What is the research gap in evaluating the hyomental distance ratio as a predictor of difficult airway using ultrasonography in patients undergoing surgery under general anesthesia?

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Last updated: October 2, 2026 • View editorial policy

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Research Gaps in Hyomental Distance Ratio (HMDR) as an Ultrasound Predictor for Difficult Airway

The primary research gap is that HMDR shows inconsistent and often poor diagnostic performance as a standalone ultrasound predictor, with recent high-quality evidence demonstrating it fails to reliably predict difficult intubation, particularly in morbidly obese patients, and is consistently outperformed by other ultrasound parameters.

Key Evidence Gaps Identified

1. Contradictory Diagnostic Performance

The most recent 2025 study 1 directly challenges HMDR's utility, showing:

  • Non-significant correlation with Cormack-Lehane grade [Kendall's tau: -0.0614; P = 0.211]
  • No correlation with intubation difficulty score [Kendall's tau: -0.0405; P = 0.409]
  • HMDR was explicitly stated to "poorly correlate" and "failed to predict difficult intubation"

This contradicts earlier 2024 findings 2 where HMDR showed significant association with difficult laryngoscopy, revealing inconsistent reproducibility across different populations and settings.

2. Population-Specific Limitations

Critical gaps exist in understanding HMDR performance across patient subgroups:

  • Morbid obesity: The 2020 study 3 revealed negligible correlation between clinical and ultrasound HMDR measurements in morbidly obese patients (Pearson coefficient 0.14), with difficulties in palpating anatomical structures
  • Elderly patients: The 2026 study 4 found HMDR was selected by LASSO regression but showed inferior performance compared to mandibular condylar translation distance (MCTD), which achieved AUC 0.89-0.91
  • Pediatric populations: The 2026 pediatric study 5 did not find HMDR significant, with skin-to-epiglottis distance demonstrating superior performance

3. Inferior Performance Compared to Alternative Parameters

Multiple recent studies demonstrate other ultrasound measurements consistently outperform HMDR:

  • Pre-epiglottic space/epiglottis-to-vocal cord ratio (PreE/E-VC): Showed strong correlation with both CL grade [Kendall's tau: 0.423] and IDS [Kendall's tau: 0.391] with 77.8% sensitivity and 86% specificity 1
  • Mandibular condylar translation distance (MCTD): Achieved highest AUC (0.89-0.91) in elderly patients 4
  • Anterior neck soft tissue thickness (ANS-VC): Demonstrated 78.9% sensitivity and 71.1% specificity (AUC 0.816), superior to HMDR (AUC 0.713) 6
  • Pretracheal soft-tissue thickness: Showed stronger association with difficult laryngoscopy than hyomental distance 7

4. Lack of Standardized Measurement Protocols

No consensus exists on:

  • Optimal patient positioning during HMDR measurement
  • Probe placement technique standardization
  • Whether clinical or ultrasound measurement is superior (moderate correlation in non-obese [Pearson 0.494], negligible in obese [0.14]) 3
  • Cut-off values that vary across studies without validation

5. Limited Value as Standalone Predictor

The evidence consistently shows HMDR requires combination with other parameters:

  • Combined ultrasound models (including HMDR with other parameters) showed AUC 0.848-0.944 2, 4
  • HMDR alone demonstrates only "moderately satisfactory" diagnostic profiles 2
  • The 2026 elderly patient study 4 included HMDR in the final multivariable model but it was not the primary driver of predictive performance

6. Missing Comparative Effectiveness Research

Critical gaps include:

  • Head-to-head trials comparing HMDR versus newer parameters (PreE/E-VC, MCTD, ANS-VC)
  • Cost-effectiveness analysis of ultrasound screening protocols
  • Impact on clinical outcomes (failed intubation rates, complications, mortality) rather than just diagnostic accuracy
  • Validation in emergency settings versus elective surgery
  • Interobserver reliability studies across different operator experience levels

7. Lack of Clinical Implementation Studies

No evidence addresses:

  • How HMDR findings should change clinical management decisions
  • Whether HMDR screening reduces adverse airway events in real-world practice
  • Integration into existing airway assessment algorithms
  • Training requirements for reliable measurement

Clinical Implications of These Gaps

Given the 2025 evidence 1 showing HMDR fails to predict difficult intubation, future research should:

  • Abandon HMDR as a primary ultrasound screening tool and focus validation efforts on PreE/E-VC ratio, MCTD, and ANS-VC
  • Conduct multicenter prospective trials with standardized protocols comparing these superior parameters
  • Establish whether ultrasound assessment actually improves patient outcomes (morbidity/mortality) beyond diagnostic accuracy
  • Develop population-specific prediction models, as HMDR performance varies dramatically by body habitus

The most significant gap is the absence of outcome-based evidence: all studies focus on diagnostic accuracy for laryngoscopic view, but none demonstrate whether using HMDR (or any ultrasound parameter) actually reduces failed intubations, aspiration events, hypoxic complications, or mortality in surgical patients.

References

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