Neck and Laryngeal Anatomy

1. Quick Answer

Neck and laryngeal anatomy encompasses the surface landmarks, cartilaginous framework, musculature, neurovascular supply, and fascial layers essential for safe airway management in the intensive care setting. The larynx extends from C3-C6 in adults and serves as the protective valve for the lower airway and organ of phonation.

Key Concepts:

• Surface landmarks: Hyoid bone (C3), thyroid cartilage (C4-C5), cricothyroid membrane (C5-C6), cricoid cartilage (C6), sternal notch (T2-T3)
• Laryngeal cartilages: Three unpaired (thyroid, cricoid, epiglottis) and three paired (arytenoid, corniculate, cuneiform)
• Nerve supply: Superior laryngeal nerve (SLN) and recurrent laryngeal nerve (RLN) - both from vagus
• Cricothyroid membrane: Target for emergency front-of-neck access (FONA)

ICU Relevance:

• Critical for endotracheal intubation, difficult airway management, and emergency cricothyroidotomy
• Understanding nerve supply prevents iatrogenic injury during procedures
• Surface landmarks guide emergency and elective airway access
• Pediatric differences affect equipment selection and technique

Exam Focus:

• CICM First Part examiners commonly ask about laryngeal innervation, cartilage structure, surface anatomy for procedures, and adult vs pediatric differences

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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"
• "Outline the surface anatomy relevant to emergency cricothyroidotomy"
• "Draw and label a cross-section of the larynx at the level of the vocal cords"
• "Describe the course and clinical significance of the recurrent laryngeal nerve"
• "Compare the adult and pediatric laryngeal anatomy and their implications for airway management"
• "Outline the anatomical structures relevant to percutaneous tracheostomy"

Expected depth:

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

Written MCQ:

Common topics tested:

• Laryngeal cartilages and their properties (hyaline vs elastic)
• Recurrent laryngeal nerve course (left vs right differences)
• Intrinsic laryngeal muscle actions (abductor vs adductors)
• Consequences of unilateral vs bilateral RLN injury
• Surface landmarks and vertebral levels
• Airway dimensions (adult vs pediatric)

Difficulty level:

• Applied anatomical scenarios (e.g., "After thyroid surgery, a patient develops stridor. Which nerve is injured?")
• Identification of structures from descriptions
• Clinical consequences of anatomical variants

Oral Viva:

Expected discussion flow:

1. Define/Describe - Overview of neck and laryngeal divisions and boundaries
2. Surface Landmarks - Identify structures from hyoid to sternal notch
3. Cartilages - Structure, properties, articulations
4. Muscles - Intrinsic vs extrinsic, actions on vocal cords
5. Innervation - SLN and RLN distribution, injury patterns
6. Blood Supply - Arterial supply with clinical significance
7. Apply to ICU - Cricothyroidotomy, tracheostomy, intubation
8. Compare - Adult vs pediatric differences

Common viva scenarios:

• "Describe the anatomy relevant to emergency cricothyroidotomy"
• "A patient develops hoarseness after thyroid surgery. Explain the anatomical basis"
• "Walk me through the anatomy for tracheostomy insertion"
• "Compare the pediatric and adult larynx"

Pass vs Fail Performance

Pass Standard:

• Accurate identification of surface landmarks from hyoid to sternal notch
• Correct description of all laryngeal cartilages and their properties
• Clear understanding of SLN and RLN distribution and functions
• Knowledge of intrinsic muscle actions (posterior cricoarytenoid = only abductor)
• Ability to describe landmarks for cricothyroidotomy and tracheostomy
• Understanding of adult vs pediatric airway differences
• Draws clear diagrams of laryngeal cartilages and cross-sections

Common Reasons for Failure:

• Confusing the superior and recurrent laryngeal nerves
• Not knowing the difference in RLN course (left vs right)
• Inability to describe consequences of unilateral vs bilateral RLN injury
• Confusion about which muscle is the only abductor (posterior cricoarytenoid)
• Not knowing the cricothyroid membrane location and dimensions
• Stating cricoid is narrowest point in adults (it is the glottis)
• Poor understanding of intrinsic laryngeal muscle actions

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3. Key Points

Must-Know Facts

1. Surface Anatomy - Vertebral Levels: Hyoid bone (C3), thyroid cartilage (C4-C5), cricothyroid membrane (C5-C6), cricoid cartilage (C6 - at level of carotid tubercle of Chassaignac), sternal notch (T2-T3), carina (T4-T5). These landmarks are essential for locating the cricothyroid membrane in emergency front-of-neck access (PMID: 28802559).

2. Cricoid Cartilage - Only Complete Ring: The cricoid is the only complete cartilaginous ring in the airway. Its signet-ring shape (narrow anterior arch, broad posterior lamina) provides structural support. This is clinically significant for cricoid pressure (Sellick's maneuver) and for understanding subglottic stenosis from prolonged intubation (PMID: 30020619).

3. Laryngeal Nerve Supply Division: The vagus nerve supplies the larynx through two branches: (1) Superior laryngeal nerve - divides into internal branch (sensory above cords) and external branch (motor to cricothyroid only); (2) Recurrent laryngeal nerve - motor to all other intrinsic muscles, sensory below cords. Injury patterns differ significantly (PMID: 26891953).

4. Posterior Cricoarytenoid - Only Abductor: The posterior cricoarytenoid muscle is the ONLY muscle that abducts (opens) the vocal cords. All other intrinsic muscles are adductors or tensors. Complete bilateral RLN injury causes airway obstruction because abduction is lost while adductor tone remains (PMID: 17534128).

5. RLN Course Asymmetry: The left RLN has a longer course (12-14cm), looping under the aortic arch. The right RLN is shorter (5-6cm), looping under the subclavian artery. The left is more susceptible to injury from aortic pathology, mediastinal tumors, and cardiothoracic surgery (PMID: 19405101).

6. Cricothyroid Membrane Dimensions: Height 9-10mm, width 22-30mm in adults. Located between inferior border of thyroid cartilage and superior border of cricoid arch. The midline is relatively avascular - the cricothyroid arteries run across the upper portion laterally. Target for emergency cricothyroidotomy (PMID: 28802559, 18312437).

7. Glottis is Narrowest in Adults: The glottis (rima glottidis at vocal cord level) is the narrowest point in the adult airway, NOT the cricoid. Traditional teaching that the cricoid is narrowest in children is now challenged by MRI studies showing the glottis is functionally narrowest in both populations (PMID: 12707126, 19453890).

8. Pediatric Larynx Position: The pediatric larynx is higher (C3-C4 vs C4-C6 in adults), more anterior, and the epiglottis is omega-shaped and more floppy. The subglottis (at cricoid level) is the limiting point for tube size due to the non-distensible complete cricoid ring, even if the glottis is anatomically narrower (PMID: 14838255, 19059018).

9. Vocal Cord Position in RLN Injury: Unilateral RLN injury causes paramedian position (hoarseness but adequate airway). Bilateral RLN injury causes both cords in paramedian position (stridor, airway obstruction). The cricothyroid muscle (intact, supplied by external SLN) maintains tension while abduction is lost (PMID: 21509149).

