The larynx is located in the anterior neck and serves two functions:
- Controls entry of material into the airway (allows air, prevents fluid or solid boluses)
- Speech
Structure
The larynx is a hollow structure, with two openings.
The skeleton of the larynx is formed by nine cartilages:
- The large thyroid cartilage lies anterior to most of the larynx, with two lamina that project laterally for the attachment of numerous muscles and ligaments
- The ringed cricoid cartilage lies inferior to the the thyroid cartilage, and encricles the lumen of the larynx
- The epiglottic cartilage lies posterior and superior to the thyroid cartilage. It has a drop like shape, upturned, with its broad end forming the epiglottis
- The paired arytenoid cartilages are pyramidal in shape, and important for the control of the vocal folds
- The paired corniculate and cuneiform cartilages lie in the aryepiglottic fold, a band of tissue that extends from the epiglottis to each arytenoid cartilage.
The quadrangular membrane forms the foundation of the aryepiglottic fold, attached to the epiglottis anteriorly and the arytenoids posteriorly. The space bounded by the epiglottis, the aryepiglottic folds and the arytenoids is the laryngeal inlet. During swallowing, the inlet is lifted though the actions of the hyoid muscles, opposing it with the epiglottis and preventing food from entering the airway.
The inferior part of the quadrangular membrane forms the vestibular folds or false cords. Beneath this fold is the vestibule, a blind ending cavity that lies on each side of the larynx, beneath the quadrangular membrane. The vocal folds or true cords run from the arytenoid to the median part of the posterior surface of the thyroid cartilage. Movement of the folds through actions of the arytenoid muscles causes variations in vibration and the voice.
Beneath the folds is the subglottic larynx, which is rarely involved in malignancy. At the inferior border of the cricoid cartilage, the larynx is continuous with the trachea.
Relations
The laryngopharynx lies posterior and lateral to the larynx. It is continuous with the larynx through the laryngeal inlet, and separated inferiorly by the aryepiglottic folds, the arytenoid cartilage and the cricoid cartilage. The extension of the laryngopharynx between the cricoid cartilage/ aryepiglottic folds (medially) and thyroid cartilage (laterally) is known as the piriform sinus.
The carotid sheath and its contents lie posterolateral. The hyoid bone is superior, and the thryoid cartilage is suspended from this by the thyrohyoid ligament.
Strap muscles of the neck overly the thyroid cartilage, beneath the subcutaeneous fascia and the skin.
Microscopic Appearance
The larynx is lined by a mix of stratified squamous or respiratory epithelium.
- The superior surface and border of the epiglottis, and the superior surface and outer surface of the aryepiglottic folds are lined by stratified squamous epithelium
- The inferior surface of the epiglottis, and internal surface of the aryepiglottic fold and the remainder of the larynx, with the exception of the vocal fold, is lined by respiratory epithelium (simple columnar)
- The vocal folds are lined by stratified squamous epithelium, with no submucosal layer. This means that the vocal folds have no lymphatic drainage which is important for the spread of glottic malignancies.
Numerous glands open into the vestibule of the larynx.
Physiological Variations
When swallowing, the epiglottis moves posteriorly and the remainder of the larynx is lifted through actions of the hyoid muscles and constrictors. This leads to closure of the laryngeal inlet.
For speech, the vocal folds may be held in a variety of positions based on the actions of the intrinsic laryngeal muscles. This allows different pitches to be formed, although the remainder of speech is due to motion of the tongue and lips.
Pathological Variations
Smokers generally replace the respiratory epithelium of the larynx with stratified squamous epithelium. This dysplasia may be a reason for the development of malignancy in this area, or may simply be due to the same carcinogenic effects of smoking.
Neurovascular Supply
Arterial Supply
The larynx is supplied predominately by the thyroid arteries.
The superior thyroid artery arises from the external carotid just above the hyoid bone, and passes anterior, inferior and medially to reach the lateral asepct of the larynx. It gives off the superior laryngeal artery, which pierces the thryohyoid membrane to supply the supraglottic larynx. A smaller branch, the cricothyroid artery, supplies blood to the cricothyroid muscle.
This muscle is notable for being the only intrinsic muscle not supplied by the recurrent laryngeal nerve
The inferior thyroid artery arises as a branch of the thyrocervical trunk. It passes medially from the trunk, behind the carotid sheath and anterior to the vertebral artery. It gives off an inferior laryngeal branch, which accompanies the recurrent laryngeal nerve superiorly to pass beneath the inferior constrictor.
Venous drainage
Venous drainage is via superior and inferior thyroid veins. The superior vein empties into the internal jugular; the inferior thyroid vein is variable but usually empties into the brachiocephalic trunk.
Lymphatics
The lymphatics of the larynx are variable depending on the position relative to the vocal folds.
- Lymphatics from the supraglottic larynx (above the folds) drain to superior deep cervical nodes, accompanying the superior thyroid vessels (levels III, occasionally level II)
- Lymphatics from the infraglottic larynx follow similar pathways to tracheal lymphatics, and end in pre- or para-tracheal nodes (level VI). From here they drain to deep cervical nodes (levels III, IV).
- There are no lymphatics of the vocal folds. If tumours in this region advance to the point of invading surrounding parts of the larynx, they may spread along either pathway.
Innervation
The innervation of the larynx is quite odd due to the long and roundabout course of the recurrent laryngeal nerve.
The superior laryngeal nerve usually arises from the inferior vagal ganglion (near the jugular foramen). It passes inferiorly within the carotid sheath, which it leaves at about the level of the hyoid bone. It accompanies the superior thyroid, and then the superior laryngeal artery through the thryohyoid membrane to supply the mucosa of the supraglottic larynx. A small branch innervates the cricothyroid muscle, the only intrinsic muscle innervated by this nerve.
The recurrent laryngeal nerve follows a slightly different path on each side. On the right, it arises from the vagus nerve as it passes in front of the subclavian artery. It hooks underneath the artery, and runs superiorly in the furrow between the oesophagus and the trachea. On the left, it arises anterior to the arch of the aorta (several cm lower than on the right). It hooks beneath the aorta and ascends in the contralateral furrow between the oesophagus and trachea. Some books refer to the nerve as the inferior laryngeal nerve once it approaches the larynx, although this is contested. The nerve supplies the intrinsic muscles of the larynx except cricothyroid; it also supplies the mucosa of the subglottic larynx.
Routes of Malignant Spread
Local Invasion
Laryngeal tumours can arise from the supraglottic larynx, the vocal folds, or the subglottic larynx. Local invasion may spread along the mucosal surface into adjacent regions, including the laryngopharynx / piriform sinus. Deeper invasion can lead to destruction of the laryngeal cartilages or erosion in to the carotid sheath, which is typically fatal.
Lymphatic Spread
Tumours arising from the vocal folds rarely spread unless they begin to invade the cartilage or surrounding mucosa. Supraglottic tumours will typically spread laterally to the level III nodes (occasionally level II). Subglottic tumours spread to level VI nodes before passing to inferior deep cervical nodes of level IV.
Haemotogenous Spread
Like all head and neck tumours, laryngeal tumours will typically spread to the lungs, liver and bone.
