Describe the anatomy and physiology of the larynx.

Skeleton, Cavity and Muscles

• The larynx is the organ of phonation and the sphincter guarding the lower airway.
• It extends from the epiglottis to the lower border of the cricoid (C6), where it becomes the trachea, and lies opposite C3 to C6 in the adult.
• The cartilages — three unpaired (thyroid, cricoid and epiglottis) and three paired (arytenoid, corniculate and cuneiform)
• The cricoid is the only complete ring in the entire airway.
• The cavity is divided into three parts by the vocal and vestibular folds, and the division governs the behaviour of cancer:
• Supraglottis — from the epiglottis to the ventricle; it has a rich bilateral lymphatic supply, so tumours here metastasise early and often bilaterally and present late

Nerve Supply, Physiology and Applied Anatomy

• Nerve supply is entirely from the vagus, through two branches
• The superior laryngeal nerve divides into an internal branch.
• The recurrent laryngeal nerve is motor to all other intrinsic muscles and sensory below the cords
• Their asymmetric course IS clinically decisive — the right recurrent laryngeal nerve loops around the subclavian artery.
• Because the left nerve has a longer, intrathoracic course, left vocal cord palsy is commoner, and a left palsy demands imaging of the chest as well as the neck — bronchial carcinoma, mediastinal nodes, aortic aneurysm and left atrial enlargement all reach it
• Injury to the external laryngeal nerve in thyroid surgery (it runs close to the superior thyroid artery) causes loss of high notes and vocal fatigue.
• SEMON’S law states that in a progressive lesion the abductor fibres are affected before the adductors.
• The functions of the larynx, in order of importance:
• 1. Protection of the lower airway.
• 2. Respiration — the cords abduct on inspiration
• 3. Phonation, which is the most recent and the least essential function — a point that matters when counselling a patient about laryngectomy
• 4. Fixation of the chest for coughing, straining, lifting, defaecation and parturition — the valsalva function, which is genuinely lost after laryngectomy
• Phonation works by the myoelastic-aerodynamic theory — the cords are adducted and set into vibration by the expiratory air stream, with the bernoulli effect drawing them back together after each opening; pitch depends on tension and length, loudness on subglottic pressure
• The paediatric larynx differs in ways that explain paediatric emergencies — it is higher (at C3 to C4), smaller and funnel-shaped, with the narrowest point at the cricoid (subglottis) rather than the glottis; the epiglottis is omega-shaped and floppy; and the submucosa is loose, so oedema forms rapidly
• The consequence IS quantitative and should BE stated: 1 mm of circumferential oedema in an infant subglottis reduces the cross-sectional area by 60%, but in an adult by only 20% — which is why a minor infection obstructs a child and not an adult
• The cricoid is the only complete ring in the airway, which explains subglottic stenosis, cricoid pressure and paediatric obstruction
• Posterior cricoarytenoid is the only abductor, so its bilateral paralysis is an airway emergency
• Cricothyroid is the only muscle spared by the recurrent laryngeal nerve, being supplied by the external laryngeal
• The glottis has almost no lymphatics, which is why glottic cancer presents early and has the best prognosis
• The supraglottis drains bilaterally and richly, which is why those tumours present late with nodes
• Reinke space is loose and gelatinous, which is where oedema, nodules and polyps form
• Stripping a cord destroys the mucosal wave permanently, which is why cord surgery is microsurgery
• Left cord palsy needs the chest imaged, since the left recurrent nerve loops around the aortic arch
• External laryngeal nerve injury loses the high notes.
• Semon law explains airway-first presentation, abductor fibres being affected before adductors in a progressive lesion
• Protection is the primary function, phonation being the most recent and least essential.