10. Tracheostomy Anatomy: Standard tracheostomy is performed between the 2nd and 4th tracheal rings. The thyroid isthmus typically overlies rings 2-3 and may need division. Structures at risk include the RLN in the tracheoesophageal groove, inferior thyroid veins, and the brachiocephalic artery (especially in children where it may cross higher) (PMID: 31580371).

Essential Anatomical Relationships

Cricothyroid Membrane:

• Superior border: Inferior edge of thyroid cartilage
• Inferior border: Superior edge of cricoid cartilage
• Lateral limits: Cricothyroid muscle and artery
• Contents: Cricothyroid artery (anastomosis - runs across upper lateral portion)
• Depth: Subcutaneous tissue, membrane - NO major vessels in midline

Tracheoesophageal Groove:

• Contains: Recurrent laryngeal nerve, inferior thyroid artery
• Location: Between trachea and esophagus
• Clinical significance: High-risk zone during thyroid surgery and tracheostomy

Carotid Sheath:

• Contents: Common carotid artery (medial), internal jugular vein (lateral), vagus nerve (posterior)
• Relationship to larynx: Runs alongside, vagus gives off SLN and RLN branches
• Clinical significance: Neck dissection, CVC insertion

Normal Values Table

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4. Surface Anatomy and Landmarks

4.1 Anterior Neck Landmarks

Understanding surface anatomy is essential for safe airway procedures. The anterior neck should be systematically examined from superior to inferior.

Hyoid Bone (C3 Level)

Anatomy:

• U-shaped bone located at the junction of the neck and floor of mouth
• Only bone in the body with NO skeletal articulation (suspended by muscles and ligaments)
• Consists of body (anterior) and greater and lesser horns (posterior projections)
• Greater horn palpable laterally at the level of the mandibular angle

Attachments:

• Muscles from above: Geniohyoid, mylohyoid, digastric, stylohyoid
• Muscles from below: Thyrohyoid, sternohyoid, omohyoid
• Ligaments: Thyrohyoid membrane (to thyroid cartilage), hyoepiglottic ligament (to epiglottis)

Clinical Significance:

• Landmark for assessing thyromental distance (TMD) in difficult airway prediction
• Fracture in strangulation or direct trauma
• Reference point for superior thyroid artery ligation (PMID: 24604555)

Thyroid Notch and Cartilage (C4-C5 Level)

Surface Landmarks:

• Thyroid notch: V-shaped depression at superior border of thyroid cartilage
• Laryngeal prominence (Adam's apple): Projection at junction of thyroid laminae
• More prominent in males (90° angle) than females (120° angle)
• Easily palpable in most individuals

Palpation Technique:

1. Place finger in suprasternal notch
2. Slide finger superiorly over tracheal rings
3. Feel prominent cricoid cartilage (complete ring)
4. Continue superiorly over cricothyroid membrane (depression)
5. Feel inferior border of thyroid cartilage
6. Continue to thyroid notch at superior border

Clinical Significance:

• Reference point for locating cricothyroid membrane (2-3cm below)
• Site of thyroid cartilage fracture in laryngeal trauma
• Landmark for superior laryngeal nerve block (thyrohyoid membrane above)

Cricothyroid Membrane (C5-C6 Level)

Anatomy:

• Fibroelastic membrane connecting inferior thyroid cartilage to superior cricoid cartilage
• Also called conus elasticus (median portion) or cricothyroid ligament
• Dimensions: 9-10mm height × 22-30mm width in adults

Boundaries:

• Superior: Inferior border of thyroid cartilage
• Inferior: Superior border of cricoid cartilage (anterior arch)
• Lateral: Cricothyroid muscles (cover lateral portions)

Blood Supply:

• Cricothyroid artery (anastomosis of superior and inferior laryngeal arteries)
• Runs across upper portion of membrane LATERALLY
• Midline is relatively avascular - safe zone for incision
• PMID: 28802559

Identification Technique (Laryngeal Handshake):

1. Stabilize thyroid cartilage between thumb and middle finger of non-dominant hand
2. Palpate with index finger moving inferiorly from thyroid notch
3. Feel depression between inferior thyroid cartilage and cricoid = cricothyroid membrane
4. Confirm cricoid cartilage below (hard, complete ring)
5. Mark midpoint of membrane for procedure

Difficult Anatomy Considerations:

• Obesity: Fat obscures landmarks, use laryngeal handshake technique
• Short neck: Reduced working space
• Female/pediatric: Smaller structures, less prominent landmarks
• Previous surgery/radiation: Scarring alters anatomy
• Neck flexion/extension: Changes relationship of structures
• Edema: Distorts palpable landmarks

Cricoid Cartilage (C6 Level)

Anatomy:

• The ONLY complete cartilaginous ring in the airway
• Signet ring shape: Narrow anterior arch (5-6mm), broad posterior lamina (20-25mm)
• Hyaline cartilage - begins to ossify from age 20-30
• Level corresponds to:
  • Carotid tubercle of Chassaignac (C6 transverse process)
  • Transition from larynx to trachea
  • Transition from pharynx to esophagus

Articulations:

• Cricoarytenoid joints (synovial): Superior surface of lamina articulates with arytenoid cartilages
• Cricothyroid joints (synovial): Lateral surface articulates with inferior horns of thyroid cartilage

Clinical Significance:

• Cricoid pressure (Sellick's maneuver): Posterior displacement occludes esophagus against vertebral body (C6) to prevent regurgitation during RSI. Efficacy now debated (PMID: 15165789)
• Subglottic stenosis: Mucosal injury at cricoid level from prolonged intubation with overinflated cuffs
• Tracheostomy: Performed BELOW cricoid cartilage (never through cricoid)
• Complete ring: Narrowest non-distensible point - limits tube size in children

Tracheal Rings (C7-T4 Level)

Anatomy:

• 16-20 C-shaped hyaline cartilage rings
• Incomplete posteriorly (membranous trachea against esophagus)
• Each ring 4-5mm height, separated by fibroelastic membrane
• Progressively narrow toward carina

Surface Palpation:

• Palpable below cricoid cartilage in thin individuals
• First 2-4 rings accessible for tracheostomy (rings 5+ deep to sternum)
• Thyroid isthmus typically overlies rings 2-3

Sternal Notch (T2-T3 Level)

Anatomy:

• Jugular (suprasternal) notch at superior border of manubrium
• Trachea palpable immediately above
• Carina approximately 5cm below sternal notch in adults

Relationships:

• Brachiocephalic artery: Crosses anterior to trachea at this level
• Left brachiocephalic vein: Crosses anterior at level of 1st rib
• Thymus: In children, may extend into superior mediastinum

Clinical Significance:

• Inferior limit of neck access
• Reference for tracheostomy tube tip position (should be 2-4cm above carina)
• In children, brachiocephalic artery may cross higher, increasing risk during tracheostomy

4.2 Neck Triangles

Understanding neck triangles is essential for surgical anatomy and localization of structures.

