A 24-year-old male presents for emergency open reduction of a mandibular fracture. Airway assessment reveals full range of neck motion, mouth opening of 4 cm, thyromental distance of 7 cm, and Mallampati class II. The anaesthetist recalls the '3-3-2 rule' for predicting difficult laryngoscopy. According to this rule, the three assessments are:

• A Inter-incisor distance 3 fingerbreadths, hyoid-mental distance 3 fingerbreadths, thyroid-to-mouth distance 2 fingerbreadths ✓
• B Mouth opening 3 cm, hyoid-mental distance 3 cm, sternomental distance 2 fingerbreadths
• C Incisor gap 3 cm, thyromental distance 3 cm, Mallampati class 2 or less
• D Neck flexion 30 degrees, mouth opening 3 cm, sternomental distance 2 fingerbreadths

Explanation

The 3-3-2 rule (originally described by Khan et al.) evaluates three proportions: inter-incisor distance at least 3 fingerbreadths (adequate mouth opening), hyoid-mental distance at least 3 fingerbreadths (adequate mandibular space), and thyroid-to-mouth distance at least 2 fingerbreadths (adequate atlanto-occipital extension and neck length for alignment). These proportions predict the ability to achieve a direct line of sight during laryngoscopy.

Reference: Clinical Anesthesia, 8th (Barash) ed.

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Written and medically reviewed by the StethoPrep medical team.