Accuracy of Sonographic Airway Parameters in Difficult Laryngoscopy Prediction: A Prospective Observational Cohort Study from Central India
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Original Article
P: 434-442
October 2023

Accuracy of Sonographic Airway Parameters in Difficult Laryngoscopy Prediction: A Prospective Observational Cohort Study from Central India

Turk J Anaesthesiol Reanim 2023;51(5):434-442
1. Department of Anaesthesiology and Critical Care, All India Institute of Medical Sciences, Bhopal, India
No information available.
No information available
Received Date: 09.02.2023
Accepted Date: 10.07.2023
Publish Date: 24.10.2023
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ABSTRACT

Objective:

Though airway ultrasonography (USG) is used to assess difficult laryngoscopy (DL), there is still ambiguity about approach followed and parameters assessed. There is need of a simple, stepwise sonographic assessment with clearly defined parameters for DL prediction. The primary objective of this study was to find diagnostic accuracy of sonographic parameters measured by a stepwise Airway-USG in DL prediction (DLP).

Methods:

This prospective, observational cohort study was done in 217 elective surgical adult patients administered general anaesthesia with tracheal intubation using conventional laryngoscopy from 1st May 2019 to 31st July 2020, after ethical approval. A sagittal Airway-USG was done using 2-6 Hz transducer in three steps specifying probe placement and head position. Demographic, clinical and Airway-USG measurements were noted. Correlation of the clinical/sonographic parameters was made with Cormack-Lehane score on DL. After receiver operating characteristic curve plotting, the sensitivity, specificity, positive predictive value, negative predictive value (NPV) of DL was calculated for each parameter using open-epi software.

Results:

DL was observed in 19/217 patients. Airway-USG parameters of skin to epiglottis distance >2.45 cm, hyomental distance with head extension <5.13 cm, head neutral <4.5 cm, their ratio <1.18, maximum tongue thickness >3.93 cm and maximum skin to tongue distance >5.45 cm were statistically significant in predicting DL. DLP score with presence of >3 positive parameters showed 98% specificity, 98% NPV and 96% diagnostic accuracy to predict DL.

Conclusion:

DLP score derived from Airway-USG may be used as a screening and diagnostic tool for DL.

Main Points

• Three-step Airway-ultrasonography (USG) can be used to assess occipital-atlantoaxial extension, submandibular space compliance, epiglottis position and tongue size.

• Difficult laryngoscopy (DL) predictor (DLP) score can be derived from measured parameters of Airway-USG.

• DLP score has a good screening and diagnostic potential to predict DL when more than 2 and 3 parameters, respectively are positive.

Introduction

Airway associated complications are the most common anaesthesia-related adverse outcomes.1 Intubation failure is usually attributed to difficult laryngoscopy (DL).2,3 The low sensitivity, high inter-observer variation of morphometric screening tests like Mallampati classification, upper lip bite test, thyromental distance, cervical spine movements has led to continued search for more accurate airway examination tool.4,5 Airway-ultrasonography (USG) is a non-invasive, portable bedside method which can visualize anatomical airway structures, confirm placement of endotracheal (ET)/double-lumen tube and guide invasive procedures like percutaneous tracheostomy and cricothyroidotomy.6,7,8,9,10

A recent meta-analysis has highlighted the heterogeneity in performance of airway sonography.11,12,13,14 There needs to be more literature on accuracy of a step-wise sonographic airway assessment to predict DL better. With this research gap, we conducted this study with the primary objective of studying accuracy of sonographic airway assessment using a three-step approach of protocolized stepwise Airway-USG examination in prediction of DL seen by Cormack-Lehane (CL) Scoring system in patients administered general anaesthesia with ET intubation for elective surgery.

Methods

Results

During the study period, 280 patients were assessed for eligibility, 220 patients were enrolled, and data analysis was possible in 217 patients. (Figure 4). The median age of this study population was 37 (IQR: 22) years, 60% of them female. The study included general surgical (25.8%), gynecological (17.1%), neuro-surgical (13.8%) and onco-surgical (10.6%) patients operated under general anaesthesia. We observed an 8.8% incidence of DL (19/217). The measured and derived sonographic parameters were noted to have a normal distribution, and homogenous variance except TTB, STDB and R2.

