III.H.1 Airway anatomy and assessmentLaryngeal innervationDifficult airway prediction
Airway anatomy and assessment
Know the larynx and its nerves, run a focused airway exam, and predict which airway tool is likely to fail.
By the end you can
- Name the laryngeal cartilages and the nerve that supplies each part of the airway
- Perform and grade a bedside airway exam (Mallampati, thyromental distance, upper lip bite, neck motion)
- Predict difficult mask ventilation, laryngoscopy, SGA placement, and front-of-neck access
Your case
Preop holding · Laparoscopic hernia repair
Mr. K, 64, 118 kg, BMI 38. Full beard. OSA on CPAP. Neck radiation for a tongue base cancer 3 years ago. His neck skin feels firm and woody.
Your exam: Mallampati III, thyromental distance 5.5 cm, mouth opening 3.5 cm, neck extension limited.
SpO2 94% RANeck 46 cmSTOP-BANG 6ASA III
Hold that thought
- Which of his findings predict difficult mask ventilation, and which one is strongest?
- Can you count on an SGA or a scalpel cricothyrotomy as your rescue?
- Awake or asleep: where does the ASA 2022 guideline point you?
You'll answer all three at the end of the lesson.
Concept 1 · The upper airway
From nose to carina
The path a breath takes
- Nose: warms and humidifies. Pass a nasal tube along the floor, below the inferior turbinate
- Pharynx: nasopharynx, oropharynx, and hypopharynx (laryngopharynx), which leads to the larynx and the esophagus
- Larynx: the valve that protects the trachea. Adult larynx spans roughly C3 to C6
- Trachea: C-shaped cartilage rings. It divides at the carina, near the sternal angle (T4 to T5)
- Right mainstem is shorter, wider, and more vertical, so a deep tube or aspirate goes right
Why the airway closes under anesthesia
- Pharyngeal muscle tone drops, so the soft palate, tongue base, and epiglottis fall back
- Jaw thrust and chin lift pull the hyoid and epiglottis forward
- An oral or nasal airway splints the passage open
Pediatric contrast: infants have a large occiput and tongue, a floppy omega-shaped epiglottis, and a higher, more anterior larynx (glottis near C3 to C4).
Concept 2 · The larynx
Nine cartilages, one complete ring
Three unpaired
- Thyroid: the shield. Its prominence is the "Adam's apple"
- Cricoid: the only complete ring, at C6
- Epiglottis: the flap that folds over the inlet when you swallow
Three paired
- Arytenoids: anchor the back of the vocal cords and pivot to open or close them
- Corniculates and cuneiforms: small, in the aryepiglottic folds
Landmarks you will use
- Vallecula: the pocket between tongue base and epiglottis. A curved blade tip sits here
- Glottis: the vocal cords and the opening between them. The narrowest part of the adult airway
- Cricothyroid membrane: between thyroid and cricoid, roughly 1 cm tall. Your emergency front door
- Pyriform recesses: beside the larynx. Where a blind tube or foreign body likes to hide
Quick check 1 of 4
You cannot intubate or oxygenate. For a scalpel cricothyrotomy, you cut the membrane between:
The cricothyroid membrane lies between the thyroid and cricoid cartilages, is superficial, and has few vessels. The thyrohyoid membrane (hyoid to thyroid) sits above the cords, so a cut there would not bypass a glottic obstruction.
Concept 3 · Airway nerves
Who feels what, who moves what
Tap each nerve. The colored zone shows where it supplies sensation or motor function.
Start at the top
Tap a nerve above to see its territory, its reflex, and how you would block it.
Glossopharyngeal nerve (CN IX)
Sensation to the posterior third of the tongue, vallecula, front of the epiglottis, tonsils, and pharyngeal walls. It is the afferent limb of the gag reflex. Blocked at the base of the anterior tonsillar pillar for awake intubation.
Superior laryngeal nerve, internal branch
Pierces the thyrohyoid membrane and supplies sensation from the epiglottis down to the vocal cords. Afferent limb of laryngospasm and cough. Blocked just below the greater horn of the hyoid.
Superior laryngeal nerve, external branch
Pure motor to the cricothyroid muscle, which tenses and lengthens the cords (pitch). Runs with the superior thyroid vessels, so thyroid surgery can injure it: a weak, tiring voice that cannot reach high notes.
