Anatomy of larynx for anesthesia resident

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Larynx anatomy for the anesthesia resident

The larynx extends from approximately C3 to C6, connecting the laryngopharynx to the trachea. It functions in phonation, airway patency, aspiration prevention, cough, and pressure generation during Valsalva. During swallowing it elevates and moves anteriorly, helping close the inlet and open the upper esophageal sphincter. Gray's Anatomy for Students, p. 1192.

Laryngoscopic view: recognize epiglottis, aryepiglottic folds, false cords, true vocal cords, arytenoids, and the glottic opening. Miller's Anesthesia, 10e, p. 5843.

1. Cartilaginous framework

There are nine cartilages:

Unpaired: thyroid, cricoid, epiglottis
Paired: arytenoid, corniculate, cuneiform

Structure	High-yield anatomy	Relevance to anesthesia
Hyoid bone	Suspends the larynx superiorly	Relevant surface landmark for superior laryngeal nerve block
Thyroid cartilage	Largest cartilage; laryngeal prominence is palpable	Its superior notch is a useful anterior-neck landmark
Epiglottis	Leaf-shaped, attached to thyroid cartilage by thyroepiglottic ligament	Key landmark in direct/video laryngoscopy; its shape and position may make exposure difficult
Cricoid cartilage	At about C6; signet-ring shaped, narrow arch anteriorly and broad lamina posteriorly; only complete cartilaginous ring	Landmark for cricothyrotomy orientation and cricoid pressure. The posterior lamina is adjacent to the esophagus
Arytenoids	Sit on the posterior cricoid lamina; vocal folds attach anteriorly to thyroid cartilage and posteriorly to vocal processes	Posterior structures visible during laryngoscopy. Arytenoid trauma/dislocation may mimic vocal-fold palsy
Corniculate and cuneiform	Small cartilages within the aryepiglottic folds	Produce the posterior and lateral contours of the laryngeal inlet

The cricothyroid membrane (CTM) lies between the inferior thyroid cartilage and superior cricoid arch. It is the target for emergency front-of-neck access. The cricoid is the only complete cartilaginous airway ring. Miller's Anesthesia, 10e, p. 5841-5842.

Cartilaginous and membranous framework. Miller's Anesthesia, 10e, p. 5843.

2. Cavities and laryngoscopic divisions

From superior to inferior:

Laryngeal inlet (aditus)
- Anterior: epiglottis
- Lateral: aryepiglottic folds
- Posterior: interarytenoid region and arytenoids
Supraglottis
- Epiglottis, aryepiglottic folds, false vocal folds, laryngeal ventricles
- In adults, supraglottic structures commonly obstruct dynamically during induction or deep sedation.
Glottis
- True vocal folds plus the rima glottidis.
- Narrowest part of the adult larynx at the level of the vocal cords in a functional sense.
Subglottis
- From lower margin of the vocal folds to inferior border of cricoid.
- A fixed, non-distensible region due to the complete cricoid ring. Important in edema and acquired subglottic stenosis.

Pediatric distinction: Traditionally the cricoid was described as the narrowest region. Modern imaging indicates the glottis is often the narrowest transverse level, but the subglottic cricoid remains clinically important because it is less distensible and susceptible to pressure injury from an oversized tube.

3. Vocal folds and intrinsic muscles

Vocal folds

True vocal folds: mucosa over the vocal ligament and vocalis muscle.
Attach from the inner thyroid cartilage anteriorly to the vocal processes of arytenoids posteriorly.
False vocal folds lie superiorly and do not normally contribute to phonation.

Intrinsic muscle actions

Muscle	Principal action	Airway relevance
Posterior cricoarytenoid (PCA)	Only abductor of the vocal folds	Opens glottis during inspiration. Bilateral weakness causes a dangerously narrow airway
Lateral cricoarytenoid (LCA)	Adducts anterior vocal folds	Closure for phonation and airway protection
Interarytenoid, transverse and oblique	Adduct arytenoid cartilages, close posterior glottis	Improves glottic closure
Thyroarytenoid/vocalis	Relaxes/shortens and fine-tunes vocal folds	Voice control
Cricothyroid (CT)	Tenses and elongates vocal folds by tilting thyroid cartilage forward	Only intrinsic muscle not supplied by RLN

Exam pearl:
PCA is the only vocal-fold abductor. If both recurrent laryngeal nerves are injured, the folds may rest near the midline, causing stridor and possible inability to ventilate or extubate safely.

4. Innervation: the anesthesia-critical section

All motor and sensory laryngeal innervation is via the vagus nerve (CN X).

