Comparison of Airway Ultrasound Indices and Clinical Assessment for the Prediction of Difficult Laryngoscopy in Elective Surgical Patients: A Prospective Observational Study.

BACKGROUND: Ultrasound is evolving as a probable tool in airway assessment. The upper airway is a superficial structure and has sonographically identifiable structures which makes it ideal for evaluation with the ultrasound. AIMS: The aim of this study was to evaluate the role of skin to hyoid and skin to thyrohyoid membrane distance in prediction of difficult laryngoscopy. SETTINGS AND
DESIGN: This is a prospective observational study included 150 patients aged 18-60 years of American Society of Anesthesiologists Physical Status I and II scheduled to undergo surgery under general anesthesia requiring laryngoscopy and endotracheal intubation.
MATERIALS AND METHODS: The modified Mallampati score, mouth opening, mentohyoid distance, thyromental distance were noted. Skin to hyoid bone distance and skin to thyrohyoid membrane distance were measured by ultrasound. Patients were clubbed retrospectively into easy and difficult laryngoscopy groups on the basis of Cormack Lehane grading, and the characteristics of both groups were compared. STATISTICAL ANALYSIS: Statistical Package for the Social Sciences, Version 23 was used for statistical analysis. Independent samples t-test was used to compare the means between difficult and easy laryngoscopy patients. Diagnostic accuracy of the significant (P < 0.05) variables between difficult and easy laryngoscopy patients was calculated using receiver operating characteristics curve in terms of their area under curve. Appropriate cutoff values (with corresponding sensitivity, specificity, and overall accuracy) were also identified.
RESULTS: Out of 150 patients, 13 (8.7%) were identified as difficult laryngoscopy whereas 137 patients (91.3%) were identified as easy laryngoscopy. The demographics of both groups were comparable. Mentohyoid distance, skin to hyoid bone distance, and skin to thyrohyoid distance were statistically different between easy and difficult laryngoscopy patients, with lower mentohyoid distance and higher skin to hyoid bone distance and skin to thyrohyoid distance in difficult laryngoscopy patients. Diagnostic accuracy of the mentohyoid distance (70.3%) was slightly superior to skin to hyoid bone distance (67.1%) and skin to thyrohyoid distance (68.1%).
CONCLUSION: Ultrasound measurements of skin to hyoid bone and skin to thyrohyoid membrane distance fail to eclipse clinical parameters in accurately predicting a difficult laryngoscopy. Copyright:

INTRODUCTION

Almost every clinician has faced an unanticipated difficult laryngoscopy despite the clinical airway examination indicating otherwise. The incidence of difficult laryngoscopy varies from 1.5% to 13% in the general population.[123] This leads to a delay in securing the airway, multiple laryngoscopic attempts, airway trauma, and the need to mobilize expensive equipment on an emergent basis. This process is time-consuming, interferes with the smooth flow of patient care and suboptimal utilization of expensive airway equipment, and contributes to increased morbidity among patients as well as physician stress.

Routine clinical parameters for the assessment of the airway such as Mallampati grade and thyromental distance are of value in predicting difficult airway; however, studies have shown that they lack sufficient negative predictive value in excluding difficult airway.[345] There are various proposed models and scores which try to combine these clinical tests and anatomical characteristics, and these have illustrated greater value in predicting difficult airway.[67] In view of the rapid turnover of patients, it would be of great value to identify a single rapid bedside test that could conclusively help us identify the presence of a difficult airway with precision, ensuring that appropriate planning for a difficult airway is ensured.

The ultrasound is a tool that has gained increasing popularity in recent years. Earlier Ultrasound was an exclusive tool for the use of radiologists but now it is commonly used in the emergency department, the intensive care unit and the operating theatre. The upper airway is a superficial structure formed of a predictable arrangement of sonographically identifiable structures, and this makes it ideal for evaluation with the ultrasound for features which might help identify a difficult airway.

