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AN38.1-3 | Larynx — Part 2
Nerve Supply and Blood Supply of the Larynx (AN38.1)
Nerve supply (both from CN X):
Figure: Nerve Supply and Blood Supply of the Larynx (AN38.1)
| Nerve | Origin | Supplies |
|---|---|---|
| Superior laryngeal nerve (SLN) | CN X at nodose ganglion | Divides into: |
| — External SLN | Motor: cricothyroid only | |
| — Internal SLN | Sensory: larynx above vocal cords (supraglottic); pierces thyrohyoid membrane with superior laryngeal vessels | |
| Recurrent laryngeal nerve (RLN) | CN X in the thorax | Motor: ALL intrinsic muscles EXCEPT cricothyroid; Sensory: larynx below vocal cords (subglottic) |
Course of the RLN:
- Right RLN: loops under the right subclavian artery, ascends in the right tracheo-oesophageal groove
- Left RLN: loops under the arch of the aorta at the ligamentum arteriosum, ascends in the left tracheo-oesophageal groove (longer intrathoracic course)
- Both enter the larynx posterior to the cricothyroid joint (between the inferior horn of the thyroid cartilage and the cricoid)
- The RLN crosses the inferior thyroid artery in the neck — the relationship is variable (RLN anterior, posterior, or between branches of the artery)
Blood supply:
- Superior laryngeal artery: from the superior thyroid artery (ECA); accompanies internal SLN through thyrohyoid membrane
- Inferior laryngeal artery: from the inferior thyroid artery (thyrocervical trunk); accompanies RLN
Lymphatics:
- Supraglottis: rich lymphatics → to deep cervical nodes (Levels II–IV) bilaterally (supraglottic tumours → early bilateral nodal metastasis)
- Glottis (true cords): almost no lymphatics → T1 glottic tumours rarely metastasise (best prognosis in the larynx)
- Subglottis: drains to paratracheal nodes (Level VI)
SELF-CHECK
A. Left recurrent laryngeal nerve (cord adducted = paramedian position)
B. Left external branch of the superior laryngeal nerve (cricothyroid paralysis)
C. Left vagus nerve in the carotid sheath
D. Left glossopharyngeal nerve
Reveal Answer
Answer: .
External SLN injury paralyses the cricothyroid → the vocal cord becomes lax, short, and cannot tense for high pitch. The cord may appear on the same level or slightly bowed on laryngoscopy. The voice is breathy and weak at high pitch ('cannot speak above a whisper' or voice fatigue). The cord still adducts (moves medially) because the RLN is intact. This is distinct from RLN injury where the cord is paralysed in paramedian position.
Laryngitis — Anatomical Basis (AN38.2)
Acute laryngitis:
- Inflammation of the laryngeal mucosa; most common cause = viral URTI (parainfluenza, rhinovirus, influenza)
- Hoarseness: mucosal swelling of the vocal cords alters their vibration
- The vocal cord mucosa is stratified squamous (not ciliated columnar) → limited mucociliary clearance; retained secretions worsen swelling
- Treatment: voice rest, steam inhalation, treat underlying cause
Figure: Laryngitis — Anatomical Basis (AN38.2)
Laryngitis — Anatomical Basis of Clinical Presentations
| Condition | Anatomical Basis | Key Clinical Feature |
|---|---|---|
| Acute epiglottitis | Elastic fibrocartilage + loose submucosa → rapid oedema → airway obstruction | Cherry-red swollen epiglottis; do NOT examine throat with spatula |
| Croup (laryngotracheobronchitis) | Subglottis encircled by rigid cricoid ring → oedema causes severe narrowing | Barking cough, inspiratory stridor in children |
| Chronic laryngitis | Vocal cord mucosal changes (oedema, keratosis) | Hoarseness; leukoplakia → risk of malignancy |
| Reinke oedema | Oedema in Reinke space (subepithelial layer of vocal cord) | Deep husky voice; associated with smoking |
| Vocal cord nodules | Bilateral nodules at junction of anterior 1/3 and posterior 2/3 | Singer's/screamer's nodules; vocal abuse |
Acute epiglottitis:
- Most common cause: Haemophilus influenzae type B (Hib) in unvaccinated children; Group A Streptococcus in adults
- The epiglottis is elastic fibrocartilage with loose submucosa → oedema develops rapidly → 'cherry-red' swollen epiglottis → obstructs the laryngeal inlet
