The importance of neck circumference to thyromental distance ratio (NC/TM) as a predictor of difficult intubation in obstructive sleep apnea (OSA) patients

Background
The trial to find a bedside examination that is helpful for foreseeing difficult intubation is quite inspiring. It was reported that thyromental distance (TM), body mass index (BMI), neck circumference (NC) and a Mallampati score >3 were the only helpful bedside test predictors. By using magnetic resonance imaging, it was established that more fat was present in areas around the collapsible parts of the pharynx in OSA patients. So distribution of fat may provide a better suggestion of difficult intubation than neck circumference. The current work studied the neck circumference to thyromental distance ratio as a new predictor of difficult intubation in obstructive sleep apnea (OSA) patients and comparing it to the formally standard Mallampati score.
Patients and methods
After approval of the ethical committee in Kasr Al Ainy University hospital and patients consent 50 ASA class 1 and II OSA patients, 18–60 years of both sexes, undergoing surgery under general anesthesia with tracheal intubation were enrolled in the study. It was a single group study. Body mass index, neck circumference (cm), thyromental distance (cm) and the ratio of the NC to TM (NC/TM) and Mallampati classification were recorded. Difficulty of intubation was assessed using the Intubation Difficulty Scale (IDS). Correlation between all variables and IDS, comparison between NC/TM ratio and Mallampati score as reliable tests for predicting difficult intubation and also measuring sensitivity and specificity of both.
Results
Among the 50 patients, 11 patients experienced difficult intubation according to IDS scale. The Mallampati score and NC/TM were the only statistically significant variables that were associated with a difficult intubation. NC/TM ratio was strongly correlated to difficult intubation where value = 0.01 odds ratio 37.5 with cut-off point is NC/TM ⩾5.15, but the correlation of Mallampati score was weaker value = 0.05 and odds ratio was 14.5. Moreover, NC/TM ratio showed higher sensitivity (100%) and a negative predictive value (82%), than the Mallampati score sensitivity (90%) and specificity (61%).
Conclusion
Difficult intubation in OSA obese patients was independently associated with a Mallampati score of III or IV, and NC/TM ⩾5.15. Moreover, NC/TM yielded a high sensitivity, specificity and a negative predictive value.