Many studies have been done in various group of patients evaluating various ultrasonically measured upper airway anatomical parameter's ability to predict difficult laryngoscopy. According to systematic review done by Fulkerson et al., skin to hyoid and skin to thyrohyoid membrane distance were the best predictors of difficult laryngoscopy among all ultrasonically measured parameters.[8]

Hence, we hypothesized that bedside measurement of both these parameters as a point-of-care ultrasound examination will be useful in predicting unanticipated difficult laryngoscopy in different clinical settings, and they can be more effective than commonly used clinical airway examination parameters.

MATERIALS AND METHODS

The study was conducted in accordance with the Revised Helsinki Declaration (2013). It is a prospective observational study, conducted after obtaining institutional ethics committee approval (IEC 2018-194-MD-EXP-4; dated January 17, 2019) and registering the trial in ctri.nic.in (CTRI/REF/2019/04/025169; dated May 7, 2019). Patients aged 18–60 years of American Society of Anesthesiologists Physical Status I and II scheduled to undergo surgery under general anesthesia requiring laryngoscopy and endotracheal intubation were evaluated to be included in the trial. Patients with any abnormalities or illness preventing the use of clinical airway screening tests, tracheostomy or with any obvious sign of difficult airway like maxillofacial anomalies, restricted neck movements, obesity (body mass index >35 kg/m2), and limited mouth opening (inter-incisor distance <3 cm) were excluded. After obtaining written informed consent from 150 evaluated patients who met inclusion criteria, all patients then underwent a detailed preoperative airway evaluation on the day before surgery. The modified Mallampati class, mouth opening, mentohyoid distance, and thyromental distance were noted and recorded for all patients. All patients also underwent a detailed prospective sonographic assessment by the same anesthesiologist who was experienced in airway ultrasound, and skin to hyoid and skin to thyrohyoid membrane distance were recorded. For sonographic assessment of the airway, the patient was made to lie in the supine position with head in the extended position without a pillow, with the mouth closed and the tongue on the floor of the mouth without any movement [Figures 1 and 2]. The curvilinear low-frequency probe (frequency 2–5 MHz) of an ultrasound machine (GE, USA) was used. Hyoid bone and thyrohyoid membrane were identified. Skin to hyoid bone distance was measured and skin to thyrohyoid membrane distance was measured at the midpoint of thyrohyoid membrane.

Figure 1

Position of the patient and probe

Figure 2

Sagittal ultrasound of the upper neck demonstrating A – skin to hyoid bone membrane distance and B – skin to thyrohyoid membrane distance

In this study, laryngoscopy was defined as easy if Cormack Lehane (CL) grading on laryngoscopy was 1 or 2 while it was defined as difficult if CL grading was 3 or 4. All laryngoscopies were performed by a single experienced anesthesiologist who was not privy to preoperative ultrasound airway examination.

The next day, the patient was shifted to the operating room, baseline monitors of electrocardiogram, noninvasive blood pressure, and pulse oximetry were connected, and values were noted. After preoxygenation for 3 min, intravenous midazolam 1 mg and fentanyl 2 ug.kg−1 were administered. Anesthesia was induced with injection propofol 2 mg.kg−1. After muscle relaxation with injection (Inj.) vecuronium 0.1 mg.kg−1 or Inj. atracurium 0.5 mg.kg−1 and ventilation with oxygen and sevoflurane 2% for 3 min, direct laryngoscopy was performed by an experienced anesthesiologist using a curved Macintosh blade of appropriate size, and the CL grading of laryngoscopic view was noted. Patients were then intubated with an appropriate size endotracheal tube and surgery was allowed to proceed. At the end of the surgery, the patient was extubated following reversal of neuromuscular blockade or shifted intubated to the postoperative ICU for surgical or anesthetic reasons.

The patients were clubbed retrospectively into easy and difficult laryngoscopy groups on the basis of CL grading, and the characteristics of both groups were compared.

