Evaluation of Airway Assessment Parameters: Hyomental Distance versus Ultrasound-guided Pretracheal Tissue Thickness as Predictors of Difficult Laryngoscopy

Abstract

Background:

Difficult airway remains a major cause of anesthesia-related morbidity and mortality despite advances in airway management. Conventional clinical predictors have limited predictive value. Airway ultrasound has emerged as a noninvasive tool that may improve preoperative airway assessment.

Aim:

The study aimed to evaluate the utility of ultrasonography as a preoperative assessment tool for predicting difficult airway and to compare its predictive accuracy with hyomental distance.

Materials and Methods:

This prospective observational study included 100 adult patients undergoing elective surgery under general anesthesia requiring endotracheal intubation. Preoperative airway assessment involved measurement of hyomental distance and ultrasonographic measurement of pretracheal soft-tissue thickness at the level of the vocal cords. After induction of anesthesia, direct laryngoscopy was performed and graded using the Cormack–Lehane classification.

Results:

The mean hyomental distance was significantly lower in patients with difficult laryngoscopy. Pretracheal soft-tissue thickness measured by ultrasound was significantly higher in this group (P = 0.0001) and showed a stronger association with poor glottic visualization.

Conclusion:

Ultrasonographic measurement of pretracheal soft-tissue thickness is a reliable and superior predictor of difficult laryngoscopy and may enhance routine preoperative airway assessment and patient safety.

INTRODUCTION

Airway management is a key duty of anesthesiologists. It is crucial for providing adequate oxygen and ventilation during general anesthesia. If the airway is not secured properly, it can lead to serious problems such as low oxygen levels, brain injury, or even death. Difficult airway management is still a significant cause of complications and deaths related to anesthesia around the world, despite improvements in airway devices and techniques. Studies suggest that difficult laryngoscopy happens in about 1.5% to 8.5% of cases, while failed intubation occurs in roughly 0.13% to 0.3% of cases.[1]

The airway includes structurally complex parts that stretch from the nasal and oral cavities to the trachea and lungs. It comprises various cartilaginous, muscular, and soft-tissue elements. Proper alignment of the oral, pharyngeal, and laryngeal axes is vital for good visibility during laryngoscopy. Differences in these structures, such as increased soft-tissue thickness, limited neck movement, or changed anatomical relationships, can make laryngoscopy challenging.[2,3]

Accurate prediction of difficult laryngoscopy is essential in preoperative airway assessment. Conventional clinical predictors, such as the modified Mallampati classification, thyromental distance, sternomental distance, and hyomental distance, are commonly used. However, several studies have shown that these clinical predictors have limited sensitivity and specificity when used alone. Hyomental distance measures the distance between the hyoid bone and the mentum, showing the flexibility of the submandibular space and mandibular movement. However, its measurement can vary based on patient positioning, anatomical differences, and operator technique.

Recently, ultrasonography has become a useful tool for airway evaluation. Ultrasound offers real-time, noninvasive visualization of airway structures, including the hyoid bone, thyroid cartilage, vocal cords, and pretracheal soft tissues. It allows direct measurement of airway sizes and tissue thickness, which conventional clinical assessments cannot provide. Multiple studies have shown the effectiveness of ultrasound-guided airway assessment. Pretracheal soft-tissue thickness measured at the level of the vocal cords is one of the most promising ultrasound parameters.[,5,6,7,8,9,10]

This study was conducted to evaluate and compare hyomental distance and ultrasound-guided pretracheal soft-tissue thickness as predictors of difficult laryngoscopy.

MATERIALS AND METHODS

Study design and ethical approval

This prospective comparative study was conducted in the Department of Anaesthesiology at Vinayaka Mission’s Medical College and Hospital, Karaikal, over 2 years after obtaining approval from the Institutional Ethics Committee (IEC approval number: VMMC/ANESTH/2019/). The study adhered to the ethical principles outlined in the Declaration of Helsinki. Written informed consent was obtained from all participants before enrollment.

Study population

A total of 100 adult patients (50 in each group), aged 18–60 years, belonging to American Society of Anesthesiologists (ASA) Physical Status I–III, scheduled for elective surgeries under general anesthesia requiring intubation, were included. Patients with hemodynamic instability, pediatric age group, known airway abnormalities, neck masses, requiring emergency surgeries, known allergy to ultrasonography (USG) gel, and those refusing to participate were excluded.

