Upper Airway Anatomy

1. Quick Answer

Upper airway anatomy encompasses the nasal cavity, pharynx (nasopharynx, oropharynx, hypopharynx), and larynx - the structures extending from the external nares to the lower border of the cricoid cartilage at approximately C6.

Key Concepts:

• The nasal cavity provides humidification, warming, and filtration of inspired air
• The pharynx is a muscular conduit shared by respiratory and digestive systems
• The larynx protects the lower airway and enables phonation
• The cricothyroid membrane is the anatomical target for emergency front-of-neck access

ICU Relevance:

• Critical for endotracheal intubation, difficult airway management, and tracheostomy
• Understanding nerve supply prevents iatrogenic injury during procedures
• Surface landmarks guide emergency cricothyroidotomy

Exam Focus:

• CICM First Part examiners commonly ask about laryngeal innervation, cartilage structure, and applied anatomy for airway procedures

---

2. CICM First Part Exam Focus

What Examiners Expect

Written SAQ:

Common question stems:

• "Describe the anatomy of the larynx with particular reference to its innervation"
• "Draw and label a cross-section of the larynx at the level of the vocal cords"
• "Outline the blood supply to the nasal cavity and its clinical significance"
• "Describe the surface anatomy relevant to emergency cricothyroidotomy"
• "Compare the adult and pediatric upper airway anatomy"

Expected depth:

• Detailed anatomical knowledge with named structures and relationships
• Nerve supply (sensory and motor) with clinical consequences of injury
• Blood supply with arterial territories and anastomoses
• Clear diagrams with accurate labeling
• Explicit ICU application (intubation, tracheostomy, nerve blocks)

Written MCQ:

Common topics tested:

• Laryngeal cartilages and their properties (hyaline vs elastic)
• Recurrent laryngeal nerve course and consequences of injury
• Boundaries and contents of anatomical spaces
• Airway dimensions (adult vs pediatric)
• Surface landmarks and vertebral levels

Difficulty level:

• Applied anatomical scenarios (e.g., "During thyroid surgery, which nerve is at risk?")
• Identification of structures from descriptions
• Clinical consequences of anatomical variants

Oral Viva:

Expected discussion flow:

1. Define/Describe - Overview of upper airway divisions and boundaries
2. Detail Structure - Cartilages, muscles, ligaments, spaces
3. Innervation - Vagus nerve branches, sensory/motor distribution
4. Blood Supply - Arterial supply with anastomoses
5. Apply to ICU - Intubation, tracheostomy, cricothyroidotomy
6. Compare - Adult vs pediatric differences, clinical implications

Common viva scenarios:

• "Walk me through the anatomy relevant to inserting an endotracheal tube"
• "A patient develops hoarseness after thyroid surgery. Explain the anatomical basis"
• "Describe the landmarks for emergency cricothyroidotomy"

Pass vs Fail Performance

Pass Standard:

• Accurate naming of laryngeal cartilages and their properties
• Correct description of vagus nerve branches and their functions
• Clear understanding of sensory/motor innervation above and below vocal cords
• Ability to describe surface landmarks for procedures
• Draws clear diagrams of laryngeal cartilages and cross-sections

Common Reasons for Failure:

• Confusing the superior and recurrent laryngeal nerves
• Inability to describe the course of the recurrent laryngeal nerve
• Not knowing the difference between adult and pediatric airways
• Cannot identify the cricothyroid membrane location
• Poor understanding of intrinsic laryngeal muscle actions

---

3. Key Points

Must-Know Facts

1. Nasal Cavity Blood Supply: Dual supply from internal carotid (anterior/posterior ethmoidal arteries) and external carotid (sphenopalatine, greater palatine arteries). Kiesselbach's plexus (Little's area) on anterior septum is the most common epistaxis site (PMID: 31194354).

2. Pharynx Divisions: Nasopharynx (skull base to soft palate), oropharynx (soft palate to epiglottis tip), hypopharynx (epiglottis to cricoid). The piriform fossae in the hypopharynx are key landmarks for superior laryngeal nerve block.

3. Laryngeal Cartilages: Three unpaired (thyroid, cricoid, epiglottis) and three paired (arytenoid, corniculate, cuneiform). The cricoid is the only complete cartilaginous ring in the airway (PMID: 30020619).

4. Cricothyroid Membrane: Located between the inferior border of thyroid cartilage and superior border of cricoid. Measures 9-10mm height × 22-30mm width in adults. Target for emergency front-of-neck access (PMID: 28802559).

5. Superior Laryngeal Nerve: Divides into internal branch (sensory above cords, enters thyrohyoid membrane) and external branch (motor to cricothyroid, travels with superior thyroid artery) (PMID: 26891953).

6. Recurrent Laryngeal Nerve: Motor to all intrinsic laryngeal muscles except cricothyroid; sensory below vocal cords. Left loops under aortic arch; right loops under subclavian artery. Non-recurrent variant occurs in 0.5-1% on right side (PMID: 19405101).

7. Glottis Dimensions: Adult male glottic aperture 23mm anteroposterior × 13mm transverse; narrowest part in adults. Subglottis diameter 17-18mm in adults (PMID: 12167576).

8. Pediatric Differences: Larynx at C3-C4 (vs C4-C6 adult), omega-shaped epiglottis, narrowest point historically at cricoid (now debated - functional narrowing at subglottis), obligate nasal breathers until 3-5 months (PMID: 19318998).

9. Cormack-Lehane Classification: Grade I (full glottic view), II (posterior glottis only), III (epiglottis only), IV (no glottic structures visible). Grades III-IV indicate difficult laryngoscopy (PMID: 6344656).

10. Intrinsic Laryngeal Muscles: Posterior cricoarytenoid (only abductor), lateral cricoarytenoid, transverse/oblique arytenoids, thyroarytenoid, vocalis (all adductors), cricothyroid (tensor). All supplied by RLN except cricothyroid (external SLN).

Essential Anatomical Relationships

Cricothyroid Membrane:

• Superior border: Inferior edge of thyroid cartilage
• Inferior border: Superior edge of cricoid cartilage
• Contents: Cricothyroid artery (anastomosis of superior and inferior laryngeal)
• Depth: Subcutaneous tissue, membrane - NO major vessels in midline

Tracheoesophageal Groove:

• Contains: Recurrent laryngeal nerve, inferior thyroid artery
• Clinical significance: High-risk zone during thyroid surgery

Normal Values Table

---

4. Detailed Anatomy

4.1 Nasal Cavity

Boundaries and Framework

Bony Framework:

• Roof: Nasal bones, frontal bone, cribriform plate of ethmoid, body of sphenoid
• Floor: Palatine process of maxilla, horizontal plate of palatine bone
• Lateral wall: Maxilla, lacrimal bone, ethmoid (labyrinth), inferior concha, palatine bone, medial pterygoid plate
• Medial wall (septum): Perpendicular plate of ethmoid, vomer, septal cartilage

Nasal Septum:

• Divides nasal cavity into two halves
• Cartilaginous portion (quadrangular cartilage) anteriorly
• Bony portion (vomer, perpendicular plate of ethmoid) posteriorly
• Deviation common (up to 80% of population), may impede nasotracheal intubation

Turbinates (Conchae) and Meati

Inferior Turbinate (separate bone):

• Largest turbinate
• Contains cavernous erectile tissue
• Inferior meatus beneath - nasolacrimal duct opens here
• Most important for humidification and warming

Middle Turbinate (part of ethmoid):

• Semilunar hiatus beneath
• Frontal sinus, anterior ethmoid cells, maxillary sinus drain here
• Bulla ethmoidalis visible

Superior Turbinate (part of ethmoid):

