Predicting Difficult Laryngoscopy: Truly Complicated!

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We read with great interest the article by Yao et al,1 titled “Can Mandibular Condylar Mobility Sonography Measurements Predict Difficult Laryngoscopy?” It is obvious that improvement of laryngoscopy is the first step toward the optimization of tracheal intubation. However, there is still no single, simple, but reliable method to predict difficult laryngoscopy. While their findings are interesting, we have several concerns and commentary.
As the authors pointed out, the temporal mandibular joint is a very complex structure. The mouth opening movement involves both condylar rotation and anterior–inferior sliding. As illustrated in the Figure, if mouth opening could occur without translation of the condyle along the articular eminence (short broken line), it would be closely related to the mandibular length (the distance between the mandibular condyle and the lower incisors) and angle of mouth opening. However, because considerable condylar translation occurs during normal mouth opening in a manner of sliding anteriorly and inferiorly, a smaller mouth opening is the result (mouth opening with condylar translation < mouth opening with only condylar rotation).2 The longer the translation distance (long broken line), the smaller the mouth opening will be (mouth opening in patients with longer translation distance < mouth opening with condylar translation). Since mouth opening is an important predictor for difficult laryngoscopy, we are curious how a smaller mouth opening caused by longer translation distance could actually predict easier intubation.
The authors also pointed out that during direct laryngoscopy, the jaw is lifted anteriorly and inferiorly, and thus, this anterior–inferior translation distance is important for predicting difficult laryngoscopy. This translation distance should be measured during lower jaw protrusion, rather than during mouth opening without additional jaw protrusion, as was done in this study. As a matter of fact, additional jaw protrusion with maximal mouth opening is difficult in awake patients unless the patient has excess temporal mandibular joint mobility, but it becomes possible during laryngoscopy, in which passive advancement of the lower jaw is performed when the patient is anesthetized. The translation distance measured in this study consisted of 2 components: the distance created by anterior and inferior sliding, and the distance created by condylar rotation. As a second question, we wonder how the predictive value of condylar translation distance with mouth opening in this study can be translated to the predictive value of condylar translation distance under normal intubation conditions in anesthetized patients, for whom additional lower jaw protrusion is frequently achieved.
The authors also reported that the interincisor distance, upper lip bite test, and mandibular protrusion distance were not as predictive of difficult laryngoscopy compared with the condylar translation distance. While mouth opening assessed by interincisor distance is determined by angle of mouth opening, which involves condylar rotation, both the upper lip bite test and mandibular protrusion distance involve mainly anterior–inferior sliding. It is not surprising that condylar translation distance, which assesses both condylar rotation and sliding mobility, has a higher predictive value.
In conclusion, predicting difficult laryngoscopy is a complicated assessment. Clinicians need to understand the anatomy and physiology involved with each predicting variable. Condylar translation distance might offer a higher predictive value since it involves 2 different mechanisms; although the study condition in this article is different from direct laryngoscopy in an anesthetized patient.
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