Sternomental distance and sternomental displacement as predictors for difficult laryngoscopy in adult patients

Failed tracheal intubation can have potentially life threatening consequences. Several preoperative airway assessment tests have been suggested to predict difficult laryngoscopy. Sternomental distance (SMD) is an indicator of head and neck mobility and it has been suggested that SMD is the best single test for ruling out difficult intubation amongst forced protrusion of the mandible, inter incisor gap, modified Mallampati grade and thyromental distance (TMD).  Previous studies have addressed the correlation between SMD and difficult laryngoscopy. The cut-off point of SMD for difficult laryngoscopy was 12.5cm2 and 13.5 cm.

SMD is conventionally measured with the head extended on the neck (SMD extension).  A modification of measurement of SMD is the measurement obtained with the head in neutral position (SMD neutral). It has been suggested that a difference < 5 cm between SMD extension and SMD neutral, (henceforth referred to as sternomental displacement), is associated with a difficult laryngoscopy; 3 conversely, a sternomental displacement of more than 5 cm is indicative of easy laryngoscopy. 3

SMD is likely to vary with patient size and proportion. The study hypothesis is that racial differences would affect SMD threshold values for predicting difficult laryngoscopy. The aim of the study is 1) to determine the optimum threshold value of SMD for difficult laryngoscopy (primary outcome measure) and also for difficult intubation in the Indian population; 2) to determine the extent of SMD displacement that correlates with difficult laryngoscopy.