Ultrasonography relative to Cormack-Lehane in predicting difficult laryngoscopy: a systematic review of diagnostic test accuracy : JBI Evidence Synthesis

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Ultrasonography relative to Cormack-Lehane in predicting difficult laryngoscopy: a systematic review of diagnostic test accuracy

Ferszt, Peter; Hill, Jeremy; Larson, Sandra

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JBI Evidence Synthesis 19(6):p 1444-1451, June 2021. | DOI: 10.11124/JBIES-20-00141
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Difficult airway arguably presents the most critical challenge for anesthesia providers and broadly encompasses scenarios involving difficult face-mask ventilation, supraglottic airway placement, limited glottic view upon laryngoscopy, multiple attempts at tracheal intubation, or obscured surgical airway landmarks.1,2 When difficult mask ventilation and artificial airway placement present together, lack of adequate oxygenation can rapidly progress to significant morbidity or mortality.2-8 Evidence also shows that when difficult airway is not anticipated, the provider's management of the event deteriorates, contributing to worse outcomes.4,7

The published incidence of difficult airway varies across in-hospital location, patient demographics, and type of difficulty.1,9 For example, a review conducted by the Canadian Airway Focus Group found the incidence of difficult direct laryngoscopy encountered in the operating room was 0.8% to 7% compared to 11% in the intensive care unit.1 In a meta-analysis involving nearly 51,000 participants, which evaluated the ability of physical examination screening tests to predict difficult intubation, researchers noted that a difficult intubation occurred in 5.8% (95% confidence interval [CI], 4.5–7.5%) of participants overall and in 15.8% (95% CI, 14.3–17.5%) of participants with obesity.9 In a more recent study involving 2254 patients, 6.3% experienced difficult direct laryngoscopy and 2.3% experienced difficult intubation.10

Analysis of closed malpractice claims involving difficult airway management in anesthesia practice show these claims are associated with significant patient morbidity or mortality. The American Association of Nurse Anesthetists Foundation's database of claims that occurred between 2003 and 2012 showed difficult airway management claims resulted in death (9/13), permanent and significant cognitive impairment (2/13), and mild cognitive deficit (2/13).5 The American Society of Anesthesiologists’ database of anesthesia-related claims that occurred between 2000 and 2012 included 79 difficult airway claims. Among these claims, 78% of claims occurring within the operating room or recovery room (62/79) resulted in death or brain damage, and 100% of claims occurring outside the operating room (17/79), in locations such as the emergency department or intensive care unit, resulted in death or brain damage.4 Finally, among the 67 difficult airway claims reported to the UK National Health Service Litigation Authority between 1995 and 2007, 48 resulted in death or brain damage.3

Preoperative assessment to determine each patient's risk of difficult airway is a fundamental safety routine in anesthesia practice.2,5,7 Identification of high-risk patients enables providers to pre-emptively prepare an individualized airway management plan that mitigates risk and promotes safety.2,5,7,11 The standard of care for preoperative assessment requires that a detailed physical-airway examination be performed on every patient.12,13 Physical examination characteristics suggestive of difficult airway include the following findings: uvula not visible with tongue protruded (Mallampati class III or IV); long upper incisors; overbite or inability to protrude the mandible; small inter-incisor gap; arched or narrow hard palate; short or thick neck; short thyromental distance; and limited range of motion of head or neck.9

Current physical examination assessment techniques lack sufficient sensitivity and specificity to correctly predict the risk of difficult airway in every patient. Single physical examination tests have a sensitivity of 20% to 62% and a specificity of 82% to 97%.9,14 Evidence suggests that the best single test may be the sternomental distance (n = 1085), with a sensitivity of 62% and specificity of 82%, while the most commonly used test, Mallampati classification (n = 41,193), has a sensitivity of 49% and a specificity of 86%.9 Combining multiple physical examination tests does not significantly improve their clinical value in terms of sensitivity and specificity.9 Thus, there is a need for better techniques to more accurately predict difficult airway and allow for safer airway management plans.9,14,15

Ultrasound, a technique that employs sound wave reflection to create real-time imaging, has many significant and evolving health care applications. Point-of-care ultrasound (POCUS) gives providers real-time images that support more accurate clinical decision-making and enhance patient safety.16 For example, in anesthesia practice, POCUS is now used for performance of nerve blocks and has been shown to reduce the incidence of local anesthetic systemic toxicity and hemi-diaphragmatic paresis, and it is also recognized to reduce complications associated with central line placement.17

Newer research is focusing on how POCUS airway measurements correlate with Cormack-Lehane scores, the gold standard in identifying difficult laryngoscopy.10,18-20 Predicting difficult laryngoscopy is important because roughly 33% to 40% of difficult laryngoscopies are also difficult intubations.10,21 Cormack-Lehane is a four-point scale based on the laryngeal structures visualized during direct laryngoscopy. Providers interpret a non-difficult laryngoscopy as grade 1 or 2, and a difficult laryngoscopy as grade 3 or 4.22,23 Direct laryngoscopy is uncomfortable, invasive, and requires anesthesia in order to be effective; POCUS is comparatively non-invasive and pain-free. If POCUS airway measurements are highly sensitive and specific for predicting difficult laryngoscopy, they may be able to offer a significant improvement in preoperative difficult airway detection.

