Accessory Nerve

by S. Serkan Sezer MD

The accessory nerve is the eleventh of the twelve pairs of cranial nerves and occupies a special place in anatomy and clinical medicine. Although it consists of only a few thin fibres, the muscles it controls — those of the neck and shoulders — are involved in many of the movements we make every day, from turning the head to shrugging the shoulders. Its anatomy makes the nerve vulnerable to injury, and damage to it can lead to lifelong functional problems.

Definition

In English medical dictionaries, the term is transcribed as /əkˈsesəri nɜːv/ and defined as “the eleventh cranial nerve which supplies the muscles in the neck and shoulders.”

In more detail, the accessory nerve (Latin nervus accessorius) is the eleventh cranial nerve (CN XI) and innervates two important neck and shoulder muscles: the sternocleidomastoid and the trapezius. It is also known as the spinal accessory nerve.

The name “accessory” reflects the historical view that the nerve was a “supplementary” or “added” component to the vagus nerve, and the name has persisted to the present day.

Anatomy

The anatomy of the accessory nerve is unusual: unlike most other cranial nerves, it has two distinct roots and a complex pathway.

Two Components

Classical anatomy describes the nerve as having two parts:

1. Cranial Component (Cranial Root)

  • Arises from the nucleus ambiguus in the medulla oblongata
  • Has a short course
  • Joins the vagus nerve (CN X)
  • Innervates the muscles of the soft palate, pharynx, and larynx
  • In modern anatomy, this part is often regarded as a component of the vagus nerve

2. Spinal Component (Spinal Root)

  • Arises from the upper five or six cervical spinal segments (C1–C5/C6)
  • Originates from the lateral horn of the spinal cord
  • Travels upwards
  • Enters the cranium through the foramen magnum
  • Leaves the cranium through the jugular foramen
  • Innervates the sternocleidomastoid and trapezius muscles

Modern Anatomical View

According to current anatomical thinking, the “true” accessory nerve consists only of the spinal component. The cranial component is now considered part of the vagus nerve. Throughout this article, “accessory nerve” refers principally to the spinal component.

Course of the Nerve

The course of the accessory nerve can be traced through several stages:

  1. Spinal root — fibres from the C1–C5/C6 segments unite to form a trunk
  2. Foramen magnum — the nerve enters the cranium
  3. Inside the skull — runs briefly together with the cranial root
  4. Jugular foramen — exits the cranium together with cranial nerves IX (glossopharyngeal) and X (vagus)
  5. Carotid triangle — passes lateral to the internal jugular vein
  6. Sternocleidomastoid — enters the deep surface of the muscle, gives off a branch to it
  7. Posterior triangle of the neck — emerges from the posterior border of the sternocleidomastoid, runs across the posterior triangle
  8. Trapezius — enters the deep surface of the muscle and innervates it

Surface Anatomy

The route of the nerve in the posterior triangle of the neck is clinically important:

  • Emerges at roughly the midpoint of the posterior border of the sternocleidomastoid
  • Crosses the posterior triangle in an oblique direction
  • Enters the trapezius about 2–5 cm above the clavicle
  • In this region the nerve runs very superficially, just under the skin, and is therefore vulnerable to surgical injury

Function

The accessory nerve is essentially a motor nerve and innervates two important muscles:

Sternocleidomastoid Muscle

Functions:

  • Turning the head to the opposite side — unilateral contraction
  • Tilting the head to the same side — unilateral contraction
  • Flexing the head forwards — bilateral contraction
  • Accessory respiratory function — particularly during deep breathing
  • Stabilisation of the head

Trapezius Muscle

A large, triangular muscle with three functional parts:

Upper Part

  • Shoulder elevation (shrugging)
  • Upward rotation of the scapula
  • Support of the head

Middle Part

  • Retraction of the scapula (pulling the shoulder blade towards the spine)
  • Stabilisation of the scapula

Lower Part

  • Depression of the scapula
  • Upward rotation of the scapula
  • Stabilisation of the scapula

The trapezius is critical for normal positioning and movement of the scapula and for raising the arm above the head.

Sensory Function

Although the accessory nerve is generally regarded as a pure motor nerve, some studies suggest that it also carries a small number of proprioceptive fibres — sensory signals from the muscles to the brain about their position and state of contraction.

Clinical Examination

Assessment of the function of the accessory nerve is a fundamental part of the neurological examination.