Airway assessment

Preoperative clinical airway assessment was performed by an experienced anesthesiologist. Hyomental distance was measured from the upper border of the hyoid bone to the lower border of the mentum using a rigid measuring scale with the patient supine, head in neutral position, and mouth closed. Ultrasound assessment was performed using a high-frequency linear ultrasound probe (6–13 MHz) positioned transversely at the level of the vocal cords. Pretracheal soft-tissue thickness was measured as the distance from the skin surface to the anterior tracheal wall. All measurements were recorded before induction of anesthesia.

Laryngoscopic assessment

Following induction of general anesthesia and adequate neuromuscular blockade, direct laryngoscopy was performed using a Macintosh laryngoscope blade by an experienced anesthesiologist blinded to the preoperative ultrasound findings. The laryngoscopic view was graded according to the Cormack–Lehane classification. Grade I and II were considered easy laryngoscopy, while Grade III and IV were considered difficult laryngoscopy.

Statistical analysis

Statistical analysis was performed using SPSS version 23 (IBM Corp., Armonk, NY, USA). Continuous variables were expressed as mean ± standard deviation. Categorical variables were displayed as frequency and percentage. Comparison of continuous variables between groups was performed using the unpaired Student’s t-test for normally distributed data. Categorical variables were analyzed using the Chi-square test or Fisher’s exact test as appropriate. P <0.05 was considered statistically significant.

RESULTS

The age distribution between Group A (hyomental distance) and Group B (ultrasound assessment) was comparable, with no statistically significant difference (P = 0.844). The majority of patients belonged to the 36–45 years of age group (29%). There was no statistically significant difference in gender distribution between the groups (P = 0.316). Females constituted 53% and males 47% of the study population [Table 1].

Table 1.

Age distribution

Age group (years)	Group A (n=50), n (%)	Group B (n=50), n (%)	Total (n=100), n (%)
≤25	11 (22.0)	10 (20.0)	21 (21.0)
26–35	12 (24.0)	14 (28.0)	26 (26.0)
36–45	13 (26.0)	16 (32.0)	29 (29.0)
46–55	11 (22.0)	7 (14.0)	18 (18.0)
>55	3 (6.0)	3 (6.0)	6 (6.0)

χ²=1.401, P=0.844 (P>0.05)

Most patients (56%) had hyomental distance >6 cm, indicating a predicted easy airway. However, 44% had reduced hyomental distance (<6 cm), which is associated with increased airway difficulty. The majority of patients had pretracheal tissue thickness below 11.4 mm, which was linked to easy laryngoscopy. Increased tissue thickness >11.5 mm was observed in 20% of patients and was connected to difficult airway.

There was a statistically significant difference in difficulty prediction between the groups (P = 0.029). Ultrasound assessment (Group B) demonstrated improved predictive accuracy compared to hyomental distance assessment (Group A). The association between hyomental distance and difficult laryngoscopy was not statistically significant (P = 0.308), suggesting limited predictive accuracy.

There was a statistically highly significant association between pretracheal tissue thickness and Cormack–Lehane grading (F = 22.765, P = 0.0001) [Table 2]. Patients with easy laryngoscopy (Grade I) had significantly lower mean pretracheal tissue thickness (10.24 ± 0.56 mm), whereas patients with difficult laryngoscopy (Grade III) demonstrated markedly higher thickness (12.80 ± 0.42 mm). This finding indicates that increasing pretracheal tissue thickness is strongly associated with worsening laryngoscopic view [Figure 1].

Table 2.

Association between mean pretracheal tissue thickness and Cormack–Lehane Grade

CL grade	Number of patients (n)	Mean pretracheal thickness (mm)±SD	F	P
Grade I	37	10.24±0.56	22.765	0.0001
Grade IIa	7	12.08±0.72
Grade IIb	4	11.05±1.39
Grade III	2	12.80±0.42
Total	50	-

CL=Cormack–Lehane, SD=Standard deviation

Figure 1.