Point-of-care ultrasound airway assessment measures distances and angles between anatomical structures in the neck and oropharynx. Anterior soft tissue quantification (ASQ) techniques currently under investigation include measures of the distance from skin to the epiglottis and the skin to hyoid bone; the angle between the epiglottis and glottis; and tongue thickness.18 By correlating ASQ measurements with Cormack-Lehane grades, new insights will be gained regarding its utility in supporting airway assessment strategies for difficult airway prediction. For example, in a recent study, POCUS tongue thickness measurement had a sensitivity of 71% and specificity of 72% for predicting Cormack-Lehane grade 3 or 4, while modified Mallampati had a sensitivity of 45% and specificity of 75%.18

Short of direct laryngoscopy, which is both invasive and uncomfortable, providers do not have a mechanism to confidently predict difficult airway. Therefore, a systematic review of the diagnostic efficacy of newer POCUS ASQ techniques to diagnose difficult laryngoscopy is warranted and has potential to advance clinical practice, patient safety, and future research related to predicting the difficult airway.

On March 1, 2020, the authors conducted a preliminary search of MEDLINE (Ovid) and identified studies that meet the inclusion criteria for this review.10,18-20 As part of this preliminary search, Cochrane Library, the JBI Database of Systematic Reviews and Implementation Reports, and PROSPERO were also searched. The authors found no evidence that a systematic review was in progress or had been conducted related to the use of ultrasound to predict difficult laryngoscopy. The authors have more recently found two protocols, which were submitted to PROSPERO in late April 2020 and are related to this topic. However, neither protocol states that it will include positive and negative predictive values, which are the standard metric for establishing diagnostic test accuracy. The authors have attempted to contact the authors of these two protocols but received no response.

Review question

What is the diagnostic test accuracy of POCUS ASQ techniques for predicting difficult laryngoscopy in relation to Cormack-Lehane scores?

Inclusion criteria


This review will consider adults 18 years and older undergoing elective surgery who have a POCUS ASQ airway examination while awake and prior to any sedation. Patients with known upper airway anatomical abnormalities, cervical instability, or undergoing rapid sequence anesthetic induction and intubation will be excluded.

Index test

This review will include all reported POCUS ASQ measurements currently investigated to predict a difficult laryngoscopy as its index test.

Reference test

This review will use the Cormack-Lehane score obtained during direct laryngoscopy as its reference test.

Diagnosis of interest

The diagnosis of interest is difficult laryngoscopy as defined in the literature by a Cormack-Lehane grade 3 or 4.

Types of studies

This review will consider prospective, observational studies of diagnostic test accuracy that include sensitivity and specificity outcome measures in relation to the established reference test. Positive predictive values, negative predictive values, positive and negative likelihood ratios, diagnostics odds ratios, and receiver operating characteristics will also be included when provided.


Search strategy

The search strategy will aim to find published and unpublished studies using Medical Subject Headings (MeSH) and relevant keywords in a five-step process. An initial limited search of Ovid MEDLINE and Trip Database will be undertaken using the following MeSH terms: “ultrasonography,” “ultrasound,” “anaesthesia,” “anesthesia,” “airway management,” “intubation,” “laryngoscopy,” “laryngeal,” and “preoperative care” (step 1). Subsequently, an analysis of the text words contained in the title and abstract and of the index terms used to describe the articles will be undertaken to expand the list of search terms (step 2). This information will be used to inform the development of a search strategy, which will be tailored for each information source as outlined in Appendix I (step 3). A second search using identified keywords and key terms will then be undertaken across all included databases (step 4). Finally, the reference lists of all studies selected for critical appraisal will be screened for additional studies (step 5). Methodological search filters or terms will not be applied to the search strategy as they are not recommended for reviews of diagnostic accuracy.24

The databases to be searched will include MEDLINE (Ovid), Trip Database, Embase, and CINAHL Complete (EBSCO). The trial register to be searched will be Clinicaltrials.gov. The search for unpublished studies will include Google Scholar, MedNar, Grey Literature Report, and ProQuest Dissertations and Theses.

