Paralysis of one of four main nerves serving the larynx, and two of the four if the paralysis is bilateral. SLN paralysis may be idiopathic or perhaps the result of neck surgery. The SLN internal branch supplies sensation to the interior of the upper part of the larynx; the external branch supplies motor innervation to the cricothyroid muscle.

Individuals with SLN paralysis, whether unilateral or bilateral, seem to compensate for the sensory deficits, provided that motor swallowing abilities are intact. On the other hand, they have symptoms of loss of upper voice and poor vocal projection. These two symptoms are easily verified by clinician elicitation and judgment of response.