Statistical analysis

Normality of the continuous variables was assessed. As data were normally distributed, continuous variables are presented in mean ± standard deviation, while categorical variables are presented in frequency and percentage. Independent samples t-test was used to compare the means between difficult and easy laryngoscopy patients. Diagnostic accuracy (to detect difficult laryngoscopy) of the significant (P < 0.05) variables between difficult and easy laryngoscopy patients was calculated using receiver operating characteristics (ROC) curve in terms of their area under curve (AUC). Appropriate cutoff values (with corresponding sensitivity, specificity, and overall accuracy) were also identified and corresponding cutoff values were selected such a way so that both measures (sensitivity and specificity) should be >50%. Error bar graph and ROC curve were also used to present the data. A P < 0.05 was considered as statistically significant. Statistical Package for the Social Sciences, Version 23 (SPSS 23, IBM, Chicago, Illinois, USA) and Med Calc software were used for calculation.

Sample size estimation

Assuming a 10% incidence of difficult laryngoscopy at two-sided minimum 95% confidence interval and 5% margin of error in assumed incidence, the estimated sample size came out to be 140.[9] Finally, in this study, we included 150 patients. Sample size was estimated using Software Power Analysis and Sample Size Version 8 (PASS-2008).

RESULTS

In our study, 150 patients were assessed with ultrasonography for markers of difficult intubation and underwent general anesthesia. The mean and median age were 42.84 years and 45 years, respectively (range: 18–78 years) with the majority being female (n = 87, 58%) [Table 1]. Out of 150 patients, 13 (8.7%) were identified as difficult laryngoscopy, whereas 137 patients (91.3%) were identified as easy laryngoscopy. Demographic and radiological measurements were compared in the patients with easy and difficult laryngoscopy [Table 1]. The demographics of both groups were comparable.

Table 1

Distribution of demographic and radiological measurements between easy and difficult laryngoscopy patients (n=150)

Variable’s	Mean±SD		P
	Easy (n=137, 91.3%)	Difficult (n=13, 8.7%)
Age (years)	42.84±14.18	47.00±12.75	0.310
Weight (kg)	62.04±10.68	56.23±7.50	0.057
BMI	25.88±3.89	25.48±4.04	0.522
Mentohyoid distance (cm)	5.85±0.68	5.19±1.01	0.038
Thyromental distance (cm)	6.80±0.63	6.62±0.82	0.326
Skin to hyoid bone distance (cm)	0.70±0.18	0.82±0.23	0.022
Skin to thyrohyoid distance (cm)	0.80±0.23	1.01±0.31	0.030

Independent samples t-test used, P<0.05 significant. SD=Standard deviation, BMI=Body mass index

Mentohyoid distance, skin to hyoid bone distance, and skin to thyrohyoid distance were statistically different between easy and difficult laryngoscopy patients, with lower mentohyoid distance and higher skin to hyoid bone distance and skin to thyrohyoid distance in difficult laryngoscopy patients as compared to easy laryngoscopy patients [Table 1]. Nonparametric Mann–Whitney U-test was used to assess for any significant difference in the MPG grading between the easy and the difficult intubation group. The difference between these two groups was insignificant.

Using the above inputs, diagnostic accuracy of the above three significant variables was calculated in terms of their AUC by using ROC curve. Results showed that diagnostic accuracy of the mentohyoid distance, skin to hyoid bone distance, and skin to thyrohyoid distance was 70.3% [Figure 3 and Table 2], 67.1%, and 68.1% (each P < 0.05) [Figure 4 and Table 2]. Cutoff values for these variables were calculated individually, and selection of the cutoff value was done with a strategy to achieve at least >50% sensitivity and specificity each at the chosen corresponding cutoff value.

Figure 3

Receiver operating characteristics curve showing diagnostic accuracy of the mentohyoid distance to detect difficult laryngoscopy

Table 2

Diagnostic accuracy of the radiological measurements to detect difficult laryngoscopy (n=150)

Variable’s	AUC			P
	Value (%)	95% CI
		Lower	Upper
Mentohyoid distance*	70.3	53	87.6	0.016
Skin to hyoid bone distance†	67.1	50.6	83.7	0.042
Skin to thyrohyoid distance†	68.1	51.8	84.3	0.032

*Smaller value indicates difficult laryngoscopy, †Larger value indicates difficult laryngoscopy, P<0.05 significant. CI=Confidence interval, AUC=Area under curve computed by receiver operating characteristics

Figure 4

Receiver operating characteristics curve showing diagnostic accuracy of the skin to hyoid bone distance and skin to thyrohyoid distance to detect difficult laryngoscopy

For mentohyoid distance, when the cutoff value is ≤5.65 cm, sensitivity and specificity to detect difficult laryngoscopy were 76.9% and 62%, respectively. Other cutoff values for this variable could not be identified as no other cutoff value had both measures at least >50%.

