Proximal spinal accessory nerve lesions cause weakness of the sternocleidomastoid and the trapezius muscles. Damage within the posterior triangle of the neck spares the sternocleidomastoid, resulting in trapezius weakness. With sternocleidomastoid weakness, there is weakness of turning the head to the opposite side. Involvement of the trapezius muscle manifests as drooping of the shoulder and mild upper scapular winging away from the chest wall, with slight lateral displacement. Weakness in shoulder elevation and arm abduction above horizontal is typical. Most individuals with accessory neuropathies also present with shoulder and neck pain.
The most common site of isolated accessory neuropathy is in the neck. The close association of the accessory nerve with the superficial cervical lymph nodes makes it vulnerable to iatrogenic damage during lymph node biopsy or radical neck dissection. The accessory nerve can also be directly compressed by swollen lymph nodes or other solid tumors. Rarely, accessory neuropathy occurs after blunt or penetrating neck trauma, or it is due to radiation injury with treatment of neck tumors. Damage can occur after carotid endarterectomy or jugular vein cannulation because of the nerve’s proximity to large neck vessels. Accessory neuropathy is sometimes seen as part of brachial plexitis or Parsonage-Turner syndrome.
Intrinsic spinal cord lesions, posterior fossa meningiomas, or metastases near the jugular foramen or foramen magnum may injure the intraspinal and intracranial portions of the accessory nerve but usually also affect the glossopharyngeal and vagal nerves. At times, the hypoglossal nerve exiting through the adjacent hypoglossal foramen is involved, as well as the adjacent sympathetic chain fibers, resulting in an associated Horner syndrome. Disorders of the anterior horn cell, including motor neuron disease, syringomyelia, and poliomyelitis, may involve the nuclei of the accessory nerve.