Figure 4

Discussion

Several anatomical and pathophysiological components, independently or in combination, can influence the laryngoscopic view. The main anatomical structures obscuring the glottic vision are the tongue, hyoid bone, and epiglottis.18 Extension at Occipito-Atlanto-Axial (OAA) joint during laryngoscopy brings the oral axis in near alignment with laryngopharyngeal axes, aiding the glottic vison.19 The morphometric screening tests investigate one or a few of these components, hence need better sensitivity. A meta-analysis by Shiga et al.5 have confirmed their poor sensitivity with fair specificity. Our results for modified Mallampati score, thyromental distance, and inter-incisor distance were consistent with Shiga et al.5 study.

USG has been studied to visualize and quantify upper airway anatomical structures with good precision.11,12,17 In our study, we have demonstrated the accuracy of a simple, three step approach of protocolized step-wise Airway-USG examination in anticipating DL. The measured sonographic parameters of HMDN, HMDE, skin-to-tongue distance at a maximum vertical distance from the dorsum of the tongue (STDM), tongue thickness at maximum vertical distance from the dorsum of tongue (TTM), DSE and the derived values of HMDR, Delta_HMD were significantly associated with DL.

Conclusion

Direct laryngoscopy predictor score derived from a three-step sonographic airway assessment may be utilized as a screening and diagnostic tool for DLP in patients undergoing elective surgery to avoid unanticipated difficult airway. We recommend further studies in different populations to validate the DLP score derived in our study.

Subjects and Methods

This single-centre, prospective observational study was conducted in an academic tertiary care hospital in Central India in American Society of Anesthesiologists Physical Status (ASA-PS) I-III patients, aged 18-70 years, undergoing elective surgery under general anaesthesia with ET intubation from 1st May 2019 to 31st July 2020. Ethical clearance was given by the Institutional Ethics Committee (IHEC-LOP/2019/MD0049). All study participants gave written informed consent. We excluded patients with any airway abnormality preventing the use of clinical screening tests and Airway-USG like head and neck surgery/trauma/tumors/burns/scars/radiotherapy injuries/neck abscess/hematoma/beard, medical conditions like rheumatoid arthritis, ankylosing spondylitis, pregnancy, extreme obesity [body mass index (BMI) ≥40 kg m2-1], previous history of DL and where laryngoscopy was not part of anaesthesia plan.

Data Collection

On the preoperative day, a trained anaesthesiologist collected the demographic variables and clinical airway parameters like inter-incisor gap, modified Mallampati score and thyromental distance. A single trained study investigator, blinded to the clinical airway parameters, performed the Airway-USG examination using a 2-6 Hz curvilinear transducer of the SonoSite M Turbo portable ultrasound machine. During Airway-USG, all patients were positioned supine with mouth closed and were instructed to keep their tongue relaxed and touch the lower incisors, without phonation or deglutination.

Several upper airway anatomical components influence the glottic view during laryngoscopy. Tongue and oral cavity volume, submandibular space compliance, epiglottis and extension at occipito-atlanto-axial joint are important. To assess and quantify these components ultrasonographically, yet keep it simple to perform, we proposed a three-step approach of protocolized step-wise Airway-USG examination. The three steps of as follows:

Step 1: With the patient’s head in a neutral position, the transducer was placed in the midline of suprahyoid region in sagittal plane, as shown in Figure 1a, and adjusted to bring the hyoid bone, muscles of the floor of the mouth (geniohyoid and mylohyoid), the entire tongue and mentum in a single frame (Figure 1b). The following parameters were measured.

Figure 1

Tongue thickness was measured at the base of tongue (TTB) and at a maximum vertical distance (TTM), from the tongue’s dorsum to the geniohyoid muscle’s dorsum.

