Intraoperative Neuromonitoring (IONM) in thyroid surgery is a complement that can reduce recurrent laryngeal nerve (RLN) injury. In that setting, it can have a positive impact on patients’ recovery after thyroidectomy. Although not all papers in worldwide literature report that advantage, many do report a decrease in nerve palsy with the use of IONM in thyroid surgery. So, in my opinion, it is a very useful tool that can help surgeons achieve better outcomes and avoid severe complications.
On May the 13th 2023, Porto Thyroid Meeting was held in Porto, Portugal. Besides being part of the organizing committee with Prof. Jaime Vilaça, Prof. Oscar Vidal, Dr. Susana Graça, and Prof. Carlos Serra, I lectured about neuromonitoring in thyroid surgery. The following words will be a small resume of that presentation.
Lesion to the RLN occurs in 2 to 13% of standard thyroidectomies and increases in surgery for thyroid cancer. A rate of 1 to 2% of permanent injury has been reported in expert hands. Since Lahey in 1938 that intraoperative visualization of the RLN is the standard of care, associated with a reduction in both temporary and permanent nerve paralysis. Additionally, surgeons must not forget that most injuries occur due to traction, heat, and compression and that in these scenarios the RLN can be visually intact but malfunction. IONM has a significant role in thyroid surgery because it gives the surgeon information on the nerve function, working as a complement to the visual identification of the nerve.
IONM has several benefits in thyroid surgery:
Neural mapping
rates of nerve identification up to 98-100%
Insight into pathological states of the nerve
an invaded nerve with vocal cord paralysis (VCP) can have a residual EMG activity – resect, or do not resect the nerve in case of need?
Identification of impending injury
modify surgical maneuvers (e.g., stop applying traction to the thyroid)
Detection of intraoperative injury
extremely important to avoid bilateral VCP in total thyroidectomy – loss of signal (LOS) on the first side can make you not proceed with contralateral lobectomy.
So, IONM has prognostic power and can change operative strategy to avoid bilateral VCP and its significant morbidity (e.g., tracheostomy). To perform a correct neuromonitoring it is essential to have a pre- and postoperative laryngoscopy, and follow the bellow sequence of stimulation:
V1 – presurgical vagal stimulation (3 mA)
this gives you the true positive
you can only accept a negative stimulation as a true negative after getting a true positive
tip: never cut tissue based on a negative stimulation if a true positive has not been obtained yet
R1 – RLN stimulation before (2 mA) and during dissection (1 mA)
R2 – RLN stimulation after dissection (1 mA)
V2 – postsurgical vagal stimulation (3 mA)
most accurate prognostic test of RLN function (if you stimulate only the RLN, you can miss an injury in a nerve segment proximal to the point of stimulation)
Many times, a LOS is not a true LOS but only a malfunctioning of the neuromonitoring system. When to consider a true LOS?
V1 amplitude > 500 µV at the beginning
No or low response (<250 µV) with stimulation at 1 or 2 mA in a dry field
Absence of laryngeal twist on ipsilateral vagal stimulation
When a LOS occurs during a procedure, the surgeon must keep calm and work with the anesthesiologist and nurse to find out if there is a stimulation problem or a recording site problem. An algorithm to follow in the case of LOS and confirm neural injury is presented in Figure 1.
Another advantage of neuromonitoring is to identify a non-recurrent laryngeal nerve (NRLN). Stimulate the vagal nerve in two points:
At the level of the superior border of the thyroid cartilage;
At the level of the fourth tracheal ring.
While in a RLN you get a positive EMG in both points, in the case of a NRLN you have no EMG when stimulating at the level of the fourth tracheal ring. An NRLN is a rarity (0.5-1%), but it is a possibility surgeons can diagnose easily with the advent of IONM.
Finally, there are data supporting the use of IONM in thyroid surgery. In 2009, Barczynski et al. concluded that IONM is associated with lower rates of temporary paralysis compared to visual identification alone. The paralysis rate decreases from 19% to 7.8% in reoperations (Chan et al. 2016). In 2017, a paper from Wong et al. pointed out that IONM has benefits in high-risk thyroid surgeries (cancer, revision surgery, large goiters, and substernal goiters). Additionally, papers from 2002 and 2007 concluded that IONM is associated with significantly lower RLN paralysis in the benign subgroup.
Has take-home messages I presented the following two:
IONM is a complement to visual identification of the RLN and reduces postoperative paralysis.
IONM is strongly indicated for high-risk surgeries but should be considered for all cases.
Whether you use IONM or not, carefully identify the nerve in all cases. If you have neuromonitoring available, use it. If you do not have it, ask for it. IONM can make a difference.
Do you enjoy adrenal surgery?
Must have this new book!
Costa Almeida CE, editor. Posterior Retroperitoneoscopic Adrenalectomy. Indications, Technical Steps and Outcomes. Switzerland: Springer; 2023.
Dr. Carlos Eduardo Costa Almeida
General Surgeon
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