• Smallest turbinate
• Superior meatus beneath
• Posterior ethmoid cells drain here
• Sphenoethmoidal recess above - sphenoid sinus drainage

Clinical Application:

• Preferred passage for nasotracheal tube: Floor of nose (inferior meatus)
• Avoid: Direct perpendicular insertion (causes trauma)
• Vasoconstriction: Xylometazoline or cocaine reduces turbinate engorgement

Paranasal Sinuses

Blood Supply

External Carotid Contribution (major supply):

1. Sphenopalatine Artery (terminal branch of maxillary artery):
   • Enters via sphenopalatine foramen (posterior to middle turbinate)
   • Branches: Posterior lateral nasal arteries, posterior septal artery
   • "Artery of epistaxis"

• source of posterior bleeds
  • PMID: 29489215

2. Greater Palatine Artery (branch of maxillary):

   • Ascends through incisive canal
   • Supplies anterior septum

3. Superior Labial Artery (branch of facial):

   • Supplies nasal vestibule and anterior septum

Internal Carotid Contribution:

1. Anterior Ethmoidal Artery (branch of ophthalmic):

   • Exits orbit through anterior ethmoidal canal
   • Traverses anterior cranial fossa
   • Descends through cribriform plate
   • Supplies superior septum and lateral wall
   • PMID: 24604555

2. Posterior Ethmoidal Artery (branch of ophthalmic):

   • Smaller contribution
   • Supplies posterior superior nasal cavity

Kiesselbach's Plexus (Little's Area):

• Location: Anteroinferior nasal septum
• Anastomosis of 5 arteries:
  1. Anterior ethmoidal artery
  2. Sphenopalatine artery
  3. Greater palatine artery
  4. Superior labial artery
  5. (Variable) Posterior ethmoidal artery
• Source of 90% of epistaxis (anterior bleeds)
• PMID: 31194354

Woodruff's Plexus:

• Location: Posterior lateral nasal wall
• Posterior epistaxis site
• Sphenopalatine artery territory
• PMID: 28414498

Nerve Supply

Olfactory (CN I):

• Olfactory epithelium on superior septum, superior turbinate, cribriform area
• Olfactory nerve filaments pass through cribriform plate

Trigeminal (CN V):

Ophthalmic Division (V1):

• Anterior ethmoidal nerve - anterior septum, anterior lateral wall

Maxillary Division (V2):

• Posterior superior nasal nerves (via pterygopalatine ganglion)
• Nasopalatine nerve - most of septum
• Greater palatine nerve - inferior meatus
• Parasympathetic secretomotor via pterygopalatine ganglion

Autonomic Supply:

• Parasympathetic: Pterygopalatine ganglion (facial nerve via greater petrosal)
• Sympathetic: Superior cervical ganglion via deep petrosal nerve

Clinical Application - Nasal Anaesthesia:

• Anterior 2/3: Block anterior ethmoidal nerve (lidocaine-soaked pledget in olfactory cleft)
• Posterior 1/3: Block sphenopalatine ganglion (spray greater palatine foramen or inject via submucosa)

Lymphatic Drainage

• Anterior cavity: Submandibular nodes
• Posterior cavity: Retropharyngeal and deep cervical nodes
• Important route for infection spread to CNS (via cribriform plate)

---

4.2 Pharynx

The pharynx is a muscular tube extending from the skull base to the lower border of the cricoid cartilage (C6 level), approximately 12-14cm in length.

Nasopharynx

Boundaries:

• Superior: Skull base (sphenoid body, basilar part of occipital bone)
• Inferior: Soft palate (communicates with oropharynx via pharyngeal isthmus)
• Anterior: Posterior nasal apertures (choanae)
• Posterior: C1-C2 vertebral bodies

Contents:

1. Pharyngeal Tonsil (Adenoid):

   • Lymphoid tissue on posterior wall
   • Enlarges in childhood, regresses after puberty
   • Hypertrophy causes nasal obstruction, sleep apnea, eustachian dysfunction
   • Located at pharyngeal recess (fossa of Rosenmüller)

2. Eustachian Tube Orifice:

   • Opens on lateral wall, behind inferior turbinate level
   • Torus tubarius (cartilaginous elevation) above opening
   • Salpingopharyngeal fold below
   • Equalizes middle ear pressure

3. Fossa of Rosenmüller (Pharyngeal Recess):

   • Depression behind torus tubarius
   • Common site of nasopharyngeal carcinoma

Clinical Significance:

• Obligate nasal breathing in neonates - nasopharyngeal obstruction causes respiratory distress
• Adenoid hypertrophy impedes nasotracheal intubation in children
• Passage of nasogastric tube through this region

Oropharynx

Boundaries:

• Superior: Soft palate
• Inferior: Upper border of epiglottis (or hyoid bone level)
• Anterior: Posterior 1/3 of tongue (base), palatoglossal arch (anterior pillar)
• Posterior: C2-C3 vertebral bodies
• Lateral: Palatine tonsils, palatopharyngeal arch (posterior pillar)

Contents:

1. Palatine Tonsils:

   • Lymphoid tissue in tonsillar fossa (between anterior and posterior pillars)
   • Blood supply: Tonsillar artery (branch of facial), ascending palatine, dorsal lingual
   • Venous drainage to pharyngeal plexus (risk of hemorrhage during tonsillectomy)
   • Peritonsillar abscess (quinsy) may cause airway obstruction

2. Base of Tongue (Lingual Tonsil):

   • Posterior 1/3 of tongue
   • Lymphoid tissue (Waldeyer's ring component)
   • Valleculae - paired depressions between tongue base and epiglottis
   • Critical landmark for Macintosh blade placement during laryngoscopy

3. Uvula and Soft Palate:

   • Muscular valve separating nasopharynx and oropharynx
   • Muscles: Tensor veli palatini, levator veli palatini, musculus uvulae
   • Collapse contributes to obstructive sleep apnea

Clinical Significance:

• Mallampati classification based on visibility of oropharyngeal structures
• Macroglossia (down syndrome, acromegaly, amyloidosis) causes difficult intubation
• Peritonsillar abscess may deviate uvula and cause trismus

Hypopharynx (Laryngopharynx)

Boundaries:

• Superior: Upper border of epiglottis (C3 level)
• Inferior: Lower border of cricoid cartilage (C6 level) - continuous with esophagus
• Anterior: Posterior surface of larynx (aryepiglottic folds, arytenoids, posterior cricoid)
• Posterior: C3-C6 vertebral bodies

Contents:

1. Piriform Fossae (Recesses):

   • Paired recesses lateral to laryngeal inlet
   • Bounded by: Aryepiglottic fold medially, thyroid cartilage and thyrohyoid membrane laterally
   • Floor formed by thyrohyoid membrane (internal branch of SLN passes deep to mucosa here)
   • Site for superior laryngeal nerve block - pooled local anesthetic blocks sensation above cords
   • Foreign bodies commonly lodge here

2. Posterior Cricoid Region:

   • Junction with esophagus (cricopharyngeus muscle = upper esophageal sphincter)
   • Narrowest part of pharynx
   • Pharyngoesophageal junction (Killian's dehiscence) - site of Zenker's diverticulum

Clinical Significance:

• Piriform fossa mucosa overlies internal SLN - block provides anesthesia above vocal cords
• Cricopharyngeal spasm may impede nasogastric tube passage
• Aspiration of hypopharyngeal secretions common in obtunded patients

Pharyngeal Muscles

Outer Circular Layer (Constrictors):

Inner Longitudinal Layer (Elevators):

Pharyngeal Plexus:

• Location: Posterior pharyngeal wall
• Components: Pharyngeal branch of vagus (motor), glossopharyngeal (sensory), sympathetic
• Motor supply: Vagus (nucleus ambiguus) via pharyngeal branch
• Sensory supply: Glossopharyngeal (pharynx), vagus (hypopharynx)

---

4.3 Larynx

The larynx extends from the tip of the epiglottis (C3 level) to the lower border of the cricoid cartilage (C6 level) in adults. It functions as the valve protecting the lower airway and as the organ of phonation.

