Cernea et al. [9] and Kierner et al. [33] describe the nerve path in relation to the STA, Friedman et al. [19] use as landmark the inferior constrictor muscle (ICM), and Selvan et al. [53] analyze the relationship between the entries of the nerve into the cricothyroid muscle, the superior thyroid vessels (STV), and the cricoid cartilage. Table 17.2 summarizes these classifications.

The Cernea et al.’s [9] classification divides EBSLN into type 1, that is, the nerve crosses the STA more than 1 cm above the margin of the superior thyroid pole (68 % in case of small goiter; 23 % in case of large goiter); type 2A, that is, the nerve crosses the STA <1 cm above the margin of the superior thyroid pole (18 % in case of small goiter; 15 % in case of large goiter); and type 2B, that is, when the nerve crosses the superior thyroid pedicle below the margin of the superior thyroid pole (14 % in case of small goiter; 54 % in case of large goiter).

Cernea 2A and 2B types are particularly frequent (15–55 %) and prone to be injured during the upper thyroid pedicle dissection and ligation.

Cernea classification is nowadays the most commonly used EBSLN surgical classification in the clinical practice and research, for its simplicity and ease of use.

Knowing this anatomical space, otherwise called Jolles space, is useful to find the EBSLN. The triangular region has the base at the bottom, represented by the upper edge of the superior thyroid pole; the medial margin is composed of the thyroid and cricoid cartilages, the pharyngeal constrictors and cricothyroid muscle, and the retracted sternothyroid muscle which normally lies above laterally. In the medial portion of the area, it is usually possible to find the EBSLN. The STV are located laterally and the STA between the vein and the nerve [42].

Another important aspect about the EBSLN is the presence of anastomoses between its trunk and the other nerves, particularly with the IBSLN and the recurrent laryngeal nerve (RLN). The connection between IBSLN and EBSLN runs through the thyroid foramen (an opening inside the thyroid cartilage) when present, often followed by the blood vessels [52]. The human communicating nerve (HCN) [61], also called pyriform nerve [51] or cricothyroid connection branch [38], is the anastomosis which connects the EBSLN to the RLN in one or both sides. Its position between these nerves suggests that it could be the nerve of the fifth branchial arch [51, 61]. It starts from the medial surface of the cricothyroid muscle and enters into the lateral surface of the thyroarytenoid (TA) muscle. Sometimes it is double branched, but more frequently it is a single trunk [38].

Depending on the position where the human communicating nerve joins the RLN, four variants of anastomoses have been described [38]:

This nerve is composed of sensory fibers, passing into the TA muscle to terminate near the cricoarytenoid joint in the subglottic mucosa, and motor fibers joining the RLN or entering the TA muscle directly [61].

RLN and EBSLN share certain monitoring standards of equipment setup, endotracheal tube placement, anesthesia, and correct tube positioning verification tests [45]. Monitoring is set with event threshold of 100 mcV (150 mcV usually for RLN) and pulsatile stimulus duration of 100 mcs at 4 Hz [5]. We commonly use the intermitted IONM manual stimulator probe as dissecting instrument, disposable or reusable. We prefer the ball tip stimulator probe which is atraumatic during dissection (Fig. 17.2). On the other hand, stimulating electrodes may be monopolar or bipolar. Bipolar stimulator may be preferred to monopolar one for EBSLN monitoring due to its small nerve diameter, less stimulation artifacts, convergence, and precision of stimulation (Fig 17.3). Bipolar stimulator may provide the potential greater sensitivity [5]. If a bipolar device is used, the precise orientation of the anode and cathode stimulating electrodes placed on the nerve is of crucial importance. The bipolar probe may not be the best option for nerve mapping since the stimulation is more focal in comparison with the monopolar probe, which provides more diffuse current spread and may facilitate the mapping of a wider region [5]. The stimulator probe should be set at 1–2/3 mA. Stimulation is set with 2–3 mA for nerve identification, mapping, and dissection; with 1 mA for nerve confirmation, monitoring, S1 and S2 stimulation (Table 17.3). In particular, for confirmation of visually identified EBSLN, a 1 mA current should be preferred but higher values (up to 2 mA) should be used for EBSLN mapping. Multiple and heterogeneous different nerve-monitoring formats have been studied but for safety and simplicity reasons; endotracheal tube-based surface electrode systems are the most commonly used monitoring system [45]. De facto, endotracheal tube-based systems with a graphic monitor documentation are preferred for neural monitoring. New audio and graphic monitor IONM equipment must be preferred today to only audio monitors due to the possibility of print documentation, amplitude and latency quantification, storage/record, differentiation between signal and artifact, forensic issues, research, and justifications of surgical deliberations. In fact, audio-only systems lack EMG response stimulation assessment (threshold, amplitude, latency, and waveform shape) [5, 45]. We routinely adopt EMG electrode surface endotracheal tube. Endotracheal EMG tube electrodes gain more popularity recently due to its availability, safety, noninvasive nature, ease of setup, ease of use, and the capacity to derive larger areas of evoked muscle potentials as for EBSLN monitoring. Finally, the SLN is the first branch of the vagal nerve after skull base exit; thus continuous monitoring (C-IONM) probe electrode cannot monitor EBSLN; however C-IONM may provide simultaneous and permanent monitoring of RLN during upper pole dissection.

An accurate surgical technique must be adopted to spare the cricothyroid muscle and the EBSLN. The EBSLN course must be visually identified and monitored in order to avoid neural damage when approaching the superior pole by identification of its course. For this reason, a perfect anatomical knowledge and the classification of the different EBSLN variations are mandatory [5]. In the avascular space between the superior pole and the cricothyroid muscle, a blunt dissection of the superior pole should be performed to obtain clear exposure of the sternothyroid–laryngeal triangle where the EBSLN is located (Fig. 17.4). In the vast majority of cases, there is no need for transverse division of the strap muscles if the thyroid has a normal size or it is only slightly enlarged. However, in cases of large masses or if a short neck is present, a partial division of the sternothyroid muscle may improve the surgical access. Moreover, a better exposure of the sternothyroid–laryngeal triangle can be achieved via gentle latero-caudal traction of the thyroid lobe [5]. The transverse division of the cranial portion of the sternothyroid muscle should be performed with caution due to possible proximity of EBSLN. Previous stimulation/mapping is suggested to exclude EBSLN proximity with a stimulation set between 2 and 3 mA.

Via a blunt dissection, the superior thyroid vessels should be identified, and individual branches exposed [5]. Notably, transverse division of sternothyroid muscle (its superior edge) and a latero-caudal traction of the superior thyroid pole, followed by blunt dissection within the avascular plane of sternothyroid–laryngeal triangle, provide improved EBSLN exposure. The nerve is usually descending parallel to the superior thyroid artery and it is located on the inferior constrictor muscle before its termination within the cricothyroid muscle [5].

It is vital to visually search for the nerve, but of note, almost 20 % of EBSLNs cannot be visually identified because of their subfascial/intramuscular course [19, 34]. Selvan et al. have shown that in many cases, non-neural tissue can be misidentified for the EBSLN [53].