Sperior laryngeal nerve block is a valuable technique to facilitate upper airway anesthesia. The standard “blind” approach uses the close anatomical relationship of the superior laryngeal nerve to the hyoid bone and thyrohyoid membrane.1–3 Even though ultrasonography is capable of providing detailed imaging of the airway and adjacent anatomy, visualization of the superior laryngeal nerve has not been possible.4–6 There has only been a single case report describing superior laryngeal nerve block facilitated by ultrasound guidance without nerve visualization.7 The purpose of this study was to develop an approach for ultrasound-guided superior laryngeal nerve block in humans by defining the ultrasound technique, and sonographic landmarks in volunteers followed by cadaver simulation of real-time ultrasound-guided superior laryngeal nerve injection with dye.
After approval by the University of British Columbia Ethics committee, written informed consent was obtained from 20 volunteers. Inclusion criteria were healthy volunteers with no previous surgery or implants in the neck region. Exclusion criteria were allergy to ultrasound gel, age <25 or >75 years, body mass index >45 kg/m2, neck immobility, or pathology. All subjects were placed supine with the neck extended. All ultrasound scans were performed by one investigator (B.K.) using an 8 to 15 MHz transducer (HST15 to 8/20 linear probe, Ultrasonix, Richmond, BC, Canada) to identify the superior laryngeal nerve bilaterally. The transducer was initially placed in the sagittal plane to identify the greater cornu of hyoid bone and then rotated transversely to identify the superior lateral aspect of the thyrohyoid membrane. By rotating the medial aspect of the transducer cephalad, the superior laryngeal nerve was identified (Fig. 1). Sonographic views were considered feasible when key anatomical structures (hyoid bone, thyrohyoid membrane, thyroid cartilage, superior laryngeal artery and superior laryngeal nerve) could be delineated. Sonographic examinations were recorded, the best image captured and graded, based on a 0 to 2 scale (Fig. 2). In each volunteer, a minimum of 2 captured images (bilateral scans per volunteer) were obtained. The primary outcomes were the abilities to (a) visualize the superior laryngeal nerve, (b) identify adjacent landmarks, (c) trace the superior laryngeal nerve medially as it pierced the thyrohyoid membrane, and (d) time to scan (seconds).
A convenience sample of 20 volunteers with bilateral ultrasound scanning yielding a sample size of 40 was used to obtain reliable data on the outcomes of interest. Data were analyzed using SPSS 19.0 stats program and GraphPad software to calculate the confidence limits for scan times (Shapiro-Wilks then Wilcoxon’s test for nonparametric data) and percentage visualization of structures.
Two female cadavers (weight, 43 and 45 kg; height, 173 and 175 cm, respectively) were obtained and studied in accordance with the Ethics Review Board guidelines and after institutional approval. An 8- to 15-MHz transducer was used to identify the superior laryngeal nerve (bilateral scans per cadaver) using the technique developed in volunteers. An 18-gauge block needle was inserted in-plane to the ultrasound beam and advanced to each superior laryngeal nerve and 1 mL of green dye was injected.
An anatomist then performed a dissection to the superior laryngeal nerve area in a layer-by-layer fashion. For each cadaver, the location and spread of dye was determined, photographed, and referenced with respect to the intended target structures.
Superior laryngeal nerve sonography was feasible in all 20 volunteers (7 female, 13 male) (Table 1). Forty sonographic video recordings of the superior laryngeal nerve were obtained. Sonographic visualization of the key landmarks and superior laryngeal nerve are summarized in Figure 2. The hyoid bone, thyrohyoid cartilage, thyrohyoid membrane, and superior laryngeal nerve were visible in 40/40 scans (95% CI, 0.896 to 1.0). The superior laryngeal artery was visualized in 4/40 scans (95% CI, 0.04 to 0.24), not visualized in 18/40 scans (95% CI, 0.31 to 0.60) and equivocal in 18/40 scans (95% CI, 0.31 to 0.60).
In both cadavers, the targeted structures in the superior laryngeal nerve area and the superior laryngeal nerve were identified on 4 of 4 sides. Successful dye placement around the superior laryngeal nerve was confirmed by anatomical dissections in 4 of 4 attempts. The ultrasonographic view of the superior laryngeal nerve and relevant structures is illustrated in Figure 3.
In this study, we present a method for the ultrasonographic visualization of the superior laryngeal nerve followed by successful cadaver simulation of real-time ultrasound-guided superior laryngeal nerve injection.
Even though the superior laryngeal nerve space can be visualized by ultrasonography, the superior laryngeal nerve has proven difficult to visualize.4–7 Factors that are important for successful ultrasonographic nerve visualization and block include nerve size, depth of nerve from skin, imaging technology, and needle technology.8–10 We successfully visualized the superior laryngeal nerve using a small hockey stick-shaped 8 to 15 MHz transducer. The probe was small enough to be manipulated correctly and aligned in both transverse and longitudinal planes without image compromise. We found it easier to place the transducer in a sagittal position to obtain a good image of the hyoid. Subsequently, the transducer was rotated obliquely to obtain a consistent ultrasonographic image of the superior laryngeal nerve because the nerve travels medially and caudally toward the thyrohyoid membrane.1 We also found it easy and useful to trace the nerve as it pierced the thyrohyoid membrane to confirm superior laryngeal nerve visualization. We noted that visualization of the superior laryngeal artery was inconsistent and difficult, despite earlier work suggesting the use of this landmark in finding the superior laryngeal nerve.6–7 We demonstrated that in-plane needle advancement toward the superior laryngeal nerve can be performed in real-time, and that 1 mL of dye injection in cadavers resulted in a clear spread around the superior laryngeal nerve, confirmed by anatomical dissection. These features are essential attributes for the avoidance of complications related to needle malposition. Although our sonographic views did not permit accurate measurement of the superior laryngeal nerve to hyoid distance, we concur with previous cadaveric studies which found the mean distance from the internal branch of the superior laryngeal nerve to the greater horn of the hyoid bone in a craniocaudal direction to be 2.4 mm.3
There are some limitations of this study. We only performed a cadaver simulation of real-time ultrasound-guided superior laryngeal nerve injection. We did not include a prospective clinical series. Controlled clinical trials of our method are definitely warranted.
In conclusion, we have demonstrated that ultrasound visualization and injection of the superior laryngeal nerve is feasible with a small hockey stick-shaped linear probe. We propose that this method has clinical application for blockade of the superior laryngeal nerve.
Name: Balvindar Kaur, MBBS, FANZCA.
Contribution: This author helped conduct the study, analyze the data, and prepare the manuscript.
Name: Raymond Tang, MSc, MD, FRCPC.
Contribution: This author helped design the study and conduct the study.
Name: Andrew Sawka, MD, FRCPC.
Contribution: This author helped conduct the study.
Name: Claudia Krebs, MD, PhD.
Contribution: This author helped with anatomical dissections.
Name: Himat Vaghadia, MBBS, FRCPC.
Contribution: This author helped design the study and prepare the manuscript.
This manuscript was handled by: Terese T. Horlocker, MD.
We thank Dr. Henrik Huttunen for helping us with images for the study.
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