Larynx

What it normally does

• The larynx is the narrowest part of the upper airway, and in a small child it is the least forgiving. On imaging in both children and adults the glottis and immediate subglottis are the narrowest points, but the subglottis is ringed by the cricoid — the only complete ring of cartilage in the airway — so it is the narrowest part that cannot stretch. Poiseuille: for laminar flow, resistance is inversely proportional to the fourth power of the radius (halve the radius, sixteen times the resistance), and once flow turns turbulent it worsens faster still.

  a millimetre of circumferential swelling an adult would barely notice halves the radius of a 4 mm infant subglottis — about a 75% loss of cross-sectional area and roughly a sixteen-fold rise in resistance — and that is stridor and visible work of breathing.

• Contact with the laryngeal inlet reflexly closes the cords (the laryngeal adductor reflex, exaggerated as laryngospasm), and the reflex is blunted by sleep, sedation and anaesthesia. Sensation comes from the internal branch of the superior laryngeal nerve above the cords and from the recurrent laryngeal nerve below. A cough uses the same machinery — close the cords, build pressure, release it explosively.

  lose that sensation or that closure and food and saliva go straight into the trachea, with no effective cough available to clear them.

• One nerve moves almost everything. The recurrent laryngeal nerve supplies every intrinsic muscle of the larynx except cricothyroid (external branch of the superior laryngeal nerve), and only one of those muscles opens the cords — posterior cricoarytenoid.

  one nerve out leaves a cord that will not move and a breathy voice, usually with an adequate airway; both nerves out leaves nothing that can abduct the cords at all.

• The recurrent laryngeal nerves take a long detour before they reach the larynx — the left hooks under the arch of the aorta at the ligamentum arteriosum, the right under the right subclavian artery — and both then ascend towards the larynx in or near the tracheo-oesophageal groove, closely related to the thyroid gland and its inferior thyroid artery.

  hoarseness can be the first sign of disease in the chest (the left nerve has far more thorax to be caught in), and surgery — thyroidectomy above all — is the commonest way these nerves are injured.

What goes wrong

• Croup (laryngotracheobronchitis)← from “The larynx is the narrowest part of the upper …”

  Parainfluenza virus (most often type 1) inflames the lining of the larynx and trachea. The mucosa just below the cords swells, and because the cricoid is a rigid complete ring it can only swell inwards. A millimetre of oedema takes a large fraction of a toddler's subglottic radius, and since resistance is inversely proportional to the fourth power of the radius, the work of breathing rises steeply and flow through the narrowed segment turns turbulent and audible.

  Subglottic oedema inside a rigid cricoid ring. Barking cough, hoarse voice, inspiratory stridor. A single dose of oral corticosteroid is recommended at all severities, including the mild child going home; nebulised adrenaline is added when there is stridor at rest. Keeping the child calm matters — distress worsens the obstruction.

  You would find: A barking, seal-like cough with a hoarse voice and inspiratory stridor in a child, typically 6 months to 6 years with a peak in the second year, worse at night. Stridor only when upset is mild; stridor at rest with chest recession is not. A toxic-looking child who drools, sits forward and will not cough is a different disease — epiglottitis (now rare since Hib vaccination, though invasive Hib disease still falls disproportionately on under-vaccinated Aboriginal and Torres Strait Islander children) or bacterial tracheitis, which is not Hib-related and is not prevented by that vaccine.

• Unilateral recurrent laryngeal nerve palsy← from “The recurrent laryngeal nerves take a long det…”

  The nerve is injured out along its detour, not at the larynx — caught during thyroidectomy, invaded by a left hilar or aortopulmonary-window lung tumour, or stretched by an aortic arch aneurysm. Every intrinsic muscle on that side except cricothyroid is denervated, so that cord stops moving and usually comes to rest in the paramedian position (close to, but not at, the midline; a complete vagal lesion, which also takes out cricothyroid, tends to leave it further out in the intermediate or cadaveric position). The working cord cannot quite close against it, so the glottis does not seal: air leaks during speech and pressure cannot be built for a cough.

