Injury, Spinal Accessory Nerve
From WikiCNS
Description
Injury to the spinal accessory nerve is often iatrogenic, classically occuring when a lymph node is removed from the posterior cervical chain. Significant nerve injury causes muscular wasting and shoulder weakness. Other causes of an accessory nerve injury include, blunt and sharp trauma to the neck, as well as skull base and foramen magnun pathology.
Anatomy
The accessory nerve is a "purely" motor nerve; it consists of a cranial root and spinal root. The cranial root arises from the caudal segment of the nucleus ambiguous and runs laterally toward the jugular foramen where it becomes united with the spinal root. The spinal root originates from the nucleus of the spinal accessory nerve, which extends from C1-C6. The spinal root runs between the dorsal roots of the spinal nerves and the dentate ligament, upwards through the foramen magnum where it meets the cranial root. After exiting the cranium, the accessory nerve passes deep to the sternocleidomastoid and then passes under the trapezius muscle. It innervates these two muscles; the sternocleiomastoid rotates the head to the opposite side, and the trapezius stabilizes the scapula, elevates (shrugs) the shoulder, and assists with scapular adduction, and arm abduction at the shoulder.
Clinical Presentation
Patients usually present with a dull ache of the shoulder region, an inability to use the affected arm overhead, scapular winging, and atrophy of the trapezius. A history of cervical trauma or recent surgery should be reviewed. Although a lower, drooping, shoulder is often present, many patients do not complain of this per se, being more concerned with the pain or weakness.
Diagnosis
The diagnosis of a spinal accessory nerve palsy usually relies on a pertinent history and examination. Weakness and atrophy that begins after a surgical procedure or penetrating trauma is often the case. On examination the affected shoulder is in a lower position (drooping) compared to the normal shoulder, trapezial atrophy is present, and some winging of the scapula may be evident at rest. These are some of the more useful examination findings; therefore the diagnosis can often be made with careful observation alone. Theere is weakness of shoulder shrug, however, the patient can often still shrug the shoulder, and some patients even have a symmetrical shrug because this movement is also performed by the levator scapulae, which is innervated by the dorsal scapular nerve. The trapezius assists in arm abduction above 90 degrees, therefore patients have trouble with this movement. When the patient windmills their arms in abduction overhead, incoordination of the affected scapula can be seen. Scapular winging also occurs, which unlike a serrartus anterior palsy, does not presist when the arm and shoulder are protracted forward. When the strenocleidomastoid muscle is weak or atrophic, skull base and foramen magnum pathology should be excluded with an MRI. Electrodiagnostic testing confirms injury to the spinal accessory nerve, and can help evaluate partial injuries, or those with early reinnervation. Nevertheless, some partial injuries without denervation may be difficult to diagnosis.
Treatment
Because the spinal accessory nerve is superficial and readily exposed, early surgical exploration and repair should be considered, especially with iatrogenic or sharp, lacerating injuries. Alternatively. when nerve continuity is likely, or if partial function is present, it may be prudent to observe these patients for 3-6 months with serial electrodiagnostic tests, and explore those patients who fail to recover. Transected nerves should be repaired directly, or with an interposition nerve graft obtained from the greater auricular or sural nerve. If the nerve not found transected and positive nerve action potentials are present, then an external neurolysis should be performed. For patients who do not have a viable proximal accessory nerve stump to repair (e.g., after it was removed with a skull base tumor), a split thickness nerve transfer from the hypoglossal to the spinal accessory nerve should be considered. For chronic palsies (older than 1-2 years), where nerve surgery is no longer an option, one may undergo a tendon transfer where the levator scapulae and rhomboids are advanced from under the scapular to over the margin of the scapula.
Outcome
As with other peripheral nerve injuries, graft length and the timing of repair determine outcome. Because this nerve is a "pure" motor nerve, an early repair has an excellent chance of good recovery. The tendon tranfer procedure described above provides scapular stability, and to a much lesser extent, active movement. A complete spinal accessory palsy is very debilitating and therefore should be treated aggressively.