Anterior Triangle

Boundaries:

• Anterior: Midline of neck
• Posterior: Anterior border of sternocleidomastoid (SCM)
• Superior: Inferior border of mandible

Subdivisions:

1. Submental triangle (single, midline)
2. Submandibular (digastric) triangle
3. Carotid triangle - contains carotid sheath, larynx, thyroid
4. Muscular (ompohyoid) triangle - contains thyroid, larynx, trachea

Contents Relevant to Airway:

• Thyroid and cricoid cartilages
• Cricothyroid membrane
• Trachea (upper rings)
• Thyroid gland
• Carotid sheath and contents

Posterior Triangle

Boundaries:

• Anterior: Posterior border of SCM
• Posterior: Anterior border of trapezius
• Inferior: Middle third of clavicle

Contents Relevant to Airway:

• Accessory nerve (CN XI)
• External jugular vein
• Transverse cervical and suprascapular vessels
• Brachial plexus (supraclavicular portion)
• Subclavian artery (3rd part)

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5. Laryngeal Cartilages

5.1 Unpaired Cartilages

Thyroid Cartilage

Structure (PMID: 30252327):

• Largest laryngeal cartilage
• Two laminae meeting anteriorly at acute angle
• Thyroid angle: 90° in males (prominent Adam's apple), 120° in females
• Forms protective shield for underlying structures

Anatomical Features:

• Superior notch: V-shaped depression at upper border (palpable landmark)
• Inferior notch: Shallow notch at lower border
• Superior horns: Project superiorly to articulate with hyoid via thyrohyoid membrane
• Inferior horns: Project inferiorly to articulate with cricoid (cricothyroid joint)
• Oblique line: Ridge on lateral surface for muscle attachment
• Laryngeal prominence: Anterior projection where laminae meet

Muscle Attachments:

• Oblique line: Thyrohyoid, sternothyroid, inferior pharyngeal constrictor
• Inner surface: Thyroarytenoid, vocalis, thyroepiglottic

Articulations:

• Thyrohyoid membrane: Connects to hyoid bone superiorly
• Cricothyroid joint (synovial): Inferior horns articulate with cricoid - allows rocking motion that tenses vocal cords

Cartilage Type: Hyaline - begins ossification at age 20-30, may become brittle

Cricoid Cartilage

Structure (PMID: 30020619):

• ONLY complete cartilaginous ring in the airway
• Signet ring shape: Narrow anterior arch, broad posterior lamina
• Provides structural support for larynx and upper trachea

Anatomical Features:

• Anterior arch: 5-6mm height, narrow
• Posterior lamina: 20-25mm height, broad
• Facets for arytenoids: On superolateral surface of lamina
• Facets for thyroid: On lateral surface for cricothyroid joints

Dimensions:

• Internal anteroposterior diameter: 11-13mm (adult)
• Internal transverse diameter (at arch): 13-17mm
• Internal transverse diameter (at lamina): 17-23mm

Articulations:

• Cricoarytenoid joints (synovial): Allow rotation and gliding of arytenoids
• Cricothyroid joints (synovial): Allow rocking of thyroid on cricoid

Clinical Significance:

• Complete ring prevents expansion - cuff overinflation causes mucosal ischemia and stenosis
• Sellick's maneuver target (posterior compression against C6)
• Level of transition from larynx to trachea
• In children, the subglottis at cricoid level is the limiting point for tube size

Cartilage Type: Hyaline - ossifies with age

Epiglottis

Structure (PMID: 30969678):

• Leaf-shaped elastic cartilage
• Projects superiorly behind tongue base
• Forms anterior wall of laryngeal inlet

Anatomical Features:

• Petiole (stalk): Attaches to inner angle of thyroid cartilage via thyroepiglottic ligament
• Free edge: Curves posteriorly, covered by mucosa
• Lingual surface: Faces tongue, covered by stratified squamous epithelium
• Laryngeal surface: Faces larynx, covered by respiratory epithelium
• Tubercle: Small projection on posterior surface at petiole

Shape Variations:

• Adult: Flat, flexible
• Pediatric: Omega (U)-shaped, narrower, more angled, relatively stiffer

Attachments:

• Thyroepiglottic ligament: To inner thyroid cartilage
• Hyoepiglottic ligament: To body of hyoid bone
• Median glossoepiglottic fold: To tongue
• Lateral glossoepiglottic folds: To pharyngeal wall (form valleculae)
• Aryepiglottic folds: To arytenoid cartilages

Cartilage Type: ELASTIC - does NOT ossify (unlike hyaline cartilages)

Clinical Significance:

• Target for Macintosh (curved) blade during laryngoscopy - tip placed in vallecula
• Lifted directly by Miller (straight) blade
• Omega shape in infants may require straight blade
• Epiglottitis causes life-threatening swelling

5.2 Paired Cartilages

Arytenoid Cartilages

Structure (PMID: 30422502):

• Paired pyramidal cartilages sitting on posterosuperior border of cricoid lamina
• Most important cartilages for vocal cord function
• Movement controls vocal cord position

Anatomical Features:

• Apex: Articulates with corniculate cartilage superiorly
• Base: Articulates with cricoid via cricoarytenoid joint
• Vocal process (anterior projection): Attachment of vocal ligament (true vocal cord)
• Muscular process (lateral projection): Attachment of intrinsic laryngeal muscles

Cricoarytenoid Joint:

• Synovial joint allowing:
  • "Rotation: Around vertical axis (abduction/adduction of vocal cords)"
  • "Gliding: Medially and laterally (further adjusts cord position)"
• Movement controlled by intrinsic muscles attaching to muscular process

Clinical Significance:

• Arytenoid dislocation: Complication of traumatic intubation
• Arytenoid cartilage visibility on laryngoscopy (posterior commissure landmarks)
• Fixed arytenoid may mimic vocal cord paralysis

Cartilage Type: Hyaline - may ossify with age

Corniculate Cartilages

Structure:

• Small, conical elastic cartilages
• Sit on apices of arytenoid cartilages
• Embedded in aryepiglottic folds

Function:

• Provide support to aryepiglottic folds
• Form corniculate tubercles (visible on laryngoscopy as small elevations posterior to cuneiform tubercles)

Cartilage Type: Elastic - do NOT ossify

Cuneiform Cartilages

Structure:

• Small, elongated elastic cartilages
• Embedded in aryepiglottic folds, anterior to corniculate cartilages
• Not attached to other cartilages

Function:

• Provide support to aryepiglottic folds
• Form cuneiform tubercles (visible on laryngoscopy as whitish elevations)

Cartilage Type: Elastic - do NOT ossify

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6. Laryngeal Muscles

6.1 Extrinsic Muscles

Extrinsic muscles attach the larynx to surrounding structures and move the larynx as a whole during swallowing and phonation.

Suprahyoid Muscles (Elevate Larynx)

Infrahyoid (Strap) Muscles (Depress Larynx)

Clinical Significance:

• Strap muscles separated in midline during tracheostomy (minimal bleeding)
• Extrinsic muscle function important for swallowing and protecting airway
• Elevation of larynx during swallowing brings epiglottis over laryngeal inlet

6.2 Intrinsic Muscles

Intrinsic muscles alter the position, shape, and tension of the vocal cords. All are supplied by the recurrent laryngeal nerve EXCEPT the cricothyroid (supplied by external branch of SLN).

