What is the rationale and justification for conducting a prospective study evaluating the head‑mouth distance ratio (HMDR) as an ultrasonographic predictor of a difficult airway?

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Last updated: September 25, 2026 • View editorial policy

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Rationale and Justification for Prospective HMDR Evaluation

The rationale for conducting a prospective study on HMDR as an ultrasonographic predictor of difficult airways is compelling because existing evidence shows contradictory results—with some studies demonstrating strong predictive value while others show poor correlation—and reported cut-off values vary substantially across populations, necessitating prospective validation to determine its true clinical utility.

Need for Simple and Objective Airway Prediction

Traditional clinical airway assessment tools (Modified Mallampati, thyromental distance, upper lip bite test) suffer from significant limitations:

  • Subjectivity and interobserver variability reduce reliability 1
  • Inconsistent accuracy in specific populations, particularly pediatric patients 2, 3
  • No single clinical predictor achieves 100% sensitivity and specificity 4

Ultrasonography offers objective, reproducible measurements that are non-cooperation-dependent—a critical advantage in pediatric and uncooperative patients 2.

Contradictory Evidence on HMDR Performance

The existing literature reveals striking inconsistencies that justify further prospective evaluation:

Studies Supporting HMDR:

  • In elderly patients: HMDR was selected by LASSO regression as a significant predictor and remained independently associated with difficult laryngoscopy in multivariable models 5
  • In obese and non-obese populations: HMDR was statistically significant in both groups, with cut-offs of ≤1.23 (non-obese: 68.8% sensitivity, 72.4% specificity) and ≤1.20 (obese: 85.7% sensitivity, 91.7% specificity), proving reliable irrespective of BMI 6
  • General surgical population: HMDR showed significant association with difficult laryngoscopy 4

Studies Refuting HMDR:

  • Most recent 2025 study: HMDR showed non-significant correlation with Cormack-Lehane grade [Kendall's tau: -0.0614; P = 0.211] and failed to correlate with intubation difficulty score [Kendall's tau: -0.0405; P = 0.409], leading authors to conclude it "poorly correlated" and "failed to predict difficult intubation" 7

Variation in Reported Cut-off Values

The substantial variation in HMDR thresholds across studies highlights the need for population-specific validation:

  • ≤1.23 in non-obese patients 6
  • ≤1.20 in obese patients 6
  • No established pediatric cut-offs despite anatomical differences 2, 3

This variation suggests that locally applicable evidence is essential for clinical implementation, as cut-offs derived from one population may not generalize to others with different anthropometric characteristics.

Clinical Utility Considerations

Advantages of HMDR as a Parameter:

  • Simple measurement technique: Requires only two positions (neutral and extended) 7, 6
  • Objective and reproducible: Unlike Mallampati classification 1
  • Can be integrated into combined models: When incorporated with other ultrasound parameters (MCTD, tongue thickness) and clinical predictors, diagnostic performance improves substantially (AUC 0.924-0.944) 5

Critical Pitfalls Requiring Prospective Evaluation:

  • Measurement standardization: Head positioning (neutral vs. extended) must be precisely defined
  • Operator training requirements: Ultrasound technique consistency affects reliability
  • Population-specific performance: HMDR may perform differently across age groups, BMI categories, and ethnic populations

Why Individual Evaluation is Necessary

While other ultrasound parameters show promise—mandibular condylar translation distance demonstrated the strongest individual performance (AUC 0.89-0.91) 5, and PreE/E-VC ratio showed superior correlation (AUC 0.77-0.86) 7—HMDR deserves dedicated prospective evaluation because:

  1. It's the simplest measurement requiring minimal anatomical landmarks
  2. Conflicting evidence demands resolution through rigorous methodology
  3. Combined models require validated individual components for optimal performance 5, 4
  4. Cost-effectiveness analysis requires prospective data on implementation feasibility

A well-designed prospective study with standardized measurement protocols, blinded outcome assessment, and adequate sample size across diverse populations is essential to definitively establish whether HMDR should be incorporated into routine preoperative airway assessment or abandoned in favor of superior alternatives like MCTD or PreE/E-VC ratio.

References

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