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The accessory nerve is the eleventh of what are known as the cranial nerves and is divided into a main part, known as the spinal accessory nerve, and a lesser part which is referred to as the cranial portion. Twelve pairs of cranial nerves arise from the tissue of the brain and travel out of the skull carrying information concerning the senses, muscles and glands. The spinal parts of the accessory nerves travel to the neck and shoulder area, where they supply muscles known as sternocleidomastoid and trapezius. Spinal accessory nerve damage leads to these muscles becoming weak or paralyzed, causing pain and loss of function.
Sternocleidomastoid acts to bend the neck, and tilts and rotates the head, while trapezius lifts the shoulder and pulls the head back. Spinal accessory nerve injury can produce different symptoms depending on which section of the nerve is damaged, but problems relating to trapezius are more noticeable because other muscles apart from sternocleidomastoid are involved in neck flexion and head rotation. Loss of normal trapezius function causes shoulder pain, which can extend to the upper back, neck and arm. Typically, the shoulder becomes weak and sometimes droops, it is difficult to raise the arm, and the shoulder blade may stick out.
In cases of accessory nerve damage where the trapezius muscle no longer functions, the site of the injury is usually an area, through which the nerve passes, known as the posterior triangle of the neck. This area lies behind the sternocleidomastoid, which is the diagonal strap of muscle running down from the ear to the inner end of the collar bone. The posterior triangle is often affected by surgical procedures such as removal of lymph nodes in the neck, and this type of operation is a common cause of accessory nerve injury. Other causes include stabbing or gunshot injuries, and pressure, which could be the result of a heavy blow, whiplash, or even an overenthusiastic neck massage.
Treatment of accessory nerve damage affecting the trapezius muscle is important, because the pain and loss of normal shoulder function can have a great effect on a person's everyday life. If the accessory nerve is injured during a surgical operation, then ideally it can be repaired as part of the same procedure. In cases where the damage is discovered later, the outlook is better if a repair is made within three months of the original injury.