Vocal Cord Abductor (Opens Glottis)

Clinical Pearl: The PCA is the only muscle that can open the vocal cords. In RLN injury, this muscle is paralyzed while the adductors (supplied by same nerve) are also paralyzed. However, the cricothyroid (external SLN) remains functional, maintaining cord tension. The cords assume a paramedian position due to passive elastic recoil and residual cricothyroid action.

Vocal Cord Adductors (Close Glottis)

Vocal Cord Tensor

Cricothyroid Muscle Special Features:

• Only intrinsic muscle on OUTSIDE of larynx (visible externally)
• Supplied by external branch of SLN (NOT RLN)
• "Singer's muscle" or "voice pitch muscle"
• Two parts: Pars recta (vertical fibers) and pars obliqua (oblique fibers)
• Action: Rocks thyroid cartilage forward on cricoid, stretching vocal cords

Clinical Significance: In complete RLN injury, the cricothyroid continues to function (external SLN intact), maintaining cord tension. This explains why voice may be preserved (cords can still vibrate) but airway is compromised (cords cannot abduct).

6.3 Muscle Actions Summary

During Inspiration:

• Posterior cricoarytenoid (PCA) contracts → abducts cords → opens glottis

During Phonation:

• Lateral cricoarytenoid + transverse arytenoid contract → adducts cords
• Cricothyroid adjusts tension (pitch)
• Thyroarytenoid adjusts thickness and relaxation
• Vocalis fine-tunes tension

During Valsalva/Coughing:

• All adductors contract maximally → tight glottic closure
• Oblique arytenoids + aryepiglottic muscles narrow inlet

Mnemonic for RLN Muscles: "All intrinsic muscles except cricothyroid are supplied by RLN"

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7. Nerve Supply to the Larynx

7.1 Vagus Nerve Overview

The larynx receives its entire innervation from the vagus nerve (CN X) through two branches:

1. Superior laryngeal nerve (SLN)
2. Recurrent laryngeal nerve (RLN)

Vagus Nerve Nuclei:

• Nucleus ambiguus: Motor (branchial motor to laryngeal muscles)
• Dorsal motor nucleus: Parasympathetic
• Nucleus tractus solitarius: Sensory (general visceral afferent)

Vagus Course:

• Exits skull via jugular foramen with CN IX and XI
• Descends in carotid sheath between internal jugular vein (lateral) and carotid artery (medial)
• Gives off SLN at inferior (nodose) ganglion
• Continues into thorax, gives off RLN at different levels on each side

7.2 Superior Laryngeal Nerve (SLN)

Origin: Arises from inferior (nodose) ganglion of vagus at C1-C2 level (PMID: 26891953)

Course:

• Descends posterior to internal carotid artery
• Divides at level of hyoid bone into internal and external branches

Internal Branch (Sensory)

Course:

• Pierces thyrohyoid membrane with superior laryngeal artery
• Entry point: 2cm below hyoid bone, 1cm anterior to superior horn of thyroid cartilage

Function:

• Sensory to mucosa ABOVE vocal cords (supraglottic larynx)
• Sensory to base of tongue, valleculae, epiglottis
• Afferent limb of cough reflex from supraglottic region
• Afferent for laryngeal chemoreflex (glottic closure with laryngeal stimulation)

Clinical Applications:

• Superior laryngeal nerve block: Topical anesthetic in piriform fossa or injection through thyrohyoid membrane blocks sensation above cords
• Injury causes loss of supraglottic sensation → aspiration risk

External Branch (Motor)

Course (PMID: 30113310):

• Smaller branch, runs with superior thyroid artery on inferior constrictor
• Variable relationship to artery (medial, lateral, or between branches)
• High risk during superior pole thyroid ligation

Classification (Cernea Classification):

• Type 1: Crosses superior thyroid artery >1cm above superior pole (low risk)
• Type 2 a: Crosses <1cm above superior pole (moderate risk)
• Type 2 b: Crosses below superior pole (HIGH RISK - 20%)

Function:

• Motor to cricothyroid muscle (ONLY muscle it supplies)
• Also contributes to inferior pharyngeal constrictor (cricopharyngeus part)

Clinical Consequences of Injury:

• Loss of ability to produce high-pitched sounds
• Voice fatigue, monotonous voice, reduced projection
• Less dramatic than RLN injury - often unrecognized
• "Opera singer's nerve"
• devastating for professional voice users
• May compensate with contralateral cricothyroid over time

7.3 Recurrent Laryngeal Nerve (RLN)

Origin: Vagus nerve in thorax (left) or root of neck (right) (PMID: 19405101, 27506925)

Right RLN Course

• Arises from vagus anterior to right subclavian artery
• Loops under right subclavian artery, posterior to artery
• Ascends obliquely (lateral to medial) toward tracheoesophageal groove
• Course length: 5-6cm (shorter than left)
• More variable relationship to inferior thyroid artery
• Enters larynx deep to inferior constrictor, at cricothyroid joint

Left RLN Course

• Arises from vagus anterior to aortic arch
• Loops under aortic arch, posterior to ligamentum arteriosum
• Ascends in tracheoesophageal groove (more medial position)
• Course length: 12-14cm (longer than right)
• More consistent anatomical position
• More susceptible to injury from aortic pathology, mediastinal tumors

RLN Function

Motor Supply:

• ALL intrinsic laryngeal muscles EXCEPT cricothyroid:
  • Posterior cricoarytenoid (only abductor)
  • Lateral cricoarytenoid (adductor)
  • Transverse arytenoid (adductor)
  • Oblique arytenoid (adductor)
  • Thyroarytenoid (adductor/relaxor)
  • Vocalis (fine-tuning)

Sensory Supply:

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

Relationship to Inferior Thyroid Artery

The RLN has a highly variable relationship to the inferior thyroid artery - this is the critical danger zone during thyroid surgery.

