Vocal cord paralysis (VCP) is the loss of mobility of the true vocal cords secondary to a mechanical cause infiltrating the glottis or a neural lesion affecting the vagus (CN X) or recurrent laryngeal nerves (RLN). These nerves provide motor innervation to the intrinsic muscles of the larynx and travel a long distance from the skull base into the upper mediastinum.
Most cases of VCP are unilateral due to a compressive mass, although up to 1/3 may be bilateral. VCP can be classified in central or peripheral, with central causes being about 10% and the vast majority are peripheral.
The main symptom of unilateral VCP is dysphonia but it can also manifest as hoarseness, aspiration episodes, liquid dysphagia, vocal fatigue and shortness of breath.
It is important to note that up to 40% of people with unilateral VCP are asymptomatic at the time of diagnosis.
ANATOMY OF THE LARYNX
The larynx is a cervical hollow structure between the oropharynx and trachea, anchored on an external frame formed by the hyoid bone and six cartilaginous structure, the first three are unpaired and the rest are paired.
- Epiglottis
- Thyroid
- Cricoid
- Arytenoid cartilages
- Cornicuated cartilages
- Cuneiform cartilages
This cartilaginous “skeleton” forms the support of the larynx and is coated by an inner mucosal layer, shaping several prominent ridges and folds.
The vocal cords are a double layered mucous membrane infolding that stretch horizontally across the laryngeal cavity.
Three paralell structures, the false vocal cord, laringeal ventricles and true vocal cords, divide the larynx into (from cranial to caudal order):
1- Supraglottis: extends from the tip of the epiglottis to the laryngeal ventricle and it contains:
- Epiglottis
- PE fat
- Arytenoid cartilages
- AEF
- False vocal cords = superior layer of infolded membrane
2- Glottis: extends towards an arbitrary plane located 1 cm below the level of the true vocal cords. It contains:
- True vocal cords = formed from the inferior layer of infolded membrane and contain the thyroarytenoid (lateral fibers) and vocalis muscles (medial fibers).
- Laryngeal ventricles
- Cricoarytenoid joints
- Anterior/posterior comissures
3- Subglottis: extends from the plane under the true vocal cords (1 cm under the laryngeal ventricle) till the inferior margin of the cricoid cartilage.
Vocal cord movement is controlled by the intrinsic laryngeal muscles (ILM), which have a complex and hard to remember nomenclature. We must emphasize on these 4 anatomic references in order to help us categorize them according to their point of origin:
- Epiglottis
- Aryepiglottic muscle (within the aryepiglottic fold)
- Thyroid cartilage
- Thyroepiglottic muscle
- Thyroarytenoid muscle (lateral fibers of true vocal cords)
- Cricoid cartilage
- Cricothyroid muscle
- Posterior cricoarytenoid muscle
- Lateral cricoarytenoid muscle
- Arytenoid cartilages
- Oblique arytenoid muscle
- Transverse or interarytenoid muscle
- Vocalis muscle
The muscles that control the inlet of the larynx are the aryepiglottic, oblique arytenoid and thyroepiglottic muscles and the ones that move the vocal ligaments are the cricothyroid, thyroarytenoid, posterior cricoarytenoid, lateral cricoarytenoid, transverse arytenoid and vocalis muscles.
The only abductor muscle amongst this group is the posterior cricoarythenoid muscle.
All ILM are innervated by the RLN, except for the cricothyroid muscle which is innervated by the superior laryngeal nerve (branch of the NC X).
ANATOMY OF THE VAGUS NERVE (CN X)
Tenth and longest cranial nerve, originating in the medulla oblongata and has a sensitive, motor, sensorial and parasympathetic components.
It can be divided into three segments:
1) Intracranial:
Nucleus ambiguus gives rise to the motor fibers that innerve the ILM and is located:
- between the medullary pyramids and the inferior cerebellar peduncle
- posteriorly to the inferior olive
- anterolaterally to the lower part of the fourth ventricle.
The CN X exits the medulla oblongata through its posterolateral margin, between the olivar sulcus and inferior cerebellar peduncule.
It continues allong the lateral cerebelomedullary cisterns and emerges from the skull base via the pars vascularis located in the posterolateral half of the jugular foramen, in close proximity with the glossopharyngeal (NC IX) and accesory cranial nerves (NC XI).
2) Cervical:
Descends throughout the neck within the carotid sheath:
- Posteromedial to the internal jugular vein
- Posterolateral to the internal/common carotid artery
3) Upper mediastinum:
At this level right and left vagus nerve pathways differ:
Right CN X =
- Runs anterior to the origin of the right subclavian artery and posterior to the sternoclavicular artery and brachiocephalic vein.
- Below the right subclavian artery, the RLN branches from the CN X.
Left CN X =
- Runs between the common carotid artery/left subclavian artery and posterior to the sternoclavicular artery and brachiocephalic vein.
- Crosses anterior to the aortic arch, where the RLN branches from the CN X.
Both CN X enter the upper abdomen through the esophageal hiatus (at the level of T10).
ANATOMY OF RLN
The course and length of both RLN differs making the left almost twice as long as the right.
Right RLN =
- loops posteromedially under the origin of the right subclavian artery/brachiocephalic trunk
Left RLN =
- loops posteromedially under the aortic arch (through the aortopulmonary window)
Both of them ascend along the tracheoesophageal groove towards the larynx, however the right RLN only reaches the tracheoesophageal groove at a more distal level (near the cricothyroid joint) because of its more lateral origin and oblique course