# Comparison of Neck Circumference at the Hyoid and Thyroid Levels, Neck Circumference to Thyromental Distance Ratio, and Hyomental Distance Ratio in Predicting Difficult Laryngeal Visualization in Obese and Non-Obese Populations

## TL;DR

This study compares different neck measurements to predict difficult laryngeal visualization in obese and non-obese patients, finding that hyomental distance ratio is a reliable predictor regardless of BMI.

## Contribution

The study identifies hyomental distance ratio as a reliable predictor of difficult laryngeal visualization across both obese and non-obese populations.

## Key findings

- Hyomental distance ratio (HMDR) was a statistically significant predictor of difficult laryngeal visualization in both obese and non-obese groups.
- Neck circumference at the thyroid level to thyromental distance ratio (NCt/TMD) predicted difficult laryngeal visualization only in the obese group.
- Modified Mallampati grade (MMG) was significant only in the obese group for predicting difficult laryngeal visualization.

## Abstract

Introduction: Several preoperative predictive factors for difficult visualization of the larynx (DVL) have been identified, and these factors have significant predictive value in the obese population. This prospective observational study was conducted to compare the predictive abilities of different airway predictors between obese and non-obese populations.

Material and methods: Our study included a total of 90 patients, divided into two groups based on body mass index (BMI): group O (obese) with BMI >30 kg/m² and group NO (non-obese) with BMI <30 kg/m². The patients were evaluated using the modified Mallampati grade (MMG), neck circumference at the level of the hyoid (NCh) and thyroid (NCt), thyromental distance (TMD), and hyomental distance (HMD) in both neutral and extended head positions in both groups. The modified Cormack and Lehane (MCL) grading for visualization of the larynx was observed during direct laryngoscopy prior to intubation. MCL grades 2b, 3, and 4 were considered indicative of DVL in this study.

Results: DVL was observed in 41.1% of the population, 46.7% of the obese (O) group, and 35.6% of the non-obese (NO) group. MMG was statistically significant only in the O group for predicting the DVL with a specificity of 85.1%. In the NO group, MMG ≥3 was not significant statistically and showed a specificity of 70.8%. NCh and NCt had no statistically significant difference in differentiating the DVL between obese and non-obese groups. NCt/TMD in the O group with a cut-off value of 5.135 had a sensitivity of 61.9% and a specificity of 58.3% and predicted DVL only in the obese group. Hyomental distance ratio (HMDR) was a statistically significant index in both groups and has a good predictive utility for the DVL. An HMDR ≤1.23 had a sensitivity of 68.8% and a specificity of 72.4% in the non-obese population. An HMDR ≤1.20 in the obese group had a sensitivity of 85.7% and a specificity of 91.7%.

Conclusion: The higher incidence of DVL (MCL grades 2b, 3, and 4) in the obese population when compared to the non-obese population was not statistically significant. NCt and NCh had minimal value as predictors of DVL. This study concludes that HMDR was a reliable airway assessment tool irrespective of BMI, while MMG and NCt/TMD were reliable difficult airway predictors but only in the obese population.