Discuss role of usg in Airway assessment. What are the criteria for difficult airway prediction . As an md anaesthesia exam question . With possible viva questions .
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https://pmc.ncbi.nlm.nih.gov/articles/PMC10177245
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Role of USG in Airway Assessment & Criteria for Difficult Airway Prediction
(MD Anaesthesia — Long Essay / Short Note format)
1. Introduction
Difficult airway management remains a leading cause of anaesthesia-related morbidity and mortality. Traditional bedside screening tests (Mallampati grade, thyromental distance, etc.) have only moderate sensitivity and specificity and are examiner-dependent. Point-of-care airway ultrasonography (USG) has emerged as a non-invasive, radiation-free, reproducible "fifth pillar" of airway examination that adds objective anatomical data to clinical assessment - Reddy P et al., Indian J Anaesth, 2016.
2. Role of USG in Airway Assessment
A. Pre-operative / Predictive role (difficult laryngoscopy & intubation)
USG measures soft-tissue and bony landmarks that correlate with Cormack-Lehane grade and difficulty of laryngoscopy:
| USG Parameter | Probe placement / View | What it predicts |
|---|---|---|
| Tongue thickness (TT) | Submental/sub-mandibular, midline sagittal | Tongue volume relative to oral cavity - TT > 6.1 cm predicts difficult intubation (sensitivity ~75%, specificity ~72%) |
| Hyomental distance (HMD) - neutral and extended | Submental, sagittal, measuring hyoid-to-mentum | Considered the single most reliable USG predictor of difficult laryngoscopy across recent meta-analyses; HMD ≤ 5.29 cm shows sensitivity ~96.7%, specificity ~71.6% |
| Hyomental distance ratio (HMDR1, HMDR2) = ratio of HMD in extension/neutral or ramped position | Same submental view, two head positions | Corrects for individual variation; HMDR1 ≤ 1.12 (Se 75%, Sp 76.2%); HMDR2 ≤ 1.085-1.23 depending on cut-off (Se 75-100%, Sp 85-90%) |
| Distance from skin to epiglottis (DSE) | Thyrohyoid membrane, transverse/sagittal | Predicts a difficult, anteriorly-placed larynx; correlates with difficult laryngoscopy |
| Distance from skin to hyoid bone / vocal cords | Submandibular / thyroid cartilage level | Soft tissue thickness overlying airway structures |
| Pre-epiglottic space ratio | Thyrohyoid membrane | Airway soft tissue bulk (obesity, OSA correlation) |
A standardized combination of these views has been proposed as the DARES protocol (Difficult Airway Evaluation with Sonography) - using thyrohyoid and suprahyoid windows to combine DSE, HMD, HMDR1/2 and tongue thickness into one structured scan (Dabo-Trubelja A, J Vis Exp, 2023; Alsumali et al./PMC10177245).
B. Peri-operative and other established roles
- Confirmation of endotracheal tube placement - tracheal vs oesophageal intubation (real-time "bullet sign"/tracheal shadow with posterior wall reverberation vs "double tract sign" if oesophageal).
- Confirmation of correct ETT depth / carinal position and detection of endobronchial migration.
- Identification of the cricothyroid membrane before elective or emergency front-of-neck access (particularly useful in obese patients where landmarks are impalpable) - improves first-pass success of cricothyroidotomy.
- Selection of appropriate ETT / double-lumen tube size - by measuring subglottic diameter (useful in paediatrics to reduce post-extubation stridor).
- Prediction of post-extubation laryngeal oedema - air-column width difference test (comparing tracheal air column width with cuff deflated vs inflated); a large difference correlates with lower risk of post-extubation stridor.
- Assessment of vocal cord mobility (pre/post thyroid surgery, suspected recurrent laryngeal nerve injury) - avoids awake laryngoscopy.
- Gastric ultrasound for aspiration risk assessment before induction (related airway safety application).
- Guiding regional techniques relevant to airway - superior laryngeal nerve block, percutaneous tracheostomy guidance.
C. Advantages over clinical bedside tests
- Objective, quantitative, reproducible; not dependent on patient cooperation (useful in trismus, cervical spine immobilization, obesity, where mouth opening/neck extension cannot be assessed).
- Radiation-free, portable, real-time, repeatable at bedside/ICU/ER.
- Can be combined with clinical scores to improve overall predictive accuracy (multivariate/composite models outperform either alone) - Frontiers in Medicine, 2024.
D. Limitations
- Operator-dependent; needs training and standardized technique.
- Cut-off values vary across studies/populations - no single universally validated threshold.
- Systematic reviews (Giordano et al., Eur J Anaesthesiol 2023; Bhargava et al., Crit Care Med 2023) note moderate-to-low certainty of evidence and heterogeneity in protocols; USG is currently an adjunct, not a replacement, for standard clinical airway assessment.
- Cannot assess mouth opening, dentition, or neck mobility directly.
3. Criteria for Difficult Airway Prediction
A. Definitions (ASA Task Force / DAS)
- Difficult mask ventilation, difficult laryngoscopy (Cormack-Lehane grade III/IV), difficult intubation (>3 attempts or >10 min by an experienced laryngoscopist), difficult supraglottic airway placement, and difficult surgical airway.