For skin to hyoid bone distance, when the cutoff value is ≥0.705 cm, sensitivity and specificity to detect difficult laryngoscopy are 76.9% and 51.8%, respectively. Similarly, at cutoff values 0.795 cm and 0.875 cm, the corresponding sensitivity/specificity was 61.5%/72.3% and 53.8%/83.2%, respectively.

For skin to thyrohyoid distance, when the cutoff value is ≥0.846 cm, sensitivity and specificity to detect difficult laryngoscopy are 61.5% and 56.2%, respectively. Similarly, at cutoff values of 0.945 cm, corresponding sensitivity and specificity were 53.8% and 77.4%, respectively [Table 3].

Table 3

Diagnostic accuracy of the radiological measurements to detect difficult laryngoscopy (n=150)

Variable’s Cut off value	Sensitivity (%)	Specificity (%)	Overall accuracy (%)
Mentohyoid distance (cm)*
5.65	76.9	62	63.29
Skin to hyoid bone distance (cm)†
0.7050	76.9	51.8	53.99
0.7950	61.5	72.3	71.35
0.8750	53.8	83.2	80.64
Skin to thyrohyoid distance (cm)†
0.8455	61.5	56.2	56.65
0.9450	53.8	77.4	75.35

*Smaller value indicates difficult laryngoscopy, †Larger value indicates difficult laryngoscopy, P<0.05 significant. CI=Confidence Interval, AUC=Area under curve computed by receiver operating characteristics

DISCUSSION

In our study, we assessed certain variables which correlated with the thickness of the skin on the anterior aspect of the neck – namely, the skin to thyrohyoid membrane distance and the skin to hyoid distance. These were measured with sonography and compared to certain standard assessments of the preoperative airway – namely – the Mallampati grade, the thyromental distance, and the mentohyoid distance. Intubation was performed and the CL grading was noted – allowing us to divide patients into easy and difficult laryngoscopy groups as detailed above.

During preliminary evaluations before the study, we found that when a low-frequency curvilinear probe is placed sagittally in midline of the neck just cephalad to thyroid cartilage, it allows quick identification of hyoid bone and thyrohyoid membrane and measurement of their distance from skin. We found that sagittal placement had a rapid learning curve with good interobserver variability. Hence, all our measurements were made with a low-frequency curvilinear probe in the sagittal plane.

Since we wanted to determine the efficacy of ultrasound indices in predicting unanticipated difficult laryngoscopy, we excluded patients with anticipated difficulty.

There have been several studies evaluating various other parameters for screening of the airway – skin to epiglottis distance,[1011] the tongue volume,[12] tongue thickness, ratio of the thickness from the skin to the hyoid bone in neutral and extended positions,[891314] distance from the epiglottis to the vocal cord midpoint and depth of the pre epiglottic space.[15] We found skin to thyrohyoid membrane distance and the skin to hyoid distance are relatively easy to measure even by a novice hence we chose these measurements for our study. We feel for ultrasound to be established as a widespread evaluation modality in airway examination it needs to be simple, reproducible and rapid. Multiple ultrasonic measurements with considerable learning curve and nebulous ultrasonic landmarks may prove effective in studies but may find lessor adoption in clinical practice, especially in emergent situations.

Among the bedside tests of airway evaluation, the only test which demonstrated any significant difference between easy and difficult intubation among our sample of 150 patients was the mentohyoid distance. This is not surprising as several studies of bedside airway tests have shown that individual tests are rarely adequate for differentiating easy and difficult intubation – clinicians need to rely on a gestalt that derives from multiple different tests.[56] Mallampati grade, the most commonly used test for airway examination, was found to be inadequate for this purpose – there was no significant difference seen between the two groups. Thyromental distance was similarly insignificant.

