subtle unilateral weakness may only be apparent with side-to-side comparison 4

Imaging When Dysfunction is Detected

Primary Imaging Approach

• MRI of the head without and with IV contrast combined with MRI of the orbit, face, and neck is the preferred imaging modality (rated 8/9 by ACR guidelines) when CN XI dysfunction is identified 5
• CT neck with IV contrast is an appropriate alternative (rated 7/9) and can be complementary to MRI 5
• Imaging protocols should visualize the entire nerve pathway from the brainstem nuclei through the jugular foramen to the muscles in the neck 3