Clinical Cases - Superficial Back

A 33-year-old woman undergoes a lymph node biopsy of her deep cervical nodes on the left side of her neck. Immediately following surgery, she complains of weakness in her shoulder. On exam, her left shoulder droops and she is unable to raise the point of her shoulder. She does not feel any numbness in her shoulder, back, or neck.

Questions to consider:

1. What nerve appears to have been inadvertently cut during the biopsy?
   

   It appears that this woman's spinal accessory nerve (Cranial Nerve XI) has been cut or injured. The accessory or spinal accessory nerve innervates the trapezius muscle, which is the only muscle that can elevate the point of the shoulder, or the acromion.

2. In most peripheral nerve injuries, there is usually a characteristic cutaneous sensory loss, producing numbness in a specific area. Why is there no numbness in this case?
   

   The spinal accessory nerve is unique among peripheral nerves in that it does not carry any sensory fibers whatsoever. Many peripheral nerves in the limbs, for instance, innervate both skeletal muscle and skin. A few peripheral nerves, however, are described as purely sensory or motor. In all cases, except for CN XI and a few other cranial nerves, this is a misnomer, however. Cutaneous sensory nerves all carry motor fibers that innervate sweat glands (secretomotor), blood vessel walls (vasomotor), and arrector pili muscles (pilomotor). Motor nerves to skeletal muscle all carry proprioceptive (sensory) fibers. These fibers return sensory information, concerning the state of contraction and position of joints, back to the central nervous system, providing feedback that enables coordination of movement. However, CN XI is truly only motor - it does not carry proprioceptive fibers.

3. If the spinal accessory nerve carries no proprioceptive fibers, how can there be coordinated movement of the trapezius muscle?
   

   Sensory branches from cervical spinal nerves C3 and C4 combine with branches of the spinal accessory nerve beneath the trapezius muscle. This is referred to as the subtrapezial plexus. These sensory branches return the proprioceptive information back to the cervical spinal cord , which is where the spinal accessory nerve originates. Hence, the motor and sensory signals combine for coordinated motion of trapezius.

4. If the spinal accessory nerve arises from the upper cervical spinal cord, why is it called the eleventh cranial nerve?
   

   Cranial nerve XI actually has two portions. The larger portion arises within the upper cervical spinal cord, ascends up through the foramen magnum in the base of the skull, and then leaves the skull again through the jugular foramen. So, it merely travels through the cranium briefly, and yet it is designated a cranial nerve. The other, smaller portion of the accessory nerve, referred to as the cranial accessory nerve, arises from the medulla oblongata and joins the vagus nerve (CN X), providing that nerve with the skeletal motor fibers destined for the larynx and pharynx.