In 1970, Riddell (1) stated that, in a bilateral procedure, the secondary lobe should not be resected until integrity of the recurrent laryngeal nerve (RLN) has been confirmed by stimulation with an electrical current. Electrical stimulation can also reveal whether or not an unidentified strand of tissue is RLN tissue.

Dissecting and identifying the RLN are considered routine procedures now that safe and effective methods have been established (23). Preventing RLN injury during thyroid surgery requires good visualization and exposure of the RLN, adequate knowledge of RLN anatomy, adequate surgical experience and training, and pre- and post-operative laryngoscopy (Table 1) (23).

Whereas these requirements are widely accepted for routine thyroid surgery, new technical developments have emerged in the past 15 years. Specifically, intraoperative neural monitoring (IONM) has been widely adopted by thyroid surgeons, by general practitioners, and by specialists in endocrine medicine, head and neck cancer, and ear, nose and throat medicine (Table 2). For example, IONM is now used in 90% and 70% of all thyroid procedures performed in the USA and in Germany, respectively (45).

Currently, both intermittent IONM (I-IONM) (23) and continuous IONM (C-IONM) (78) are recognized as effective techniques for RLN management. Studies also show that IONM can facilitate functional preservation of the external branch of the superior laryngeal nerve (SLN) (6). Specific advantages of IONM include the following:

Below we discuss possible future directions for use of IONM in thyroid and parathyroid surgery (Table 3).

Notwithstanding these advantages, surgeons must be aware that current IONM technology has several important limitations (Table 4). First, currently available IONM techniques (including both neurosurgical and orthopedic techniques) cannot provide class I evidence. Using IONM for class II and class III studies also has major limitations (811) (Table 5). Ideally, IONM should provide the level of evidence needed for the thyroid surgery procedures most commonly performed in clinical practice. Outcomes of IONM applied in evidence based medicine are no worse than those obtained by conventional thyroid surgery. Class I evidence is rarely obtained by IONM because IONM is rarely used to prevent RLN deficit and because the incidence of permanent RLN complications is low (811). Table 4 shows the future directions of IONM research that are needed to improve its outcomes.

Currently, C-IONM procedures are performed more frequently than I-IONM procedures. One reason for the relatively greater use of C-IONM is that it enables continuous VN monitoring. That is, function can be monitored throughout the entire RLN. Additionally, C-IONM provides comprehensive electromyography (EMG) documentation, detects imminent RLN failure, and reveals RLN stress mechanisms. Finally, C-IONM can reveal malposition of an EMG tube (7812).

A major advantage of C-IONM is that, by alerting the surgeon to the potential for irreversible neural damage, it can reduce the risk of permanent RLN injury (78). For example, C-IONM can provide an early warning of impending neural injury by revealing adverse changes in EMG. Nerve function can then be preserved by taking appropriate corrective action and by aborting maneuvers that can potentially cause adverse EMG changes and permanent vocal cord paralysis (VCP) (78). However, thyroid surgeons require effective methods for identifying important adverse events and for differentiating between true adverse events and artifacts (7812). Additionally, amplitude reductions and latency increases detected by C-IONM can reveal a mild combined events (mCEs) or severe combined events (sCEs) (Table 6). Notably, mCEs and isolated amplitude or latency changes are not associated with VCP whereas sCEs denote electrophysiologic events that can be reversed by aborting the related surgical maneuver. However, both mCEs and sCEs can cause a loss of signal (LOS), which typically has much lower reversibility, and postoperative VCP (78). Thus, modification of a surgical maneuver associated with sCEs can prevent subsequent neural injury leading to postoperative VCP. However, surgeons should consider that any IONM modality is more effective for preventing impending neural damage secondary to a stretch or compression injury than for preventing neural damage caused by inadvertent neural transection or thermal injury.

For the above reasons, C-IONM performed via continuous permanent stimulation of the RLN via the VN is clearly an important future direction of IONM. That is, C-IONM is likely to become a standard procedure in monitored thyroidectomy.

Endoscopic thyroidectomy is a new and increasingly adaptive technique that demands full control of nerve function. Surgeons cannot apply new endoscopic approaches without full control of the laryngeal nerves. This would be in contrast with commonly performed routine open surgery. It would be a step back. Endoscopic thyroidectomy has to be in the same security area of open procedure for RLN management. To achieve a net benefit, all new approaches to remote thyroidectomy require an effective method of nerve monitoring.

In comparison to open procedures, acceptance of endoscopic thyroidectomy monitoring has been relatively slow (13). Some studies of monitoring methods for endoscopic or robotic procedures indicate that standard I-IONM (V1, R1, R2, and V2) is feasible and effective (13). In comparison, the use of C-IONM in endoscopic or robotic thyroidectomy is not well reported. One reason is the difficulty of using currently available C-IONM probe electrodes during endoscopic or robotic thyroidectomies.

Similarly, the ideal stimulating probes for I-IONM in endoscopic/robotic thyroidectomy have not yet been identified. I-IONM probes can be percutaneous using conventional open probes, or long probe (from spinal surgery), adapting dissecting endoscopic instruments to the IONM monitor, flexible wires electrodes, or integrating neural monitoring features with energy based devices (EBDs).

Technological developments in endoscopic thyroidectomy present an exciting opportunity to improve IONM outcomes and to simplify IONM techniques and accessories.

The first guidelines for neural monitoring of the RLN were published in 2010 (3). Guidelines for SLN monitoring were published in 2012 (6). The tremendous advances in IONM technologies and techniques in the past 5 years include new monitoring equipment, EMG tubes, intermittent and continuous probe stimulators, and software. New clinical evidence and outcomes have also been reported. Therefore, current guidelines must be updated. In addition to the updated guidelines required by health professionals, patients require updated information about monitored procedures, including technological failure rates and staged procedures.

Furthermore, with the introduction of IONM in endoscopic thyroidectomy, troubleshooting algorithms must also be reassessed because laryngeal twitch is difficult to assess by endoscopy. The same is true for contralateral VN stimulation, which are essential in the problem-solving moment. Based on the clinical experience of endoscopic thyroid surgeons, professional organizations (e.g., The American Society of Neurophysiological Monitoring) must clarify and refine guidelines for monitored endoscopic and robotic approaches, identify and prioritize important areas of research and technological development (e.g., further simplification of monitoring technology, use of SLN monitoring technology, use of C-IONM technology, and nerve injury mechanisms), explore the full utility of monitoring in endoscopic surgery, refine standards for equipment set up (e.g., endotracheal tube placement), refine standards for evaluating LOS events, and refine intraoperative problem-solving algorithms.