Thyroid Surgery and
Nerve Monitoring Course
The Recurrent Laryngeal Nerves
Dr Frank CT Voon
Frank_Voon@nuhs.edu.sg
http://courseware.nus.edu.sg/som/Anatomy
Advanced Surgery Training Centre
National University Hospital
2 June 2010
Innervation of the Larynx
The larynx receives motor and sensory supply from the right and left
vagus nerves through 3 nerves on each side.
These are the:
Internal laryngeal nerve
External laryngeal nerve
Recurrent laryngeal nerve.
The internal laryngeal nerve contains afferent or sensory fibers.
The external laryngeal nerve contains efferent or motor fibers.
The recurrent laryngeal nerve contains both sensory and motor fibers.
The Vagus Nerves
Each vagus nerve initially gives off 2 branches, the superior laryngeal
nerve and the recurrent laryngeal nerve.
The superior laryngeal nerve then subsequently divides into an
internal laryngeal nerve and the external laryngeal nerve.
The external laryngeal nerve pierces the inferior constrictor of the
pharynx and enters the cricothyroid.
It supplies both muscles.
The internal laryngeal nerve carries sensory fibers from the mucosa of
the larynx above the level of the vocal folds.
The terminal branches of the internal laryngeal nerve communicate
with the sensory branches of the recurrent laryngeal nerve.
The Recurrent Laryngeal Nerves
The sensory fibers in the recurrent laryngeal nerve supply the mucosa
of the larynx below the level of the vocal folds.
The motor fibers in the recurrent laryngeal nerve supply all the
intrinsic muscles of the larynx except cricothyroid.
The right and left recurrent nerves branch off from the vagus nerves at
different levels following the development of the aortic arches in the
embryo.
The right recurrent laryngeal nerve hooks under the first part of the
subclavian artery.
The left recurrent laryngeal nerve hooks under the arch of the aorta
and the ligamentum arteriosum.
The Tracheo-Esophageal Groove
The right and left recurrent laryngeal nerves then ascend near or in
the groove between the trachea and the esophagus to reach the
larynx and the lower lobe of the thyroid gland.
Both nerves are closely related to the thyroid gland.
Both nerves are closely related to the inferior thyroid artery.
Both recurrent laryngeal nerves therefore pose a danger in thyroid
operations.
Each recurrent laryngeal nerve divides into two or more branches
before passing deep to the lower border of the inferior constrictor of
the pharynx and behind the cricothyroid joint to enter the larynx.
The Right Recurrent
Laryngeal Nerve
At the level of the lower border of the thyroid, it is usually no more than 1
cm lateral to (or within) the tracheo-esophageal groove.
At the level of the midportion of the thyroid, it is within the tracheoesophageal groove, and is usually found either anterior or posterior to a
main branch of the inferior thyroid artery.
Here, it may divide into 2 or more branches. The most important branch
travels beneath the inferior border of the cricothyroid muscle (note that the
cricothyroid muscle is supplied by the external laryngeal nerve).
The other branches enter in the spaces above or below the first or second
rings of the trachea.
The Left Recurrent
Laryngeal Nerve
The right and the left recurrent laryngeal nerves are consistently found in
the tracheo-esophageal groove when they are within 2.5 cm of their entry
into the larynx.
The recurrent laryngeal nerve passes either below or behind a branch of
the inferior thyroid artery before entering the larynx at the level of the
cricothyroid joint (lower border of the cricothyroid muscle).
The nerve is closely related to the superior parathyroid gland, the inferior
thyroid artery and the posterior part of the thyroid gland.
Careful dissection is needed to avoid stretching of the nerve as it is
basically tethered to the cricothyroid muscle when it passes below the
muscle.
Nerve Injuries
Injury to a recurrent laryngeal nerve leads to paralysis of the vocal cord on
the affected side, with the vocal cord remaining in a median position or just
lateral to the midline.
Speech is still possible if the contralateral vocal cord can approximate the
paralyzed cord though the voice is weakened.
Severe impairment of the voice and ineffectiveness of coughing will result
when closure is not possible due to the paralyzed cord being in an
abducted position.
Complete loss of voice can result with obstruction to the airway that may
require emergency intubation and tracheostomy if there is bilateral
damage to the recurrent laryngeal nerves.
At times, bilateral injury may lead to the cords being in an abducted
position which will allow air flow though it may result in upper respiratory
tract infections from ineffective coughing.
The Internal Laryngeal Nerve
The superior laryngeal nerve begins to separate from the vagus nerve at
the base of the skull.
It descends alongside the internal carotid artery to reach the superior pole
of the thyroid gland.
At the level of the cornu of the hyoid, the superior laryngeal nerve divides
into the internal laryngeal nerve and the external laryngeal nerve.
The internal laryngeal nerve is the larger branch of the superior laryngeal
nerve.
It is afferent and contains sensory fibers.
It enters the thyrohyoid membrane to innervate the mucosa of the larynx,
above the level of the vocal fold.
The External Laryngeal Nerve
The external laryngeal nerve is efferent and contains motor fibers.
It is the smaller branch of the superior laryngeal nerve and continues to
travel on the lateral surface of the inferior constrictor of the pharynx.
It usually descends anteriorly and medially together with the superior
thyroid artery.
Within 1 cm of the entrance of the superior thyroid artery into the capsule
of the thyroid gland, the external laryngeal nerve usually turns medially to
enter and innervate the cricothyroid muscle.
During thyroid lobectomy, the external laryngeal nerve may not be seen if
it has already entered the fascia covering the inferior constrictor of the
pharynx.
If the blood vessels are ligated at too great a distance from the superior
pole of the thyroid gland, the external laryngeal nerve may be damaged.
This can result in a severe loss in quality and strength of the voice.
Intrinsic Muscles of the Larynx
Cricothyroid - stretches and abducts the vocal cords
Posterior cricoarytenoid - abduction and external rotation of the
arytenoid cartilages, resulting in abducted vocal cords
Lateral cricoarytenoid - adduction and internal rotation resulting in
adducted vocal cords
Transverse arytenoid - adduction, resulting in adducted vocal cords
Oblique arytenoid - adduction and narrowing of the laryngeal inlet
Vocalis increases the thickness of the cords and changes the tone
Thyroarytenoid - narrows the laryngeal inlet.
Note that the posterior cricoarytenoids are the only muscles that
abduct the vocal cords.
Their paralysis (eg from bilateral injury to the recurrent laryngeal
nerves) will lead to breathing difficulties.
The ligament of Berry
The thyroid gland has its own capsule.
The pretracheal fascia surrounding the trachea also encloses the capsule.
The part of the capsule of the thyroid gland that fuses with the pretracheal
fascia laterally and posteriorly is known as the ligament of Berry.
The ligament is closely attached to the cricoid cartilage.
Important surgical implications
The recurrent laryngeal nerves ascend in the tracheo-esophageal groove
and enter the larynx just lateral to the ligament of Berry.
Both recurrent laryngeal nerves are usually in the tracheo-esophageal
groove when they are within 2.5 cm of their entrance into the larynx.