Laryngeal Cartilages

Unpaired Cartilages:

1. Thyroid Cartilage (Hyaline):

   • Largest laryngeal cartilage
   • Two laminae meeting anteriorly at thyroid angle (90° male, 120° female = laryngeal prominence/Adam's apple)
   • Superior horns articulate with hyoid via thyrohyoid membrane
   • Inferior horns articulate with cricoid cartilage (cricothyroid joint - synovial)
   • Oblique line on lateral surface - attachment for thyrohyoid, sternothyroid, inferior pharyngeal constrictor
   • Superior thyroid notch - palpable landmark
   • PMID: 30252327

2. Cricoid Cartilage (Hyaline):

   • Only complete cartilaginous ring in the airway
   • Signet ring shape - narrow anterior arch, broad posterior lamina
   • Level: C6 (carotid tubercle of Chassaignac)
   • Cricoid pressure (Sellick's maneuver) - occludes esophagus against vertebral body
   • Articulates superiorly with arytenoids (cricoarytenoid joints)
   • Articulates laterally with thyroid cartilage (cricothyroid joints)
   • PMID: 30020619

3. Epiglottis (Elastic cartilage):

   • Leaf-shaped, attached to inner angle of thyroid cartilage (thyroepiglottic ligament)
   • Projects superiorly behind tongue base
   • Elastic - does NOT ossify (unlike hyaline cartilages)
   • Omega-shaped (U-shaped) in infants vs flat in adults
   • Forms anterior wall of laryngeal inlet
   • Pre-epiglottic space - fat-filled space in front of epiglottis
   • PMID: 30969678

Paired Cartilages:

1. Arytenoid Cartilages (Hyaline):

   • Pyramidal shape, sit on posterosuperior border of cricoid lamina
   • Vocal process (anterior) - attachment of vocal ligament
   • Muscular process (lateral) - attachment of intrinsic muscles
   • Apex articulates with corniculate cartilage
   • Cricoarytenoid joints - synovial, allow rotation and gliding
   • Movement controls vocal cord position (abduction/adduction)
   • PMID: 30422502

2. Corniculate Cartilages (Elastic):

   • Small, conical
   • Sit on apices of arytenoid cartilages
   • Form cuneiform tubercles in aryepiglottic folds

3. Cuneiform Cartilages (Elastic):

   • Small, elongated
   • Embedded in aryepiglottic folds anterior to corniculates
   • Form cuneiform tubercles (visible on laryngoscopy)

Cartilage Ossification:

• Hyaline cartilages (thyroid, cricoid, arytenoids) begin to ossify from age 20-30
• Epiglottis (elastic) does NOT ossify
• Clinical relevance: Calcified cartilages more brittle, fracture during trauma

Laryngeal Membranes and Ligaments

Extrinsic Membranes (connect larynx to surrounding structures):

1. Thyrohyoid Membrane:

   • Connects superior border of thyroid cartilage to hyoid bone
   • Pierced by: Internal branch of superior laryngeal nerve, superior laryngeal artery/vein
   • Thickened centrally as median thyrohyoid ligament
   • Thickened posterolaterally as lateral thyrohyoid ligaments (contain triticeal cartilages)

2. Cricotracheal Membrane:

   • Connects cricoid cartilage to first tracheal ring
   • Continuous with tracheal cartilaginous rings

3. Hyoepiglottic Ligament:

   • Connects epiglottis to body of hyoid bone

Intrinsic Membranes (within larynx):

1. Quadrangular Membrane:

   • Upper portion of fibroelastic larynx
   • From lateral epiglottis to arytenoid and corniculate cartilages
   • Free upper edge = aryepiglottic fold
   • Free lower edge = vestibular ligament (false vocal cord)

2. Cricothyroid Membrane (Conus Elasticus):

   • Lower portion of fibroelastic larynx
   • From upper border of cricoid arch to inferior surface of thyroid cartilage anteriorly and vocal processes posteriorly
   • Free upper edge = vocal ligament (true vocal cord)
   • Median part (cricothyroid ligament) = site for cricothyroidotomy
   • PMID: 28802559

3. Cricothyroid Ligament (Median Cricothyroid Membrane):

   • Thickened central portion of cricothyroid membrane
   • Subcutaneous between thyroid and cricoid cartilages
   • Dimensions: Height 9-10mm × Width 22-30mm (adults)
   • Relatively avascular in midline (cricothyroid arteries are lateral)
   • Target for emergency front-of-neck access (FONA)

Interior of the Larynx

Laryngeal Inlet (Aditus):

• Opening from pharynx into larynx
• Bounded by: Epiglottis anteriorly, aryepiglottic folds laterally, interarytenoid notch posteriorly
• Faces posterosuperiorly
• "Keyhole" appearance on laryngoscopy

Vestibule:

• Space from laryngeal inlet to vestibular folds
• Bounded laterally by quadrangular membrane

Vestibular Folds (False Vocal Cords):

• Formed by free lower edge of quadrangular membrane covered by mucosa
• Pink, vascular
• Do NOT vibrate during phonation
• Protect true cords, sphincteric function

Laryngeal Ventricle (Sinus of Morgagni):

• Fusiform recess between vestibular and vocal folds
• Saccule extends superiorly (contains mucous glands that lubricate vocal cords)

Vocal Folds (True Vocal Cords):

• Formed by: Vocal ligament (free edge of conus elasticus) + vocalis muscle + mucosa
• White, avascular appearance (stratified squamous epithelium)
• Vibrate during phonation
• Anterior commissure: Where cords meet at thyroid cartilage angle
• Posterior commissure: Gap between arytenoid cartilages

Rima Glottidis (Glottic Aperture):

• Opening between vocal folds and arytenoid cartilages
• Intermembranous part (between cords): 60% of length
• Intercartilaginous part (between arytenoids): 40% of length
• Narrowest part of adult larynx
• Dimensions: Male 23mm AP × 13mm transverse (during inspiration)

Infraglottic Cavity (Subglottis):

• Below vocal cords to inferior border of cricoid
• Conical shape, wider below
• Continuous with trachea

Intrinsic Laryngeal Muscles

Actions on Vocal Cords:

Clinical Pearl - RLN Injury:

• Unilateral RLN injury: Cord in paramedian position (adductors unopposed by abductor, but cricothyroid still tensing). Voice hoarse but airway adequate.
• Bilateral RLN injury: Both cords in paramedian position. Airway obstruction - stridor, respiratory distress. Voice may be normal but "whispery".
• Only posterior cricoarytenoid (PCA) abducts - if denervated, cord cannot open

Cricothyroid Muscle Special Features:

• Only intrinsic muscle on OUTSIDE of larynx
• Supplied by external branch of SLN (not RLN)
• "Singer's muscle"
• increases pitch by tensing cords
• Acting alone after RLN injury - cords tense but immobile (good voice, poor airway)

---

4.4 Nerve Supply to the Larynx

The larynx receives its entire innervation from the vagus nerve (CN X) through two branches: the superior laryngeal nerve and the recurrent laryngeal nerve.

