Method

Thyroidectomy

In general and visceral surgery, intraoperative neuromonitoring (IONM) can be of great value, especially during thyroidectomy for the surgical treatment of thyroid disease, and during the resection of colorectal cancer.

IONM method during thyroidectomy

Prior to thyroidectomy, a surface electrode should be attached to the endotracheal tube to record the neuromonitoring signals at the vocal muscle, unless a prepared prefabricated tube with electrodes is available. The nerve to be monitored during thyroidectomy is the (inferior) recurrent laryngeal nerve, a branch of the vagus nerve that innervates all laryngeal muscles (with the exception of the cricothyroid muscle) as well as the oesophagus and trachea. This nerve is stimulated either directly or indirectly via the vagus nerve.

IONM during thyroidectomy

Please note that the following recommendations on neuromonitoring during thyroidectomy are of a general nature. The individual vital parameters of the patients, the existing surgical situation as well as the technology used should always be considered.

During thyroidectomy, intraoperative neuromonitoring is used as follows:

Frequency: The stimulation frequency can be set from 0.1–30 Hz. When stimulating the recurrent laryngeal nerve and the vagus nerve, a frequency between 1 Hz and 7 Hz is usually selected.

Current: The maximum current to be set should be 5 mA for direct nerve stimulation (DNS) and continuous stimulation of the vagus nerve. Common values are 1–3 mA for DNS and 2–5 mA for continuous stimulation of the vagus nerve.

For stimulation during thyroidectomy, a stimulation probe should be selected that allows direct stimulation of the vagus nerve and the recurrent laryngeal nerve. Either monopolar or bipolar stimulation probes in various versions can be used for this purpose. As a check, stimulation of the vagus nerve should first be carried out to ensure that the recording electrode(s) is/are placed correctly, the measuring system is correctly adjusted and the entire nerve pathway is covered.

Continuous neuromonitoring (cIONM) is performed using a vagus electrode that is placed on the nerve during the entire surgery.

The nerve can first be localised intraoperatively by stimulating the area in question with the stimulation probe. The response signals of an intact nerve have a latency in the range of approximately 2–9 ms during thyroidectomy. The exact value depends on the stimulation site and nerve. The typical latency times for thyroidectomy of the vagus nerve are around 6 ms on the left and around 4 ms on the right. For the recurrent laryngeal nerve, the usual latency values on both sides are around 2 ms.

In order to receive a response at all, the motor threshold must be exceeded during stimulation. The resulting action potential triggers a stimulus response in the innervated muscles. This response is recorded via the electrodes and displayed visually and acoustically by the device. It is possible to stimulate intermittently with the hand-held probe or to attach an electrode to the nerve for continuous stimulation.

Recording during thyroidectomy

The laryngeal electrode is a safe, reliable and less invasive intraoperative monitoring method for thyroidectomy. Monitoring is performed by recording a surface EMG at the vocal muscle using the surface electrode affixed to the endotracheal tube, which is in contact with the vocal muscle. For electromyographic recording during thyroidectomy, it is also essential to ensure correct anaesthesia protocols are followed.