Patterns:

• RLN anterior to artery: 40-45%
• RLN posterior to artery: 35-40%
• RLN between arterial branches: 15-25%
• NO reliable rule - always identify nerve before ligating artery

Ligament of Berry (Suspensory Ligament of Thyroid)

• Dense fibrous tissue attaching thyroid to cricoid and trachea
• RLN often passes through or very close to this ligament
• Most common site of RLN injury during thyroidectomy
• Only 0.5-1.0cm from thyroid capsule

Anatomical Variants

Non-Recurrent Laryngeal Nerve (NRLN) (PMID: 12660381):

• 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 (does not recur)
• Very high risk of injury if not recognized preoperatively
• Suspect if preoperative imaging shows aberrant subclavian artery

7.4 Clinical Consequences of Nerve Injury

Unilateral SLN (External Branch) Injury

Mechanism: Thyroidectomy (superior pole ligation), carotid endarterectomy

Cord Position: Normal position and movement (RLN intact)

Cord Tension: Reduced on affected side (cricothyroid paralyzed)

Symptoms:

• Lower pitch, inability to produce high notes
• Voice fatigue, monotonous voice
• Reduced vocal projection
• Often subtle, may go unrecognized

Compensation: Contralateral cricothyroid may compensate

Treatment: Voice therapy usually effective

Unilateral RLN Injury (PMID: 17534128)

Mechanism: Thyroidectomy, cardiothoracic surgery, aortic aneurysm, lung cancer, trauma

Cord Position: PARAMEDIAN (near midline) or intermediate

Mechanism of Position:

• Loss of abductor (posterior cricoarytenoid) → cannot open cord
• Loss of adductors (LCA, TA, etc.) → reduced adduction force
• Cricothyroid intact (external SLN) → maintains tension
• Passive elastic recoil → cord assumes paramedian position

Symptoms:

• Hoarseness, breathy voice
• Weak cough
• Possible aspiration (if combined with sensory loss)
• Adequate airway (contralateral cord can abduct fully)

Compensation: May improve with time (regeneration) or medialization procedures

Treatment:

• Observation for 6-12 months (spontaneous recovery possible)
• Voice therapy
• Injection laryngoplasty (temporary medialization)
• Thyroplasty type I (permanent medialization)

Bilateral RLN Injury (PMID: 21509149)

Mechanism: Bilateral thyroidectomy, re-operative surgery, extensive neck dissection

Cord Position: Both cords in PARAMEDIAN position (near midline)

Mechanism:

• Complete loss of ALL abductor function bilaterally
• Cricothyroid muscles intact → both cords tensed and adducted
• No abduction possible → rima glottidis very narrow

Symptoms:

• STRIDOR (inspiratory > expiratory)
• Respiratory distress
• AIRWAY OBSTRUCTION - may be life-threatening
• Voice may be preserved (cords can still vibrate when air passes)
• Paradoxically, voice may sound relatively normal despite airway emergency

EMERGENCY MANAGEMENT:

• Immediate reintubation
• If intubation fails → emergency tracheostomy/cricothyroidotomy
• Long-term: Tracheostomy while planning definitive surgery

Surgical Options:

• Posterior cordotomy (create airway gap posteriorly)
• Arytenoidectomy (remove arytenoid to widen glottis)
• Reinnervation procedures (experimental)
• Permanent tracheostomy if above fail

Prevention: Staged bilateral thyroidectomy preferred (assess cord function between stages)

Combined SLN and RLN Injury (Unilateral)

Mechanism: Extensive neck dissection, radical thyroidectomy

Cord Position: CADAVERIC (intermediate, more lateral than paramedian)

Explanation: Loss of ALL innervation to that side - both tension and movement lost

Prognosis: Worse prognosis for voice recovery than isolated RLN injury

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8. Blood Supply to the Larynx

8.1 Arterial Supply

Superior Laryngeal Artery

Origin: Branch of superior thyroid artery (from external carotid)

Course:

• Accompanies internal branch of SLN
• Pierces thyrohyoid membrane to enter larynx

Supply:

• Larynx above vocal cords (supraglottic)
• Epiglottis
• Major blood supply to larynx

Inferior Laryngeal Artery

Origin: Branch of inferior thyroid artery (from thyrocervical trunk of subclavian)

Course:

• Accompanies recurrent laryngeal nerve
• Enters larynx at cricothyroid joint

Supply:

• Larynx below vocal cords (subglottic)
• Posterior larynx
• Upper trachea

Cricothyroid Artery

Anatomy:

• Anastomosis between superior and inferior laryngeal arteries
• Runs across UPPER LATERAL portion of cricothyroid membrane
• May be single, double, or a plexus

Clinical Significance:

• MIDLINE is relatively avascular - safe zone for cricothyroidotomy
• Stay in lower half of membrane if possible
• Hemorrhage may obscure field if lateral incision

8.2 Venous Drainage

Superior Laryngeal Vein:

• Drains supraglottic larynx
• Accompanies superior laryngeal artery
• Drains to superior thyroid vein → internal jugular vein

Inferior Laryngeal Vein:

• Drains subglottic larynx
• Drains to inferior thyroid vein → brachiocephalic vein

8.3 Lymphatic Drainage

Understanding lymphatic drainage is important for laryngeal carcinoma staging.

Supraglottic Larynx (above vocal cords):

• Rich lymphatic network
• Bilateral drainage possible
• Drains to: Upper deep cervical nodes (levels II, III)
• Early nodal metastasis common

Glottis (vocal cords):

• Sparse lymphatic network (stratified squamous epithelium)
• Late nodal metastasis
• Drains to: Prelaryngeal (Delphian) node, pretracheal nodes

Subglottic Region:

• Drains to: Pretracheal, paratracheal nodes
• May spread to mediastinal nodes
• Poorer prognosis if subglottic involvement

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9. Cricothyroidotomy Anatomy

9.1 Indications

"Cannot Intubate, Cannot Oxygenate" (CICO) Scenario:

• Failed intubation
• Failed supraglottic airway
• Failed oxygenation via face mask
• Final step in DAS/ANZCA difficult airway algorithms

9.2 Anatomical Target

Cricothyroid Membrane (CTM):

• Location: Between inferior border of thyroid cartilage and superior border of cricoid cartilage
• Level: C5-C6
• Dimensions: 9-10mm (height) × 22-30mm (width) in adults

Safe Zone:

• Midline, lower half of membrane
• Avoids cricothyroid artery (crosses upper lateral portion)
• Avoids anterior jugular veins (1-2cm lateral to midline)

Depth to Airway:

• Skin → subcutaneous tissue → membrane = 5-10mm in thin patients
• May be significantly deeper in obesity (>15mm)

9.3 Structures at Risk

9.4 Surgical Technique (Anatomical Considerations)

Landmark Identification:

1. Laryngeal handshake: Stabilize thyroid cartilage with non-dominant hand
2. Palpate thyroid notch, laryngeal prominence
3. Slide finger inferiorly to feel depression (CTM)
4. Confirm cricoid cartilage below

Incision Options:

Vertical Skin Incision (preferred in emergency):

• Allows adjustment if landmarks unclear
• 3-4cm midline incision
• Blunt dissection through fat if obese
• Then horizontal stab through membrane

Horizontal Skin and Membrane Incision:

• Faster if landmarks clear
• May be preferred in elective/semi-elective setting

Membrane Incision:

• Horizontal stab through membrane (low and central)
• Rotate scalpel 90° OR insert tracheal hook to maintain opening
• Bougie insertion → railroad 6.0mm cuffed ETT

9.5 Pediatric Considerations

Children < 8-10 years (PMID: 19318998):

• Cricothyroid membrane TOO SMALL for surgical technique
• Height only 2.5-5mm, width 6-12mm
• Needle cricothyroidotomy preferred
• 14G or 16G cannula inserted through membrane
• Connect to jet ventilation (50 psi oxygen) if available
• Temporary oxygenation only (30-45 minutes)
• Convert to formal tracheostomy when stable

