Diagnostic Predictor of Difficult Laryngoscopy: The Hyomental Distance Ratio

BACKGROUND: We evaluated the usefulness of the hyomental distance (HMD) ratio (HMDR), defined as the ratio of the HMD at the extreme of head extension to that in the neutral position, in predicting difficult visualization of the larynx (DVL) in apparently normal patients, by examining the following preoperative airway predictors, alone and in combination: the modified Mallampati test, HMD in the neutral position, HMD and thyromental distance at the extreme of head extension and HMDR. METHODS: Preoperatively, we assessed the five airway predictors in 213 adult patients undergoing general anesthesia with tracheal intubation. A single experienced anesthesiologist, blinded to the results of the airway evaluation, performed all of the direct laryngoscopies and graded the views using the modified Cormack and Lehane scale. DVL was defined as a Grade 3 or 4 view. The optimal cutoff points for each test were determined at the maximal point of the area under the curve in the receiver operating characteristic curve. For the modified Mallampati test, Class >= 3 was predefined as a predictor of DVL. RESULTS: The larynx was difficult to visualize in 26 (12.2%) patients. In univariate analyses, the HMD and thyromental distance at the extreme of head extension and the HMDR were significantly related to DVL. The HMDR with the optimal cutoff point of 1.2 had greater diagnostic accuracy (area under the curve of 0.782), than other single predictors (P < 0.05), and it alone showed a greater diagnostic validity profile (sensitivity, 88%; specificity, 60%) than any test combinations. CONCLUSIONS: The HMDR with a test threshold of 1.2 is a clinically reliable predictor of DVL.