European Journal of Cardiovascular Medicine

During general anesthesia, endotracheal intubation is frequently used to secure the airway. Even highly skilled anesthesiologists may have difficulties during unexpectedly challenging intubations, which may result in unsuccessful intubations and raise morbidity and mortality rates.^1,2 The appropriate management of challenging airway problems is facilitated by routine preoperative airway examination. The sensitivity and specificity of clinical screening measures used often in airway assessment are insufficient to identify difficult laryngoscopy. Since direct laryngoscopy involves an invasive procedure, the Cormack-Lehane (CL) grading view cannot be used for routine preoperative airway examination.^3,4

A non-invasive diagnostic method for assessing airways before surgery is ultrasound, which can measure features that could indicate a challenging airway. Similar to a CT scan, ultrasound can measure nearly every aspect of airway anatomy.^5,6 Numerous recent studies have demonstrated a strong correlation between difficult laryngoscopy and numerous ultrasonic parameters, as well as the importance of these data in predicting difficult intubation.^7,8 Although their results varied, the anterior neck soft tissue thickness at the level of the vocal cord (ANS-VC) and the anterior neck soft tissue thickness at the level of the thyrohyoid membrane (ANS-TM) exhibited superior predictive values.^9,10

In this regard, our primary objective was to evaluate the capability of ANS-TM and ANS-VC in predicting difficult laryngoscopy in Indian populations. The secondary objective was to compare ANS-TM, ANS-VC, and clinical airway screening tests-body mass index (BMI), neck circumference (NC), modified Mallampati (MMP) grade, and thyromental distance (TMD)-to predict difficult laryngoscopy and intubation.

This observational study was conducted in the Anesthesia department from May 2024 to April 2025 after obtaining approval from the Institutional Ethics Committee. Written informed consent was obtained from all the patients preoperatively.

Inclusion criteria

Patients aged 18-70 years with an American Society of Anesthesiologists (ASA) physical status of I, II, or III requiring endotracheal intubation under general anesthesia for elective procedures were included in the study.

Exclusion criteria

Patients who needed rapid sequence induction and had a history of difficult intubation, obese patients with a body mass index (BMI) of more than 40, patients with notable swelling in the neck region (thyroid), pregnant patients, and patients with maxillofacial anomalies were excluded from the study.

Sample size

Based on the study conducted by Wu et al⁴, the sample size was calculated based on the correlation between anterior neck soft tissue thickness at the level of thyrohyoid membrane and vocal cord, with 80% power, an alpha error of 5%, assuming a population correlation coefficient of 0.5%, and the total sample size calculated as 200 patients.

Data collection

Clinical parameters and ultrasonography parameters were the two stages of the patient's airway evaluation. Age, sex, height, weight, and BMI were among the basic variables recorded. Clinical screening tests included NC, TMD, and MMP grading.

When evaluating MMP grading, the patient sat with their head in a neutral position, their mouth as wide open as they could, and their tongue sticking out as far as they could.
The visible buildings served as the basis for the grading. Class I: the anterior and posterior pillars, faucets, uvula, and soft palate were all visible. Class II: uvula, faucets, and soft palate were all discernible. Class III: just the base of the uvula and the soft palate were visible. Only the hard palate was seen in class IV.

The patient was ordered to lie flat in the supine position in order to measure the thyromental distance, which is the distance between the thyroid notch and the tip of the mentum using the neck's maximum extension. A measuring tape was used to measure the circumference of the neck at the level of the thyroid notch.

With the patient resting supine and the head in a neutral position, an ultrasound airway assessment was conducted using a General Electric-GE LOGIQ P7-sonography machine (GE Healthcare, USA) with a frequency of 10–13 MHz in the transverse plane. The thyrohyoid membrane level, which is halfway between the thyroid cartilage and hyoid bone, was where ANS-TM was measured.The distance from the skin surface to the middle axis of the highest part of the epiglottis through the thyrohyoid membrane was measured in centimeters.

