The spinal accessory nerve is unique among the cranial nerves in being almost entirely spinal in origin. Its motor neurons sit not in a brainstem nucleus but in the upper cervical cord, between C1 and C5 or C6. From there, the nerve ascends into the skull through the foramen magnum, joins briefly with the cranial roots of the vagus, and exits with CN IX and CN X through the jugular foramen. Once in the neck, it innervates two muscles: the sternocleidomastoid and the trapezius. That is the entire territory. The exam is brief — about a minute — and the clinical content is uncomplicated, but the few patterns of dysfunction it produces are worth recognizing because they are easy to miss.

Functional Anatomy

The traditional teaching is that CN XI has two parts — a cranial root from the nucleus ambiguus of the medulla and a spinal root from the upper cervical cord — that travel together briefly and then separate. The cranial root is now generally considered part of the vagus rather than a true CN XI component; many anatomists therefore refer to CN XI simply as the spinal accessory nerve. The clinically relevant fibers are the spinal ones.

Cell bodies of the spinal motor neurons lie in the lateral portion of the ventral horn from C1 through about C5 or C6. Axons exit the cord laterally, ascend along the spinal canal, pass through the foramen magnum, briefly contact the cranial accessory rootlets, and then exit the skull through the jugular foramen. Outside the skull, the nerve runs into the posterior triangle of the neck, where it crosses superficially and is anatomically vulnerable to surgical injury (especially during lymph node biopsy in this region). It supplies the sternocleidomastoid on its way down, then enters the deep surface of the trapezius and innervates it from above downward.

This superficial course is one of the most clinically important facts about CN XI. A surgeon retracting in the posterior triangle of the neck can transect or stretch the nerve in a procedure as routine as a lymph node biopsy, leaving the patient with trapezius weakness, shoulder droop, and chronic shoulder pain. Iatrogenic CN XI injury after cervical surgery is one of the most common causes of isolated accessory nerve palsy and is increasingly the subject of medicolegal review.

The sternocleidomastoid receives partly bilateral cortical input; the trapezius is more lateralized. A unilateral hemispheric lesion can therefore produce ipsilateral (rather than contralateral) sternocleidomastoid weakness — because the muscle’s primary action is to turn the head away from itself, and a left hemispheric lesion weakens the right-turning function, which depends on the left sternocleidomastoid. This counterintuitive innervation pattern accounts for the occasional unilateral SCM weakness ipsilateral to a stroke.

The Examination

Sternocleidomastoid

The sternocleidomastoid pulls the head down on the same side and turns the face to the opposite side. Two tests follow from this:

1. Ask the patient to turn their head against your hand placed on the jaw. Resisting head turn to the right tests the left sternocleidomastoid (the muscle that contracts to turn the head right). Compare sides for strength.
2. With the patient in the supine position, ask them to lift the head off the bed against gravity. Bilateral weakness is a sign that bilateral CN XI or bulbar dysfunction is present; neck flexion weakness is one of the early signs of generalized myasthenia gravis and of many myopathies. Marked neck flexion weakness in a patient with no obvious focal deficit raises the differential of neuromuscular junction disease, polymyositis, dermatomyositis, and motor neuron disease.

Inspect the muscle for bulk. Chronic CN XI denervation produces visible atrophy of the sternocleidomastoid, with a sunken contour at the base of the neck. Bilateral atrophy is part of the pattern in myotonic dystrophy and in motor neuron disease.

Trapezius

The trapezius is a broad sheet with three functional parts. The upper fibers elevate and brace the shoulder; the middle fibers retract the scapula; the lower fibers depress and rotate the scapula. The accessory nerve supplies all three. The classical tests:

1. Shoulder shrug: ask the patient to elevate both shoulders to the ears while you resist downward. Asymmetric shrug reveals weakness on the affected side.
2. Scapular position: from behind, look at the resting position of the scapulae. Loss of trapezius tone produces a characteristic posture — the scapula sits lower, slightly more laterally placed, and rotated so that the inferior angle moves outward. The vertebral border may flare, and the shoulder droops.
3. Arm abduction: trapezius weakness limits arm abduction above the horizontal. The patient may abduct to about 90 degrees normally but cannot continue past it because the trapezius is needed to rotate the scapula upward for full overhead movement. Pain on prolonged use, weakness on lifting overhead, and a sense of shoulder instability are typical complaints.

Trapezius weakness is sometimes confused with rotator cuff disease or shoulder impingement, and patients are sometimes referred to orthopedics or physical therapy for what is in fact a neurological problem. The distinguishing features are the scapular winging on inspection (long thoracic nerve palsy gives a different pattern of winging, see below), the visible loss of upper trapezius bulk at the angle of the neck and shoulder, and the absence of true rotator cuff signs.

