ANESTHESIA training begins with a long series of “commandments.” One of these is “Direct laryngoscopy is best performed with the patient’s head in the ‘sniffing position’ because it permits a better laryngeal view.” This dictum has rarely been questioned before Adnet et al. 1,2reassessed the value of the sniffing position in their series of clinical investigations. Using magnetic resonance imaging techniques, 1they measured the angles of three anatomic axes (mouth [MA], pharynx [PA], and larynx [LA]), and demonstrated that neither the sniffing position nor simple neck extension achieved alignment of the three axes. Conversely, both positions resulted in approximately equal angles between the line of vision and LA as compared with the neutral head position. Although that study was performed in awake human volunteers without a laryngoscope in place, the results called into question the so-called three-axes alignment theory and the superiority of the sniffing position.
In this issue of Anesthesiology, the research group of Adnet et al. 2extended their work and evaluated whether the sniffing position produced better glottic visualization during direct laryngoscopy than simple neck extension did. The study was performed in 456 anesthetized, nonparalyzed adults. They found that the incidence of difficult laryngoscopy (defined as Cormack grade 3 or 4) was 11.4% in the sniffing position and 10.7% with simple neck extension. In addition, the distribution of Intubation Difficulty Scores (a measurement also developed by this group) did not statistically differ between the two positions. This study confirmed the results of their magnetic resonance imaging assessment in clinical settings.
This kind of rigorous reassessment of a critically important clinical issue deserves our highest regard. However, their findings should not be carelessly interpreted. While I agree with inappropriateness of the three-axes alignment theory, the inferiority of the sniffing position for direct laryngoscopy was not shown in either the current trial or their previous magnetic resonance imaging study. Although they demonstrated that simple neck extension is as good as the sniffing position in most situations, they also showed that the sniffing position is advantageous in obese patients and patients in whom head extension is limited.
It was a great shock for me to learn that the sniffing position had become a “gold standard” since the historic article of Bannister and Macbeth 3published in 1944 and to realize how many anesthesia textbooks had recommended this as the best head and neck position for laryngoscopy and had adopted the three-axes alignment concept as a theoretical background. Although the sniffing position has an advantage over the neutral head position for laryngoscopy, it is surprising that a systematic search for a better head and neck position has not captured the interest of the anesthesia community. Even the excellent studies performed by Adnet et al. 1,2do not answer the question “What is the best head and neck position for direct laryngoscopy?”
The anatomy and biomechanics of the head and neck differ significantly among patients and, moreover, change differently in response to head and neck positioning. Furthermore, varying responses to external laryngeal pressure and different types of laryngoscope blades further complicate this issue. With this complexity, all possible combinations cannot be easily tested. What is needed is a new conceptual framework to understand the mechanisms of laryngoscopy, a framework that moves beyond the three-axes alignment theory. Adnet et al. 1demonstrated that in comparison with the neutral head position, the sniffing position and simple neck extension both reduced the angle between MA and PA but increased the angle between PA and LA. This suggests that approximation of the three axes is not essential for the glottic view but rather indicates that reduction of the angle between MA and PA, caused by upper cervical extension, is fundamental to aligning the line of vision and LA. Unfortunately, this kind of axial theory does not take into consideration the interaction between anatomic axes and surrounding structures, or laryngoscopy itself.
Cormack and Lehane 4explained, based on anatomic consideration, that three main factors block the line of vision during laryngoscopy: upper teeth and forward displacement of the larynx and downward displacement of the tongue. Their explanation can be developed into a more generalized concept, which may be useful in understanding the steps in dynamic structural interactions during laryngoscopy as well as mechanisms of difficult laryngoscopy. Our aim during laryngoscopy is to reach the vocal cords through the originally curved, oral airway space (fig. 1). As illustrated in figure 1a, there are two groups of obstacles between our eyes and the vocal cords: obstacles located posterior to the oral airway space (upper teeth, maxilla, head, and others) and obstacles located anterior to the airway (tongue, epiglottis, mandible, and others). Raising the head from the table in the sniffing position (anterior flexion of the lower cervical spine) produces upward movement of both obstacles (fig. 1d). Slight extension of the facial plane from the horizontal (extension of atlantooccipital joint) moves the posterior obstacles downward (fig. 1e). In contrast, simple neck extension on a flat operating table produces upward movement of the vocal cords and anterior obstacles (fig. 1b). Considering that the essential action of direct laryngoscopy is to move the anterior obstacles upward, caudally allowing complete visualization of the vocal cords (figs. 1c and f), the fundamental role of positioning is to shift the posterior obstacles downward, ensuring posterior field of vision, whereas optimal position of the laryngoscopist’s eyes may differ between positions. Furthermore, backward and upward movement of the vocal cords by externally applied forces may result in improvement of the visual field. Unfortunately, although this “obstacle theory” explains why limited neck extension prevents proper rearrangement of the obstacles during positioning, it does not clarify the mechanisms by which this is offset by the sniffing position. However, the theory does predict difficulty in visualization of the vocal cords when the obstacles are abnormally located (receding mandible, hanging epiglottis, prominent upper incisors) or increased in size (macroglossia, tumors, swelling, obesity) or when movement is limited in response to positioning and laryngoscopy (limited mouth opening, limited neck extension, obesity). For example, obesity may increase the size of the obstacles, limit the movement of the anterior obstacles, and, of course, also interfere with the laryngoscopy handle. Although conceptual understanding of the mechanisms underlying direct laryngoscopy is crucial for reducing the chances of airway misadventures and improving the safety of endotracheal intubation, this theory or any other new concept needs scientific validation.
Finally, I would like to make one comment. It is tempting to argue that recently introduced alternative intubation techniques make some of these issues obsolete. Although the introduction of new airway management techniques (e.g. , fiberoptic devices, intubating laryngeal mask airways) 5may someday render direct laryngoscopy an anachronism, that time is far in the future. In the interim, I strongly urge anesthesiologists to follow the lead of Adnet et al. 1,2in seeking better techniques through systematic and scientific examinations of the basic biomechanics of laryngoscopy.