Vagus Nerve (CN X)

Origin: Nucleus ambiguus (motor), dorsal motor nucleus (parasympathetic), nucleus tractus solitarius (sensory)

Course: Exits skull via jugular foramen, descends in carotid sheath between internal jugular vein and internal/common carotid artery

Branches to Larynx:

1. Superior laryngeal nerve (from inferior vagal ganglion)
2. Recurrent laryngeal nerve (at different levels left vs right)

Superior Laryngeal Nerve (SLN)

Origin: Arises from inferior ganglion of vagus (nodose ganglion) at C1-C2 level

Course: Descends behind internal carotid artery, divides at level of hyoid bone

Branches:

1. Internal Branch (Sensory):

   • Pierces thyrohyoid membrane with superior laryngeal artery (2cm above thyrohyoid membrane midpoint)
   • Provides sensory supply to: Laryngeal mucosa ABOVE vocal cords, base of tongue, valleculae, epiglottis
   • Afferent limb of cough reflex from supraglottic region
   • Can be blocked in piriform fossa (topical local anesthetic)
   • PMID: 26891953

2. External Branch (Motor):

   • Smaller branch, runs with superior thyroid artery on inferior constrictor
   • High risk during thyroid surgery (superior pole ligation)
   • Supplies: Cricothyroid muscle (tensor of vocal cords)
   • Also supplies inferior pharyngeal constrictor (cricopharyngeus part)
   • "External laryngeal nerve"

• misnomer as no external motor component except cricothyroid
  • PMID: 25648382

Clinical Significance of External SLN Injury:

• Loss of ability to produce high-pitched sounds
• Voice fatigue, monotonous voice
• Less dramatic than RLN injury
• Often unrecognized postoperatively
• "Opera singer's nerve"

Recurrent Laryngeal Nerve (RLN)

Origin: Vagus nerve in thorax (left) or root of neck (right)

Course - LEFT:

• Arises from vagus anterior to aortic arch
• Loops under aortic arch, posterior to ligamentum arteriosum
• Ascends in tracheoesophageal groove
• Longer course than right (12-14cm vs 5-6cm)
• More medial position
• PMID: 19405101

Course - RIGHT:

• Arises from vagus anterior to subclavian artery
• Loops under right subclavian artery
• Ascends more obliquely (lateral to medial) toward tracheoesophageal groove
• Shorter course (5-6cm)
• More variable relationship to inferior thyroid artery

Relationship to Inferior Thyroid Artery:

• May pass anterior, posterior, or between branches
• Highly variable - NO reliable rule
• Critical zone during thyroidectomy

Entry to Larynx:

• Passes deep to inferior constrictor muscle
• Enters at cricothyroid joint (behind joint, under inferior cornu of thyroid cartilage)
• Branches to all intrinsic muscles except cricothyroid

Motor Supply:

• All intrinsic laryngeal muscles EXCEPT cricothyroid
• Posterior cricoarytenoid (abductor) - most vulnerable branch
• Lateral cricoarytenoid, transverse arytenoid, oblique arytenoid, thyroarytenoid, vocalis (adductors)

Sensory Supply:

• Laryngeal mucosa BELOW vocal cords
• Tracheal mucosa (upper)
• Afferent limb of cough reflex from subglottic region

Anatomic Variants:

Non-Recurrent Laryngeal Nerve (NRLN):

• Occurs in 0.5-1% of patients
• Almost exclusively on RIGHT side
• Associated with aberrant right subclavian artery (arteria lusoria)
• Nerve arises directly from vagus and passes medially to enter larynx
• High risk of injury during thyroid surgery if not recognized
• PMID: 12660381

Ligament of Berry (Suspensory Ligament of Thyroid):

• Dense fibrous tissue attaching thyroid to trachea
• RLN often passes through or very close to this ligament
• Most common site of RLN injury during thyroidectomy

Clinical Consequences of Laryngeal Nerve Injury

Unilateral SLN (External Branch) Injury:

• Loss of cricothyroid function
• Voice changes: Lower pitch, inability to produce high notes, vocal fatigue
• Often subtle, may go unrecognized
• Voice therapy usually effective

Unilateral RLN Injury:

• Vocal cord paralysis in paramedian or intermediate position
• Symptoms: Hoarseness, breathy voice, weak cough, possible aspiration
• Adequate airway (contralateral cord can abduct)
• May improve with time (regeneration) or with medialization procedures
• PMID: 17534128

Bilateral RLN Injury:

• Both cords paralyzed in paramedian position
• AIRWAY EMERGENCY: Stridor, respiratory distress, obstruction
• Voice may be preserved (cords can still vibrate)
• Requires immediate reintubation or tracheostomy
• Staged thyroidectomy preferred to prevent this complication
• PMID: 21509149

Bilateral SLN Injury:

• Both cords relaxed (bowed)
• Voice fatigue, hoarseness, limited pitch range
• Less acute than bilateral RLN injury

Combined SLN and RLN Injury (Unilateral):

• Cord in cadaveric (intermediate) position
• Worse prognosis for voice recovery

---

4.5 Blood Supply to the Larynx

Arterial Supply

Superior Laryngeal Artery:

• Branch of superior thyroid artery (from external carotid)
• Pierces thyrohyoid membrane with internal branch of SLN
• Supplies: Larynx above vocal cords, epiglottis
• Major blood supply to larynx

Inferior Laryngeal Artery:

• Branch of inferior thyroid artery (from thyrocervical trunk of subclavian)
• Accompanies recurrent laryngeal nerve
• Supplies: Larynx below vocal cords, posterior larynx

Cricothyroid Artery:

• Anastomosis between superior laryngeal and inferior laryngeal arteries
• Crosses anterior to cricothyroid membrane (LATERALLY)
• Midline relatively avascular - safe for cricothyroidotomy

Venous Drainage

• Superior laryngeal vein: Drains to superior thyroid vein → internal jugular vein
• Inferior laryngeal vein: Drains to inferior thyroid vein → brachiocephalic vein

Lymphatic Drainage

Clinical Significance: Laryngeal carcinoma metastasis patterns

Supraglottic Larynx (above vocal cords):

• Rich lymphatic network
• Drains to: Upper deep cervical nodes (levels II, III)
• Bilateral drainage common (risk of contralateral metastasis)

Glottis (vocal cords):

• Sparse lymphatics (vocal cord mucosa - stratified squamous)
• Late nodal metastasis
• Drains to: Pre-laryngeal (Delphian node), pretracheal nodes

Subglottic Region:

• Drains to: Pretracheal, paratracheal nodes
• May spread to mediastinal nodes

---

4.6 Airway Dimensions

Adult Dimensions

Pediatric Dimensions

Poiseuille's Law Application (PMID: 25440628):

• Resistance ∝ 1/radius⁴
• 1mm edema in infant (4mm subglottis): Reduces cross-sectional area by ~75%
• 1mm edema in adult (17mm subglottis): Reduces cross-sectional area by ~44%
• Pediatric airways much more sensitive to edema

Adult vs Pediatric Differences

PMID: 19318998, 18231508, 25440628

---

4.7 Surface Anatomy and Landmarks

Anterior Neck Landmarks

From Superior to Inferior:

1. Hyoid Bone (C3):

   • Palpable at level of mandibular angle
   • Greater horn palpable laterally
   • No skeletal articulation (suspended by muscles)

2. Thyrohyoid Membrane:

   • Between hyoid and thyroid cartilage
   • Site of internal SLN entry

3. Thyroid Notch (C4-C5):

   • V-shaped notch at superior border of thyroid cartilage
   • Easily palpable in most individuals
   • Laryngeal prominence (Adam's apple) just below

4. Thyroid Lamina:

   • Lateral surface palpable
   • Oblique line felt laterally

5. Cricothyroid Membrane (C5-C6):

   • Depression between inferior border of thyroid cartilage and cricoid arch
   • Target for emergency cricothyroidotomy
   • Located 2-3cm below laryngeal prominence
   • Palpate "dip" moving finger inferiorly from thyroid cartilage