Recurrent laryngeal nerve
Sensation below the cords and to the trachea. Motor to every intrinsic laryngeal muscle except the cricothyroid. The left loops under the aortic arch, the right under the subclavian artery, so the left is longer and more vulnerable in the chest.
Cricothyroid membrane
Between thyroid and cricoid, roughly 1 cm tall. Site of the transtracheal block (lidocaine injected here triggers a cough that spreads it) and of emergency front-of-neck access. Find it by palpation or ultrasound before induction when difficulty is expected.
Concept 4 · Cord muscles and nerve injury
One opener, and what happens when nerves fail
The intrinsic muscles
- Posterior cricoarytenoid: the only abductor. It opens the cords
- Lateral cricoarytenoid and interarytenoids: adduct (close) the cords
- Cricothyroid: tenses the cords. The only one on the external SLN
- Thyroarytenoid (vocalis): relaxes and shortens the cords
| Injury | What you see |
|---|---|
| Unilateral RLN | Hoarseness. Affected cord sits paramedian |
| Bilateral RLN, acute | Stridor and respiratory distress. Both cords near midline |
| Bilateral RLN, chronic | Aphonia; airway often adequate at rest |
| External SLN | Weak voice, loss of high pitch; often subtle |
| Unilateral vagus | Hoarseness (RLN and SLN both lost) |
Watch out: after thyroid or parathyroid surgery, stridor at extubation means look at the cords. Also think neck hematoma and, a day or so later, hypocalcemia.
Quick check 2 of 4
Right after extubation from a total thyroidectomy, a patient has inspiratory stridor and falling SpO2. Most likely nerve injury?
Bilateral RLN injury leaves both cords near the midline with no abductor, so the airway obstructs. Reintubate. A unilateral RLN injury is the tempting pick, but it usually causes hoarseness, not respiratory distress.
Concept 5 · Sort the territories
Which nerve is it?
Tap an item, then tap its nerve. The vocal cords are the dividing line for the vagus branches.
Awake intubation link: topical or block the IX territory to kill the gag, the internal SLN to stop laryngeal cough and spasm, and the RLN territory (transtracheal) to tolerate the tube in the trachea. Techniques come in the airway blocks lesson.
Concept 6 · The airway history
The chart often tells you first
Ask every patient
- Prior anesthesia records: a documented difficult airway is the strongest warning you will get
- Snoring or OSA: screen with STOP-BANG (3 or more is increased risk)
- Head and neck history: radiation, surgery, tumor, trauma
- Symptoms now: stridor, voice change, dysphagia, trouble lying flat
| Condition | Airway problem |
|---|---|
| Rheumatoid arthritis | Atlantoaxial instability, cricoarytenoid arthritis, stiff TMJ |
| Ankylosing spondylitis | Fixed, flexed cervical spine |
| Down syndrome | Large tongue, atlantoaxial instability, small subglottis |
| Acromegaly | Big tongue and epiglottis, narrowed glottis |
| Diabetes (long-standing) | Stiff joints ("prayer sign"), reduced neck extension |
| Pregnancy | Mucosal edema, worse Mallampati, fast desaturation |
| Pierre Robin, Treacher Collins | Small mandible, tongue falls back |
Concept 7 · Two grading scales
What you see awake vs what you see asleep
Modified Mallampati: bedside, awake
- Technique: sitting, head neutral, mouth open wide, tongue out, no phonation
- Class I: soft palate, fauces, uvula, and pillars. II: soft palate, fauces, uvula
- Class III: soft palate and base of uvula. IV: hard palate only
- Class III or IV predicts harder laryngoscopy and is a mask ventilation predictor too
Cormack-Lehane: the view at laryngoscopy
- Grade 1: most of the glottis. Grade 2: posterior glottis or arytenoids only (2a, 2b)
- Grade 3: epiglottis only. Grade 4: not even the epiglottis
- Grade 3 or 4 defines difficult laryngoscopy. Document it for the next provider
- Mallampati predicts; Cormack-Lehane describes. They correlate only loosely
Concept 8 · The bedside numbers
Measure the space you have to work in
- Mouth opening (interincisor): under 3 cm makes blade and SGA entry hard
- Thyromental distance (neck extended): under 6 cm means a small mandibular space for the tongue
- Sternomental distance (head extended, mouth closed): under 12.5 cm
- Upper lip bite test: class III (cannot reach the upper lip) predicts difficult laryngoscopy
- Neck extension: normal atlanto-occipital extension is about 35 degrees. Limited or fixed is a red flag
Fill the blanks
The 3-3-2 rule: 3 fingers of mouth opening, 3 from chin to , and 2 from hyoid to thyroid notch.