- Do NOT examine the throat with a spatula in a suspected case (stimulation → complete laryngospasm → death)
- Management: urgent anaesthetic review, IV antibiotics, controlled airway in theatre
Croup (laryngotracheobronchitis):
- Parainfluenza virus infection in children (6 months–3 years)
- Subglottic oedema (below the vocal cords, encircled by the inelastic cricoid ring) → inspiratory stridor + barking cough
- 'Steeple sign' on AP soft tissue neck X-ray: loss of the normal subglottic shoulder → narrow, steepled subglottis
- Treatment: single dose of dexamethasone (reduces mucosal oedema); nebulised adrenaline for severe cases
Chronic laryngitis:
- Causes: tobacco smoke, chronic alcohol, vocal abuse, GERD (laryngopharyngeal reflux), pollution
- Leads to: squamous metaplasia of the laryngeal mucosa → leukoplakia (white patch on vocal cord) → carcinoma in situ → invasive SCC
- India: chronic laryngitis is the most common precancerous laryngeal condition; bidi/cigarette smoking + pan masala use are the dominant risk factors
Recurrent Laryngeal Nerve Injury — Anatomical Basis (AN38.3)
Sites of RLN injury:
Figure: Recurrent Laryngeal Nerve Injury — Anatomical Basis (AN38.3)
Recurrent Laryngeal Nerve Injury — Sites and Causes
| Level | Cause | Clinical Significance |
|---|---|---|
| Neck (tracheo-oesophageal groove) | Thyroid surgery (most common), thyroid cancer, radical neck dissection | Most common site of iatrogenic RLN injury |
| Thorax — left side | Aortic aneurysm, mediastinal tumour, left atrial enlargement (Ortner syndrome) | Left RLN has longer course (loops under aortic arch) — more vulnerable |
| Thorax — right side | Apical lung tumour (Pancoast), subclavian artery aneurysm | Right RLN loops under subclavian artery — shorter course |
| Unilateral injury | Cord in paramedian position (adducted) | Hoarseness, breathy voice; compensated by opposite cord |
| Bilateral injury | Both cords adducted (paramedian) | Stridor, airway obstruction — may need emergency tracheostomy |
| Level | Cause |
|---|---|
| Neck (tracheo-oesophageal groove) | Thyroid surgery (most common); thyroid cancer invasion; radical neck dissection; carotid endarterectomy |
| Mediastinum (right subclavian/left aortic arch level) | Left: aortic aneurysm, Ortner syndrome (mitral stenosis with large left atrium compressing left RLN), mediastinal lymphoma, lung apex carcinoma; Right: right subclavian artery anomalies |
| Direct laryngeal trauma | Intubation trauma, blunt neck trauma |
The inferior thyroid artery-RLN relationship:
- The RLN crosses the inferior thyroid artery in the neck
- The relationship is variable:
- RLN anterior to the artery (~50%)
- RLN posterior to the artery (~30%)
- RLN between branches of the artery (~20%)
- Right non-recurrent laryngeal nerve (0.5–1%): arises directly from the right vagus, courses transversely to the larynx; associated with an aberrant right subclavian artery (arteria lusoria); can be mistakenly ligated as the inferior thyroid artery
Clinical features of RLN injury:
| Type | Clinical features |
|---|---|
| Unilateral (complete) | Hoarseness; breathy voice; cord in paramedian/cadaveric position; aspiration on liquids; cough |
| Unilateral (partial, neuropraxia) | Transient hoarseness; recovers in weeks-months |
| Bilateral (complete) | Both cords paramedian → severe inspiratory stridor → respiratory failure → emergency tracheostomy |
| Bilateral (partial, SLN spared) | Whispering voice; bilateral bowing; aspiration |
Vocal cord positions:
- Median (paramedian): adducted — cadaveric position; RLN + SLN both lost; or complete RLN injury
- Lateral (abducted): open as in quiet breathing
- Paramedian: slight abduction from median; incomplete RLN injury
Figure: Clinical features of RLN injury:
Ortner's syndrome (cardiovocal syndrome): Left-sided hoarseness due to compression of the left RLN by an enlarged cardiovascular structure — classically a large left atrium from rheumatic mitral stenosis; also aortic aneurysm. Historically important in India where rheumatic heart disease is still prevalent.