6. Cricoid Cartilage (C6):

   • "Signet ring"

• prominent anterior arch
  • Level of carotid tubercle (Chassaignac's tubercle on C6 transverse process)
  • Level of cricoid pressure (Sellick's maneuver)
  • Transition to trachea

7. Tracheal Rings:

   • Palpable below cricoid
   • C-shaped, open posteriorly

8. Sternal Notch (T2-T3):

   • Jugular notch
   • Trachea palpable above
   • Carina ~5cm below in adults

Landmarks for Cricothyroidotomy

Key Measurements:

• Cricothyroid membrane: 9-10mm height × 22-30mm width
• Location: Midline, between thyroid and cricoid cartilages
• Depth: Subcutaneous tissue + membrane (total 5-10mm)

Identification Technique:

1. Palpate thyroid notch (V-shaped superior border)
2. Move finger inferiorly over laryngeal prominence
3. Feel "dip" or depression = cricothyroid membrane
4. Confirm cricoid cartilage below (hard, complete ring)

Structures at Risk:

• Cricothyroid artery: Runs laterally across upper membrane (stay midline)
• Anterior jugular veins: 1-2cm lateral to midline
• Thyroid isthmus: Usually below cricoid (may extend superiorly)
• Pyramidal lobe: May extend to hyoid in 30-50%

Difficult Anatomy:

• Obesity: Fat obscures landmarks
• Short neck: Reduced space
• Neck flexion/extension: Alters relationships
• Previous surgery/radiation: Scarring, altered anatomy
• Female/pediatric: Smaller structures

PMID: 28802559

---

4.8 Applied Anatomy for ICU

Difficult Airway Prediction

Anatomical Predictors (PMID: 29761335, 30721295):

Mallampati Classification (PMID: 3592174):

• Class I: Soft palate, fauces, uvula, tonsillar pillars visible
• Class II: Soft palate, fauces, uvula visible
• Class III: Soft palate, base of uvula visible
• Class IV: Only hard palate visible

Cormack-Lehane Classification (PMID: 6344656):

• Grade I: Full view of glottis
• Grade IIa: Partial view of glottis
• Grade IIb: Only posterior cartilages and epiglottis visible
• Grade III: Only epiglottis visible
• Grade IV: Neither glottis nor epiglottis visible

LEMON Criteria (PMID: 15982823):

• Look externally: Facial trauma, large tongue, obesity, short neck
• Evaluate 3-3-2 rule: 3 fingers mouth opening, 3 fingers TMD, 2 fingers hyoid-thyroid
• Mallampati: Class III-IV
• Obstruction: Epiglottitis, peritonsillar abscess, angioedema
• Neck mobility: Cervical spine immobilization, limited extension

Anatomy for Endotracheal Intubation

Oral Axis Alignment:

• Three axes must align: Oral, pharyngeal, laryngeal
• "Sniffing position": Neck flexion + atlantooccipital extension
• Head elevation 8-10cm above bed level

Laryngoscopy:

• Macintosh (curved) blade: Tip in vallecula, lifts epiglottis indirectly
• Miller (straight) blade: Lifts epiglottis directly (better for anterior larynx)
• View: Epiglottis → arytenoids → posterior commissure → vocal cords

ETT Placement:

• Adult male: ID 8.0-8.5mm, insert 21-23cm at teeth
• Adult female: ID 7.0-7.5mm, insert 19-21cm at teeth
• Tip position: Mid-trachea (T2-T4), 3-5cm above carina
• Cuff at subglottic level (below cords)

Complications Related to Anatomy:

• Esophageal intubation: ETT passes posterior to arytenoids
• Right mainstem intubation: Right bronchus more vertical
• Vocal cord trauma: Forced passage through closed glottis
• Arytenoid dislocation: Excessive force on arytenoid cartilages
• Posterior pharyngeal wall trauma: Tip of blade

Anatomy for Tracheostomy

Surgical Tracheostomy Landmarks:

• Incision: Horizontal or vertical at level of 2nd-4th tracheal rings
• Structures encountered:
  1. Skin and subcutaneous tissue
  2. Superficial cervical fascia and platysma
  3. Strap muscles (sternohyoid, sternothyroid) - separated in midline
  4. Thyroid isthmus (may need division if overlying)
  5. Pretracheal fascia
  6. Anterior tracheal wall

Structures at Risk:

• Thyroid isthmus: Overlies tracheal rings 2-4; may require division and ligation
• Anterior jugular veins: Lateral to midline
• Inferior thyroid veins: Descend in front of trachea
• Thyroid ima artery: Present in 10%, ascends anterior to trachea
• Brachiocephalic (innominate) artery: Crosses trachea at sternal notch level in children
• Recurrent laryngeal nerves: In tracheoesophageal groove bilaterally

High Tracheostomy Complications (above 2nd ring):

• Subglottic stenosis
• Damage to cricoid cartilage
• Risk to recurrent laryngeal nerve

Percutaneous Dilational Tracheostomy (PDT):

• Preferred between rings 2-4
• Use bronchoscopic guidance to confirm position
• Avoid if landmarks difficult (obesity, previous surgery)

Anatomy for Cricothyroidotomy

Indications:

• Cannot Intubate, Cannot Oxygenate (CICO) scenario
• Final step in difficult airway algorithms (ANZCA, DAS)

Surgical Technique:

1. Identify landmarks (thyroid cartilage, cricothyroid membrane, cricoid)
2. Stabilize larynx with non-dominant hand
3. Vertical skin incision (allows adjustment if landmarks unclear)
4. Horizontal stab through membrane
5. Bougie insertion → railroading ETT (size 6.0mm) or cuffed cricothyroidotomy tube

Needle Cricothyroidotomy (temporary oxygenation):

• 14G cannula through membrane
• Jet ventilation (50 psi oxygen, 1 second inflation)
• Risk: Barotrauma if upper airway obstructed
• Only buys 30-45 minutes

Anatomical Considerations:

• Membrane dimensions limit tube size (maximum 6.0mm ID tube)
• Stay midline to avoid cricothyroid arteries
• Angle cannula/tube caudally (toward trachea)
• Smaller in females, children, elderly

Contraindications (Relative):

• Children < 8-10 years (membrane too small) → needle technique preferred
• Complete laryngeal transection
• Laryngotracheal disruption

PMID: 28802559

---

5. Australian/NZ Context

ANZCA/CICM Airway Guidelines

ANZCA PS61: Guidelines on Checking Anaesthesia Delivery Systems:

• Pre-use check of airway equipment
• Difficult airway trolley requirements

ANZCA/CICM PS56: Guidelines on the Conduct of Major Regional Analgesia in Obstetrics:

• Airway considerations in obstetric patients
• Failed intubation protocols

All4Kids Difficult Airway Guidelines (Australian Paediatric):

• Specific pediatric airway algorithms
• Equipment sizing

Vortex Approach (Australian origin):

• Cognitive aid for airway management
• Three "lifelines": Face mask, supraglottic device, endotracheal tube
• Green zone (adequate oxygenation) vs Red zone (CICO)

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Peoples:

• Higher rates of conditions affecting airway:
  • Obstructive sleep apnea (obesity prevalence)
  • Rheumatic heart disease (difficult airway if cardiac failure)
  • Chronic kidney disease (fluid overload, pulmonary edema)
  • Diabetes (cervical spine stiffness, limited neck mobility)
• Potential for difficult airway assessment and planning
• Communication through Aboriginal Hospital Liaison Officers for consent
• Cultural considerations for tracheostomy (altered appearance)

Māori (New Zealand):

• Similar chronic disease burden affecting airway
• Whānau involvement in airway planning and consent
• Tikanga considerations for tracheostomy
• Te Tiriti o Waitangi obligations in healthcare delivery

Remote/Rural Considerations

RFDS (Royal Flying Doctor Service) Airway Management:

• Limited equipment in remote settings
• Pre-hospital RSI protocols
• Telemedicine support for difficult airway
• Standardized drug kits for intubation

Challenges in Remote Australia:

• Delayed access to definitive airway management
• Limited backup if primary approach fails
• Video laryngoscopy increasingly available in retrieval services
• Surgical airway skills essential for remote practitioners

---

6. Clinical Application

ICU Scenario 1: Failed Intubation Leading to CICO

Case: A 52-year-old obese male with known OSA presents for emergency laparotomy. After induction, bag-mask ventilation is difficult, two attempts at intubation fail (Cormack-Lehane Grade IV), and supraglottic airway fails to provide ventilation. SpO2 drops to 70%.

