Airway Ultrasound as Predictor of Difficult Direct Laryngoscopy

A Systematic Review and Meta-analysis

Andrea Carsetti, MD; Massimiliano Sorbello, MD; Erica Adrario, MD; Abele Donati, MD, PhD; Stefano Falcetta, MD


Anesth Analg. 2022;134(4):740-750. 

In This Article

Abstract and Introduction


Background: Despite several clinical index tests that are currently applied for airway assessment, unpredicted difficult laryngoscopy may still represent a serious problem in anesthesia practice. The aim of this systematic review and meta-analysis was to evaluate whether preoperative airway ultrasound can predict difficult direct laryngoscopy in adult patients undergoing elective surgery under general anesthesia.

Methods: We searched the Medline, Scopus, and Web of Science databases from their inception to December 2020. The population of interest included adults who required tracheal intubation for elective surgery under general anesthesia without clear anatomical abnormalities suggesting difficult laryngoscopy. A bivariate model has been used to assess the accuracy of each ultrasound index test to predict difficult direct laryngoscopy.

Results: Fifteen studies have been considered for quantitative analysis of summary receiver operating characteristic (SROC). The sensitivity for distance from skin to epiglottis (DSE), distance from skin to hyoid bone (DSHB), and distance from skin to vocal cords (DSVC) was 0.82 (0.74–0.87), 0.71 (0.58–0.82), and 0.75 (0.62–0.84), respectively. The specificity for DSE, DSHB, and DSVC was 0.79 (0.70–0.87), 0.71 (0.57–0.82), and 0.72 (0.45–0.89), respectively. The area under the curve (AUC) for DSE, DSHB, DSVC, and ratio between the depth of the pre-epiglottic space and the distance from the epiglottis to the vocal cords (Pre-E/E-VC) was 0.87 (0.84–0.90), 0.77 (0.73–0.81), 0.78 (0.74–0.81), and 0.71 (0.67–0.75), respectively. Patients with difficult direct laryngoscopy have higher DSE, DSVC, and DSHB values than patients with easy laryngoscopy, with a mean difference of 0.38 cm (95% confidence interval [CI], 0.17–0.58 cm; P = .0004), 0.18 cm (95% CI, 0.01–0.35 cm; P = .04), and 0.23 cm (95% CI, 0.08–0.39 cm; P = .004), respectively.

Conclusions: Our study demonstrates that airway ultrasound index tests are significantly different between patients with easy versus difficult direct laryngoscopy, and the DSE is the most studied index test in literature to predict difficult direct laryngoscopy. However, it is not currently possible to reach a definitive conclusion. Further studies are needed with better standardization of ultrasound assessment to limit all possible sources of heterogeneity.


Unsuccessful airway management leads to serious morbidity and mortality, and the unanticipated difficult intubation is a potentially life-threatening event during anesthesia. Several bedside screening tests are used in clinical practice to identify patients at the risk of difficult airway. Despite their accuracy and benefit were well proven in the literature and daily practice, a small number of patients classified to an easy airway may still present an unexpected difficulty. Predicting a "difficult airway" is not at all an easy task for all patients:[1–7] many structures and functional units are involved in the pathogenesis of a difficult airway, which is a dynamic phenomenon and highly dependent on the operator's experience. We then need to consider that many studies have been performed with different definitions and criteria for difficult airway, including the interobserver variability during the assessment, given that not all measurements are provided with objective parameters. Finally, we need to consider that all the factors involved in the genesis of a difficult airway may be differently combined in a large number of possibilities.[8] This means that predicting a difficult airway represents the attempt to adopt a quantitative assessment of many qualitative and quantitative parameters, with the final conclusion that "any difficult airway is difficult its own way."[9] This may also explain why the incidence of difficult airway and difficult intubation varies from 5% to 22%,[10–12] with important implications for clinical practice and patients' outcomes. Nowadays, several clinical tests recommended by current guidelines for airway assessment[13] make patients with difficult airways easily identifiable. On the other hand, a minority of subjects classified with easy airways will be instead unexpectedly difficult to manage.[9] Hence, the need to develop adequate tools to successfully predict not a difficult airway but an unexpectedly difficult airway in patients previously classified as easy, possibly including in the clinical evaluation some objective index tests to increase sensibility and specificity and reduce interobserver variability. Although this may seem a problem interesting to a very small number of patients, it may have serious life-threatening consequences when it occurs.

For many years, ultrasounds have been used as a complementary tool to predict difficulty in airway management, both from a qualitative and quantitative perspective.

To date, many studies have been published with the aim to find an effective ultrasound indicator to predict a difficult airway, but with significant limitations due to large variability of the sample homogeneity, to the kind of population included, and to the absence of a standardized protocol for ultrasound assessments.

The aim of this systematic review and meta-analysis was to evaluate whether preoperative upper airway ultrasound (UA-US) can predict a difficult airway in adult patients undergoing elective surgery under general anesthesia without clear anatomical evidence of difficult airway on standard clinical examination. Moreover, the mean difference (MD) of UA-US index tests between patients with easy and difficult direct laryngoscopy has been investigated.