Spinal
accessory nerve block is indicated for severe tonic or clonic spasm of the
trapezius and sternocleidomastoid muscles. Together with the cervical
plexus, it can be blocked for carotid surgery.
The
accessory nerve is a cranial nerve that emerges from the jugular foramen.
The cranial portion separates and joins the vagus nerve. The spinal
accessory nerve continues into the upper part of the sternocleidomastoid
muscle, emerging from its posterior border at the junction of the lower and
middle thirds of this muscle. There, the nerve crosses the neck to supply
the trapezius muscle. The spinal accessory nerve can be blocked in the
substance of the sternocleidomastoid muscle.
A 2.5 cm, 23
G needle is inserted into the sternocleidomastoid muscle, 2 cm below the tip of
the mastoid process, with the patient's head turned to the opposite side to
make the muscle prominent. A quantity of 5 ml local anesthetic injected
at this site blocks the branches to the sternocleidomastoid and trapezius
muscles. This block results in weakness of the muscles innervated by the spinal
accessory nerve; for example, difficulty in lifting the head off the bed,
turning the head, or raising the arm above the head.
A successful
block is recognized by the absence of contraction of the sternocleidomastoid
muscle when the patient turns the head to the opposite side and weakness of the
trapezius muscle when the patient attempts to shrug his or her shoulders.
No serious
complications occur when the local anesthetic is placed in the substance of the
muscle. Block of the lesser occipital nerve may result in numbness behind
the ear.