Skin-to-tongue distance was measured at the base of tongue (STDB), and a maximum vertical distance (STDM), from the dorsum of the tongue to the skin surface.

The hyomental distance also measured in a neutral position (HMDN) from the hyoid bone’s upper border to the mentum’s lower border.

Step 2: The patient’s head was extended (Figure 2a) without changing the probe position. Hyomental distance in extension (HMDE) was measured from hyoid bone’s upper border to mentum’s lower border (Figure 2b).

Figure 2

Step 3: With the head back in a neutral position, the transducer was slowly moved caudally in the midline to the infrahyoid region, keeping the hyoid bone in frame (Figure 3a), to trace the entire length of epiglottis, which appeared as a hypoechoic structure with hyperechoic air-mucosa interface on its posterior surface. Distance from skin to epiglottis (DSE) was measured just below the hyoid bone from skin surface to the posterior surface of epiglottis (Figure 3b).

Figure 3

On the day of surgery, standard institutional protocols were followed for induction of general anaesthesia with ET intubation done by an independent conventionally trained anaesthesiologist with more than 5 years of experience using Macintosh laryngoscopes of appropriate size blinded to preoperative airway sonography findings. The CL grading was noted.15

For each case, the study’s end point was the difficulty in laryngoscopy judged by the CL grading, where Grades 1 or 2 and Grades 3 or 4 were considered easy and DL, respectively. The demographic, clinical and Airway-USG parameters were compared between easy and DL patients.

Statistical Analysis

Based on previous studies, the sensitivity of USG parameters was reported from 65% to 85% (average 75%) and clinical screening tests was reported from 20-62% (average 41%).16,17 To estimate at least 30% higher sensitivity of USG over clinical parameters with 80% power to detect this change, considering a prevalence of 9.5% to 12% (average of 11) DL for the Indian population, with 95% confidence interval (CI), the estimated sample size calculated using PASS software was 200. We assumed an attrition rate of 10% and calculated the final sample size as 220 patients.

Data was entered, cleaned, and coded in Microsoft Excel 2013. Data was analysed using IBM Statistical Package for the Social Sciences version 23. The Shapiro-Wilk method was used to test the distribution normalcy of numerical variables and presented as mean [standard deviation (SD)] when normally distributed, while non-normally distributed variables presented as median [interquartile range (IQR)]. Categorical variables were expressed as absolute numbers and percentages. Ratios were expressed as values and their 95% CI.

Pearson’s chi-square and Fisher’s exact tests were used as significance tests for the association between categorical variables. Using Levene’s test for equality of variances, numerical variables were checked for homogeneity between the difficult and easy laryngoscopy groups. Independent samples t-test and ANOVA test were used as tests of significance for homogenous numerical variables, while Mann-Whitney U test was used as test of significance for non-homogenous numerical variables. Correlation analysis was performed using the Pearson test. Receiver operating characteristic (ROC) curves were plotted and optimal cut-off values were determined using Youden’s index.

Four derived parameters were calculated from the measured values.

Hyomental distance ratio (HMDR) is defined as the ratio of HMDE divided by HMDN head position.

Delta_HMD is defined as percentage change in Hyomental distance during Occipito-Atlanto-Axial joint (Neck) extension.

ΔHMD =[HMDEHMDNHMDE]X 100

R1 defined as ratio of tongue thickness (TTM) to skin to tongue distance (STDM) at maximum tongue width.

R2 defined as ratio of tongue thickness (TTB) to skin to tongue distance (STDB) at base of the tongue.

“Difficult Laryngoscopy Prediction (DLP)” Scoring System

Since DL is influenced by complex upper airway anatomy, a composite DLP score was developed using statistically significant USG parameters measuring different static and dynamic upper airway components. Diagnostic parameters such as sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), likelihood ratio (LR) and diagnostic accuracy were calculated for individual and composite parameters using open-epi software.