B. History
- Previous difficult intubation/anaesthetic records, obstructive sleep apnoea, snoring, stridor, syndromic/congenital facial anomalies, radiotherapy to neck, acromegaly, diabetes (stiff joint syndrome), rheumatoid arthritis (atlanto-axial subluxation), tumours/abscess of oral cavity or neck, trauma, burns, pregnancy.
C. Bedside clinical examination criteria
| Test | Positive (predictive of difficulty) cut-off |
|---|---|
| Modified Mallampati Class (Barash, Table 23-6) | Class III/IV |
| Thyromental distance (TMD) | < 6-6.5 cm |
| Sternomental distance | < 12.5 cm |
| Inter-incisor / mouth opening | < 3 cm (< 2 finger-breadths) |
| Upper lip bite test (ULBT) | Class III (lower incisors cannot reach upper lip/vermillion) |
| Neck circumference | > 43 cm (particularly obese patients) |
| Neck movement / extension | < 80-90 degrees, or reduced atlanto-occipital extension |
| Ratio of height to thyromental distance (RHTMD) | > 23-25 |
| Mandibular protrusion test | Inability to protrude mandible |
| Wilson Risk Sum Score | Score ≥ 2 (weight, head/neck movement, jaw movement, receding mandible, buck teeth - each scored 0-2) |
| El-Ganzouri Multivariate Risk Index | Combines mouth opening, TMD, Mallampati, neck movement, ability to prognath, weight, history of difficult intubation - score ≥ 4 predicts difficulty |
| LEMON (emergency airway) | Look externally, Evaluate 3-3-2 rule, Mallampati, Obstruction, Neck mobility |
D. Anatomical/pathological red flags
Micrognathia/retrognathia, macroglossia, short thick neck, receding mandible, prominent incisors, high arched palate, limited cervical spine mobility, morbid obesity, beard (mask seal), oropharyngeal/laryngeal mass, airway oedema, previous neck irradiation or surgery.
E. Radiological/USG-based criteria (as above)
HMD ≤ 5.2-5.3 cm, HMDR1 ≤ 1.12, HMDR2 ≤ 1.08-1.23, tongue thickness > 6.1 cm, increased DSE - used increasingly as adjuncts, especially in obese or ICU/emergency patients where clinical tests are unreliable.
F. Composite approach
No single test has adequate sensitivity/specificity alone (Mallampati sensitivity ~40-60%). Current best practice combines multiple clinical predictors + USG parameters to improve pre-operative risk stratification, in line with the 2022 ASA Practice Guidelines and Difficult Airway Society (DAS) recommendations for a structured, multi-component airway assessment before anaesthesia.
4. Possible Viva Questions
- What are the "fifth pillar" and "four pillars" of airway assessment, and where does USG fit in?
- Which two USG views form the basis of the DARES protocol? What structures do they visualise?
- Define hyomental distance ratio. Why is a ratio preferred over an absolute distance?
- How does airway USG differ in its application between the ICU/emergency setting and the elective OT setting?
- How would you confirm correct ETT placement using ultrasound without capnography?
- What is the "air column width difference test" and its clinical significance?
- How would you identify the cricothyroid membrane sonographically in an obese patient with an emergency airway?
- Compare the sensitivity/specificity of Mallampati grading versus USG-measured tongue thickness for predicting difficult laryngoscopy.
- What are the components of the Wilson Risk Sum Score and El-Ganzouri Risk Index?
- What is the LEMON mnemonic used for, and in which clinical setting is it most applicable?
- What are the limitations of ultrasound in predicting a difficult airway, and why is there no universally accepted cut-off?
- How does obesity affect both clinical and sonographic airway assessment?
- Outline the ASA/DAS difficult airway algorithm and where pre-operative assessment fits into it.
- How can ultrasound help decide endotracheal or double-lumen tube size in a paediatric patient?
- What sonographic finding would suggest post-extubation laryngeal oedema risk, and how would you manage a positive finding?
Key sources referenced: Barash, Cullen & Stoelting's Clinical Anesthesia, 9e (The Difficult Airway); Morgan & Mikhail's Clinical Anesthesiology, 7e (Difficult Airway Algorithm); Reddy P et al., "Ultrasonography - A viable tool for airway assessment," Indian J Anaesth 2016;60:807-13; and recent systematic reviews/meta-analyses: Bhargava V et al., Crit Care Med 2023 (PMID 36519985); Giordano G et al., Eur J Anaesthesiol 2023 (PMID 36748275); Benavides-Zora D et al., J Cardiothorac Vasc Anesth 2023 (PMID 37012134); Soni VM et al., Anaesthesia 2025 (PMID 40891437).
Evidence note: Systematic reviews consistently rate the certainty of evidence for USG-based difficult airway prediction as low-to-moderate due to heterogeneous protocols and cut-offs (PMID 36748275, 36519985) - answer this in exam by acknowledging USG as a promising adjunct, not a stand-alone replacement for clinical bedside assessment.
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