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AN38.1-3 | Larynx — Part 1
CLINICAL SCENARIO
A 55-year-old male singer from Chennai presents with progressive hoarseness of 4 months' duration. He is a non-smoker. Laryngoscopy reveals a white plaque on the right true vocal cord (leukoplakia). Three months later, he returns with worsening hoarseness and stridor. A biopsy confirms squamous cell carcinoma (T2N0M0).
Why does laryngeal cancer cause hoarseness so early? Why does a T1 tumour of the glottis have an excellent prognosis while a supraglottic tumour of the same stage has a worse prognosis? What is the anatomical basis of the lymphatic drainage differences between the glottis and supraglottis?
The larynx is an exquisitely engineered organ — a valve that protects the airway, controls breathing, and produces voice. Its anatomy directly predicts the clinical behaviour of laryngeal disease.
WHY THIS MATTERS
The larynx is central to three of the most critical clinical encounters in surgery, ENT, and medicine:
- Recurrent laryngeal nerve (RLN) injury — the most common serious complication of thyroid surgery, thoracic surgery, and cardiac surgery; must be understood in every surgeon
- Laryngeal cancer — 3,000–5,000 cases/year in India; strongly linked to tobacco and alcohol; early disease = curable; late disease = devastating
- Croup and epiglottitis — common and potentially fatal ENT emergencies in Indian children; the anatomy of the subglottis (cricoid ring) and the epiglottis explain why these are life-threatening
- Intubation — every doctor intubating a patient must understand the laryngoscopic anatomy (rima glottidis, arytenoids, epiglottis, pyriform fossae)
- Laryngitis — extremely common in India (pollution, tobacco, vocal abuse); chronic laryngitis → leukoplakia → malignancy
- Emergency tracheostomy and cricothyroidotomy — the cricothyroid membrane is the landmark for emergency airway access
RECALL
Before we begin, recall:
- The larynx extends from C3 to C6; it sits between the pharynx (above) and the trachea (below)
- It is composed of cartilages (3 unpaired + 3 paired), connected by ligaments and membranes, and moved by intrinsic and extrinsic muscles
- The vocal cords (vocal folds) produce voice by vibration; they are attached anteriorly to the thyroid cartilage and posteriorly to the arytenoid cartilages
- The rima glottidis is the space between the true vocal cords and the arytenoids; it is the narrowest part of the adult larynx
Laryngeal Cartilages, Ligaments, and Membranes (AN38.1)
Unpaired cartilages:
Figure: Laryngeal Cartilages, Ligaments, and Membranes (AN38.1)
| Cartilage | Type | Features |
|---|---|---|
| Thyroid | Hyaline | Largest; two laminae fuse at the thyroid notch + laryngeal prominence (Adam's apple); superior and inferior horns; inferior horn articulates with cricoid |
| Cricoid | Hyaline | Only complete ring in the respiratory tract; narrow anterior arch + broad posterior lamina; articulates with thyroid (inferior horns) and arytenoids |
| Epiglottis | Elastic (fibrocartilage) | Leaf-shaped; attached to inner angle of thyroid anteriorly (thyroepiglottic ligament); folds down during swallowing to protect the airway; petiolus = narrow lower end |
Paired cartilages:
| Cartilage | Type | Features |
|---|---|---|
| Arytenoids | Hyaline | Pyramid-shaped; apex, base (sits on cricoid), muscular process (posterior, for posterior cricoarytenoid), vocal process (anterior, attachment of vocal ligament) |
| Corniculate | Elastic | Sits on apex of arytenoid; in the aryepiglottic fold |
| Cuneiform | Elastic | Anterolateral to corniculate in aryepiglottic fold |
Key membranes and ligaments:
Figure: Paired cartilages:
| Structure | Location | Clinical Importance |
|---|---|---|
| Thyrohyoid membrane | Thyroid cartilage → hyoid bone | Internal laryngeal nerve + superior laryngeal vessels pierce it |
| Cricothyroid membrane (conus elasticus) | Cricoid → thyroid cartilage; free upper edge = vocal ligament | Cricothyrotomy site for emergency airway access (below vocal cords, above cricoid) |
| Cricotracheal membrane | Cricoid → 1st tracheal ring | Tracheostomy performed below this |
| Quadrangular membrane | Upper free edge = aryepiglottic fold (supraglottic); lower free edge = vestibular ligament (false cord) | Defines the vestibule and ventricle |
Interior of the Larynx — Compartments (AN38.1)
Three compartments:
Figure: Interior of the Larynx — Compartments (AN38.1)