Association of DL with Demographic and Clinical Airway Parameters

Patients with DL were observed to have higher age [43 (IQR: 16) years vs 36 (IQR: 20) years, P=0.002] and BMI [26.62±3.13 (95% CI: 25.11-27.12) kg m2-1 vs 22.77±3.91 (95% CI: 22.31-23.31) kg m2-1, P=0.002] in comparison to those with easy laryngoscopy.

MMP and TMD were the only clinical test observed to be statistically significant in patients with DL. Though both parameters showed poor sensitivity, the specificity was good (Table 1).

Table 1

Association of DL with Protocolized Stepwise Airway-USG Examination Parameters

Amongst the measured parameters, HMDN, HMDE, skin-to-tongue distance maximum (STDM), tongue thickness maximum (TTM), and DSE were statistically significant in differentiating easy and DL. Pearson correlation analysis showed a strong positive correlation between DL and DSE (r=0.71, P < 0.001), moderate negative correlation between DL and HMDE (r=-0.42, P < 0.001), small correlation between DL and STDM (r=0.27 P=0.01) but minimal correlation between DL and TTM, HMDN. Amongst the derived, HMDR and delta hyomental distance (Delta_HMD) were statistically significant in differentiating easy and DL. Mean±SD, area under the ROC curve, optimal cut-off value along with their sensitivity, specificity and odds ratio of statistically significant measured and derived variable is mentioned in Table 2 and Table 3, respectively.

Table 2
Table 3

One-way ANOVA test and post-hoc analysis (using Dunnett’s t3 multiple comparisons of means) revealed two Airway-USG parameters, namely HMDE and DSE, exhibited statistically significant difference between different CL grades (Table 4).

Table 4

Predictor of Difficult Laryngoscopy on Logistic Regression

Multivariate logistic regression showed DSE, HMDE, STDM and Delta_HMD were independent predictors of DL, their cut-off values were used to develop the Difficult Laryngoscopy Prediction Score. Each of them scored 1 and 0 for satisfying and not satisfying the cut-off criteria, respectively. DLP score=DSE + HMDE + STDM + Delta_HMD. The diagnostic profile of DLP score ≥2 and ≥3 shown in Table 5.

Table 5

Difficult Laryngoscopy Prediction Score

Since DL is influenced by complex airway anatomy involving both static and dynamic components of the upper airway, the diagnostic accuracy of a test could be improved by investigating multiple factors affecting DL. The composite DLP score combines 4 crucial anatomical aspects of DL-DSE for anterior neck soft tissue thickness and thyroid-epiglottic angle, HMDE for submandibular compliance, STDM for tongue and floor of mouth thickness and Delta_HMD for OAA joint extension.

With a 100% sensitivity, 100% NPV, LR- 0.01 and 81% DA, the DLP score ≥2 can be employed as a screening test for DL, thus warning the intubating team about the possibility of DL. DLP score ≥3 had a 98% specificity, 79% PPV, LR+ 39 and 96% DA for DLP and can be employed as a diagnostic test in anticipating DL.

Strengths and Limitations

The main strength of our study is the simplified three-step Airway-USG assessment method, which may be used in future studies to decrease heterogeneity in the sonographic airway parameters assessed. It systematically examines both static and dynamic components of airway anatomy responsible for DL with good precision. Second, we have highlighted the diagnostic accuracy of the composite DLP score derived for the first time in our study, which encompasses four independent anatomical factors responsible for DL.

Our study has many limitations.

- It is a single center study with limited patients.

- Due to the low incidence of DL, the two study groups had an unequal sample size, which may have impacted the diagnostic profile of the USG parameters.

- We excluded patients with known anticipated DL, like pregnant, morbidly obese, and patients with airway anatomical abnormalities to avoid confounding factors.

- In our study, we never encountered someone with MMP4 score (large tongue to oral cavity ratio); this might have underscored the tongue related USG parameters in anticipating DL.

- We did not have a USG parameter to measure mouth opening, hence lacking complete independence of protocolized step-wise Airway-USG examination in anticipating DL.

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