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Ultrasound-based assessment of hyomental distances in neutral, ramped, and maximum hyperextended positions, and derived ratios, for the prediction of difficult airway in the obese population: a pilot diagnostic accuracy study.

BACKGROUND
Ultrasonography-assessed hyomental distance (HMD) ratio has been found to discriminate between obese patients with Cormack-Lehane grades 1 or 2 vs. those with grades 3 or 4. The aim of our study is to evaluate the performance of the HMD evaluated ultrasonographically in neutral, ramped, and maximum hyperextended positions, as well as for the ratios obtained by dividing the HMD in the ramped position to that in the neutral position (HMDR1) and by dividing the HMD in maximum hyperextension to that in the neutral position (HMDR2), in order to predict the occurrence of Cormack-Lehane grades 3 or 4 during direct laryngoscopy.


METHODS
Ultrasound measurements were performed preoperatively in 25 patients with morbid obesity, measuring the HMD in the neutral, ramped, and maximum hyperextended positions. Pre-epyglotic soft tissue thickness, Mallampati and upper lip bite test scores were recorded. Cormack-Lehane grading was considered as an outcome variable in the Receiver Operating Characteristics curve analysis.


RESULTS
HMD in the neutral, ramped and maximum hyperextended positions presented similar sensitivities, 100% [39.8-100.0]. For HMD, specificity was 42.86% [21.8-66.0] in the neutral position, 61.9% [38.4-81.9] in the ramped postion, and 71.4% [47.8-88.7] in the maximum hyperextended position. For HMDR1, the cut-off value was 1.12. This threshold provides 75% [19.4-99.4] sensitivity and 76.2% [52.8-91.8] specificity. For HMDR2, a cut-off value of 1.23 provides 100% [39.8-100.0] sensitivity and 90.5% [69.6-98.8] specificity.


CONCLUSION
HMDR2 seems to have superior diagnostic accuracy in predicting difficult laryngoscopy in the obese population compared to HMDR1, as well as compared to the HMD in the neutral, ramped, and maximum hyperextended positions.