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The superior laryngeal nerve (SLN) is a part of the vagus nerve located near the larynx in the throat. It is separated into two branches, the internal branch and the external branch. The internal superior laryngeal nerve (IBSLN) enervates the larynx’s mucus membranes, and the external superior laryngeal nerve (EBSLN) controls the cricothyroid and inferior constrictor muscles found in the throat. Damage to the superior laryngeal nerve can cause paralysis of the nerve, changing the sound of a person’s voice or causing an inability to talk. Speech therapy may help a person with a SLN injury to regain some normal voice functions.
Two functions of the superior laryngeal nerve are sensory and motor. The sensory part of the nerve’s function is related to the afferent signals sent from the supraglottic larynx. Motor signals are sent through the SLN to the ipsilateral cricothyroid muscle located in the throat.
Vocal pitch is determined by the superior laryngeal nerve. Impulses from the nerve are relayed to the cricothyroid muscles causing them to contract; the contraction moves the cricoid lamina, which then moves the vocal folds. The pitch of a person’s voice is changed by the tensioning and lengthening of the vocal folds when the cricothyroid muscle contracts.
A serious complication of thyroid surgery is caused by an injury to the SLN during the operation. Located near the superior thyroid artery, the nerve is difficult to locate at the beginning of surgery because it is in a slightly different location in each person. The trauma to the EBSLN can cause the voice to sound different or result in modulation problems.
Other causes of damage to the superior laryngeal nerve that may cause paralysis are varied. A tumor located at the base of the skull may cause the SLN to become compressed and non-functional. Tumors found in the esophagus, known as oesophageal tumors, may put pressure on the SLN and compromise its functionality. Blunt force trauma to the throat may cause injury to the SLN that can cause paralysis of the nerve.
Symptoms of SLN paralysis are voice related. A person’s voice may become raspy and weak after a short period of time talking, and the ability to increase the volume of the voice may be affected by the paralysis of the nerve. Another effect of SLN damage occurs when a person is singing; the paralysis causes an inability to increase the volume of the singing voice.
Diagnostic tests may need to be performed to evaluate the function of the superior laryngeal nerve. A test called a laryngeal electromyograhic exam is used to measure the action potentials sent out from the muscles controlled by the SLN. Electrodes are placed on the throat, chest, and clavicle, and a small thin needle is inserted on both sides of the cricothyroid muscle during the test. No measurable action potential indicates the nerve is not functioning.