ANS‑VC was measured from the skin to the anterior commissure of true vocal cords with the linear probe placed transversely in the submandibular region; it was calculated by averaging the depth measured in the central axis of the neck in centimeters and 15 mm to the left and right

The patient was then shifted to the operation theater; electrocardiogram, non-invasive blood pressure, pulse oximetry, and capnography monitors were connected. The patient was premedicated with intravenous injections of glycopyrrolate 0.005 mg/kg, midazolam 0.3 mg/kg, and fentanyl 2 mcg/kg, and then pre-oxygenated with 100% oxygen. The patient was induced with intravenous injections of propofol 2 mg/kg and atracurium 0.5 mg/kg and ventilated with oxygen and sevoflurane 2% for three minutes. A well-experienced anesthesiologist performed a direct laryngoscopy. The vocal cord was viewed without any external maneuver, and the anesthesiologist graded it as per CL grading. Intubation was graded easy (CL Grades 1 and 2) or difficult (CL Grades 3 and 4).

Cormack-Lehane's grading was based on visible structures. Grade 1: Entire vocal cords were visible. Grade 2: Partial view of vocal cords or arytenoids. Grade 3: Epiglottis alone was visible. Grade 4: Glottic structures were not visible at all.

Bilateral equal air admission and capnography were used to confirm the endotracheal tube location. The preoperative ultrasonography airway examination was unknown to the anesthesiologist who performed the patient's intubation. Nitrous oxide, 50:50 oxygen, 1%–2% sevoflurane, atracurium, and fentanyl were used to maintain anesthesia. The patient was reversed and extubated at the conclusion of the procedure after a sufficient amount of recuperation. The information gathered was noted and examined.

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Statistical analysis

Microsoft Excel (Microsoft Corp., New York, USA) was used to tabulate the data, and SPSS version 23 (IBM Corp., New York, USA) was used for analysis. For continuous data, descriptive statistics included mean±standard deviation, and for categorical variables, frequency and percentage. When comparing the results for categorical variables, the chi-square test was used to determine the link between easy and difficult laryngoscopy. Throughout the study, a P-value of less than 0.05 was deemed significant. The sensitivity and specificity of the results were predicted using the receiver operating characteristics curve (ROC curve). The AUC is a metric used to evaluate the test's validity. A perfect diagnostic test has an AUC of 1.

A total of 200 patients (118 males and 82 females) were included in this study. The basic demographic characteristics are shown in Table 1. Using the CL grades assigned during direct laryngoscopy, the patients were divided into 164 patients (82%) in the easy group and 36 patients (18%) in the difficult group. Among the demographic details compared, body weight and BMI values were higher in the difficult laryngoscopy group. The mean body weight was 63.4±10.66 kg for the easy laryngoscopy group and 69.63±9.14 kg for the difficult laryngoscopy group (P < 0.05). The mean BMI value was 23.8±3.4 kg/m² for the easy laryngoscopy group and 26.8±2.81 kg/m² for the difficult laryngoscopy group (P < 0.05).

Table 1. Clinical parameters for predicting difficult laryngoscopy.

NC: neck circumference, MMP: modified Mallampati grade, TMD: thyromental distance.

Clinical parameters such as NC, MMP, and TMD are shown in Table 1. The mean value of NC for the easy laryngoscopy group was 35.1±3.1 cm and 42.1±4.1 cm for the difficult laryngoscopy group (P < 0.001). A statistically significant difference was found between both groups for MMP grading (P < 0.001). The mean TMD for the easy laryngoscopy group was 6.61±1.04 cm, and 6.89±0.81 cm for the difficult laryngoscopy group (P = 0.49), which was not significant.

Table 2. Ultrasound parameters for predicting difficult laryngoscopy.

ANS-TM: anterior neck soft tissue thickness at the level of thyrohyoid membrane; ANS-VC: anterior neck soft tissue thickness at the level of vocal cord.