Anatomical Basis:

• Obesity causes excessive soft tissue in pharynx
• Large tongue base obscures view of larynx
• Cervical spine immobility reduces axis alignment
• Short neck makes landmark identification difficult

Applied Anatomy for CICO Management:

1. Identify Cricothyroid Membrane:

   • Palpate inferior border of thyroid cartilage
   • Feel "dip" before cricoid cartilage
   • In obesity, use laryngeal handshake technique

2. Surgical Cricothyroidotomy:

   • Vertical skin incision (8cm) to improve landmark identification
   • Blunt dissection through fat to membrane
   • Horizontal stab through membrane
   • Bougie → 6.0mm cuffed ETT

3. Post-Procedure Anatomy:

   • Tube tip in subglottic space
   • Limited length of tube in trachea
   • Convert to tracheostomy within 72 hours

ICU Scenario 2: Post-Thyroidectomy Stridor

Case: A 45-year-old woman is extubated 6 hours after total thyroidectomy. She develops progressive stridor, voice changes, and respiratory distress.

Differential Diagnosis Based on Anatomy:

1. Bilateral RLN Injury:

   • Both cords in paramedian position
   • Stridor (inspiratory > expiratory)
   • Voice may be preserved but weak
   • Most critical diagnosis - requires immediate reintubation

2. Unilateral RLN Injury:

   • One cord paralyzed, may cause stridor if edema also present
   • Hoarseness predominant symptom

3. External SLN Injury:

   • Loss of cricothyroid function
   • Voice changes (lower pitch) without stridor
   • Less urgent

4. Laryngeal/Surgical Site Hematoma:

   • External compression of airway
   • May require bedside hematoma evacuation
   • Rapidly progressive

Anatomical Considerations for Reintubation:

• Edema may distort anatomy
• Previous surgical trauma increases difficulty
• Video laryngoscopy preferred
• Prepare for surgical airway if reintubation fails

ICU Scenario 3: Nasal Intubation

Case: A 28-year-old male with mandibular fracture requires nasotracheal intubation for theatre.

Anatomical Considerations:

1. Nasal Preparation:

   • Vasoconstrict with xylometazoline or cocaine
   • Preferred side: Right (bevel on left avoids septum)
   • Avoid with basilar skull fracture (cribriform plate injury)

2. Tube Passage:

   • Inferior meatus (floor of nose) - largest passage
   • Aim perpendicular to face plane, then advance horizontally
   • Gentle rotation if resistance (turbinates)

3. Blood Supply Risks:

   • Kiesselbach's plexus (anterior septum) - epistaxis
   • Sphenopalatine artery (posterior) - severe bleeding if injured
   • Pressure necrosis of septum with prolonged intubation

4. Laryngoscopy and Advancement:

   • Visualize tube entering pharynx
   • Magill forceps to guide tip through cords
   • Avoid arytenoid trauma

---

7. SAQ Practice Questions

SAQ 1: Laryngeal Anatomy and Innervation (15 marks)

Time: 15 minutes

Question:

A 55-year-old woman undergoes total thyroidectomy. Postoperatively, she develops stridor and respiratory distress requiring reintubation.

(a) Describe the anatomy of the recurrent laryngeal nerve, including its origin, course, and structures it supplies. (5 marks)

(b) Explain the anatomical basis for the difference in position of the vocal cords in unilateral versus bilateral recurrent laryngeal nerve injury. (5 marks)

(c) Outline the anatomical landmarks and technique for emergency cricothyroidotomy if reintubation fails. (5 marks)

---

Model Answer:

(a) Recurrent Laryngeal Nerve Anatomy (5 marks):

Origin (1 mark):

• Branch of vagus nerve (CN X)
• Arises in thorax (left) or root of neck (right)

Course - Differences (2 marks):

• Left RLN: Arises anterior to aortic arch, loops under arch posterior to ligamentum arteriosum, ascends in tracheoesophageal groove. Longer course (12-14cm), more medial position.
• Right RLN: Arises anterior to subclavian artery, loops under it, ascends obliquely toward tracheoesophageal groove. Shorter course (5-6cm), more lateral then medial.
• Both enter larynx deep to inferior constrictor, at cricothyroid joint

Structures Supplied (2 marks):

• Motor: All intrinsic laryngeal muscles EXCEPT cricothyroid:
  • Posterior cricoarytenoid (only abductor)
  • Lateral cricoarytenoid, transverse/oblique arytenoids, thyroarytenoid, vocalis (adductors)
• Sensory: Laryngeal mucosa below vocal cords, upper trachea

---

(b) Vocal Cord Position in RLN Injury (5 marks):

Unilateral RLN Injury (2.5 marks):

• Affected cord in paramedian or intermediate position
• Mechanism:
  • Loss of posterior cricoarytenoid (abductor) → cannot open cord
  • Cricothyroid (external SLN) still functioning → maintains tension
  • Residual adductor tone from intact contralateral RLN and passive elastic recoil
• Result: Hoarseness, weak voice, but airway usually adequate as contralateral cord can abduct fully

Bilateral RLN Injury (2.5 marks):

• Both cords in paramedian position (close to midline)
• Mechanism:
  • Complete loss of all abductor function (bilateral posterior cricoarytenoid paralysis)
  • Cricothyroid muscles intact bilaterally → both cords tensed and adducted
  • No abduction possible → rima glottidis very narrow
• Result: Stridor, respiratory distress, airway obstruction
  • Voice may be preserved (cords can still vibrate)
  • Requires immediate intervention (reintubation or tracheostomy)

---

(c) Emergency Cricothyroidotomy (5 marks):

Anatomical Landmarks (2 marks):

• Palpate thyroid notch (V-shaped depression) at superior border of thyroid cartilage
• Move finger inferiorly over laryngeal prominence (Adam's apple)
• Feel "dip" = cricothyroid membrane (between inferior thyroid cartilage and cricoid arch)
• Confirm cricoid cartilage below (complete ring, more prominent)
• Membrane: 9-10mm height × 22-30mm width, midline relatively avascular

Surgical Technique (3 marks):

1. Stabilize larynx: Grasp thyroid cartilage with non-dominant hand (laryngeal handshake)
2. Vertical skin incision: 3-4cm over membrane (allows adjustment if landmarks unclear)
3. Horizontal stab incision: Through membrane with scalpel
4. Caudal direction: Rotate blade 90° or insert tracheal hook to maintain opening
5. Bougie insertion: Advance bougie into trachea, confirm position
6. Tube insertion: Railroad 6.0mm cuffed ETT over bougie
7. Confirm: ETCO2, bilateral chest movement
8. Secure: Tape and plan for definitive airway (tracheostomy within 72 hours)

---

SAQ 2: Nasal Cavity and Pharyngeal Anatomy (15 marks)

Time: 15 minutes

Question:

A 65-year-old man with severe epistaxis requires nasal packing and possible surgical intervention.

