Ultrasound Based Prediction of Difficult Airway: A Prospective Observational Evaluation of Skin-to-Thyroid Cartilage and Skin-to-Hyoid Bone Distances

Abstract Background and Aims: Accurate prediction of difficult tracheal intubation (DTI) remains a challenge in anaesthetic practice. This study aimed to evaluate the diagnostic accuracy of ultrasound-measured skin-to-hyoid bone (SHB) and skin-to-thyroid cartilage (ST) distances in predicting DTI. Secondary objectives included determining the sensitivity and specificity of neck ultrasound measurements and identifying which sonographic parameter serves as a better predictor of a difficult airway. In addition, ultrasound-based predictors were compared with conventional clinical airway assessment tools. Materials and Methods: This prospective observational study included 66 adult ASA I-II patients undergoing elective surgery under general anaesthesia with endotracheal intubation. Preoperative clinical predictors – Modified Mallampati classification, thyromental distance, sternomental distance and interincisor gap – were assessed. Ultrasound measurements of SHB and ST distances were obtained in both neutral and sniffing positions. DTI was defined as Cormack–Lehane Grade 3b or 4 (Cook’s modification). Diagnostic performance was analysed using receiver operating characteristic curves, and inter-observer agreement was measured by Cohen’s kappa. Results: Clinical predictors had poor accuracy (AUC <0.7). The ST distance achieved AUC >0.7, with sensitivity 48.5%, specificity 84.8%, positive predictive value (PPV) 76.2% and negative predictive value (NPV) 62.2%. By comparison, clinical assessment yielded sensitivity 15.2%, specificity 87.9%, PPV 55.6% and NPV 50.9%. SHB and ST/SHB ratios ≥1 were not significantly predictive. Inter-observer agreement was higher for ultrasound (κ = 0.333, P = 0.007) than for clinical assessment (κ = 0.03, P = 1). Conclusion: Neck ultrasonography – particularly ST distance – is a superior non-invasive predictor of DTI compared to clinical methods. SHB distance and ST/SHB ratio offer no additional predictive value. Incorporation of ultrasound in pre-anaesthetic airway evaluation may enhance patient safety.