SELF-CHECK
A. Permanent loss of singing voice
B. Development of aspiration pneumonia
C. Acute respiratory obstruction requiring tracheostomy
D. Hypocalcaemia from parathyroid removal
Reveal Answer
Answer: .
With bilateral RLN injury, both posterior cricoarytenoid muscles (the only abductors of the vocal cords) are paralysed. Both vocal cords lie in the paramedian position → the rima glottidis is severely narrowed → acute inspiratory stridor → respiratory failure. Emergency tracheostomy (or endoscopic cordotomy/arytenoidectomy) is required. Hypocalcaemia is also a concern after total thyroidectomy, but it is not the immediately life-threatening problem here.
SELF-CHECK
A. Glottic tumours are more radiosensitive
B. The true vocal cord has almost no lymphatic drainage → late nodal metastasis → T1 glottic cancer is typically N0 at presentation
C. Glottic tumours are histologically more differentiated
D. The glottis is more accessible to endoscopic laser surgery
Reveal Answer
Answer: .
The true vocal cords (glottis) have a paucity of lymphatics — there is almost no lymphatic drainage of the vocal cord itself. Therefore, T1 glottic tumours (confined to the cord) rarely spread to lymph nodes at presentation. In contrast, the supraglottis has rich lymphatics that drain bilaterally to deep cervical nodes → T1 supraglottic tumours are frequently N1/N2 at diagnosis. This anatomical difference explains the markedly different prognosis.
SELF-CHECK
A. Space between the thyroid cartilage and the hyoid bone (thyrohyoid membrane)
B. Space between the thyroid cartilage and the cricoid cartilage (cricothyroid membrane)
C. Space between the cricoid cartilage and the first tracheal ring (cricotracheal membrane)
D. Space between the 2nd and 3rd tracheal rings (standard tracheostomy site)
Reveal Answer
Answer: .
The cricothyroid membrane (conus elasticus) between the inferior border of the thyroid cartilage and the superior border of the cricoid cartilage is the correct site for cricothyrotomy. It is the most accessible avascular midline structure for emergency airway access, accessed inferior to the vocal cords (the upper airway obstruction). Tracheostomy is the elective procedure performed lower down (between the 2nd and 3rd tracheal rings). The thyrohyoid membrane is above the laryngeal inlet and would not bypass the obstruction.
CLINICAL PEARL
Right non-recurrent laryngeal nerve — a surgical trap: In approximately 0.5–1% of patients, the right RLN does not loop under the subclavian artery but instead takes a direct transverse course from the right vagus to the larynx, passing at the level of the inferior thyroid artery. This is ALWAYS associated with an aberrant right subclavian artery (arteria lusoria) arising from the descending aorta and passing posterior to the oesophagus. A surgeon who is unaware of this variant may mistake the non-recurrent RLN for the inferior thyroid artery and ligate it — causing permanent right vocal cord paralysis. Pre-operative CT angiogram identifies the vascular anomaly. Key rule: if you cannot find the right RLN in the tracheo-oesophageal groove, consider a non-recurrent variant and carefully trace the vagus for a direct transverse branch before applying any ligature near the inferior thyroid artery.