Normal Hyoid-Mental Distance Measurement
The normal hyoid-mental distance (hyomental distance) in adults measured on a lateral neck view with the head in neutral position and mouth closed is ≥3 cm, and a decreased distance of <3 cm is considered a predictor of potential difficult airway. 1
Clinical Context and Measurement Standards
The hyoid-mental distance is a critical anatomical measurement used in airway assessment, particularly for predicting difficult intubation and airway management. 1
Standard Measurement Parameters
- Normal threshold: ≥3 cm (30 mm) in adults 1
- Abnormal/concerning: <3 cm indicates decreased hyoid-mental distance and potential difficult airway 1
- Measurement technique: Distance measured from the hyoid bone to the mental prominence (symphysis of the mandible) with head in neutral position 1
The American Society of Anesthesiologists (ASA) specifically identifies decreased hyoid-mental distance (≤3 cm in an adult) as one of the head and neck anatomical features associated with difficulty in airway management. 1
Anatomical Variations and Clinical Significance
Research-Based Normal Values
Recent research provides more granular data on hyomental distance measurements:
- General adult population: Mean hyomental distance of approximately 41.8 mm (range 27-60.3 mm) 2
- Gender considerations: Female subjects tend to have slightly different measurements, though the 3 cm threshold remains clinically applicable 3
Relationship to Difficult Airway
A longer mandibulohyoid distance (the vertical component of this measurement) is substantially associated with difficult laryngoscopy and intubation. 4 This occurs because:
- A relatively short mandibular ramus positions the mandibular angle more rostrally 4
- A relatively caudal (lower) larynx position increases the distance 4
- These anatomical variations create unfavorable geometry for direct laryngoscopy 4
Integration into Airway Assessment
Comprehensive Evaluation Framework
The ASA guidelines emphasize that multiple anatomical measures should be assessed together, as no single characteristic consistently predicts difficult airways. 1 The hyomental distance should be evaluated alongside:
- Thyromental distance 1
- Sternomental distance 1
- Mallampati score 1
- Mouth opening (≥3 cm normal) 1
- Neck mobility and extension 1
Clinical Measurement Technique
When measuring hyoid-mental distance clinically:
- Position the patient with head in neutral alignment (not extended or flexed) 1
- Ensure mouth is closed 1
- Measure the linear distance from the hyoid bone to the mentum (chin prominence) 1
- Document if distance is <3 cm, as this warrants heightened airway management preparation 1
Important Clinical Caveats
Limitations of Single Measurements
The 2022 ASA Difficult Airway Guidelines note that anatomical measurements including hyomental distance show "very high predictive and comparative variability, with sensitivity, specificity, and significance values ranging from low to very high." 1 Therefore:
- Never rely on hyomental distance alone for airway risk stratification 1
- Use it as part of a multiparameter assessment 1
- Consider patient-specific factors (obesity, cervical spine disease, facial trauma) that may alter measurements 1
Special Populations
In patients with mucopolysaccharidosis, the hyomental angle (not just distance) becomes more acute despite similar absolute distances, indicating a high larynx that may be missed by distance measurements alone. 2 This highlights that the geometric relationship matters as much as the absolute measurement. 2
Practical Application
Before sedation or anesthesia, if hyoid-mental distance is ≤3 cm:
- Ensure skilled assistance is immediately available 1
- Prepare difficult airway equipment 1
- Inform the patient of special airway management risks 1
- Consider awake intubation techniques if multiple difficult airway predictors are present 1
The 3 cm threshold remains the clinically validated standard for identifying patients who may require enhanced airway management strategies. 1