Reliability of the thyromental height test for prediction of difficult visualisation of the larynx: A prospective external validation

Background and Aims: Thyromental height (TMH) has been reported to be useful for prediction of difficult visualisation of the larynx (DVL), defined as Cormack--Lehane (C&L) grade III or IV. The aim of this study was to compare the diagnostic accuracy of the TMH test for DVL with that of other clinically used tests in Japanese patients. Methods: Six hundred and nine surgical patients undergoing endotracheal intubation under general anaesthesia were enrolled in this prospective observational study. TMH, thyromental distance (TMD), and Samsoon and Young's modified Mallampati (MMT) tests were performed in all patients. The C&L grades for the laryngoscopic view with and without external backward, upward, rightward pressure (BURP) were determined by designated airway assessors. The cutoff value for the TMH test was calculated using receiver-operating characteristic (ROC) curve analysis. The sensitivity, specificity, positive predictive value, accuracy, positive likelihood ratio, and area under the ROC curve (AUROC) for each predictive test were calculated and compared. Results: ROC curve analysis indicated that 54 mm is the optimal cutoff value for the TMH test. However, both this value and the conventional cutoff value of 50 mm, which has been reported as having good diagnostic accuracy in the literature, had poor diagnostic accuracy. The AUROC for the TMH test was 0.631 without BURP and 0.592 with BURP; these values were not superior to those for the TMD test or MMT. Conclusion: The TMH test is not a good predictor of DVL in Japanese patients