Assessment of epiglottic depth at three different points with ultrasound in predicting difficult laryngoscopy: A prospective clinical study

Objective: The aim of our study was to evaluate whether preoperative airway ultrasonography can predict difficult direct laryngoscopy and intubation in adult patients undergoing elective surgery under general anesthesia. Therefore we evaluated both clinical and ultrasonographic tests to predict difficult laryngoscopy and intubation and sought to find the most sentisitive ultrasonography measurements. Material and Methods: Airway assessment of 150 ASA 1-3 adult patients undergoing elective surgery under general anesthesia was evaluated by means of clinical and ultrasonography techniques. Demographic data, body mass index (BMI), thyromental distance, hyomental distance, neck movements, distance between incisors, upper lip bite test class, Mallampati score, Cormack-Lehane classification were recorded. The distance between the skin and hyoid bone, depth of epiglottis from the skin at three points in the parasagittal plane, thickness of the tongue root, tongue area and volume, neutral and hyomental distance were measured with ultrasonography. The obtained ultrasonography data and clinical airway assessment tests were compared among the difficult (Group D) and easy laryngoscopy groups (Group E) and the relationship between them was examined. Mann-Whitney U test, Pearson Chi-Square test and Fisher's Exact tests were used for the comparisons between groups. Multivariate logistic regression modeling was used to measure the effect of independent variables on dependent variables. In addition, cut-off, sensitivity and specificity values were calculated by ROC (Receiver Operating Characteristic) analysis, p<0,05 was considered statistically significant. Results: Patients in group D were older (easy 39,6 ± 15, difficult 46,5 ± 12; p=0,002) and the incidence of patients with high body mass index were higher compared to group E (easy %15 vs. difficult %41; p=0,007). The incidence of patients with high grade upper lip bite test (grade 2-3) (easy %45 vs. difficult %91; p<0,001) was significantly higher in group D. The most predictive ultrasonography measurement was epiglottis depth measured at the upper hyoid border [AUC 0,86 sensitivity %91, specificity %68, cut-off>24 mm], ratio of epiglottis depth at upper hyoid border / thyrohyoid membrane level [AUC 0,86, sensitivity %75, specificity %81, cut-off>1,32 mm], ratio of epiglottis depth at upper hyoid border / neck circumference [AUC 0,84; sensitivity %56; specificity %92; cut-off >0,67] and ratio of epiglottis depth at upper hyoid border / lower hyoid border [AUC 0,82; sensitivity %84; specificity %66, cut-off >1,08]. Four independent clinical and ultrasonograpy variables were found to be correlated with difficult laryngoscopy. According to regression analysis, model identification coefficient were R2 (Nagelkerke) = 0.53, which means that 53% of the dependent variable variance could be explained by independent variables. Our model was found to be reliable at confidence interval of 95%. Conclusion: Our study shows that epiglottis depth from the skin evaluated at 3 different levels is a valid criterion in predicting difficult laryngoscopy. Epiglottis depth measured at the level of upper hyoid border was found to be the most sensitive ultrasonography method in predicting difficult laryngoscopy. Clinical and ultrasonography based airway evaluation model revealed that patients over 40 years of age, with moderate and high risk of OSAS, epiglottis depth measured at the upper hyoid border, ratio of epiglottis depth at upper hyoid border / lower hyoid border, ratio of epiglottis depth at upper hyoid border / thyrohyoid membrane level were the most predictive parameters to predict difficult laryngoscopy. Key-words: airway ultrasound, difficult laryngoscopy, difficult airway, difficult intubation, skin-epiglottis depth.