2.13 Innervation of Larynx

Question 1

Sensory Innervation

Answer 1

1. 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.

2. The superior branch divides thereafter
   into internal and external laryngeal nerves.
3. 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.
4. The recurrent laryngeal nerve provides the sensory supply
   to the laryngeal mucosa below the vocal cords.

Question 2

Motor Innervation

Answer 2

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.

Question 3

One obvious clinical area for discussion is the provision of anaesthesia for awake
fibreoptic intubation.

Nebulized Lidocaine

Answer 3

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.

Question 4

Topical Anaesthetic

Answer 4

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.

Question 5

Spray as You Go’ Technique

Answer 5

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

Question 6

Clinical Consequences of Injury to the Laryngeal Nerves

Answer 6

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.