Anatomical Differences:

• Larynx more anterior and cephalad
• Structures softer and more mobile
• Higher risk of posterior wall injury
• Smaller margin of error

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10. Tracheostomy Anatomy

10.1 Indications

• Prolonged mechanical ventilation
• Upper airway obstruction
• Pulmonary toilet
• Airway protection (aspiration risk)
• Failed extubation

10.2 Anatomical Site

Optimal Level: Between 2nd and 4th tracheal rings

Avoid:

• Above 2nd ring (high tracheostomy): Risk of subglottic stenosis, cricoid damage
• 1st tracheal ring: Direct cricoid injury
• Below 4th ring: Approaching thoracic inlet, increased complications

10.3 Structures Encountered (Superficial to Deep)

10.4 Structures at Risk

10.5 Percutaneous Dilational Tracheostomy (PDT)

Anatomical Considerations:

• Performed at bedside with bronchoscopic guidance
• Preferred between rings 2-3 (or 3-4)
• Needle insertion in midline, confirmed with bronchoscopy
• Serial dilation over guidewire

Contraindications (Anatomical):

• Difficult landmarks (obesity, short neck)
• Abnormal anatomy (previous surgery, radiation)
• Overlying vessels (thyroid ima, aberrant vessels)
• Unstable cervical spine (relative)
• Pediatric (<12 years typically)

10.6 Pediatric Tracheostomy Considerations

Brachiocephalic Artery:

• Crosses trachea at higher level in children
• May be anterior to trachea at 6th-8th rings in infants
• Risk of erosion into artery with tracheostomy cuff
• Must consider when positioning tube

Higher Larynx:

• Trachea is shorter
• Fewer rings available above thoracic inlet
• Preference for lower tracheostomy in children is dangerous

Softer Tissues:

• Greater risk of posterior wall injury
• May need stay sutures to maintain stoma

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11. Adult vs Pediatric Airway Differences

11.1 Anatomical Comparison

11.2 The "Narrowest Point" Debate

Traditional Teaching (PMID: 14838255 - Eckenhoff 1951):

• Pediatric airway is "funnel-shaped"
• Cricoid cartilage is the narrowest point
• Subglottis is limiting for tube size
• Led to recommendation for uncuffed tubes in children

Modern Evidence (PMID: 12707126 - Litman 2003):

• MRI studies show glottis is anatomically narrowest in both children and adults
• Airway is elliptical, not funnel-shaped
• The cricoid is the narrowest NON-DISTENSIBLE structure
• Cuffed tubes are now acceptable in children if cuff pressure monitored

Clinical Reconciliation:

• The glottis (vocal cords) is the narrowest point anatomically
• The cricoid (being a complete ring) is the narrowest FIXED point
• The cricoid determines maximum tube size that will pass without trauma
• Modern low-pressure cuffs allow safe cuffed tube use in children

11.3 Clinical Implications

Laryngoscopy:

• Pediatric larynx is anterior - use straight blade or hyperangulated videolaryngoscope
• May need to lift epiglottis directly (Miller technique)
• Omega-shaped epiglottis may obscure view

Intubation:

• Use appropriately sized tube (Cole formula: (age/4) + 4 for uncuffed)
• Tube may catch on anterior commissure
• Cuffed tubes with low-pressure cuffs now preferred (PMID: 31174259)

Cricothyroidotomy:

• Surgical technique difficult in children < 8-10 years
• Needle cricothyroidotomy with jet ventilation preferred
• Small membrane dimensions limit access

ETT Position:

• Short trachea → higher risk of endobronchial intubation or accidental extubation
• Head movement significantly affects tube position
• Verify position with chest X-ray and regular assessment

11.4 Poiseuille's Law Application (PMID: 25440628)

Resistance to flow through a tube: R = 8μl / (π × r⁴)

Resistance is inversely proportional to the fourth power of the radius.

Clinical Significance:

• 1mm of mucosal edema in an infant (4mm subglottis):
  • Reduces cross-sectional area by ~75%
  • Increases resistance by ~16-fold
• 1mm of mucosal edema in an adult (17mm subglottis):
  • Reduces cross-sectional area by ~44%
  • Increases resistance by ~3-fold

Conclusion: Pediatric airways are MUCH more sensitive to edema, inflammation, or foreign body than adult airways.

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12. Australian/NZ Context

12.1 Guidelines

ANZCA PS61: Guidelines on Checking Anaesthesia Delivery Systems

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

ANZCA/CICM Airway Guidelines:

• Standardized approach to difficult airway
• Vortex cognitive aid (Australian origin)
• Emphasis on early front-of-neck access in CICO

Vortex Approach:

• Three "lifelines": Face mask, supraglottic device, endotracheal tube
• Green zone (adequate oxygenation) vs Red zone (CICO)
• Decision to perform FONA based on inability to oxygenate, not inability to intubate

All4Kids (Paediatric Airway Guidelines):

• Specific pediatric airway algorithms
• Age-appropriate equipment sizing
• Different threshold for surgical airway in children

12.2 Indigenous Health Considerations

Aboriginal and Torres Strait Islander Peoples:

• Higher rates of conditions affecting airway:
  • Obesity and obstructive sleep apnea
  • Rheumatic heart disease
  • Diabetes (cervical spine stiffness)
  • Chronic kidney disease
• Potential for difficult airway - thorough assessment essential
• Cultural considerations for tracheostomy (altered appearance, speaking valve importance)
• Communication through Aboriginal Hospital Liaison Officers for consent
• Family involvement in decision-making

Māori (New Zealand):

• Similar chronic disease burden
• Te Tiriti o Waitangi obligations
• Whānau involvement in airway planning
• Tikanga considerations for permanent tracheostomy
• Māori Health Workers for cultural support

12.3 Remote and Rural Considerations

RFDS (Royal Flying Doctor Service) Airway Management:

• Limited equipment in remote settings
• Pre-hospital RSI protocols
• Telemedicine support for difficult airway decisions
• Video laryngoscopy increasingly available
• Surgical airway skills essential for remote practitioners

Challenges:

• Delayed access to definitive care
• Limited backup if primary approach fails
• Retrieval may take hours
• Need for self-sufficiency

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13. SAQ Practice Questions

SAQ 1: Laryngeal Innervation and Consequences of Injury (15 marks)

Time: 15 minutes

Question:

A 52-year-old woman develops stridor and respiratory distress 4 hours after total thyroidectomy for multinodular goitre.

