Need of intraoperative laryngeal nerve monitoring in head and neck surgeries.

Recurrent laryngeal nerve (RLN) and external branch of superior laryngeal nerve injuries associated with thyroid and parathyroid surgeries may lead to complications such as stridor, dyspnea, and voice changes. Traditionally, visualizations of vocal cords in the perioperative period have remained the sole method to identify intraoperative nerves injuries. The availability of intraoperative laryngeal nerve monitoring (IOLNM) and its added advantages of guided and graded surgical resection has been validated as a new tool in thyroid surgeries in many studies.[1]

However, owing to the low incidence of RLN injuries, most of the studies are underpowered and routine use of IOLNM remains controversial. In 2011, International Neural Monitoring Study Group standardized the use of IOLNM to identify nerve injuries as an adjunct to conventional direct visualization.[2] Identification of RLN and superior laryngeal nerves by IOLNM was associated with better preservation of short- and long-term voice function.[3]

IOLNM can be done by electromyography during which the electrodes are placed adjacent to vocal cords by needling of cricothyroid. There has been evolution of new noninvasive methods of vagal nerve monitoring during neck endocrine surgeries.[4] Noninvasive method averts the need of intraoperative vagal electrodes and solely depends on endotracheal electrodes to stimulate laryngeal mucosa and elicit laryngeal adductor reflex (LAR) whose response is continuously monitored intraoperatively. LAR monitoring detects the entire vagal reflex arc and has an advantage of preventing vagal nerve injury during surgery of neck.

These electrodes can be either attached to the surface of endotracheal tube (ETT) or specialized ETT with embedded electrodes can be used.[5] The embedded electrodes increase the outer diameter of ETT, an orotracheal intubation, and may make it difficult to rail road the ETT over the fiberoptic bronchoscope.

IOLNM during endoscopic thyroidectomies is done by placing a percutaneous neural monitoring probe through the tract made by 18G syringe after ultrasound scanning of the neck to prevent any vessel injury.[6] This described technique had no instances of probe malpositioning and interferences with other endoscopic instruments. Percutaneous IOLNM improves quality dissection and safety in endoscopic procedures. It has also added advantages of laryngeal nerve mapping and identification, assessment of information about nerve function and prognosis, maintains the standardization of monitoring technique, and provides confidence during thyroidectomies.

During monitoring, loss of signal (LOS) is defined as decrease in nerve amplitude to 100 μV from a baseline amplitude of more than 500 μV with suprathreshold vagal stimulation and is subdivided into segmental, focal type 1, and global type 2.[78] IOLNM can facilitate staged thyroidectomies and prevent bilateral RLN injuries whenever a LOS is detected intraoperatively on one side.[67] IOLNM has also been a useful adjunct in identifying abnormal nerves such as non-RLN and prevented its damage intraoperatively and hence postoperative complications.[9]

IOLNM has also been applied in central nerve dissections to identify RLN, but even with IOLNM RLN palsy may occur during exposure of the laryngeal nerves during central neck dissection.[6] However, owing to rarity of nerve injuries in central nerve dissection, a large-scale study is required to further validate the effect of IOLNM.

Re-exploration surgeries, large malignancies, and post radiation surgical neck dissection are associated with higher incidence of nerve injuries and many surgical organizations have suggested routine use of IOLNM.[1011121314]

RLN monitoring during thyroidectomies and other cervical procedures in special populations of children and adolescent groups facilitated the identification of RLN and predicted postoperative nerve paresis.[15]

Even with the widespread applications of IOLNM, many authors have not supported the use of this technique owing to its limitations. IOLNM has not decreased the operative time and has shown to increase the cost of surgery. In some studies, the technique has not been proven to prevent postoperative nerve paresis compared with gold standard nerve visualization.[16] There are also disparities with the use of IOLNM among various centers, ethnic groups, and surgeons’ choice.[17]

A large meta-analysis to know the protective effects of IOLNM in thyroid surgery can reduce the incidence of transient, total, or permanent RLN injuries during thyroidectomies. The result of meta-analysis also recommends its use in bilateral surgeries and malignant cases, but insisted the need of further exploration of use of IOLNM in re-exploration cases.[18]

There has been evolution and advancement of surgical technique to improve short and long-term surgical outcome in neck surgeries over years. Although IOLNM use has been controversial among various clinicians, many centers have incorporated it in routine patient care. Despite its limitations and conflicting results in routine thyroid surgeries, there is enough evidence to support its use during complicated neck surgeries such as large malignancies, preoperative nerve paresis, and postradiation neck explorations.