Intrathoracic injury to the motor nerve supply of the larynx.

A previously reported experimental study demonstrated that the intermediate (partly abducted) position of a paralysed vocal cord may be due to physiological inactivation, rather than paralysis, of the cricothyroid muscle. This inhibition was shown to be caused by interruption of vagal afferent impulses originating in pulmonary pressure receptors. The case reported here offered an unusual opportunity of studying this condition by serial section of a human larynx. A patient's left vocal cord was paralysed by cancer in the left pulmonary hilum and apex. There was aspiration and loss of voice. The larynx was obtained at autopsy and studied by serial sections. These showed that the P.C.A- muscle on the paralysed side was, in fact, completely atrophic and degenerated, as were the other intrinsic muscles but that the cricothyroid muscle was morphologically normal. Invasion of both the recurrent laryngeal nerve and the vagus at the thoracic inlet interrupts afferent impulses and inactivates the cricothyroid muscle, with resultant glottic incompetence.