5.11. Cranial Nerve XI
- Anatomy:
- Purely motor nerve
- Cranial portion: caudal part of the N. ambiguus; spinal portion: (accessory nucleus C1–C5) of the ventral horn:
- C1–C2 innervates the ipsilateral SCM muscle
- C3–C4 innervates the ipsilateral trapezius
- Cranial part of nerve (N. ambiguus) supplies the pharynx and larynx; the internal ramus
- External ramus: innervates SCM and trapezius; receives branches from C2, C3, C4 (anterior roots)
- Corticobulbar fibers to the trapezius are primarily crossed; to the SCM muscle ipsilateral termination; possible that there is a decussation to the opposite pons and then a decussation back to terminate on the side of origin (ipsilateral hemisphere), the second decussation occurs at C1 of the cord
- Sternal head of SCM may receive bilateral cortical innervation (ipsi > contralateral) and double decussation (pons/cervical cord); clavicular head (tilts the head ipsilaterally)
- Corticobulbar innervation to the SCM is located dorsally in the brainstem tegmentum; fibers to the trapezius are located in the ventral brainstem
- Trapezius muscle:
- Retracts the head
- Raises the abducted arm above the horizontal
- Bilateral SCM weakness; weakness of neck flexors; head falls backward
- Unilateral trapezius weakness (accessory nerve lesion); upper trapezius fibers (lower fibers supplied by cervical plexus):
- There is a lower shoulder on the affected side
- The scapula is displaced down and laterally
- Paresis of shoulder elevation and retraction; cannot raise the arm above horizontal after it has been abducted (supraspinatus and deltoid)
- Bilateral trapezius weakness; weakness of neck extension
Anatomical Syndromes of XI
- Weakness of the SCM or trapezius:
- Lesions of accessory nerve distal to its bifurcation
- SCM and trapezius weakness same side:
- Contralateral brainstem lesion
- Ipsilateral high cervical cord lesion (interrupts second decussation from the pons)
- Proximal accessory nerve lesion
- SCM weakness with spared trapezius:
- Lesion of dorsal brainstem tegmentum (SCM represented dorsally in the tegmentum)
- Lesion of upper cervical accessory roots (lower cervical roots innervates the trapezius)
Nuclear Lesions of XI
- Atrophy and fasciculations of affected SCM and trapezius muscles
- Intraparenchymal tumor (astrocytoma, metastases)
- AVM or cavernous hemangioma
- Syringobulbia
Peripheral Lesion (within Skull and Foramen Magnum)
- Syndromic associations:
- Vernet's (jugular foramen); tumor, fracture, infection
- Schmidt's X, XI
- Collet-Sicard IX, X, XI, XII (retroparotid or retropharyngeal space)
- Garcin: all cranial nerve on one side (tumor); following chemotherapy; diabetes
Lesions within the Neck
- Posterior triangle surgery (lymph node biopsy)
- Internal jugular vein catheterization
- Blunt trauma to the shoulder
- Radiation therapy
- Dislocation of the shoulder
- X-RT (myokymia; hypertrophy)
- Brachial plexus traction injury with associated cervical plexus injury
- Partial hanging (suicide attempt)
- Posterior triangle injury (primarily occurs with lymph node biopsy for lymphoma; causes isolated trapezius weakness
- Aberrant vessel (cervical dystonia); pressure on the nerve
- Cervical cord trauma
Differential Diagnosis of Neck Extensor Weakness
- MG
- Motor neuron disease (MND); dropped head
- Polymyositis
- Dermatomyositis
- Inclusion body myositis
- FSH dystrophy
- Congenital absence SCM (flexor weakness)
Tumors
- Meningioma
- Schwannoma
- Chordoma (at jugular foramen)
- Glomus jugulare
- Base of the skull metastases
- Cervical syrinx with astrocytoma
- Hemangioblastoma with cervical syrinx
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