(a) Describe the anatomy of the recurrent laryngeal nerve (RLN), including its origin, course (comparing left and right), and structures it supplies. (6 marks)

(b) Explain the anatomical basis for the clinical presentation in this patient, assuming bilateral RLN injury. (5 marks)

(c) Outline the immediate management and long-term surgical options for bilateral RLN injury. (4 marks)

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Model Answer:

(a) Recurrent Laryngeal Nerve Anatomy (6 marks):

Origin (1 mark):

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

Course - RIGHT RLN (1.5 marks):

• Arises from vagus anterior to right subclavian artery
• Loops UNDER subclavian artery, posterior to it
• Ascends obliquely (lateral to medial) toward tracheoesophageal groove
• Shorter course: 5-6cm
• Variable relationship to inferior thyroid artery
• Enters larynx at cricothyroid joint

Course - LEFT RLN (1.5 marks):

• Arises from vagus anterior to aortic arch
• Loops UNDER aortic arch, posterior to ligamentum arteriosum
• Ascends in tracheoesophageal groove (more medial, consistent position)
• Longer course: 12-14cm
• More susceptible to injury from intrathoracic pathology
• Enters larynx at cricothyroid joint

Structures Supplied (2 marks):

• Motor: All intrinsic laryngeal muscles EXCEPT cricothyroid:
  • Posterior cricoarytenoid (ONLY abductor)
  • Lateral cricoarytenoid (adductor)
  • Transverse and oblique arytenoid (adductors)
  • Thyroarytenoid (adductor/relaxor)
  • Vocalis (fine-tuning)
• Sensory: Laryngeal mucosa below vocal cords (subglottic), upper trachea

---

(b) Anatomical Basis of Clinical Presentation (5 marks):

Bilateral RLN Injury Mechanism (2 marks):

• Both posterior cricoarytenoid muscles paralyzed → complete loss of abductor function
• Both lateral cricoarytenoid, transverse arytenoid, thyroarytenoid muscles paralyzed → loss of active adduction
• Cricothyroid muscles INTACT (supplied by external SLN) → maintains vocal cord tension

Vocal Cord Position (2 marks):

• Both cords assume PARAMEDIAN position (near midline)
• Mechanism:
  • Passive elastic recoil of cords toward midline
  • Intact cricothyroid action maintains tension and slight adduction
  • No active abduction possible (PCA paralyzed)
• Rima glottidis (glottic aperture) very narrow

Resulting Symptoms (1 mark):

• Stridor: Narrow glottis causes inspiratory (and expiratory) noise
• Respiratory distress: Inadequate airway diameter for ventilation
• Airway obstruction: Progressive, potentially life-threatening
• Voice may be relatively preserved (cords can vibrate)

---

(c) Management of Bilateral RLN Injury (4 marks):

Immediate Management (2 marks):

• Recognition and calling for help
• Supplemental oxygen
• Immediate reintubation (priority)
• If intubation fails → emergency tracheostomy or cricothyroidotomy
• ICU admission for observation and definitive airway
• Laryngoscopy to confirm diagnosis (both cords immobile in paramedian position)

Long-Term Surgical Options (2 marks):

• Tracheostomy: Temporary or permanent to bypass obstruction
• Posterior cordotomy: Laser division of posterior cord to widen glottis
• Arytenoidectomy: Removal of arytenoid cartilage (unilateral) to widen glottis
• Lateralization procedures: Suture fixation of cord in abducted position
• Laryngeal reinnervation: Experimental, nerve transfer to restore function
• Trade-off: Widening airway improves breathing but may compromise voice quality

---

SAQ 2: Surface Anatomy for Emergency Cricothyroidotomy (15 marks)

Time: 15 minutes

Question:

A 45-year-old obese male (BMI 42) presents with angioedema causing complete upper airway obstruction. All attempts at intubation and supraglottic airway insertion have failed. You must perform an emergency cricothyroidotomy.

(a) Describe the surface anatomy of the anterior neck from the hyoid bone to the cricoid cartilage, including vertebral levels. (5 marks)

(b) Describe the anatomical features of the cricothyroid membrane, including its dimensions, boundaries, and vascular anatomy. (5 marks)

(c) Outline the anatomical challenges of cricothyroidotomy in obese patients and how to overcome them. (5 marks)

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Model Answer:

(a) Surface Anatomy - Hyoid to Cricoid (5 marks):

Hyoid Bone (1 mark):

• U-shaped bone at C3 level
• Palpable at level of mandibular angle
• Greater horn palpable laterally
• Suspended by muscles (no skeletal articulation)

Thyroid Cartilage (2 marks):

• Largest laryngeal cartilage at C4-C5 level
• Thyroid notch: V-shaped depression at superior border (easily palpable)
• Laryngeal prominence (Adam's apple): Anterior projection where laminae meet
• More prominent in males (90° angle) than females (120° angle)
• Thyrohyoid membrane connects to hyoid above

Cricothyroid Membrane (1 mark):

• Depression between inferior thyroid cartilage and cricoid cartilage
• Located at C5-C6 level
• 2-3cm below laryngeal prominence
• TARGET for emergency front-of-neck access

Cricoid Cartilage (1 mark):

• Only complete cartilaginous ring at C6 level
• Signet ring shape (narrow anterior arch, broad posterior lamina)
• Corresponds to carotid tubercle of Chassaignac (C6 transverse process)
• Transition from larynx to trachea

---

(b) Cricothyroid Membrane Anatomy (5 marks):

Dimensions (1 mark):

• Height: 9-10mm (vertical extent)
• Width: 22-30mm (transverse extent)
• Depth to airway: 5-10mm in thin patients (may be >15mm in obese)

Boundaries (2 marks):

• Superior: Inferior border of thyroid cartilage
• Inferior: Superior border of cricoid cartilage (anterior arch)
• Lateral: Cricothyroid muscles (cover lateral portions of membrane)
• Deep: Opens into infraglottic cavity (subglottic airway)

Vascular Anatomy (2 marks):

• Cricothyroid artery: Anastomosis of superior and inferior laryngeal arteries
• Runs across UPPER portion of membrane LATERALLY (not midline)
• MIDLINE is relatively avascular - safe zone for incision
• Anterior jugular veins: Located 1-2cm lateral to midline
• Thyroid isthmus: Usually below cricoid (rarely overlies membrane)
• Avoid upper lateral quadrants to minimize bleeding

---

(c) Challenges in Obesity and Solutions (5 marks):

Anatomical Challenges (2.5 marks):

• Obscured landmarks: Subcutaneous fat obscures thyroid cartilage and cricoid
• Increased depth: Membrane may be >15-20mm deep (vs 5-10mm in normal BMI)
• Skin folds: Neck flexion creases may mislead
• Short neck: Reduced working space between mandible and sternum
• Mobile landmarks: Fat allows structures to move during palpation

Techniques to Overcome Challenges (2.5 marks):

1. Laryngeal handshake technique:

   • Stabilize thyroid cartilage between thumb and middle finger
   • Palpate structures with index finger
   • Maintains orientation during procedure

2. Vertical skin incision (preferred):

   • 8-10cm midline incision (longer than usual)
   • Allows identification of landmarks after skin incision
   • Permits adjustment if initial incision is not over membrane

3. Blunt dissection through fat:

   • Use fingers or blunt instrument to separate fat
   • Continue until cartilage structures palpated

4. Ultrasound guidance (if time permits):

   • Pre-procedural identification of cricothyroid membrane
   • Particularly useful in semi-elective FONA

5. Bougie-assisted technique:

   • Once membrane entered, bougie confirms tracheal placement
   • Railroad small (6.0mm) cuffed tube

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14. Viva Scenarios

Viva 1: Laryngeal Anatomy and Nerve Injury

Examiner: "A 48-year-old woman is referred to ICU following right hemithyroidectomy. She has a hoarse voice. Tell me about the anatomy of the laryngeal nerves."