Distinguishing the Two Causes of Scapular Winging

Scapular winging is one of the most commonly confused findings in clinical practice. The two principal causes are CN XI palsy (trapezius weakness) and long thoracic nerve palsy (serratus anterior weakness). The pattern of winging is different, and once you see both, they are not easily confused.

Patterns of Dysfunction

Isolated CN XI Palsy

The most common cause is iatrogenic injury during cervical surgery, especially posterior triangle lymph node biopsy. Patients present with shoulder droop, weakness on arm abduction above the horizontal, chronic shoulder pain, and on exam — visible trapezius atrophy with scapular winging on resisted abduction. The diagnosis is often delayed for months because the patient is referred for orthopedic evaluation first. The neurological history of recent neck surgery, combined with the characteristic scapular pattern, makes the diagnosis at the bedside.

Non-surgical causes include radiation to the neck, malignant infiltration of the nerve in head and neck cancer, blunt trauma to the lateral neck, and rare neuralgic amyotrophy involving the accessory nerve. Compression in the posterior triangle by a benign mass — lipoma, lymphadenopathy — also occurs.

Combined CN IX, X, XI Lesions (Jugular Foramen Syndromes)

Discussed in the CN IX and CN X page. A mass at the jugular foramen affects all three nerves passing through it. Recognition depends on combining the palatal weakness, pharyngeal sensory loss, and accessory nerve findings into a single clinical picture and imaging the skull base.

Motor Neuron Disease

ALS commonly involves the cervical motor neurons that supply CN XI, and trapezius and sternocleidomastoid weakness can be part of the upper limb and bulbar onset patterns. Wasting of the sternocleidomastoid and shoulder girdle, fasciculations, and brisk reflexes in adjacent muscles are typical.

Spasmodic Torticollis (Cervical Dystonia)

This is not a CN XI weakness but a dystonia involving the muscles supplied by CN XI (and others). The head is involuntarily turned, tilted, or pulled, often with associated tremor of the neck. The clinical pattern is named for the dominant abnormal posture: torticollis (rotation), laterocollis (tilt), retrocollis (extension), antecollis (flexion). The diagnosis is clinical, the treatment is botulinum toxin injection of the involved muscles, and the natural history is one of slow progression in many patients with stable or self-limited courses in others. A geste antagoniste — the sensory trick by which a light touch to the chin or face transiently reduces the dystonia — is highly characteristic.

🔍 Did You Know?

The “sleeping” CN XI is a recognized clinical situation in which the sternocleidomastoid is selectively spared while the trapezius is profoundly weak — pointing to a lesion in the posterior triangle of the neck, below the SCM branch. Conversely, lesions above the SCM branch (at the jugular foramen or higher) weaken both muscles together. The split pattern therefore has localizing value: pure trapezius weakness without SCM involvement implies a posterior triangle lesion until proven otherwise.

Pitfalls and Pearls

• The sternocleidomastoid turns the head to the opposite side. Weakness on the left causes difficulty turning the head to the right.
• Test neck flexion against gravity. Bilateral weakness raises the differential of myasthenia gravis, polymyositis, motor neuron disease, and other neuromuscular conditions before it is “CN XI.”
• Trapezius weakness is misdiagnosed as rotator cuff disease. A patient with a shoulder problem and recent neck surgery deserves a careful CN XI exam.
• Scapular winging has two principal causes: trapezius weakness (CN XI) and serratus anterior weakness (long thoracic nerve). Inspect at rest and during the appropriate provocation maneuver — abduction or wall-push — to tell them apart.
• Iatrogenic injury during posterior triangle lymph node biopsy is the single most common cause of isolated CN XI palsy. Ask about recent neck surgery in every patient with a new shoulder droop.
• Sternocleidomastoid asymmetry in a patient with a stroke may be ipsilateral to the lesion — the muscle turns the head to the opposite side, so the muscle weakened by a contralateral cortical lesion is the ipsilateral one. Pure CN XI lesions weaken the SCM on the same side as the trapezius.
• Cervical dystonia is not weakness. The muscles are overactive, not paretic. Botulinum toxin to the dystonic muscles is the standard treatment.

References

1. Campbell WW. DeJong’s The Neurologic Examination. 7th ed. Philadelphia: Lippincott Williams & Wilkins; 2013. Chapter 19.
2. Kelley MJ, Kane TE, Leggin BG. Spinal accessory nerve palsy: associated signs and symptoms. J Orthop Sports Phys Ther. 2008;38(2):78-86.
3. Wiater JM, Bigliani LU. Spinal accessory nerve injury. Clin Orthop Relat Res. 1999;(368):5-16.
4. Albanese A, Bhatia K, Bressman SB, et al. Phenomenology and classification of dystonia: a consensus update. Mov Disord. 2013;28(7):863-873.
5. Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 7th ed. Philadelphia: Wolters Kluwer; 2017.