(a) Describe the blood supply to the nasal cavity, including the sources from both the internal and external carotid systems. (5 marks)

(b) Describe the anatomy of Kiesselbach's plexus and explain why it is the most common site of epistaxis. (4 marks)

(c) Outline the anatomy of the pharynx relevant to airway management, including its divisions and the location of the piriform fossae. (6 marks)

---

Model Answer:

(a) Nasal Cavity Blood Supply (5 marks):

External Carotid Contribution (2.5 marks):

1. Sphenopalatine Artery (terminal branch of maxillary artery):
   • Enters via sphenopalatine foramen (behind middle turbinate)
   • Branches: Posterior lateral nasal arteries (lateral wall, turbinates), posterior septal artery (septum)
   • "Artery of epistaxis"

• major source of posterior bleeds

2. Greater Palatine Artery (maxillary artery branch):

   • Ascends through incisive canal
   • Supplies anterior septum

3. Superior Labial Artery (facial artery branch):

   • Supplies nasal vestibule and anterior septum

Internal Carotid Contribution (2.5 marks):

1. Anterior Ethmoidal Artery (ophthalmic artery branch):

   • Exits orbit via anterior ethmoidal canal
   • Traverses anterior cranial fossa
   • Descends through cribriform plate
   • Supplies superior and anterior septum, anterior lateral wall

2. Posterior Ethmoidal Artery (ophthalmic artery branch):

   • Smaller contribution
   • Supplies posterior superior nasal cavity

---

(b) Kiesselbach's Plexus (4 marks):

Anatomy (2 marks):

• Location: Anterior-inferior nasal septum (Little's area)
• Anastomosis of 5 arteries:
  1. Anterior ethmoidal artery
  2. Sphenopalatine artery (posterior septal branch)
  3. Greater palatine artery
  4. Superior labial artery
  5. Posterior ethmoidal artery (variable)
• Rich vascular network in thin overlying mucosa

Why Most Common Site of Epistaxis (2 marks):

• Exposed location: Anterior position subject to trauma (nose picking, dry air)
• Thin mucosa: Minimal protection of underlying vessels
• Dual arterial supply: Anastomosis from both ICA and ECA systems increases vascular density
• Kiesselbach's plexus disruption: Minor trauma causes significant bleeding from multiple small vessels
• Account for 90% of all epistaxis cases (anterior epistaxis)

---

(c) Pharynx Anatomy for Airway Management (6 marks):

Divisions and Boundaries (3 marks):

1. Nasopharynx (skull base to soft palate):

   • Contains: Adenoids (pharyngeal tonsil), Eustachian tube orifices, fossa of Rosenmüller
   • Level: C1-C2 vertebrae
   • Relevance: Adenoid hypertrophy obstructs nasotracheal route

2. Oropharynx (soft palate to tip of epiglottis):

   • Contains: Palatine tonsils, base of tongue, uvula, valleculae
   • Level: C2-C3 vertebrae
   • Relevance: Mallampati assessment, supraglottic device seating, Macintosh blade in vallecula

3. Hypopharynx (epiglottis to lower border of cricoid):

   • Contains: Piriform fossae, posterior cricoid region (cricopharyngeus = upper esophageal sphincter)
   • Level: C3-C6 vertebrae
   • Relevance: SLN block, aspiration risk

Piriform Fossae Anatomy (3 marks):

• Location: Paired recesses lateral to laryngeal inlet
• Boundaries:
  • "Medial: Aryepiglottic fold"
  • "Lateral: Thyroid cartilage lamina and thyrohyoid membrane"
  • "Floor: Thyrohyoid membrane (internal SLN passes deep to mucosa)"
• Clinical Relevance:
  • "Superior laryngeal nerve block: Topical local anesthetic pooled in piriform fossa anesthetizes internal branch of SLN → provides sensory block above vocal cords for awake intubation"
  • Foreign bodies commonly lodge here
  • Entrance to esophagus posterior to fossae

---

8. Viva Practice Scenarios

Viva Scenario 1: Laryngeal Anatomy for Airway Management

Stem: "You are the ICU registrar called to assist with a 'cannot intubate' scenario in theatre. The patient is a 48-year-old male undergoing emergency surgery."

---

Examiner: "Tell me about the anatomical landmarks you would use to perform an emergency cricothyroidotomy."

Candidate: "The cricothyroid membrane is the anatomical target for emergency front-of-neck access. It is located between the inferior border of the thyroid cartilage and the superior border of the cricoid cartilage.

To identify it, I would:

1. Palpate the thyroid notch - the V-shaped depression at the superior border of the thyroid cartilage
2. Move my finger inferiorly over the laryngeal prominence (Adam's apple)
3. Feel the depression or 'dip' which is the cricothyroid membrane
4. Confirm the cricoid cartilage below - this is a complete cartilaginous ring and feels more prominent

The membrane measures approximately 9-10mm in height and 22-30mm in width. The midline is relatively avascular, though the cricothyroid artery crosses laterally."

---

Examiner: "Good. What structures are at risk during cricothyroidotomy?"

Candidate: "The structures at risk include:

Vascular:

• Cricothyroid artery - anastomosis of superior and inferior laryngeal arteries, runs laterally across the membrane, so staying midline avoids this
• Anterior jugular veins - located 1-2cm lateral to midline

Thyroid:

• Thyroid isthmus - usually lies below the cricoid, but may extend superiorly
• Pyramidal lobe - present in 30-50% of patients, may extend to the hyoid

Laryngeal Structures:

• Vocal cords - if incision too high, may damage cords
• Posterior tracheal wall and esophagus - if too much depth or posterior pressure

Neurological:

• Recurrent laryngeal nerves are protected in the tracheoesophageal groove, away from the anterior midline approach"

---

Examiner: "The patient has been reintubated successfully. What are the anatomical differences between the adult and pediatric airway that would have made cricothyroidotomy more difficult in a child?"

Candidate: "There are several key anatomical differences:

Cricothyroid Membrane Size:

• In a neonate, the membrane is only 2.5-3mm high and 6-8mm wide, compared to 9-10mm × 22-30mm in adults
• This makes surgical cricothyroidotomy technically very difficult and needle cricothyroidotomy is preferred in children under 8-10 years

Larynx Position:

• The pediatric larynx is at C3-C4 level (versus C4-C6 in adults)
• This means it is more cephalad and anterior, altering the angle of approach

Airway Shape and Narrowest Point:

• Pediatric airway is traditionally described as funnel-shaped with the narrowest point at the cricoid
• Adult airway is more cylindrical with the narrowest point at the glottis

Other Differences:

• Relatively larger tongue and occiput
• Omega-shaped, floppy epiglottis
• Smaller tracheal diameter (4mm in neonate vs 15-20mm in adult)
• Softer, more compliant cartilages"

---

Examiner: "Describe the nerve supply to the larynx and what would happen if the recurrent laryngeal nerve was damaged bilaterally."

Candidate: "The larynx is innervated entirely by the vagus nerve through two branches:

Superior Laryngeal Nerve:

• Divides into internal and external branches
• Internal branch: Sensory to laryngeal mucosa above the vocal cords, passes through thyrohyoid membrane
• External branch: Motor to cricothyroid muscle (the only tensor of the vocal cords)

Recurrent Laryngeal Nerve:

• Motor to all intrinsic laryngeal muscles except cricothyroid
• This includes the posterior cricoarytenoid (the only abductor), and all adductors
• Sensory to mucosa below the vocal cords

Bilateral RLN Injury:

• Both cords would be paralyzed in a paramedian or adducted position
• The posterior cricoarytenoid muscles (only abductors) are denervated bilaterally
• The cricothyroid muscles remain intact (external SLN), maintaining tension on the cords
• Clinical result: Stridor, respiratory distress, and airway obstruction
• The voice may actually be preserved because the cords can still vibrate
• This is an airway emergency requiring immediate reintubation or tracheostomy

This differs from unilateral injury where only one cord is paralyzed and the contralateral abductor can still open the airway."