A thyromental distance under predicts difficult laryngoscopy.
In the upper lip bite test, class means the lower incisors cannot bite the upper lip at all.
Mallampati is graded with the tongue out and .
Quick check 3 of 4
Your patient can bite her upper lip with her lower incisors, but only below the vermilion line. Upper lip bite class?
Class I covers the vermilion completely, class II bites the lip below the vermilion line, and class III cannot reach the upper lip at all. Class I is the tempting pick, but she cannot reach above the vermilion line. There is no class IV in this test.
Concept 9 · Four ways to fail
Predict each rescue, not just the tube
ASA 2022 asks you to judge each airway task separately. Classic mnemonics from Walls and Murphy's Manual of Emergency Airway Management cover all four. Newer editions revise two (ROMAN for mask, SMART for front of neck); the classic versions below are still widely taught.
Mask: MOANS
- Mask seal (beard, facial trauma)
- Obesity, obstruction
- Age over 55
- No teeth
- Snorers, sleep apnea, stiff lungs
Laryngoscopy: LEMON
- Look externally
- Evaluate 3-3-2
- Mallampati
- Obstruction, obesity
- Neck mobility
SGA: RODS
- Restricted mouth opening
- Obstruction, obesity
- Disrupted or distorted airway
- Stiff lungs or cervical spine
Front of neck: SHORT
- Surgery on the neck
- Hematoma or infection
- Obesity (hard to find landmarks)
- Radiation
- Tumor
What the outcome studies found
- Langeron 2000 (difficult mask): age over 55, BMI over 26, beard, no teeth, snoring. Two or more is a warning
- Kheterpal 2009 (impossible mask, about 0.15%): neck radiation changes was the strongest, plus male sex, OSA, Mallampati III or IV, and beard
- Kheterpal 2013: difficult mask plus difficult laryngoscopy together in about 0.4% of cases
Watch out: every bedside test has modest sensitivity. In a 2018 Cochrane review, the upper lip bite test performed best for laryngoscopy, yet no single test was a good screen. Combine findings and always have a backup plan.
Signature interactive
Difficult airway predictor
Set the exam findings. Watch which mnemonic letters light up for each task, and how the plan changes. This is a teaching model, not a validated score.
Load a patient
Mallampati
Upper lip bite
Neck extension
Also present
Suggested plan
Teaching model only: the point weights and cutoffs are this lesson's own. Findings follow the classic mnemonics and outcome studies (Langeron 2000, Kheterpal 2006 and 2009). It is not a validated score.
Quick check 4 of 4
A 71-year-old man with a full beard, no teeth, and untreated OSA. Mallampati II, normal neck. Which task is most likely to be difficult?
Beard, no teeth, age, male sex, and OSA are MOANS and Kheterpal predictors of difficult mask ventilation. His LEMON findings are reassuring, and missing teeth often make laryngoscopy easier. Leave dentures in for masking if you can.
Practice
True or false, fast
Rapid fire
The external branch of the SLN supplies sensation above the cords.
That is the internal branch. The external branch is motor to the cricothyroid muscle.
The posterior cricoarytenoid is the only muscle that opens the cords.
It is the sole abductor, supplied by the RLN. Lose it on both sides and the cords sit near midline.
A pregnant patient's Mallampati class can worsen during labor.
Airway edema increases through pregnancy and labor. Reassess the airway right before a labor anesthetic.
A Mallampati class I rules out difficult laryngoscopy.
Sensitivity is modest, so many difficult laryngoscopies occur in class I or II. A normal test does not rule it out.
Neck radiation changes predict impossible mask ventilation.
In Kheterpal 2009 it was the strongest independent predictor. It also distorts SGA fit and the front of the neck.
Edentulous patients are usually harder to intubate than to mask ventilate.
