A comparison of the ratio of patient's height to thyromental distance with the modified Mallampati and the upper lip bite test in predicting difficult laryngoscopy

Background: The aim of the present study was to compare the ability to predict difficult visualization of the larynx from the following preoperative airway predictive indices, in isolation and combination: modified Mallampati test (MMT), the ratio of height to thyromental distance (RHTMD) and the Upper-Lip-Bite test (ULBT). Methods: We collected data on 603 consecutive patients scheduled for elective surgery under general anesthesia requiring endotracheal intubation and then evaluated all three factors before surgery. An experienced anesthesiologist, not informed of the recorded preoperative airway evaluation, performed the laryngoscopy and grading (as per Cormack and Lehane's classification). Sensitivity, specificity, and positive and negative predictive value, Receiver operating characteristic (ROC) Curve and the area under ROC curve (AUC) for each airway predictor in isolation and in combination were determined. Results: Difficult laryngoscopy (Grade 3 or 4) occurred in 41 (6.8%) patients. The main endpoint of the present study, the AUC of the ROC, was significantly lower for the MMT (AUC, 0.511; 95% CI, 0.470–0.552) than the ULBT (AUC, 0.709; 95% CI, 0.671–0.745, P=0.002) and the RHTMD score (AUC, 0.711; 95% CI, 0.673–0.747, P=0.001). There was no significant difference between the AUC of the ROC for the ULBT and the RHTMD score. By using discrimination analysis, the optimal cutoff point for the RHTMD for predicting difficult laryngoscopy was 21.06 (sensitivity, 75.6%; specificity, 58.5%). Conclusion: The RHTMD is comparable with ULBT for prediction of difficult laryngoscopy in general population.