Comment on “Clinical roles of point-of-care ultrasonography in airway management”

TO THE EDITOR: I read with great interest the recently published review article “Clinical roles of point-of-care ultrasonography in airway management” by Seo et al. [1]. The article brilliantly elaborated the role of point of care ultrasonography (POCUS) in airway assessment and management, however, some irregularities in the article needed immediate attention.

First, in the probe selection section, the authors have mistakenly mentioned “a high-frequency curvilinear probe is preferred for imaging deeper structures…” which should be corrected to “a low-frequency curvilinear probe”.

Secondly, they recommended the use of a curvilinear probe in suprasternal view, which gives us information on anatomical structures surrounding the trachea in a panoramic view; however, I would like to point out that a linear probe in this view is more commonly preferred, as mentioned in reference number 10 cited by the authors themselves [2]. One study compared the two probes and concluded that the diagnostic accuracy for endotracheal tube confirmation was not significantly different between the two transducer types; however, curvilinear probes were associated with longer time to confirmation and lower operator confidence level [3].

Third, there is a discrepancy between the description in the text and the depiction in Fig. 1 regarding the measurement of tongue thickness. In Fig. 1, the tongue thickness is shown to be measured from suprahyoid muscle to the tongue surface, but in text it is described from skin to the dorsal surface of tongue. Although there is variability in the literature and there is no single universally accepted definition of tongue thickness, some studies define it as the distance from the skin to the dorsal surface of the tongue, while others measure it from the suprahyoid muscle to the dorsal surface of the tongue [4,5]. Nonetheless, this inconsistency in the text and the figure could be confusing to readers.

Fourth, in Fig. 2, the thyrohyoid membrane (THM) is visualized between the two strap muscles (SM) in a transverse view; the same has also been described by Lin et al. [6]. I would like to mention here that THM can also be identified in parasagittal view from the hyoid bone to thyroid cartilage and is a useful landmark for superior laryngeal nerve block, given for awake fibreoptic intubation. Also, in Fig. 4A, the POCUS view is mistakenly mentioned as longitudinal instead of transverse plane view.

Finally, the echogenicity of various tissues also needs correction, e.g., cartilages like the trachea and muscular bands like vocal cords are mentioned as hyperechoic instead of hypoechoic. It is the underlying air-mucosal interface in these structures that imparts a hyperechoic appearance. Hence, it would be more appropriate to be more precise and use accurate terminology regarding the specific components contributing to hyperechoic appearances in airway ultrasound. Similarly, the THM, which is hyperechoic in appearance [6], is mentioned as hypoechoic in the article.

Despite these minor errors, the authors have given an excellent insight into the clinical roles of airway POCUS, which needs to be incorporated into our routine teaching and learning curriculum. The American Society of Regional Anesthesia and Pain Medicine expert panel has given recommendations for education and training of POCUS skills, including those for airway ultrasound [7]. They have defined the standardized ways of gaining competencies, the maintenance of which requires regular practice and staying abreast with the up-to-date technologies and applications.