Only studies published in English will be included. Only studies published since 1984 will be included, as this time period correlates with the advent of the use of the Cormack-Lehane reference test.23

Study selection

Following the search, all identified citations will be collated and uploaded into RefWorks (ProQuest LLC, Ann Arbor, USA) and duplicates removed. Titles and abstracts will then be reviewed by each author (PF, JH, SL) against the established inclusion criteria. Following this preliminary review, the full text of selected studies will be assessed against the inclusion criteria by each author. Any disagreements that arise among the authors during the preliminary review or during full-text review will be resolved through discussion. Full-text studies that do not meet the inclusion criteria will be excluded, and reasons for exclusion will be provided in an appendix in the final systematic review report. Included studies will be imported into the JBI System for the Unified Management, Assessment and Review of Information (JBI SUMARI; JBI, Adelaide, Australia) and undergo a process of critical appraisal. Search results will be reported in full and presented in a Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) flow diagram as part of the final report.25

Assessment of methodological quality

Selected studies will be critically appraised by each author (PF, JH, SL) for methodological quality using the most recently updated Quality Diagnostic Accuracy Studies (QUADAS-2) checklist.26 Any disagreements that arise will be resolved through discussion. Following critical appraisal, studies that do not meet the quality threshold will be excluded. The quality threshold will require that, in order to be included in this review, studies have low risk of bias in all domains specified in the QUADAS-2 tool. The domains include patient selection, index tests, reference standard, and flow and timing.26

Data extraction

Two authors (PF, JH) will independently extract data from each included study using a standardized data extraction matrix developed by the authors (Appendix II). The matrix contains over 50 data points and includes details about participant demographics, exclusion criteria, research methodology, types of ultrasound equipment, types of ultrasound airway views and measurements, research findings, and limitations of each study. Any disagreements that arise between the authors will be resolved through discussion. Where required, authors of included studies will be contacted to request missing or additional data.

Data synthesis

Data synthesis will be conducted using RevMan v.5 (Copenhagen: The Nordic Cochrane Centre, Cochrane) software. For each index test, the main outcomes will be the positive and negative predictive values and the pooled sensitivity and specificity data with 95% confidence intervals. Where possible, study results will be pooled in statistical bivariate meta-analysis. If the same diagnostic thresholds are used across studies for each index test, then these will be displayed on a paired forest plot. If they vary, they will be displayed on a summary receiver operator characteristic (SROC) curve. Heterogeneity will be assessed visually based on how closely the studies on the paired forest plot align or how closely they map to the curve of the SROC. Tufanaru et al.27 will guide the reviewers’ choice of model (random or fixed effects) and method for meta-analysis. If heterogeneity is suspected, it will be investigated through subgroup analysis. When statistical pooling is not possible, narrative summary will be provided including tables and figures to aid in data presentation.

Assessing certainty in the findings

A Summary of Findings, which includes all of the contributing studies, will be developed using GRADEPro GDT (McMaster University, ON, Canada) software. The Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach28 for grading the quality of evidence for diagnostic test accuracy will be followed. Information presented in the summary table for each study will include sample size, study design, and measures of diagnostic test accuracy. Also included will be a ranking of the quality of evidence based on study limitations (risk of bias), imprecision, inconsistency, and publication bias.

