Methods of Airway Assessment

A pre-procedure airway assessment is mandatory before every anaesthetic and any anticipated intubation. No single test predicts a difficult airway with 100% accuracy; a combination of history, physical examination, and scoring systems is the standard approach.

1. History

The single most predictive factor for difficult intubation is a previous history of difficult intubation. The history should cover:

Prior anaesthetic records and any documented difficulty
Symptoms of airway-related pathology: stridor, dysphagia, hoarseness, positional dyspnoea
History of snoring / obstructive sleep apnoea (predictive of difficult mask ventilation)
Changes in weight or neck size since last anaesthetic
Head and neck surgery, radiation, or trauma
Conditions that restrict mouth opening: rheumatoid arthritis, temporo-mandibular joint disease, scleroderma

2. External Visual Inspection

Inspect the face and neck for:

Obvious facial deformities, asymmetry, or dysmorphic features
Short, thick, or bull neck
Receding or small mandible (micrognathia)
Large goitre or neck masses
Facial burns, angioedema, Ludwig's angina, airway tumours
Beard (increases risk of poor mask seal)
Cervical collar or traction devices (restrict laryngoscopy)
Neck circumference >43 cm (17 inches) is more predictive of difficult intubation than BMI alone

3. Mouth Opening

Measure inter-incisor distance with maximal mouth opening
< 3 cm (or < 2 fingerbreadths) between upper and lower incisors predicts difficult intubation
Inspect the oropharynx for neoplasm, high arched palate, macroglossia

4. Mallampati Classification

The most widely used scoring test. Performed with the patient seated, mouth fully open, tongue protruded maximally, and phonation avoided. It relates tongue size to the oral cavity.

Class	Structures Visible
I	Soft palate, uvula, fauces, tonsillar pillars all visible
II	Soft palate, uvula, upper fauces visible; pillars obscured
III	Only soft and hard palate visible
IV	Only hard palate visible

Class III or IV predicts a potentially difficult laryngoscopy
Best used in combination with other tests - sensitivity alone is ~39% but specificity ~86%
The modified Mallampati test (Samsoon & Young, 1987) adds Class IV to the original three-class system

5. Thyromental Distance (TMD)

Measured from the thyroid notch to the mentum (chin) with the neck fully extended
< 6.5 cm (< 3 fingerbreadths) indicates reduced submandibular space ("anterior larynx") and predicts difficult intubation
During laryngoscopy, the tongue must be displaced into this space; a small or non-compliant space limits glottic exposure

6. Sternomental Distance (SMD)

Measured from the sternal notch to the point of the chin with the head in full extension, mouth closed
< 12.5 cm is associated with difficult intubation
Reflects the combined contribution of neck mobility and mandibular space

7. Neck Range of Motion

Optimal laryngoscopy requires cervical flexion + atlanto-occipital extension (the "sniffing position")
< 80° of total neck flexion-extension is predictive of difficult intubation
Conditions that restrict movement: cervical spondylosis, rheumatoid arthritis, ankylosing spondylitis, Halo brace, collar

8. Upper Lip Bite Test (ULBT)

Patient is asked to bite the upper lip with the lower incisors:

Class I: Lower incisors bite above the upper lip vermilion line - easy intubation likely
Class II: Lower incisors bite below the vermilion line
Class III: Lower incisors cannot reach the upper lip - difficult intubation predicted

Higher specificity and less interobserver variability than Mallampati alone. Also assesses temporo-mandibular joint mobility and mandibular prognathism.

9. Mandibular Prognathism (Jaw Protrusion Test)

Ask the patient to slide the lower jaw forward so the lower incisors protrude beyond the upper
Inability to prognath (lower incisors cannot reach or exceed the upper) predicts difficult laryngoscopy
Reflects temporo-mandibular joint laxity and range of motion

10. Dentition Assessment

Long upper incisors impair direct laryngoscopy
Loose or poor dentition increases risk of dental trauma and aspiration of teeth
Edentulous patients are usually easier to intubate but harder to mask-ventilate (poor seal)
Veneers, crowns, and bridges are at high risk of damage

11. Mnemonics for Difficult Airway Prediction

LEMON (Emergency Medicine / Difficult Intubation)

Letter	Assessment
L - Look externally	Facial trauma, obesity, large incisors, large tongue, dysmorphic features
E - Evaluate 3-3-2 rule	Mouth opening ≥3 fingers; hyomental distance ≥3 fingers; thyroid-to-hyoid ≥2 fingers
M - Mallampati	Class I-IV
O - Obstruction	Stridor, foreign body, tumour, epiglottitis, angioedema
N - Neck mobility	Extension and flexion assessed

MOANS (Difficult Bag-Mask Ventilation)

Letter	Factor
M	Mask seal difficulty (beard, facial trauma)
O	Obesity / Obstruction
A	Age > 55
N	No teeth (edentulous)
S	Stiff lungs (asthma, COPD)

ROMAN (Difficult Bag-Mask Ventilation - Rosen's EM)

R - Radiation/Resistance to ventilation
O - Obstruction, Obesity, OSA
M - Mallampati, Male, Mask seal
A - Aged
N - No teeth

12. Multivariate Scoring Systems

Individual tests have poor sensitivity; combined scores perform better:

Score	Components
Wilson Risk Sum Score	Weight, head/neck movement, jaw movement, receding mandible, buck teeth
El-Ganzouri Risk Index	Mouth opening, TMD, Mallampati, neck movement, prognathism, weight, previous difficult intubation

13. Imaging Adjuncts

Point-of-care ultrasound (POCUS): Skin-to-epiglottis distance (US-DSE) shows higher sensitivity (0.80) and specificity (0.77) than most bedside tests in a 2024 systematic review - now increasingly used pre-operatively
CT/MRI neck: For known masses, subglottic stenosis, or complex pathology
Lateral neck X-ray: Useful for assessing tracheal deviation, subglottic narrowing, retropharyngeal masses

14. Factors Associated With Difficulty (Summary)

From Tintinalli's Emergency Medicine, the key risk factors are:

Obesity
History of snoring / sleep apnoea
Short neck / limited neck mobility
Small or large chin
Prominent incisors
High arched palate
Edentulous anatomy
Facial or airway trauma
Head and neck tumours
Angioedema, Ludwig's angina
Airway burns / inflammation

Key Principle

No single test is sufficient. The ASA difficult airway guidelines and anaesthesia texts (Morgan & Mikhail, Miller's) recommend a combined assessment: history + visual inspection + mouth opening + Mallampati + thyromental distance + neck mobility + prognathism. Absence of all abnormal findings is reassuring but never guarantees an easy airway - the clinician must always have a plan B and C ready.

Sources:

Morgan and Mikhail's Clinical Anesthesiology, 7e, p. 570-571
Miller's Anesthesia, 10e, Box 40.1, pp. 5845-5851
Tintinalli's Emergency Medicine, Table 29A-2, pp. 221-222
2024 systematic review (BMC Anesthesiology) - ultrasound-based US-DSE outperforms traditional bedside tests

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