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Spinal Accessory Nerve (Cranial Nerve XI, CN XI)

OVERVIEW
Key Points:
• Examine this nerve when patient complains of shoulder dysfunction.  This nerve can be injured in procedures such as posterior triangle of the neck lymph node biopsy.
• Examine this nerve to see if it is available to use as a donor nerve in nerve transfer procedures (especially in brachial plexus injuries where this becomes a critical source of spared motor neurons).
• With complete loss of function, patients often complain of inability to abduct the shoulder past 90 degrees.
• Note scapular winging may be seen in both spinal accessory nerve (as well as long thoracic nerve) palsy—so with scapular winging also ‘think’ spinal accessory nerve palsy!
EXAMINATION
Physical examination of the spinal accessory nerve includes the motor examination of the trapezius and sternocleidomastoid muscles.  Note, that a more distal injury to the spinal accessory nerve may spare function to the sternocleidomastoid and interrupt function to the trapezius.  Also, the trapezius derives varying levels of innervation directly from a second source--cervical roots--and function may not be completely absent even with complete transection of the spinal accessory nerve.
 
Detailed Examination Links:
Trapezius - The trapezius is more thoroughly examined by examining all three portions individually (upper
middle, and lower).
Summarized Examination:
• The sternocleidomastoid muscle is examined by exerting pressure on the contralateral side of the patient’s chin and having the patient rotate at the neck to the opposite side by pushing against the examiners hand.  (The right sternocleidomastoid causes rotation of the neck to the opposite/left side.)  It can be directly visualized/palpated on thin patients.
• Quick examination of the trapezius can be done by resisting attempted shoulder shrug but this quick test can be misleading in cases of spinal accessory nerve injury as upper trapezius function, innervated by the cervical roots, will be spared.  More complete examination (see detailed examination link) is critical in cases of suspected spinal accessory nerve injury.
• Scapular winging, occurs with loss of trapezius function as the trapezius helps in stabilizing the scapula against the thorax especially in external rotation.  (Note that scapular winging is also seen in long thoracic nerve/serratus muscle dysfunction.) In fact, the scapular winging in spinal accessory nerve palsy may be even more dramatic then that seen with long thoracic nerve palsy/serratus anterior muscle dysfunction.
Pathologic Findings:
• With a complete spinal accessory nerve injury, the patient will still demonstrate some preserved upper trapezius function (innervated by cervical motor roots) and will be able to shrug their shoulder (levator scapulae muscle contributes to this as well).  However, depending on the amount of glenohumeral joint and other muscle compensation, the patient may not be able to abduct the shoulder past 90 degrees.
• With trapezius muscle weakness, the patient can fully abduct the shoulder overhead but with eccentric contraction--as the patient lowers their arm, scapular winging will be noted.
• Note that scapular winging occurs both in long thoracic nerve (to serratus anterior muscle) palsy as well as with spinal accessory nerve (to trapezius muscle) palsy.   
• Bottom line, the trapezius helps move/stabilize the scapula to allow normal shoulder motion (powered by the other muscles) with absent/abnormal trapezius function, the patient will note abnormal shoulder function (which on examination can be picked up on as abnormal scapulothoracic motion):
• Shoulder abduction requires upward rotation of the scapula by the trapezius with the scapula adducted to the midline (to keep it out of the way).  With loss of trapezius function, shoulder abduction is limited.
• Forward flexion of the shoulder, on the other hand, requires upward rotation of the scapula by the serratus with the scapula abducted from the midline.  Therefore, with abnormal trapezius function, the patient can forward flex the shoulder more then they
Clinical Relevance:
• This nerve is prone to injury in posterior neck procedures which includes lymph node biopsy.
• It is critical to examine this nerve and the main muscle that it innervates in brachial plexus or other nerve injuries. Function of the trapezius is generally preserved even in very proximal or root level injury brachial plexus injury patterns.
• This nerve can serve as a donor nerve for use in nerve transfer procedures (specifically spinal accessory to suprascapular nerve transfer) in upper brachial plexus palsy patients.
Relevant Anatomy:
Innervation
• Roots: Brainstem nucleus ambiguus, C1, C2, C3, C4.
• Nerve: Spinal accessory nerve.
• Muscles Innervated: Trapezius, sternocleidomastoid.
• Innervation Route:
• Brainstem, (C1), C2, C3 → spinal accessory nerve → sternocleidomastoid.
• Brainstem, C2, C3, C4 → spinal accessory nerve → trapezius.
• Also, there is some direct innervation via the C2-C4 motor roots that does not travel within the spinal accessory nerve. Therefore, with a complete transection of the accessory nerve in the posterior triangle of the neck, the patient will still have intact upper trapezius function and shoulder shrug.