---

Viva Scenario 2: Nasal and Pharyngeal Anatomy

Stem: "A 70-year-old man in the ICU requires nasogastric tube insertion for enteral feeding. He has a history of recurrent epistaxis."

---

Examiner: "Describe the anatomy of the nasal cavity relevant to NG tube insertion."

Candidate: "The nasal cavity extends from the external nares to the posterior nasal apertures (choanae) which open into the nasopharynx.

Framework:

• Divided by the nasal septum (septal cartilage anteriorly, perpendicular plate of ethmoid and vomer posteriorly)
• Lateral wall contains three turbinates or conchae

Turbinates and Meati:

• Inferior turbinate is the largest and overlies the inferior meatus
• Middle turbinate overlies the middle meatus (paranasal sinus drainage)
• Superior turbinate is smallest, overlies superior meatus

Preferred Passage for NG Tube:

• Floor of the nose, along the inferior meatus
• This is the largest passage
• The tube should be directed horizontally, perpendicular to the face plane
• Avoid directing superiorly (toward cribriform plate)

Blood Supply Considerations:

• Kiesselbach's plexus on the anteroinferior septum - site of anterior epistaxis
• Sphenopalatine artery territory posteriorly
• Risk of epistaxis with nasal instrumentation, especially in anticoagulated patients

The nasopharynx is entered through the choanae, then the tube passes behind the soft palate into the oropharynx, through the hypopharynx and into the esophagus at C6 level."

---

Examiner: "The patient has a significant nosebleed during NG insertion. Describe the blood supply to the nasal cavity."

Candidate: "The nasal cavity has a dual blood supply from both the internal and external carotid systems.

External Carotid System (major supply):

1. Sphenopalatine Artery - terminal branch of maxillary artery

   • Enters via sphenopalatine foramen behind the middle turbinate
   • Major supply to septum and lateral wall
   • Called the 'artery of epistaxis' - source of posterior bleeds

2. Greater Palatine Artery - also from maxillary

   • Ascends through incisive canal to supply anterior septum

3. Superior Labial Artery - from facial artery

   • Supplies nasal vestibule and anterior septum

Internal Carotid System:

1. Anterior Ethmoidal Artery - from ophthalmic artery

   • Exits orbit, traverses anterior cranial fossa, descends through cribriform plate
   • Supplies anterior and superior nasal cavity

2. Posterior Ethmoidal Artery - smaller contribution

These vessels anastomose on the anterior septum forming Kiesselbach's plexus, which is the source of 90% of epistaxis."

---

Examiner: "If this patient required awake fibreoptic intubation, how would you provide anaesthesia to the upper airway, and what nerves would you need to block?"

Candidate: "For awake fibreoptic intubation, I need to provide topical anaesthesia to three anatomical regions:

1. Nasal Cavity (if nasotracheal approach):

• Innervation: Anterior ethmoidal nerve (V1) anteriorly, posterior superior nasal nerves via pterygopalatine ganglion (V2) posteriorly
• Technique: Topical lignocaine spray or cocaine-soaked pledgets placed along the nasal cavity

2. Oropharynx and Supraglottic Region:

• Innervation: Glossopharyngeal nerve (CN IX) for posterior tongue and pharynx; internal branch of superior laryngeal nerve for base of tongue, valleculae, epiglottis, and laryngeal mucosa above the cords
• Techniques:
  • "Glossopharyngeal block: Topical spray or inject at base of anterior pillar"
  • "Superior laryngeal nerve block: Inject local anaesthetic where internal branch pierces thyrohyoid membrane, or pool in piriform fossa to allow mucosal diffusion"

3. Larynx Below Vocal Cords and Trachea:

• Innervation: Recurrent laryngeal nerve
• Technique: Transtracheal injection through cricothyroid membrane, or 'spray as you go' through the fibrescope

The piriform fossa is particularly useful for the internal SLN block - local anaesthetic pooled here diffuses through the mucosa to anaesthetise the nerve as it passes deep to the piriform fossa floor."

---

9. MCQ Practice Questions

MCQ 1

The cricothyroid muscle is innervated by:

A. Internal branch of superior laryngeal nerve
B. External branch of superior laryngeal nerve
C. Recurrent laryngeal nerve
D. Pharyngeal branch of vagus
E. Glossopharyngeal nerve

Answer: B

Explanation: The external branch of the superior laryngeal nerve provides motor innervation to the cricothyroid muscle. This is the only intrinsic laryngeal muscle NOT supplied by the recurrent laryngeal nerve. The cricothyroid muscle is the primary tensor of the vocal cords and is located on the external surface of the larynx. The internal branch of the SLN is sensory to the larynx above the vocal cords.

---

MCQ 2

Which of the following structures forms the narrowest part of the adult upper airway?

A. Nasopharynx
B. Oropharynx
C. Glottis (rima glottidis)
D. Subglottis (cricoid ring)
E. Trachea

Answer: C

Explanation: In adults, the narrowest part of the airway is the glottis (rima glottidis) - the opening between the vocal cords. The subglottic region (cricoid ring level) is traditionally described as the narrowest point in children, though recent evidence suggests this is debatable. In adults, the glottis measures approximately 23mm AP × 13mm transverse, while the subglottis is slightly larger at 17-18mm diameter.

---

MCQ 3

Bilateral recurrent laryngeal nerve injury results in:

A. Aphonia and adequate airway
B. Hoarseness and aspiration risk
C. Stridor and respiratory obstruction
D. Voice fatigue and vocal cord atrophy
E. No clinical manifestations

Answer: C

Explanation: Bilateral RLN injury causes both vocal cords to remain in the paramedian position (adducted). This occurs because the posterior cricoarytenoid muscles (the ONLY abductors) are denervated bilaterally, while the cricothyroid muscles (supplied by external SLN) maintain cord tension. The result is stridor and respiratory obstruction - an airway emergency. Paradoxically, the voice may be preserved because the cords can still vibrate. This contrasts with unilateral injury, which causes hoarseness but maintains an adequate airway.

---

MCQ 4

Kiesselbach's plexus (Little's area) is an anastomosis of arteries from which TWO arterial systems?

A. External carotid and vertebral
B. Internal carotid and external carotid
C. Facial and maxillary only
D. Ophthalmic and ascending pharyngeal
E. Posterior ethmoidal and inferior thyroid

Answer: B

Explanation: Kiesselbach's plexus is an anastomosis of vessels from BOTH the internal carotid (anterior ethmoidal artery from ophthalmic) and external carotid (sphenopalatine, greater palatine, and superior labial arteries) systems. This dual supply converges on the anteroinferior nasal septum, making it the most common site of epistaxis (90% of cases). The rich vascular network with dual arterial territories makes even minor trauma to this area prone to significant bleeding.

---

MCQ 5

During direct laryngoscopy with a Macintosh blade, the tip of the blade is placed in the:

A. Posterior pharynx
B. Piriform fossa
C. Vallecula
D. Glottis
E. Laryngeal ventricle

Answer: C

Explanation: The Macintosh (curved) blade is placed with its tip in the vallecula - the space between the base of the tongue and the epiglottis. Anterior lift on the blade indirectly elevates the epiglottis (via tension on the hyoepiglottic ligament), exposing the glottic opening. This contrasts with the Miller (straight) blade, which is placed posterior to the epiglottis to directly lift it. The piriform fossa is lateral to the laryngeal inlet and is the site for superior laryngeal nerve block.

---