  One cord fixed in the paramedian position → breathy voice, bovine cough, aspiration risk. New persistent hoarseness means visualising the cords, and a left-sided palsy with no neck operation means looking in the chest.

  You would find: A breathy, weak voice and a soft bovine cough with no explosive start, sometimes with coughing on thin fluids. Many patients compensate over weeks as the mobile cord crosses the midline, so the voice can improve without the nerve recovering. Hoarseness persisting beyond about three weeks in an adult — particularly a smoker — warrants laryngoscopy: the cause may be lung cancer pressing on the left nerve, or cancer of the larynx itself. Head and neck cancer is diagnosed later and kills more often in Aboriginal and Torres Strait Islander people, largely on the back of higher smoking rates.

• Bilateral recurrent laryngeal nerve palsy← from “One nerve moves almost everything. The recurre…”

  Both recurrent nerves are out — classically after total thyroidectomy, where they run right beside the gland. Posterior cricoarytenoid, the only abductor, is denervated on both sides, so nothing is left to open the cords. Both cords sit near the midline with only a narrow chink between them, and that chink is now the entire airway.

  Both posterior cricoarytenoids denervated → cords stuck near midline → stridor with a surprisingly good voice. Post-thyroidectomy stridor is bilateral palsy (or an expanding neck haematoma) until proven otherwise.

  You would find: Stridor and rising work of breathing on extubation or in the hours after thyroid surgery. The trap is the voice, which can sound near normal — cords that sit together still phonate while blocking the airway. This is an airway emergency: senior airway help, reintubation, sometimes tracheostomy.

• Aspiration through an unprotected larynx← from “Contact with the laryngeal inlet reflexly clos…”

  After a stroke, with a reduced conscious state, or in bulbar disease such as motor neurone disease, laryngeal sensation and the closure reflex it drives are lost. Food, fluid or saliva crosses the cords without provoking closure or a cough — silent aspiration. Aspirate most often follows gravity down the right main bronchus, which is wider, shorter and more vertical: towards the basal segments of the right lower lobe in an upright patient, and towards the posterior segment of the right upper lobe or the superior segment of the right lower lobe in someone lying flat.

  No laryngeal sensation → no cough → silent aspiration → right-sided (gravity-dependent) pneumonia. Swallow screen before anything by mouth.

  You would find: A wet, gurgly voice after swallowing, coughing on thin fluids, or recurrent right-sided pneumonia in someone who has had a stroke. This is why nothing goes in the mouth after a stroke — not food, not fluids, not tablets — until a validated swallow screen has been passed.

• Laryngeal oedema in anaphylaxis (and ACE inhibitor angioedema)← from “The larynx is the narrowest part of the upper …”

  In anaphylaxis, mast cells and basophils degranulate and histamine and other mediators make mucosal capillaries leak. Fluid collects in the loose submucosal tissue of the larynx and pushes into the narrowest part of the airway, where there is no spare room. ACE inhibitors produce similar swelling by a different route: ACE (kininase II) normally degrades bradykinin, so blocking it lets bradykinin accumulate and make the same mucosa leak — this is not an IgE- or mast-cell-driven allergic reaction. That difference matters, because the two do not respond to the same drugs.

  Swelling at the narrowest point of the airway. Voice change plus stridor means the airway, not the skin. In anaphylaxis, intramuscular adrenaline is the immediate treatment. In ACE inhibitor angioedema the swelling is bradykinin-mediated, so adrenaline, antihistamines and steroids should not be relied on: the priorities are airway assessment and protection and permanently stopping the ACE inhibitor.

  You would find: A change in the voice, a tight throat, swelling of lips or tongue, and stridor — with urticaria, wheeze and hypotension if it is anaphylaxis. ACE inhibitor angioedema looks similar but typically has no rash, no itch and no hypotension, often favours lips, tongue and floor of mouth, and can start months or years after the drug was begun; it is more common in people of African ancestry.

What we give, and how it works

Open a drug to see what it binds, what that does to the cell, and what you then see in the patient.