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

Background. In an attempt to determine a more comprehensive and accurate as well as simple and clinically applicable day to day basis parameter for predicting difficult laryngoscopy, individual test like upper lip bite tests and mallampati classification have been compared. In this study we compared patients height to thyromental ratio (RHTMD) with upper lip bite test (ULBT) and mallampati test (MMT). We also investigated whether the combination of these tests can improve the ability to predict difficult laryngoscopy.
Methods. In this prospective study 300 patients who were scheduled for elective surgery were selected. Before induction of anesthesia, the airway was assessed for ULBT, RHTMD and MP class. Cormack Lehan(CL) grading system was used to define difficult intubation after inducing anesthesia . Finally sensitivity, specificity, positive and negative predictive values of these tests individually and in combination were calculated.
Results. The ratio of height to thyromental distance had the highest sensitivity(89.62%), specificity(94.32%) and positive predictive value(97.44%) as compared to the modified Mallampati test[having sensitivity, specificity and positive predictive value of 63.68%,52.05%, 95.07% respectively and the upper lip bite test[having sensitivity, specificity and positive predictive value of 53.3%, 77.2%, 84.96% respectively, thus making it a good predictive test for preoperative airway assessment and evaluation of laryngoscopy among the three tests used. When the three tests i.e. the modified Mallampati test (MMT), the ratio of height to thyromental distance (RHTMD) and the upper lip bite test(ULBT) were combined sensitivity was 98.55%, specificity was 2.45%, positive predictive value(PPV) was 94.3%, negative predictive value(NPV) was 32.05%
Conclusion: We conclude that the accuracy of RHTMD is significantly higher than the other two tests, and in combination these tests more reliably predict difficult intubation.