Examination of the Sternocleidomastoid

  1. Inspection — comparison of the two sides for symmetry and atrophy
  2. Palpation — feeling the muscle when it contracts
  3. Motor function
    • The patient is asked to turn the head to one side
    • The examiner provides resistance with the palm against the side of the face
    • Each side is tested in turn (the right SCM turns the head to the left, and vice versa)
  4. Strength assessment — graded on the MRC scale from 0 to 5

Examination of the Trapezius

  1. Inspection — comparison of the shoulders; observation of any drooping
  2. Palpation — feeling the upper edge of the muscle
  3. Motor function
    • The patient is asked to shrug both shoulders
    • The examiner pushes down on the shoulders to provide resistance
    • The two sides are compared
  4. Other tests
    • Inspection of the scapula
    • Resistance test for shoulder abduction
    • Wall push-up test for signs of scapular winging

Specific Findings

In accessory nerve palsy the following may be seen:

  • Asymmetry of the SCM
  • Weakness in turning the head
  • Shoulder drop
  • Atrophy of the trapezius
  • Winging of the scapula — particularly lateral
  • Limited shoulder elevation
  • Limited overhead arm movement

Pathology

Many conditions can affect the accessory nerve.

Causes of Accessory Nerve Injury

1. Iatrogenic Injury (Caused by Medical Procedures)

The most common cause:

  • Cervical lymph node biopsy — particularly biopsies from the posterior triangle
  • Radical or modified neck dissection
  • Removal of cervical masses
  • Salivary gland surgery (parotidectomy)
  • Carotid endarterectomy
  • Central venous catheter insertion

2. Traumatic Injury

  • Penetrating neck injuries — stab wounds, gunshot wounds
  • Blunt trauma — traffic accidents, sports injuries
  • Surgical complications
  • Stretch injury — sudden forceful movements

3. Tumour Compression

  • Tumours at the base of the skull
  • Jugular foramen tumours (glomus tumours)
  • Cervical lymphomas
  • Metastases
  • Neurofibromas

4. Inflammatory and Infectious Causes

  • Viral neuritis
  • Parsonage–Turner syndrome (neuralgic amyotrophy)
  • Tuberculous lymphadenitis
  • Lyme disease

5. Other Causes

  • Diabetic neuropathy
  • Vasculitis
  • Radiation damage
  • Idiopathic (no identifiable cause)

Levels of Injury

The clinical picture varies according to the level at which the nerve is injured:

Proximal Injury (Inside the Skull or at the Jugular Foramen)

  • Both SCM and trapezius affected
  • Often accompanied by injury to other cranial nerves (IX, X, XII)
  • Vernet’s syndrome — combined IX, X, XI palsy
  • Collet–Sicard syndrome — combined IX, X, XI, XII palsy

Injury in the Neck

  • The level of injury determines which muscles are involved
  • Injury proximal to the SCM: both muscles affected
  • Injury distal to the SCM: only the trapezius affected
  • Injury in the posterior triangle: only the trapezius

Clinical Features of Accessory Nerve Palsy

The symptoms depend on the affected muscle and the level of the lesion:

Symptoms of Sternocleidomastoid Weakness

  • Difficulty turning the head
  • Asymmetry of the head
  • Weakness on turning the head
  • Atrophy of the muscle

Symptoms of Trapezius Weakness

  • Shoulder drop
  • Inability to fully elevate the arm above the head
  • Winging of the scapula
  • Shoulder and neck pain
  • Heaviness in the arm
  • Limitation of the arm in elevation and abduction

Long-Term Effects

  • Chronic shoulder pain
  • Frozen shoulder
  • Tendonitis of the rotator cuff
  • Atrophy of the trapezius
  • Postural disturbance
  • Functional disability

Diagnosis

The diagnosis is established by a combination of clinical and laboratory tests:

Clinical Examination

  • Detailed neurological examination
  • Assessment of muscle strength
  • Inspection of muscle atrophy
  • Range-of-motion testing

Imaging Studies

  • MRI — for soft-tissue evaluation
  • CT — useful for bony structures
  • Ultrasonography — for nerve and muscle assessment
  • Cervical MR neurography — for direct visualisation of the nerve

Electrodiagnostic Tests

  • Electromyography (EMG) — assesses muscle activity
  • Nerve conduction studies — measures speed and amplitude of conduction
  • Repeated EMG — for follow-up

Other Tests

  • Blood tests — to exclude inflammation, infection, autoimmune disease
  • Lumbar puncture — when meningitis or other CNS disease is suspected
  • Genetic tests — for hereditary conditions

Treatment

Treatment is shaped by the cause of the injury and the time elapsed:

Conservative Treatment

Early Period

  • Rest and protection
  • Pain management — NSAIDs, in some cases corticosteroids
  • Cold application

Physical Therapy and Rehabilitation

  • Range-of-motion exercises
  • Strengthening exercises
  • Postural training
  • Manual therapy
  • Electrical stimulation
  • Ultrasound therapy