Candidate Response:

"The larynx receives its nerve supply entirely from the vagus nerve through two branches: the superior laryngeal nerve and the recurrent laryngeal nerve.

The superior laryngeal nerve arises from the inferior ganglion of the vagus at approximately C1-C2. It descends posterior to the internal carotid artery and divides at the level of the hyoid bone into two branches:

1. The internal branch is sensory. It pierces the thyrohyoid membrane with the superior laryngeal artery to supply sensation to the mucosa above the vocal cords, including the epiglottis and valleculae.

2. The external branch is motor. It runs with the superior thyroid artery and supplies the cricothyroid muscle, which is the tensor of the vocal cords."

Examiner: "And the recurrent laryngeal nerve?"

Candidate:

"The recurrent laryngeal nerve has different courses on left and right.

The left RLN arises from the vagus anterior to the aortic arch, loops under the arch posterior to the ligamentum arteriosum, and ascends in the tracheoesophageal groove. It has a longer course of 12-14cm and is more susceptible to injury from intrathoracic pathology.

The right RLN arises from the vagus anterior to the subclavian artery, loops under it, and ascends more obliquely toward the tracheoesophageal groove. It has a shorter course of 5-6cm and a more variable relationship to the inferior thyroid artery.

Both nerves enter the larynx at the cricothyroid joint.

The RLN provides:

• Motor supply to ALL intrinsic laryngeal muscles EXCEPT the cricothyroid
• Sensory supply to the mucosa below the vocal cords"

Examiner: "What is the consequence of unilateral RLN injury?"

Candidate:

"In unilateral RLN injury, the affected vocal cord assumes a paramedian position.

This occurs because:

• The posterior cricoarytenoid (the only abductor) is paralyzed - the cord cannot open
• The adductors are also paralyzed, but passive elastic recoil keeps the cord near midline
• The cricothyroid muscle remains functional (external SLN) and maintains cord tension

Clinically, the patient presents with:

• Hoarseness and breathy voice (cord cannot close completely for phonation)
• Weak cough
• Potential aspiration (if combined with sensory loss)
• But adequate airway because the contralateral cord can still abduct fully

This patient likely has unilateral RLN injury from the hemithyroidectomy."

Examiner: "How would bilateral injury differ?"

Candidate:

"Bilateral RLN injury is an airway emergency.

Both cords assume a paramedian position because:

• Complete loss of abductor function bilaterally (both PCAs paralyzed)
• Cricothyroid muscles remain intact - maintaining tension and adduction
• Neither cord can abduct

Clinically:

• Stridor (inspiratory > expiratory)
• Respiratory distress and airway obstruction
• Paradoxically, voice may be relatively preserved because the cords can still vibrate

Immediate management requires reintubation or emergency surgical airway. Long-term options include tracheostomy, posterior cordotomy, or arytenoidectomy."

Examiner: "Good. How would you distinguish between RLN injury and external SLN injury clinically?"

Candidate:

"External SLN injury presents differently:

• Voice changes are more subtle - lower pitch, inability to hit high notes, vocal fatigue
• The patient may describe a 'weak' or 'monotonous' voice
• No stridor or airway compromise
• Often unrecognized postoperatively

This is because the cricothyroid muscle is paralyzed - the 'singer's muscle' responsible for tensing the cords and increasing pitch.

In contrast, RLN injury causes hoarseness because the cord is immobile, and bilateral injury causes stridor due to airway obstruction.

The external SLN is at risk during superior pole ligation of the superior thyroid artery during thyroidectomy."

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Viva 2: Cricothyroidotomy Anatomy

Examiner: "You are called to ED for a 'can't intubate, can't oxygenate' situation. The patient is a 50-year-old man with severe angioedema. Walk me through the anatomy for emergency cricothyroidotomy."

Candidate Response:

"I would first rapidly identify the surface landmarks using the laryngeal handshake technique.

Surface Landmarks (from superior to inferior):

• Hyoid bone at C3 level - U-shaped, palpable at mandibular angle level
• Thyroid notch - V-shaped depression at superior border of thyroid cartilage (C4-C5)
• Laryngeal prominence - the Adam's apple
• Cricothyroid membrane - the depression between inferior thyroid cartilage and cricoid (C5-C6)
• Cricoid cartilage - the complete ring at C6 level

Laryngeal Handshake Technique:

1. Stabilize thyroid cartilage between thumb and middle finger of non-dominant hand
2. Slide index finger inferiorly from thyroid notch
3. Feel the 'dip' - this is the cricothyroid membrane
4. Confirm the cricoid cartilage below (hard, complete ring)"

Examiner: "What are the dimensions of the cricothyroid membrane?"

Candidate:

"In adults:

• Height: 9-10mm (vertical extent)
• Width: 22-30mm (transverse extent)
• Depth to airway: 5-10mm in thin patients, may exceed 15mm in obesity

This is adequate for placement of a 6.0mm internal diameter cuffed tube."

Examiner: "What about the blood supply? Where should you make your incision?"

Candidate:

"The cricothyroid membrane is supplied by the cricothyroid artery, which is an anastomosis between the superior and inferior laryngeal arteries.

Critically, this artery runs across the upper portion of the membrane, laterally - not in the midline.

The safe zone for incision is the midline, lower half of the membrane.

Other structures to avoid:

• Anterior jugular veins: 1-2cm lateral to midline
• Thyroid isthmus: Usually below cricoid, but may extend upward

By staying in the midline and lower half, major bleeding is avoided."

Examiner: "What about pediatric patients?"

Candidate:

"In children under 8-10 years, the cricothyroid membrane is too small for surgical cricothyroidotomy:

• Height only 2.5-5mm
• Width only 6-12mm

Needle cricothyroidotomy is preferred:

• Insert a 14-16G cannula through the membrane
• Connect to jet ventilation (50 psi oxygen) if available
• Provides temporary oxygenation only (30-45 minutes)
• Must convert to formal tracheostomy

Additionally, in children:

• The larynx is higher (C3-C4)
• Structures are softer and more mobile
• Higher risk of posterior wall injury
• Smaller margin for error"

Examiner: "What would make cricothyroidotomy difficult in this patient?"

Candidate:

"Several factors can make landmark identification difficult:

1. Obesity: Fat obscures landmarks, increases depth to membrane
2. Short neck: Reduced working space
3. Neck swelling/edema: As in this patient with angioedema
4. Previous surgery or radiation: Scarring distorts anatomy
5. Female patients: Less prominent landmarks
6. Neck positioning: Extension improves access but may be limited

For difficult landmarks, I would:

• Use a vertical skin incision (8-10cm) to allow adjustment
• Blunt dissection through fat until cartilage structures palpated
• Laryngeal handshake to stabilize structures
• Once membrane identified, horizontal stab incision through membrane
• Bougie insertion and railroad a 6.0mm cuffed ETT"