Comparative Evaluation of Height-to-Thyromental Distance Ratio, Modified Mallampati Score, and Upper Lip Bite Test as Predictors of Difficult Laryngoscopy

Abstract

Introduction: Predicting difficult laryngoscopy is essential
to prevent serious anaesthesia-related complications.
Conventional tests like the Modified Mallampati score (MMS)
and upper lip bite test (ULBT) have limitations, while the ratio
of height to thyromental distance (RHTMD) offers a more
objective, anthropometric approach. The aim of the study is
to to compare the predictive accuracy of RHTMD, MMS, and
ULBT in identifying difficult laryngoscopy in patients undergoing
elective surgery.
Material and Methods: This prospective observational study
included 80 ASA I–III adult patients scheduled for elective
surgeries under general anaesthesia. Preoperative airway
assessments included MMS, ULBT, and RHTMD, with a cut-off
value of ≥26 for RHTMD. After induction, direct laryngoscopy
was performed, and the view was graded using the Cormack-
Lehane (CL) system. Grades III and IV were classified as difficult
laryngoscopy. Sensitivity, specificity, positive predictive value
(PPV), negative predictive value (NPV), accuracy, and area
under the curve (AUC) were calculated for each parameter
using SPSS v28.
Results: The incidence of difficult laryngoscopy was 8.75%.
MMS and ULBT both demonstrated 57.14% sensitivity, 97.26%
specificity, 66.67% PPV, 95.95% NPV, and 93.75% accuracy.
RHTMD ≥26 showed 71.43% sensitivity, 69.86% specificity,
18.52% PPV, 96.23% NPV, and 70% accuracy. The ROC-AUC
for the composite predictive model was 0.737 (p = 0.039). CL
grading achieved 100% diagnostic accuracy.
Conclusion: While MMS and ULBT offer high specificity,
RHTMD provides better sensitivity. A combined approach
improves preoperative prediction of difficult laryngoscopy and
enhances airway safety.