Appendix I: Search strategy


Search conducted on March 1, 2020


Appendix II: study data extraction tool



1. Law JA, Broemling N, Cooper RM, Drolet P, Duggan LV, Griesdale DE, et al. The difficult airway with recommendations for management–part 1–difficult tracheal intubation encountered in an unconscious/induced patient. Can J Anaesth 2013;60 (11):1089–1118.
2. Apfelbaum JL, Hagberg CA, Caplan RA, Blitt CD. Practice guidelines for management of the difficult airway: an updated report by the American Society of Anesthesiologists Task Force on Management of the Difficult Airway. Anesthesiol 2013;118 (2):251–270.
3. Cook TM, Scott S, Mihai R. Litigation related to airway and respiratory complications of anaesthesia: an analysis of claims against the NHS in England 1995–2007. Anaesthesia 2010;65 (6):556–563.
4. Joffe AM, Aziz MF, Posner KL, Duggan LV, Mincer SL, Domino KB. Management of difficult tracheal intubation: a closed claims analysis. Anesthesiol 2019;131 (4):818–829.
5. Larson S, Matthews RW, Jordan L, Hirsch MT. Improving patient outcomes through closed claims analysis: salient characteristics and patterns associated with respiratory events. AANA J 2018;86 (3):201–208.
6. Peterson GN, Domino KB, Caplan RA, Posner KL, Lee LA, Cheney FW. Management of the difficult airway: a closed claims analysis. Anesthesiol 2005;103 (1):33–39.
7. Frerk C, Mitchell VS, McNarry AF, Mendonca C, Bhagrath R, Patel A, et al. Difficult Airway Society 2015 guidelines for management of unanticipated difficult intubation in adults. Br J Anaesth 2015;115 (6):827–848.
8. Cook TM, MacDougall-Davis SR. Complications and failure of airway management. Br J Anaesth 2012;109:168–185.
9. Shiga T, Wajima Z, Inoue T, Sakamoto A, Shiga T, Wajima Z, et al. Predicting difficult intubation in apparently normal patients: a meta-analysis of bedside screening test performance. Anesthesiol 2005;103 (2):429–437.
10. Yao W, Wang B. Can tongue thickness measured by ultrasonography predict difficult tracheal intubation? Br J Anaesth 2017;118 (4):601–609.
11. Cook TM, Woodall N, Frerk C. Major complications of airway management in the UK: results of the Fourth National Audit Project of the Royal College of Anaesthetists and the Difficult Airway Society. Part 1: anaesthesia. Br J Anaesth 2011;106 (5):617–631.
12. American Association of Nurse Anesthetists. Standards for nurse anesthesia practice [internet]. 2019 [cited 2019 Dec 5]. Available from: https://www.aana.com/docs/default-source/practice-aana-com-web-documents-(all)/standards-for-nurse-anesthesia-practice.pdf.
13. Crawley SM, Dalton AJ. Predicting the difficult airway. BJA Educ 2015;15 (5):253–257.
14. Roth D, Pace N, Lee A, Hovhannisyan K, Warenits AM, Arrich J, et al. Airway physical examination tests for detection of difficult airway management in apparently normal adult patients. Cochrane Database Syst Rev 2018;5:CD008874.
15. Nørskov AK, Rosenstock CV, Wetterslev J, Astrup G, Afshari A, Lundstrøm LH. Diagnostic accuracy of anaesthesiologists’ prediction of difficult airway management in daily clinical practice: a cohort study of 188 064 patients registered in the Danish Anaesthesia Database. Anaesthesia 2015;70 (3):272–281.
16. Kline JP. Ultrasound guidance in anesthesia. AANA J 2011;79 (3):209–217.
17. Shojania KG, Duncan BW, McDonald KM, Wachter RM, Markowitz AJ. Making health care safer: a critical analysis of patient safety practices. 2001; Rockville, MD: Agency for Healthcare Research and Quality, 43.
18. Yadav NK, Rudingwa P, Mishra SK, Pannerselvam S. Ultrasound measurement of anterior neck soft tissue and tongue thickness to predict difficult laryngoscopy - an observational analytical study. Indian J Anaesth 2019;63 (8):629–634.
19. Pinto J, Cordeiro L, Pereira C, Gama R, Fernandes HL, Assunção J. Predicting difficult laryngoscopy using ultrasound measurement of distance from skin to epiglottis. J Crit Care 2016;33:26–31.
20. Wang L, Feng Y-K, Hong L, Xie W-L, Chen S-Q, Yin P, et al. Ultrasound for diagnosing new difficult laryngoscopy indicator: A prospective, self-controlled, assessor blinded, observational study. Chin Med J (Engl) 2019;132 (17):2066–2072.
21. Tamire T, Demelash H, Admasu W. Predictive values of preoperative tests for difficult laryngoscopy and intubation in adult patients at Tikur Anbessa Specialized Hospital. Anesthesiol Res Pract 2019;2019:1–13.
22. Koh W, Kim H, Kim K, Ro Y, Yang H. Encountering unexpected difficult airway: relationship with the intubation difficulty scale. Korean J Anesthesiol 2016;69 (3):244–249.
23. Cormack RS, Lehane J. Difficult tracheal intubation in obstetrics. Anaesthesia 1984;39 (11):1105–1111.
24. Beynon R, Leeflang MM, McDonald S, Eisinga A, Mitchell RL, Whiting P, et al. Search strategies to identify diagnostic accuracy studies in MEDLINE and EMBASE. Cochrane Database Syst Rev 2013; (9): MR000022.
25. Moher D, Liberati A, Tetzlaff J, Altman D. The PRISMA Group. Preferred Reporting Items for Systematic Reviews and Meta-Analyses: The PRISMA Statement. PLoS Med 2009;6 (7):e1000097.
26. Whiting PF, Rutjes AWS, Westwood ME, Mallett S, Deeks JJ, Reitsma JB, et al. QUADAS-2: a revised tool for the quality assessment of diagnostic accuracy studies. Ann Intern Med 2011;55 (8):529–536.
27. Tufanaru C, Munn Z, Stephenson M, Aromataris E. Fixed or random effects meta-analysis? Common methodological issues in systematic reviews of effectiveness. Int J Evid Based Healthc 2015;13 (3):196–207.
28. Schünemann H, Brożek J, Guyatt G, OxmanA, editors. Handbook for grading the quality of evidence and the strength of recommendations using the GRADE approach [internet]. Updated October 2013. The GRADE Working Group. 2013 [cited 2020 March 18]. Available from: https://gdt.gradepro.org/app/handbook/handbook.html.

airway; anesthesia; laryngoscopy; point-of-care; ultrasound

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