What anatomical features predict an anterior larynx (voice box)?

Anatomical Features Predicting an Anterior Larynx

No single anatomical feature consistently predicts an anterior larynx position, but a combination of short thyromental distance, limited neck extension, receding chin, and increased neck circumference are the most clinically relevant predictors for difficult laryngoscopy associated with anterior laryngeal anatomy. 1

Key Anatomical Predictors

The 2022 ASA guidelines emphasize that multiple anatomical measurements should be assessed together, as no single feature demonstrates consistent predictive value across populations 1:

Primary Measurements

• Thyromental distance (shortened): Distance from thyroid notch to mentum; shorter distances correlate with more anterior laryngeal position 1
• Sternomental distance (reduced): Reflects the relationship between mandible and larynx 1
• Limited neck extension: Restricts ability to align oral, pharyngeal, and laryngeal axes, making anterior larynx more difficult to visualize 1, 2
• Receding chin (micrognathia): Anatomically positions larynx more anteriorly relative to oral cavity 2

Secondary Anatomical Features

• Increased neck circumference: Particularly when combined with short thyromental distance (ratio of neck circumference to thyromental distance) 1
• Limited mouth opening: Restricts laryngoscope blade insertion angle 1, 2
• Prominent upper incisors: Interfere with laryngoscope positioning 1
• Reduced hyomental distance: Distance from hyoid bone to mentum when neck fully extended 1

Ultrasound-Based Measurements

• Increased skin-to-hyoid distance: Suggests deeper, more anterior laryngeal position 1
• Increased tongue volume: Displaces larynx anteriorly 1
• Increased distance from skin to epiglottis: Indicates anterior positioning 1

Clinical Context and Limitations

Critical caveat: The ASA guidelines explicitly state that all anatomical predictors demonstrate "very high predictive and comparative variability, with sensitivity, specificity, and significance values ranging from low to very high across all patient demographic measures" 1. This means that while these features are associated with anterior larynx positioning, none reliably predict it in isolation.

Multivariate Assessment Approach

Assessment of multiple features together improves prediction: evaluating receding chin, neck extension, mouth opening, dentition, tongue size, and thyromental distance collectively identifies approximately 81% of difficult airways 2. However, unexpected anatomical variations (such as floppy epiglottis or unusual glottic inlet anatomy) remain difficult to predict preoperatively 2.

High-Risk Anatomical Conditions

Certain pathological conditions predictably alter laryngeal anatomy to create more anterior positioning 1:

• Large goiter with retrosternal extension: Causes distortion and elongation of anatomical relationships 1
• Ankylosing spondylitis: Limits neck mobility 1
• Klippel-Feil syndrome: Congenital cervical fusion 1
• Airway masses: Distort normal anatomy 1

Practical Clinical Algorithm

When assessing for anterior larynx risk:

1. Measure thyromental distance with neck fully extended (normal >6.5 cm in adults) 1
2. Assess neck extension range (normal >35 degrees) 1, 2
3. Evaluate mandibular anatomy for recession or micrognathia 2
4. Calculate neck circumference to thyromental distance ratio if neck circumference >40 cm 1
5. Consider ultrasound measurements if available, particularly skin-to-hyoid distance 1

If 3 or more concerning features are present, anticipate difficult laryngoscopy and prepare alternative airway management strategies 1, 2.