Usually the reverse: no teeth makes the mask seal harder but gives more room for the blade.
Lidocaine injected through the cricothyroid membrane numbs RLN territory.
The transtracheal block numbs the trachea and below the cords. The cough it causes also spreads some lidocaine upward.
Lock it in
The four T's, top to bottom
Numbers to own: say it, then tap to check
- Thyromental distance, abnormal:
- Mouth opening, abnormal:
- Sternomental distance, abnormal:
- Normal atlanto-occipital extension:
- MOANS age cutoff:
- Cricothyroid membrane height:
- Impossible mask ventilation incidence:
- STOP-BANG neck circumference:
Board traps
What the NCE will try to trick you with
Usually not. One paralyzed cord gives hoarseness. Stridor and distress point to bilateral injury, with both cords near midline.
False for adults. In adults it is the glottis. The cricoid is the classic answer for young children, though imaging studies now suggest the glottis and subglottis are functionally narrowest there too.
No. Mallampati is an awake prediction (Roman numerals I to IV). Cormack-Lehane is the actual view at laryngoscopy (grades 1 to 4). A Mallampati I can still be a grade 3 view.
Weakly at best. BMI strongly predicts difficult mask ventilation and fast desaturation. For laryngoscopy in obese patients, neck circumference and Mallampati class matter more than BMI.
Cheat card
The whole lesson on one card
Nerves
- IX: tongue base, pharynx, gag
- Internal SLN: above the cords
- External SLN: cricothyroid (pitch)
- RLN: below cords + all other muscles
- Bilateral RLN: stridor
Bedside exam
- Mallampati III to IV
- TMD under 6 cm, mouth under 3 cm
- Sternomental under 12.5 cm
- ULBT class III
- Limited neck extension
Predict each task
- Mask: MOANS
- Laryngoscopy: LEMON
- SGA: RODS
- Front of neck: SHORT
- Radiation: strongest for impossible mask
Plan
- ASA 2022: judge each task separately
- Mask and tube both hard: think awake
- Mark the CT membrane early
- No test rules difficulty out
The printable PDF version of this card is in the Cheat Sheets library.
Lesson quiz · 1 of 5
Sensation to the laryngeal surface of the epiglottis and the aryepiglottic folds comes from the:
The internal SLN covers the larynx from the epiglottis down to the cords. The glossopharyngeal is tempting, but it supplies the front (lingual) surface of the epiglottis and the vallecula, not the inside of the larynx.
Lesson quiz · 2 of 5
Seated, mouth open and tongue out, you see the soft palate and only the base of the uvula. Mallampati class?
Soft palate plus the base of the uvula is class III. Class II shows the whole uvula and fauces. Class IV shows the hard palate only.
Lesson quiz · 3 of 5
Which finding was the strongest independent predictor of impossible mask ventilation?
In Kheterpal's 2009 study of over 50,000 mask ventilation attempts, neck radiation changes carried the highest odds. A beard also predicts difficulty, but it breaks the seal, which you can often fix (a clear dressing or gel, a two-hand grip).
Lesson quiz · 4 of 5
At laryngoscopy you see only the arytenoids and the posterior commissure. Cormack-Lehane grade?
Seeing only the posterior glottis or arytenoids is grade 2 (2b when only arytenoids show). Grade 3 is the epiglottis alone, with no laryngeal structures visible.
Lesson quiz · 5 of 5
For an awake intubation, which block anesthetizes the tracheal mucosa below the cords?
Lidocaine injected through the cricothyroid membrane numbs RLN territory below the cords. The SLN block is the tempting pick, but it covers the larynx above the cords.
Wrap-up
0 / 5
Quiz score
Back to preop · Mr. K, BMI 38, radiated neck
Mask predictors? Beard, BMI 38, OSA, age over 55, male sex, and Mallampati III. The strongest is his neck radiation (Kheterpal 2009).
Reliable rescue? No. Radiation distorts the airway (the D in RODS) and the neck (the R in SHORT), and obesity adds an O to both. Mark the cricothyroid membrane, by ultrasound if you cannot feel it, and have surgical backup ready.
Awake or asleep? With mask, laryngoscopy, and rescue all predicted difficult in a cooperative patient, ASA 2022 points to an awake intubation, often with a flexible scope.
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