The RLN is a branch of the vagus nerve. As the heart descents during embryological development, the RLN on the left is dragged down by the aortic arch; however, on the right, it remains high around the subclavian artery (Fig. 7.2).

As indicated the right RLN loops round the subclavian artery and enters the base of the neck at the thoracic inlet in a more lateral oblique direction.

The left RLN loops around the aortic arch just lateral to the obliterated ductus arteriosus and ascends along the tracheo-oesophageal groove before its entry in the larynx at the level of the cricothyroid joint. It has been noted that typically the left RLN forms an angle of between 15 and 30º relative to the trachea, but the right RLN appears to cross the neck in a more oblique angle [8].

Hisham et al. found that in 60% of cases, the nerve on both sides is located in the tracheo-oesophageal groove, 5% more lateral to the trachea and 28% directly posterior to the thyroid lobe [9].

Although most RLNs are single, up to 30% of nerves can branch before entering the larynx, and this may lead to higher risk of injury [10, 11] (Fig. 7.3).

As stated the identification and preservation of the external branch of the superior laryngeal nerve (EBSLN) and the recurrent laryngeal nerves (RLN) during thyroid and parathyroid surgery are essential. In every thyroidectomy, an attempt should be made to identify the RLN. Without this attention to detail, injury rates may be unacceptably high.

The RLN and ESBLN are at risk of iatrogenic injury during thyroidectomy given their close anatomical relationship to the thyroid and parathyroid glands. Despite appropriate surgical training and experience, these nerves may be difficult to identify and protect intraoperatively, especially in cases of invasive thyroid cancer, large goitres or revision surgery. Intraoperative neuromonitoring (IONM) is a useful aid for localising and monitoring the electrophysiological status of the RLN and EBSLN during surgery; however routine use of IONM remains controversial, as will be discussed later.

Current guidelines recommend definitive visual identification of the RLN and EBSLN during thyroid surgery being this the most reliable method of reducing the rate of injury [15, 16]. Historical surgical training promoted complete avoidance of the RLN as a means of preventing nerve injury. However, following descriptions of newer surgical techniques over 70 years ago [17, 18], during which an attempt was made to systematically recognise the RLN, visual nerve identification became an acceptable practice. Further supporting evidence for routine RLN visualisation to prevent iatrogenic injury comes from Hermann et al. [19] who compared two historical cohorts (of over 26,000 patients) undergoing thyroidectomy for benign disease, from a period when RLN identification was not performed (1979–1990), to more recent times (1991–1998), when RLN visualisation was routine practice, and the authors demonstrated that the rate of permanent RLN palsy was significantly lower with RLN identification (0.4%) versus non-identification (1.1%).

In spite of relatively low rates of permanent vocal fold paralysis with RLN visualisation alone during thyroidectomy (i.e. <2%) [20], the negative impact on laryngeal function [21–24] and the medicolegal implications [25, 26] of vocal fold paralysis have prompted thyroid surgeons to seek further reductions in the incidence of RLN injury, through the use of surgical adjuncts such as IONM. Notwithstanding the purported benefits, several studies over the past 20 years have failed to demonstrate an obvious statistically significant difference in the rates of permanent vocal fold paralysis when comparing routine use of IONM versus visual RLN identification alone [20, 27–29], a result that was confirmed in a recent systematic review of 42 studies and over 64,000 nerves at risk [30]. With the lack of strong evidence for a clinical advantage of IONM in thyroid surgery, the additional costs associated with specialised equipment and consumables [31], surgeon learning curve and theatre staff training time are cited as further reasons why its routine use may not be justified. However, whilst the benefits of IONM for routine thyroid surgery are debated, IONM has been shown to reduce the rate of RLN injury during revision surgery [32] or when neck anatomy is complex, e.g. large goitres [33]. Therefore, in the context of high-risk thyroidectomies, IONM is widely accepted to be beneficial. Conversely, for surgeons to gain experience with IONM for difficult cases, it is important that they are familiar with its use during routine procedures, so that they can precisely interpret electrophysiological information acquired during complex surgeries.

There are several additional benefits of routine IONM during thyroid surgery which include earlier identification of the RLN and EBSLN and a consequent reduction in operative time [33, 34], more accurate prediction of postoperative vocal fold palsy, given that an anatomically intact RLN may not be functional (especially if neuropraxia has occurred secondary to blunt or stretch injury) and clearer identification of the site of neuronal injury. Surgeon recognition of RLN injury from visual inspection only is very poor, at around 15% of cases [35], and this is particularly relevant in the context of bilateral RLN injury which is also poorly recognised from inspection alone [36]. The use of IONM can prevent bilateral RLN paralysis and its life-threatening complication of airway obstruction. Failed IONM stimulation of the RLN after hemithyroidectomy has resulted in some surgeons changing the operative strategy and delaying contralateral thyroid lobectomy, with a significant reduction in bilateral RLN palsy rates, compared to a reliance on visual neural integrity alone [37]. Overall, the benefits of IONM appear to outweigh any potential disadvantage from increased equipment or training costs; hence current guidelines recommend the use of laryngeal electromyography (EMG) monitoring during thyroid surgery [16].

The principles of IONM are twofold:

Stimulation of the RLN or EBSLN may be achieved using low current (1–2 mA) applied directly to the nerves or indirectly by stimulation of the ipsilateral vagus nerve. Stimulator probes may be monopolar or bipolar, with the latter producing less stimulation artefact and consequently has a greater sensitivity. Bipolar stimulating electrodes must be used in the correct orientation of the anode and cathode for effective nerve stimulation [38].

