2.13 Innervation of Larynx

• The sensory innervation of the larynx is via the vagus (tenth cranial nerve), which divides into the superior laryngeal nerve and the recurrent laryngeal nerve. • The superior branch divides thereafter into internal and external laryngeal nerves. • The internal laryngeal nerve innervates the inferior surface of the epiglottis and the supraglottic region as far as the mucous membrane above the vocal folds. • The recurrent laryngeal nerve provides the sensory supply to the laryngeal mucosa below the vocal cords.

The recurrent laryngeal nerve supplies all the intrinsic muscles of the larynx, with the exception of the cricothyroid muscle. This is supplied from the external branch of the superior laryngeal nerve. The right recurrent laryngeal nerve leaves the vagus to loop beneath the subclavian artery, before ascending to the larynx in the groove between the oesophagus and the trachea. The left recurrent laryngeal nerve passes beneath the arch of the aorta and similarly ascends in the groove between oesophagus and trachea.

Nebulized Lidocaine
Nebulized local anaesthetic (such as lidocaine 4%) will provide adequate surface anaesthesia of the airway, although the procedure takes some time, and patients may therefore find the mask claustrophobic and uncomfortable. It may not anaesthetize the nasal mucosa adequately.

Topical Anaesthetic
The nasal mucosa can be anaesthetized with local anaesthetic plus vasoconstrictor to minimize risk of bleeding. Topical cocaine can be used to a maximum dose of 1.5 mg kg–1. If oral intubation is planned, the tongue and posterior pharynx can be anaesthetized using lidocaine 4% or a lidocaine 10% metered pump which delivers 10 mg with each spray.

This is another straightforward method of anaesthetising the airway, in which local anaesthetic (usually lidocaine 4%) is introduced under direct vision via the injector channel in the fibreoptic endoscope. In practice this is a simple and almost invariably effective technique. You may nonetheless be asked about supplemental blocks.
Glossopharyngeal nerve: this provides sensory innervation to the oral pharynx, the supraglottic area, the base of the tongue and the vallecula. It can be blocked by submucosal infiltration behind the tonsillar pillars.
Superior laryngeal nerve: this can be anaesthetized by bilateral injections which can be performed either by walking off the greater cornua of the hyoid to penetrate the thyrohyoid membrane, or by walking off the superior alae of the thyroid cartilage.
Recurrent laryngeal nerve: this nerve is usually blocked even if a ‘spray as you go’ technique has been used to anaesthetize the remainder of the airway. It is blocked via a transtracheal injection that is made through the cricothyroid membrane during inspiration. The inevitable cough distributes the solution (typically 4 ml of lidocaine 4%) more widely

The external branch of the superior laryngeal nerve supplies the cricothyroid muscle, which tenses the vocal cords. Damage will be followed by hoarseness. If the injury is unilateral, this hoarseness will be temporary, because in time the other cricothyroid muscle will compensate. If it is bilateral the hoarseness will be permanent.
The recurrent laryngeal nerve supplies all those muscles which control the opening and closing of the laryngeal inlet.
Partial paralysis affects the abductor muscles more than the adductors, and so with unilateral injury the corresponding vocal cord is paralysed. This also results in hoarseness.
If both nerves are damaged, then both cords oppose or even overlap each other in the midline. This leads to inspiratory stridor and has the potential to cause total respiratory obstruction.
If one or both nerves are transected, the vocal cord(s) adopt the cadaveric position in which they lie partially abducted and through which airflow is much less compromised. Phonation may be reduced to a whisper.