Short thyromental distance is a surrogate for inadequate head extension, rather than small submandibular space, when indicating possible difficult direct laryngoscopy.

CONTEXT: Airway research in anaesthesia shows that the thyromental distance (TMD) as a predictor of difficult intubation is subject to variable sensitivity and specificity. Recently, its value in this regard has been questioned with calls for a redefinition of its role.

OBJECTIVE(S): To define the role of TMD as a predictor of possible difficult laryngoscopy.

DESIGN

A prospective observational study.

SETTING

A tertiary university teaching hospital.

PATIENTS OR OTHER

PARTICIPANTS

Two hundred and thirty-five consecutive patients (137 men), planned for endotracheal intubation anaesthesia, were enrolled in the study. Those who were edentulous, or had facial asymmetry, teeth protrusion, limited mouth opening, history of head and/or neck radiation therapy or any disorder, were excluded. Nobody was excluded once enrolled.

INTERVENTION(S): Preoperative straight-line morphometric measurements of the mandible and submandibular space (SMS) were taken by a measuring device and used to form a three-dimensional model of SMS. Sagittal dimensions of the SMS that determine the TMD were derived using trigonometric laws.

MAIN OUTCOME MEASURES: Direct laryngoscopic view, assessed by a senior anaesthetist and classified according to Cormack/Lehane classification.

RESULTS

Thirty-two patients were reported to have 'limited laryngoscopic view'. The TMD had a sensitivity of 19% and a specificity of 97% as a predictor of laryngoscopic view. Among the factors that determine the magnitude of TMD, only the degree of head extension was significantly different between the two laryngoscopy groups. The other two factors (sagittal angulomental distance, representing mandibular growth, and sagittal angulothyroid distance, representing laryngeal descent in the neck) did not differ between the two groups. Also, the SMS volume did not differ between the two laryngoscopy groups.

CONCLUSION

The role of the TMD in prediction of difficult laryngoscopy should be redefined from a variable representing the SMS volume to one acting as a surrogate for inadequate head extension.