The ultrasound parameters ANS-TM and ANS-VC are recorded in Table 2. The mean values of ANS-TM were 2.65±0.38 cm and 1.73±0.45 cm for easy and difficult laryngoscopy groups. The difference was significant, with a P value of <0.001. The mean value of ANS-VC was 1.05±0.29 cm for the easy laryngoscopy group and 1.53±0.39 cm for the difficult laryngoscopy group showing a significant P value of <0.001.

We ascertained that MMP grade III is associated with difficult intubation. The optimal cut-off value for NC is 37.1 cm and for TMD is 6.65 cm for the prediction of difficult laryngoscopy. Based on the analysis of the ROC curve, the AUC for ANS-TM, ANS-VC, MMP, and NC was above 0.7 with a significant P value of 0.001, but the AUC for TMD was 0.52 and was not significant with a P value of 0.48.

While securing an airway, anesthesiologists continue to face difficulties with unexpectedly difficult airways. Although none of the risk indicators are 100% sensitive and specific, they do help predict a difficult laryngoscopy.^11,12These risk factors include a higher body mass index, facial deformities, obstructive sleep apnea, limited cervical spine movement, restricted mouth opening, MMP classes 3 or 4, a short TMD, and increased NC. One new method for anticipating troublesome airways is ultrasound airway assessment.^13,14 Any increase in soft tissue thickness at the thyrohyoid membrane, where the tissues are lifted by the laryngoscope blade tip, raises CL grading. An indirect method of determining tongue thickness is the MMP test. When the base of the tongue is expanded, laryngoscopy becomes challenging.^15,16

The thickness of the soft tissue in the anterior neck is mostly reflected in the ultrasonic measurements. Ultrasound and clinical indicators work together to improve the sensitivity and specificity of predicting unexpected difficult laryngoscopy.^17,18

In this study, we examined how clinical characteristics (BMI, MMP, NC, and TMD) and ultrasound evaluation of airway parameters (ANS-TM, ANS-VC) may predict easy and difficult laryngoscopy in the Indian population. In order to identify difficult laryngoscopy, we discovered that the AUC for the ultrasound airway evaluations was higher than that for the clinical airway assessment instruments. The anterior neck soft tissue thickness at the level of the vocal cord had an AUC of 84% with a sensitivity of 80% and a specificity of 88%, while the thickness at the level of the thyrohyoid membrane had the highest AUC of 91% with a sensitivity of 97% and a specificity of 79% among the ultrasound parameters.

Among the clinical parameters, MMP had the highest area of 81% under the ROC curve compared to other clinical parameters but lesser than that for ultrasound parameters suggesting that ultrasound airway assessments were better than the clinical screening tests in predicting difficult laryngoscopy.

We measured ANS-TM as part of our study; the values were 2.65 ± 0.38 cm and 1.73 ± 0.45 cm for easy and difficult laryngoscopy groups, respectively, with an optimal cut-off at 2.03 cm; the values above this were associated with difficult intubation and correlated significantly. In a pilot study, Adhikari et al⁷ examined the relationship between clinical screening tests (MMP, TMD, and inter-incisor gap) and ultrasound measurements (thickness of tongue and anterior neck soft tissue thickness at the levels of hyoid bone and thyrohyoid membrane). They came to the conclusion that a good independent predictor of challenging laryngoscopy was ultrasound measurement at the thyrohyoid membrane level, with a cut-off of 2.8 cm. The outcomes were similar to those of our investigation.

Saranya et al⁸ discovered that increased intubation difficulty was associated with thicker soft tissue at all three levels: the suprasternal notch, the thyrohyoid membrane, and the hyoid bone.The thickness of anterior soft tissue from the skin to the epiglottis at thyrohyoid membrane level was 1.74 ± 0.26 cm for the easy laryngoscopy group and 2.38 ± 0.32 cm for the difficult laryngoscopy group, which is statistically significant (P < 0.001).

