Correspondence  |   February 2009
Difficult Tracheal Intubation and a Low Hyoid
Author Affiliations & Notes
  • Shiroh Isono, M.D.
  • *Chiba University, Chiba, Japan.
Article Information
Correspondence   |   February 2009
Difficult Tracheal Intubation and a Low Hyoid
Anesthesiology 2 2009, Vol.110, 431. doi:
Anesthesiology 2 2009, Vol.110, 431. doi:
In Reply:—
We thank Dr. Greenland for his thoughtful comments on our article. Difficult intubation is common in obstructive sleep apnea patients and, vice versa  , obstructive sleep apnea is common in patients with difficult intubation. He suggests a low hyoid and increased submandibular angle as common anatomical features for both obstructive sleep apnea and difficult tracheal intubation. While we completely agree with the strong linkage between difficult intubation and obstructive sleep apnea, we believe these are independent pathogenic conditions caused by different structural mechanisms.
Certainly, the low lying hyoid per se  can result in difficult tracheal intubation, since it impairs an essential step during direct laryngoscopy for improving the laryngeal view, i.e.  , vertical arrangement of the mandible, tongue base, and larynx to the facial line as we recently reported.1 In contrast, obstructive sleep apnea can be developed in subjects with normal hyoid bone position. It is of note that hyoid position of apneic patients with large maxillo-mandible size did not differ from that of nonapneic persons as shown in the table 3 of the article.2 Infants have more collapsible pharyngeal airways than adults although the hyoid bone locates more cranially in infants than adults.3 Pharyngeal patency is improved by neck extension and impaired by neck flexion, whereas the hyoid bone locates more cranially during neck flexion than during neck extension.4,5 The hyoid bone is mobile only in humans; other mammals rarely have obstructive apnea during sleep while obstructive sleep apnea is common in humans, implying possible involvement of mobility of the hyoid bone in pathogenesis of obstructive sleep apnea.6 However, we do not believe that the low hyoid per se  increases pharyngeal collapsibility. Similarly, we consider that increased submandible angle alone contributes little to mechanisms of difficult tracheal intubation, since tracheal intubation is not difficult in most obese patients in whom increased submandible angle is a common feature.7 
The presence of common anatomical abnormalities does indicate strong linkage between difficult tracheal intubation and obstructive sleep apnea. However, it does not indicate presence of common pathogenesis between them, nor their significant roles in the pathogenesis.
*Chiba University, Chiba, Japan.
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