Box plot comparison of pretracheal soft-tissue thickness (mm) across Cormack–Lehane grades. Grade I shows significantly lower thickness (10.24 ± 0.56 mm) compared to Grade III (12.80 ± 0.42 mm), P = 0.0001. The symbol * indicates a highly statistically significant difference corresponding to P < 0.001. As the observed value was P = 0.0001, the use of * is appropriate

Receiver operating characteristic (ROC) curve analysis demonstrated that ultrasound-measured pretracheal tissue thickness had superior diagnostic performance compared to hyomental distance in predicting difficult laryngoscopy [Figure 2]. The scatter plot analysis revealed a strong positive correlation between pretracheal tissue thickness and Cormack–Lehane grade (r = 0.72, P < 0.0001), confirming that increased soft-tissue thickness is associated with worsening laryngoscopic view [Figure 3].

Figure 2.

Receiver operating characteristic curve comparing diagnostic performance of ultrasound-measured pretracheal tissue thickness versus clinical hyomental distance for predicting difficult laryngoscopy. Ultrasound demonstrates superior predictive accuracy with a higher area under the curve. AUC = Area under the curve, ROC = Receiver operating characteristic

Figure 3.

Scatter plot showing correlation between pretracheal soft-tissue thickness and Cormack–Lehane grade. The plot demonstrates a positive correlation with increasing tissue thickness associated with worsening laryngoscopic view (r = 0.72, P < 0.0001)

DISCUSSION

Prediction of difficult airway remains one of the most critical components of preoperative anesthetic evaluation. Failure to anticipate difficult laryngoscopy may lead to severe complications, including hypoxia, airway trauma, aspiration, and increased perioperative morbidity and mortality.[11,12]

In this study, we evaluated and compared hyomental distance and ultrasound-guided pretracheal soft-tissue thickness as predictors of difficult laryngoscopy. Our findings showed that ultrasound-guided measurement of pretracheal tissue thickness had better predictive accuracy and a statistically significant association with difficult laryngoscopy compared to hyomental distance. In our study, hyomental distance showed some connection to difficult laryngoscopy; however, this connection was not statistically significant (P = 0.308).[13,14]

Ultrasound is a valuable tool for airway assessment. It can visualize airway structures and offer objective measurements. In our study, pretracheal tissue thickness measured using ultrasound showed a statistically significant association with difficult laryngoscopy (P = 0.0001). This finding is consistent with multiple previous studies demonstrating the predictive value of anterior neck soft-tissue thickness.[,16,17,18]

When pretracheal tissue thickness increases, it adds distance between the skin surface and airway structures, making it harder to visualize the glottis during laryngoscopy. Excess soft tissue may also restrict the displacement of airway structures and impair the alignment of airway axes. Our study showed that ultrasound assessment predicted difficult airways more accurately than hyomental distance, as evidenced by the superior ROC curve performance [Figure 2]. These results align with systematic reviews and meta-analyses examining ultrasound in airway assessment.[19]

Ultrasound has several benefits that make it particularly suitable for airway assessment. It is noninvasive, portable, radiation-free, and provides real-time imaging. Moreover, ultrasound provides quantitative measurements that are less influenced by subjective views, improving the reliability and accuracy of airway assessments. Ultrasound is especially useful for patients with obesity, short necks, or complicated anatomical structures, where clinical assessments might be unclear.[20]

The clinical implications of our study are important. Adding ultrasound to regular airway assessments can improve the prediction of difficult airways and increase patient safety. Identifying difficult airways early allows anesthesiologists to prepare appropriate management strategies, helping reduce the risk of airway-related complications and improving patient outcomes.

Limitations

This study has several limitations. First, it took place at a single center with a relatively small sample size of 100 patients. Second, ultrasound measurements depend on the operator and can vary based on the examiner experience. Third, the study only assessed pretracheal soft-tissue thickness and hyomental distance, not other potentially useful ultrasound parameters such as hyomental distance ratio, tongue thickness, and preepiglottic space. Fourth, interobserver variability was not examined. Finally, the study included only elective surgical patients with ASA Physical Status I–III, which may limit the findings’ applicability to high-risk patients or emergency situations.

CONCLUSION

Preoperative prediction of a difficult airway is crucial for safe airway management and reducing anesthesia-related complications. This study demonstrates that ultrasonographic measurement of pretracheal soft-tissue thickness is a reliable and superior predictor of difficult laryngoscopy compared to hyomental distance. Ultrasound offers direct visualization and precise measurement of airway structures, addressing the limitations of subjective clinical predictions. Its noninvasive, portable, and repeatable features make it a valuable tool in routine preoperative airway evaluation. Adding airway ultrasound to standard clinical practice may help identify difficult airways earlier and improve preparation and airway management strategies, enhancing patient safety and outcomes.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.