Compensatory Strategies

  • Strengthening of accessory muscles — rhomboids, levator scapulae
  • Activity modifications
  • Use of orthoses

Surgical Treatment

Surgery is considered in selected cases:

Direct Nerve Repair

  • For sharp, recently injured nerves
  • End-to-end suture if there is no gap
  • Nerve grafting if there is a gap
  • Microsurgical technique

Nerve Transfer

  • Use of the C7 branch of the brachial plexus
  • For long-standing injuries
  • Often used in combination with other procedures

Muscle Transfer

  • Eden–Lange procedure — transfer of the levator scapulae, rhomboid major and minor to the scapula
  • For late-stage cases
  • For functional reconstruction

Scapular Stabilisation

  • Scapulothoracic fusion — in advanced cases
  • Scapulopexy
  • Used for selected indications

Timing

The timing of surgery is critical:

  • 0–3 months — observation and conservative treatment
  • 3–6 months — for cases that fail to improve, consideration of surgery
  • 6–12 months — surgery for permanent injuries
  • Beyond 12 months — direct nerve repair becomes more difficult; functional reconstruction is considered

Prognosis

The outlook depends on several factors:

Favourable Factors

  • Early diagnosis and intervention
  • Partial injury
  • Lower-level lesion
  • Younger age
  • Adherence to physical therapy
  • The cause being a reversible factor

Less Favourable Factors

  • Complete transection
  • Late diagnosis
  • High-level lesion
  • Older age
  • Coexisting medical conditions
  • Tumour-related compression

In general, the success rate of direct nerve repair within the first 3–6 months can exceed 70–80%. After longer periods, success rates fall.

Prevention

Preventing iatrogenic accessory nerve injury is critical:

In Surgical Procedures

  • Thorough anatomical knowledge
  • Cervical lymph node biopsy in the posterior triangle should generally be avoided
  • Use of nerve identification techniques
  • Intraoperative nerve monitoring
  • Careful dissection technique
  • Adequate exposure

In Lymph Node Biopsy

  • Selective biopsy where possible
  • Avoidance of the posterior triangle
  • Selection of an alternative site
  • Use of fine-needle aspiration biopsy

Patient Information

  • Information about the risks of surgery
  • Postoperative monitoring
  • Reporting of early warning signs

Living with Accessory Nerve Injury

Several considerations are important for people living with the consequences of injury:

Activities of Daily Living

  • Difficulty with overhead movements
  • Carrying heavy objects
  • Reaching activities
  • Difficulty dressing
  • Personal hygiene

Adaptation Strategies

  • Modifying activities
  • Use of assistive devices
  • Ergonomic arrangements
  • Workplace adaptations

Psychosocial Support

  • Information and education
  • Support groups
  • Psychological counselling
  • Vocational rehabilitation

Long-Term Follow-Up

  • Regular medical follow-up
  • Continuation of physical therapy
  • Pain management
  • Coordination with the rehabilitation team

Anatomical Variations

The accessory nerve can show several anatomical variations:

  • Variations in origin
  • Variations in course
  • Connections with the cervical plexus
  • Branching patterns
  • Variations in muscle innervation

These variations are important during surgical procedures.

Embryological Development

The accessory nerve develops from neural crest cells:

  • Begins to form between the 4th and 5th weeks
  • Spinal motor nuclei form
  • Migration of nerve fibres
  • Development of muscle innervation
  • Establishment of the final anatomy

Comparative Anatomy

The structure of the accessory nerve varies between species:

  • In mammals — generally similar structure
  • In primates — well developed
  • Adaptation in humans — important for upright posture
  • In animals — variations related to species-specific functions

Research and Future

Research on the accessory nerve continues in several areas:

  • Neural regeneration studies
  • Stem cell therapies
  • Bioengineering — synthetic nerve conduits
  • Robotic surgery — more delicate interventions
  • Nerve transfer techniques
  • Functional electrical stimulation

Historical Background

The history of the accessory nerve is a fascinating part of the history of medicine:

  • Galen — earliest descriptions
  • 17th–18th centuries — detailed anatomical studies
  • Thomas Willis — described the nerve in detail in 1664
  • 19th century — clinical importance recognised
  • 20th century — surgical techniques developed
  • 21st century — modern microsurgery and rehabilitation

Disclaimer

The information provided here is intended for general informational and educational purposes only and does not constitute medical advice, diagnosis, or treatment. If you have symptoms of accessory nerve injury or suspect such an injury after a surgical procedure, please consult a qualified healthcare professional (neurologist, neurosurgeon, or otolaryngologist). The treatment and rehabilitation of accessory nerve injuries require a multidisciplinary approach and should be planned by appropriately qualified specialists. Do not rely on the content of this article when making decisions about your own or another person’s medical care.