Among the three tests which we found to be significant, the AUC was greatest for mentohyoid distance and not the ultrasound indices measured in our study. However, on plotting an ROC curve and assessing the cutoff values, it becomes easy to appreciate the fact that for the given cutoff values, aiming for a sensitivity and specificity above 50%, both ultrasound indices (skin to thyrohyoid distance as well as skin to hyoid distance) were found to be more specific than mentohyoid distance for a difficult airway. Hence, they are more effective in ruling out a difficult airway compared to mentohyoid distance; however, we have no evidence from our study that these tests, when performed alone will serve to reliably predict a difficult airway.

Our study is in contrast with the findings of Adhikari et al.,[9] who demonstrated a significant utility of anterior neck soft tissue measurements in determining a difficult airway while simultaneously demonstrating the inferiority of routine bedside clinical tests. However, this difference can be explained by difference in patient population ethnicity and large sample size of our study. Yadav et al.,[16] unlike us, did find ultrasound parameters superior to clinical bedside tests, but they used a linear ultrasound probe in the transverse plane.

Results in obese patients also seem to be conflicting. Ezri et al.[10] reported a significant role for anterior soft tissue measurements in the assessment of difficult laryngoscopy in their study, while study done by Komatsu et al.[11] did not show any significant role for the same. This is a viable area for further study; however, since we wanted to evaluate the efficacy of ultrasound measurements in unanticipated difficult laryngoscopy, we excluded obese patients from our study.

Wu et al.[12] assessed three anterior neck parameters for the assessment of difficult laryngoscopy – the skin to hyoid distance, the skin to thyrohyoid distance, and the distance from the skin to the anterior commissure. This study assessed correlation and found a strong positive correlation between skin to hyoid and skin to thyrohyoid membrane distance and difficult laryngoscopy – however, neither sensitivity and specificity were assessed nor were cutoff values based on the ROC curve identified like we have performed in our study.

Although ultrasound parameters and technique used in our study were different to the study done by Parameswari et al.,[14] our results were similar that is ultrasound indices have a better negative predictive value in predicting difficult laryngoscopy.

A review of all USG studies for difficult airway assessment performed by Fulkerson et al.[8] demonstrated that the mentohyoid distance, skin to thyrohyoid distance, and skin to hyoid distance were the best predictors of a difficult laryngoscopy – similar results to our study. According to them, all these measurements should be taken in sniffing position, while we measured ultrasound parameters in the neck extended position, we did not employ a classical sniffing position.

We tried to explore the utility of ultrasound measurement of anterior neck thickness in predicting difficult laryngoscopy and how they compare to clinical bedside tests. Our quest for quick and easy measurement of an effective anterior neck thickness led to use of a low-frequency curvilinear probe in the sagittal plane to measure skin to hyoid bone and skin to thyrohyoid membrane distance. To the best of our knowledge, this is the only observational study where a curvilinear probe is used in such a fashion.

There were a few limitations of our study – the first is the fact that the anterior neck soft tissue varies greatly between males and females and depending on their age and comorbidities. Obese patients, who are at a higher risk of difficult airway events were not included in this study for the sake of uniformity. Pregnant patients were not added for the same reason. The assessment of the sonographic parameters was performed uniformly; however, in view of the minute distances measured, it would be an error if we failed to acknowledge the fact that maintaining uniform probe pressure in each patient is difficult and this might lead to inadvertent measuring errors – some distances might be measured as less than their actual value.

CONCLUSION

In our study, ultrasound measurements of skin to hyoid bone and skin to thyrohyoid membrane distance fail to eclipse clinical parameters in accurately predicting a difficult laryngoscopy. They may have some value in ruling out a difficult laryngoscopy, but still they cannot be reliably used as stand-alone predictors of difficult laryngoscopy. This is perhaps because in laryngoscopy the sum is greater than the parts. Since laryngoscopy relies on multiple parameters, perhaps it is simplistic to rely on a single parameter.

Further studies are required to identify an optimum model of airway prediction.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.