Monitoring of vocal fold response to RLN or vagal stimulation can be accomplished in a number of ways:

Of all the monitoring techniques mentioned above, endotracheal tube surface electrodes are most commonly used worldwide [38], because it is a simple, non-invasive, commercially available technique that does not require additional skill or expertise from the operating surgeon for correct electrode placement. There are several manufacturers of neuromonitoring equipment, and a popular commercially available system is the Medtronic Nerve Integrity Monitoring (NIM^®) 3.0 unit (Fig. 7.9) which consists of surface electrodes integrated into endotracheal tubes and a monitor that provides a visual EMG waveform and auditory signal of vocal fold activity following appropriate stimulation using a probe attached to the NIM system. An advantage of the NIM unit is that it is also widely used to facial nerve monitoring during parotid or otological surgery, making it readily available in most hospitals. However the use of ET tubes with integrated electrodes restricts the anaesthetist to using Medtronic supplied tubes, which reduces flexibility given the limited range of Medtronic endotracheal tube sizes. Other manufacturers, such as Inomed (Inomed Medizintechnik GmbH), produce adhesive laryngeal electrodes for use with their nerve-monitoring units that can be applied to standard endotracheal tubes.

Regardless of the ET surface electrode or EMG monitoring system employed, a number of essential aspects require consideration prior to IONM use. The surgeon must be familiar with the neuromonitoring equipment and may have to attend additional training sessions with the manufacturer to achieve this. Unambiguous communication between the operating surgeon and anaesthetist on the preoperative laryngoscopy findings, especially if abnormal, is also important. The anaesthetist should be made aware of the need for accurate placement of the ET tube so that surface electrodes are between the vocal folds, a process that should be relatively straightforward, especially if the vocal folds are clearly visualised during intubation. Long-acting neuromuscular-blocking agents must be avoided during ET intubation, so as not to interfere with neural stimulation.

During thyroid and parathyroid surgery, surgeons may use IONM to aid identification of the RLN or EBSLN. This usually involves intermittent stimulation of the area being dissected with interpretation of the auditory signal and EMG waveform from the nerve monitor to confirm whether or not the RLN has been stimulated. The International Neural Monitoring Study Group (INMSG) has produced thorough guidelines on IONM equipment set-up for thyroid/parathyroid surgery, anaesthetic considerations and interpretation of electrophysiological data and have also standardised loss of signal troubleshooting algorithms [38]. As experience with IONM increases, it has become more evident that reliance on auditory signals of RLN stimulation is unreliable with a reported positive predictive value ranging from 9.2% to 94% [38]. A four-step procedure for IONM during thyroid surgery has been advocated [38, 47]:

Quantification of the laryngeal EMG for prognostication of postoperative vocal fold function is a key advantage of neuromonitoring systems which provide a visual EMG waveform; however this requires a basic understanding of waveform amplitude, threshold and latency, for the surgeon to interpret and act on the information provided. The INMSG has attempted to standardise these EMG waveform characteristics [38]. Repeated stimulation of the RLN and vagus nerve has so far not been shown to cause permanent neurological deficits [38].

The EBSLN is closely related to the superior thyroid pole vessels and is at risk of injury during ligation of the superior thyroid pole. This may result in difficulty with voice projection and pitch [49], especially in females. Through the human communicating nerve, the EBSLN innervates the anterior half of the ipsilateral vocal fold in up to 85% of patients [50], and this forms a basis that current IONM systems use can monitor the electrophysiological status of the EBSLN [3].

As with IONM for the RLN, the INMSG has proposed an algorithm with a sequence of neural stimulation steps to minimise the risk of EBSLN injury [3]. Standards for equipment set-up, endotracheal tube placement, anaesthetic considerations etc. are the same as for RLN IONM, with a number of additional steps required for the EBSLN. Similar to the RLN, identification of the EBSLN prior to ligation of superior thyroid pole vasculature is recommended in all patients. The anatomy of the EBSLN has been described earlier. It is worth remembering that the EBSLN may not be visually identifiable in up to 20% of patients because the nerve runs in a subfascial or intramuscular plane. IONM is particularly useful in this case, because neural mapping with a stimulation current of 1–2 mA can be used to detect these otherwise “hidden” nerves.

IONM of the EBSLN is a two-step process. Firstly, upon visualisation of the nerve in the sternothyroid-laryngeal triangle, it is stimulated at a current of 1 mA whilst observing for contraction of the cricothyroid muscle (CTM). The presence of an unambiguous CTM contraction or “twitch” confirms positive identification of the EBSLN. Additionally, prior to ligation of the superior thyroid pole, stimulation of the tissue to be divided should be performed, with concurrent inspection for a negative CTM twitch to confirm the absence of neural tissue. Secondly, upon EBSLN stimulation, the nerve monitor is observed for a laryngeal EMG waveform, with an amplitude that is usually one-third that of the ipsilateral RLN in comparison. Currently, EBSLN stimulation will result in an identifiable laryngeal EMG waveform in only 70–80% of patients [3]. The reasons for this are not clearly understood, but the INMSG has proposed that it is more likely related to equipment issues, such as suppression of early evoked potential responses or the influence of endotracheal tube positioning, given that terminal branches of the EBSLN innervate only the anterior 50% of the ipsilateral vocal fold [3]. Using the two techniques described above, the thyroid surgeon should be able to identify the EBSLN in 100% of cases.