1. Vestibule (supraglottis): from the laryngeal inlet (aryepiglottic folds) to the vestibular folds (false cords)
- Rich lymphatic drainage → early nodal metastasis of supraglottic tumours
2. Laryngeal ventricle: recess between the vestibular fold (false cord) and the vocal fold (true cord)
- Saccule: appendix of the ventricle, extends superiorly into the paraglottic space
3. Subglottis (infraglottis): from below the vocal cords to the lower border of the cricoid ring
- Narrowest part of the airway in children (the subglottis is encircled by the inelastic cricoid ring) — explains why subglottic oedema (as in croup) causes stridor
- The adult narrowest point is the rima glottidis
The vocal folds (true cords):
- Vocal ligament = upper free edge of the cricothyroid membrane (conus elasticus)
- Covered by stratified squamous epithelium (unlike the rest of the larynx which has respiratory epithelium)
- Paraglottic space: lateral to the vocal cord; communicates with the saccule; pathway for deep invasion of glottic tumours
Laryngoscopic anatomy:
- On laryngoscopy looking down: epiglottis anteriorly, arytenoids posteriorly; between them = rima glottidis (open during breathing, closed during phonation/Valsalva)
- Aryepiglottic folds: from the epiglottis to the arytenoids; the lateral edges of the laryngeal inlet
- Pyriform fossae: immediately lateral to the aryepiglottic folds (in the laryngopharynx)
Intrinsic Muscles of the Larynx (AN38.1)
Intrinsic Muscles of the Larynx
| Muscle | Action | Nerve Supply | Key Feature |
|---|---|---|---|
| Posterior cricoarytenoid | Abducts vocal cords (opens glottis) | RLN | ONLY abductor — 'safety muscle' |
| Lateral cricoarytenoid | Adducts vocal cords (closes glottis) | RLN | Main adductor for phonation |
| Transverse arytenoid | Approximates arytenoids | RLN | Only unpaired intrinsic muscle |
| Oblique arytenoid | Closes laryngeal inlet | RLN | Sphincteric action |
| Thyroarytenoid (vocalis) | Relaxes and shortens vocal cords | RLN | Fine pitch control |
| Cricothyroid | Lengthens and tenses vocal cords | External laryngeal nerve (SLN) | Only muscle NOT supplied by RLN |
All intrinsic muscles are supplied by the recurrent laryngeal nerve (RLN) EXCEPT the cricothyroid muscle which is supplied by the external laryngeal nerve (external branch of the superior laryngeal nerve).
Figure: Intrinsic Muscles of the Larynx (AN38.1)
Intrinsic Muscles of the Larynx
| Muscle | Action | Key Feature / Mnemonic |
|---|---|---|
| Posterior cricoarytenoid (PCA) | Abducts vocal cords (opens glottis) | Only abductor — 'safety muscle of the larynx' |
| Lateral cricoarytenoid | Adducts vocal cords (closes glottis) | Antagonist to PCA |
| Transverse arytenoid | Adducts vocal cords (closes posterior glottis) | Only unpaired intrinsic muscle |
| Oblique arytenoid | Adducts vocal cords, narrows inlet | Continues as aryepiglottic muscle |
| Thyroarytenoid | Relaxes and shortens vocal cords | Forms body of vocal fold |
| Vocalis | Fine-tunes pitch (tenses anterior cord) | Medial part of thyroarytenoid |
| Cricothyroid | Tenses and elongates vocal cords | Only muscle supplied by external laryngeal nerve (SLN) |
| Muscle | Action | Special feature |
|---|---|---|
| Posterior cricoarytenoid (PCA) | Abducts the vocal cords (opens the glottis) | The only abductor of the vocal cords; bilateral injury → airway obstruction |
| Lateral cricoarytenoid (LCA) | Adducts the vocal cords (closes the glottis) | Main adductor |
| Transverse + oblique arytenoids | Adduct the arytenoids (close the posterior glottis) | Transverse = sole unpaired intrinsic laryngeal muscle |
| Thyroarytenoid (vocalis = medial part) | Shortens/relaxes the vocal cord | The main body of the true vocal cord |
| Cricothyroid | Lengthens/tenses the vocal cord (increases pitch) | Supplied by external SLN (NOT RLN) |
Memory aid for PCA: 'PCA Opens' — Posterior Cricoarytenoid Abducts (Opens) the cords.
Clinical application:
- Unilateral RLN injury: paralysis of all intrinsic muscles on that side EXCEPT cricothyroid (supplied by external SLN, intact); cord lies in the paramedian position (cadaveric position if complete) → hoarseness
- Bilateral RLN injury: both cords paralysed → both lie paramedian → severe stridor + respiratory distress → emergency tracheostomy
- External SLN injury: cricothyroid paralysis → cord becomes lax and short → cannot reach high pitch; voice fatigue; most common in singers