Causes of Vocal Fold Immobility
Unilateral vocal fold immobility can be caused by a variety of diseases or disorders. Almost all of these disorders prevent movement in one of two ways: they either affect the nerve that supplies stimulation to the muscles of the vocal fold, or they mechanically interfere with the movement of the vocal fold.
VOCAL FOLD PARALYSIS
Vocal fold immobility is most frequently caused by a paralysis of the nerve that controls movement of the laryngeal muscles. This nerve is called the recurrent laryngeal nerve, and its anatomy is shown in the illustation to the left.
On both the left and the right side there is a nerve called the vagus nerve that travels down from the base of the skull into the upper neck. Up high in the neck there is a branch called the superior laryngeal nerve. This superior branch then divides again into an internal branch that mainly is responsible for our sensation in the larynx, and an external branch that goes to one of the muscles of the larynx. This particular muscle (it is called the cricothyroid) is primarily responsible for increasing the tension, and therefore the pitch, of the vocal folds.
The other major branch of the laryngeal nerve is the inferiorlaryngeal nerve. During growth of the fetus, this nerve is pulled down quite far into the neck and chest and it then loops back up to the larynx. For this reason it is also (more commonly) called the recurrent laryngeal nerve. On the left, the nerve travels around the aorta. On the right, it does not drop as low, but instead loops around a big blood vessel called the subclavian artery. It is this nerve that usually produces vocal fold immobility.
Because of this anatomy, the nerve can be stretched or cut during surgery in this region. For example, cardiac surgery or other work near the aorta can lead to a left vocal fold paralysis. Other operations in the neck that can cause weakness or complete paralysis of the nerve include thyroid surgery, carotid endarterectomy (for blocked vessels) or surgery on vertebrae in the neck. Surgeons work hard to protect this nerve while operating, but sometimes it just doesn't work well after surgery.
The nerve can also be compressed by tumors or other growths anywhere along its path from the brain to the larynx. A tumor at the base of the skull or in the upper left lung can produce paralysis. In infants there is a condition called Arnold-Chiari malformation in which the lower portion of the brainstem is displaced downward, putting pressure on both nerves and sometimes causing bilateral vocal fold paralysis.
CRICARYTENOID JOINT MOBILITY PROBLEMS
The muscles that move each vocal fold are attached to a small piece of cartilage called the arytenoid. The arytenoid sits on top of another ring of cartilage called the cricoid. There is a joint that separates these two pieces of cartilage and usually allows the two to move relative to each other. In some cases this joint can become inflamed and move poorly or not at all.
One example of this is arthritis. Some people with very severe arthritis will also have problems with the cricoarytenoid joint. Another case of cricoarytenoid joint problems occurs in people who have been intubated for a long period of time. In these cases the joint can become extremely inflamed and lock up.
VOCAL FOLD TUMOR INVASION
Vocal fold immobility can also occur due to invasion of the muscles or the joint with cancer. The most common type of cancer causing this is squamous cell cancer. When this occurs there usually are other signs of the cancer seen on laryngeal examination. However, it is possible for a cancer to be predominately beneath the lining of the larynx ("submucosal") and its only sign or symptom is an immobile vocal fold.
The vocal fold can also be immobile due to a phenomenon called synkinesis. Synkinesis can occur after injury to a nerve. Many nerves in the body consist of multiple small fibers contained within a larger bundle, like bundles of fine wire in a larger wire cable. Each fiber goes to a particular muscle group. After injury, nerves can sprout new endings that attempt to attach to the other end of the broken nerve. But if small nerve fibers are very close to each other, a spouting fiber may by mistake connect with a adjacent fiber. As a result, when the signal comes from the brain to contract a certain muscle, a different muscle receives the signal. This is called synkinesis.
In the recurrent laryngeal nerve, there are nerve fibers that go to the muscles that open the vocal folds, and other fibers to the muscles that close the vocal folds. In the uninjured nerve, these muscles usually do not contract at the same time. For example, when we breathe the brain sends a signal to the muscles that open the vocal folds.
After injury to the recurrent laryngeal nerve, there can be crossing of the recovering nerves during recovery. As a result when the brain sends the signal to open the vocal folds, there is contraction of both the opening and the closing muscles. The vocal fold may not move, similar to an isometric contraction in the arm or the leg.