A pre-anesthetic ultrasonography airway examination was carried out by Bhagavan et al.⁹ They came to the conclusion that there was a positive correlation between difficult laryngoscopy and the thickness of anterior neck soft tissue in the transverse view at the level of the thyrohyoid membrane, with a cut-off at 2.03 cm, and the distance from skin to epiglottis midway (DSEM). We measured the patient's ANS-VC thickness as part of our investigation. A cut-off point of more than 1.23 was linked to difficult intubation, and measurements were higher in the difficult laryngoscopy group (1.53 ± 0.39 cm) than in the easy group (1.05 ± 0.29 cm).

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Ezri et al¹⁰ found in their study that increased neck tissue thickness at the vocal cord level was associated with difficult intubation, with a measurement of 2.7 cm for difficult and 1.8 cm for easy laryngoscopy; the difference was statistically significant (P < 0.001). Wu et al⁴ concluded that anterior neck thickness at the level of the anterior commissure was higher in the difficult laryngoscopy group, and they were significantly correlated. The results were similar to our study. As in our study, Yadav et al⁶ demonstrated a significant association between ANS-VC and difficult laryngoscopy.

In contrast to our study, Kesarwani et al¹¹ conducted a study on 50 obese patients. They concluded that anterior neck soft tissue thickness at the level of the vocal cords was not significantly correlated with difficult intubation. Our study demonstrated that MMP grading of 3 and above was associated with difficult intubation with a P value of <0.001, with an AUC being 81%, which was lower when compared with ultrasound predictors. As MMP grading depends on the patient’s posture and cooperation, many studies have noted varying results.¹² A meta-analysis of 1,77,088 patients demonstrated that the modified Mallampati score (MMS) is inadequate as a stand-alone test for predicting difficult laryngoscopy. But it may be a combined clinical and ultrasound parameter to predict difficult tracheal intubation.¹³ This conclusion was similar to our study.

Abdhelhady et al¹⁴ conducted a study similar to ours on 80 patients. They concluded that including ultrasound parameters such as MMP and TMD in routine clinical assessments helps to discriminate easy and difficult intubation compared to clinical parameters. In our study, NC of 37 cm and above were associated with difficult intubation with a P value of <0.001 and an AUC of 76%. Similar results were observed in the study conducted by Adhikari et al⁷, where mean NC of 50 ± 3.8 cm and 43.5 ± 2.2 cm were associated with difficult and easy laryngoscopy, respectively, with a P value of <0.001.

In our study, the mean TMDs for easy and difficult laryngoscopy were 6.61 ± 0.93 cm and 6.89 ± 0.81 cm, respectively, with the optimal cut-off at 6.65 cm. The AUC for TMD was 51%, with a P value of 0.48. It demonstrates unequivocally that there is no meaningful correlation between TMD and challenging laryngoscopy. According to Tripathi and Pandey, an adult's TMD should be ≥6.5 cm.¹⁵ Similar to our study, Reddy et al¹⁶ found that TMD did not aid in predicting difficult intubation. The MMP score and the TMD measurement, two widely used clinical screening tools, have mediocre accuracy and regularly fail to distinguish between patients with difficult and easy airways, according to systematic reviews.^17,18

Our study has certain limitations. The findings cannot be extrapolated to other populations because this study was limited to 200 Indian patients. Ultrasound measures and CL grading may be impacted by the anesthesiologists' training and expertise. The amount of pressure used during the ultrasound procedure has an impact on the values recorded; this must be considered. Obese patients and parturients were not included in our study because they are more likely to experience challenging laryngoscopy, which requires further research.

We conclude that ultrasound-measured ANS-TM and ANS-VC help predict difficult airways better than clinical parameters. Anterior neck soft tissue thickness measured at the level of the thyrohyoid membrane was an independent predictor of difficult laryngoscopy supported by the highest AUC (92%) and sensitivity (98%). The commonly used clinical screening tests cannot be used as stand-alone tests to predict difficult airways and should be combined with ultrasound measurements to aid in the better prediction of difficult laryngoscopy.

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