Nerve	Motor supply	Sensory supply	Key clinical implication
Internal branch of superior laryngeal nerve, iSLN	None	Supraglottic larynx: epiglottis, laryngeal vestibule, aryepiglottic folds, superior surface of vocal folds	Main afferent limb of laryngeal protective reflexes including laryngospasm. Block supports awake airway instrumentation
External branch of superior laryngeal nerve, eSLN	Cricothyroid	None of major importance	Injury produces impaired pitch control and vocal fatigue
Recurrent laryngeal nerve, RLN	All other intrinsic muscles: PCA, LCA, interarytenoids, thyroarytenoid	Infraglottic larynx and upper trachea	Damage causes vocal-fold paresis/paralysis. Block reduces cough response below cords
Sympathetic fibers	Vasomotor and secretomotor influence	-	Limited relevance in routine airway management

This is commonly summarized as:

“All intrinsic laryngeal muscles are recurrent laryngeal, except cricothyroid.”

Recurrent laryngeal nerve course

Right RLN: loops under the right subclavian artery
Left RLN: loops under the aortic arch, posterior to ligamentum arteriosum
Both ascend in or near the tracheoesophageal groove, pass near the thyroid gland, and enter the larynx posterior to the cricothyroid joint.

The longer left-sided intrathoracic course makes left RLN palsy relevant in mediastinal, cardiac, and aortic pathology. The OpenAnesthesia laryngeal anatomy review gives a useful anesthesia-oriented overview.

Reflex arcs

Laryngospasm: supraglottic stimulation -> iSLN afferent -> brainstem -> vagal/RLN-mediated adductor response.
Cough: laryngeal/tracheal sensory input, especially below the cords via RLN, produces expiratory and glottic responses.

5. Blood supply

Superior laryngeal artery, from the superior thyroid artery, accompanies the iSLN through the thyrohyoid membrane.
Inferior laryngeal artery, from the inferior thyroid artery, accompanies the RLN.

Procedural implication: During an iSLN block, the needle is advanced through the thyrohyoid membrane. Aspirate carefully because the superior laryngeal vessels travel in this area.

6. External landmarks and airway procedures

Landmark	Clinical use
Hyoid bone	Identify superior laryngeal region
Thyroid notch and laryngeal prominence	Surface orientation, especially in front-of-neck access
Thyrohyoid membrane	iSLN block access
Thyroid cartilage	Superior boundary when identifying CTM
Cricothyroid membrane	Emergency cricothyrotomy target
Cricoid cartilage	Inferior boundary of CTM; complete ring at C6
Tracheal rings	Tracheostomy orientation

Cricothyrotomy: Palpate thyroid cartilage, move caudad to the depression of the CTM, then identify the firm cricoid cartilage below it. Avoid excessively cephalad puncture, which risks laryngeal injury, and excessively caudad placement, which enters trachea rather than the membrane.

7. Anesthesia applications

Direct and video laryngoscopy

Identify: tongue base -> epiglottis -> vallecula -> aryepiglottic folds -> posterior arytenoids -> cords.
In Macintosh laryngoscopy, blade tip in the vallecula tensions the hyoepiglottic ligament and indirectly lifts the epiglottis.
A straight blade directly lifts the epiglottis.

Intubation trauma

Potential injuries include:

Edema or hematoma of vocal folds/arytenoids
Arytenoid dislocation
Mucosal ulceration and granuloma
RLN neuropraxia from cuff pressure or surgical factors
Subglottic injury from an oversized or repeatedly manipulated tube

Use a tube that passes without force, minimize repeated attempts, monitor cuff pressure, and investigate persistent postoperative hoarseness, stridor, aspiration, or weak cough.

Laryngeal blocks for awake intubation

iSLN block: anesthetizes supraglottic mucosa, decreases gagging and laryngospasm provoked by supraglottic contact.
Transtracheal injection through CTM: anesthetizes infraglottic tracheal mucosa through cough-mediated distribution of local anesthetic.
Topical local anesthetic addresses oral, pharyngeal, and laryngeal surfaces but requires dose accounting.

Extubation

High-risk anatomy or pathology includes:

Supraglottic edema
Post-intubation laryngeal edema
Subglottic stenosis
Bilateral vocal-fold immobility
Head and neck surgery or thyroid surgery
Prior neck radiation, tumor, or airway trauma

A cuff-leak test evaluates risk indirectly and cannot exclude post-extubation obstruction. Extubate only when oxygenation, ventilation, airway patency, neuromuscular recovery, and a rescue plan are satisfactory.

Rapid recall

Level: C3-C6
Only complete airway ring: cricoid
Only vocal-fold abductor: posterior cricoarytenoid
Only intrinsic muscle not supplied by RLN: cricothyroid
Supraglottic sensation: internal SLN
Infraglottic sensation: RLN
Right RLN loop: subclavian artery
Left RLN loop: aortic arch
Emergency front-of-neck target: cricothyroid membrane
Laryngospasm afferent limb: internal SLN

The cartilaginous structure, cricoid anatomy, and key surface landmarks are described in Miller's Anesthesia, 10e, pp. 5841-5843. The broader anatomical organization and laryngeal mobility during swallowing are described in Gray's Anatomy for Students, p. 1192.

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