The superior laryngeal nerve is divided into two branches: the external branch, which is the motor branch, and the internal branch, which is the sensory branch. Injury to the external branch can easily paralyze the cricothyroid muscle, causing relaxation of the vocal cords and reduction in pitch. Injury to the internal branch, the sensory branch, can cause loss of sensation in the mucous membrane of the larynx, which can lead to choking and coughing when eating, especially when drinking water. This condition is one of the main complications of thyroid surgery and occurs when the upper pole of the thyroid gland is treated too far from the gland, or when the nerve is ligated together with the surrounding tissues during surgery without careful separation. Other major complications include damage to the recurrent laryngeal nerve. Injury to one side of the recurrent laryngeal nerve mostly leads to hoarseness, while injury to the bilateral recurrent laryngeal nerve can easily lead to loss of voice and even severe respiratory distress, resulting in asphyxia.