2 The procedure
2.1.1 Removal of part or all of the thyroid gland (partial or total thyroidectomy) may be indicated in the treatment of thyroid enlargement (goitre), thyrotoxicosis or malignancy. The thyroid gland is located close to the right and left recurrent laryngeal nerves (RLNs) which innervate the vocal cords. Damage to an RLN during thyroid surgery may result in temporary or permanent hoarseness, or, in the case of bilateral damage, breathing difficulties and inability to speak.
2.1.2 Conventionally, thyroid surgery is done without continuous IONM. Under general anaesthesia, an incision is made in the front of the neck and the underlying muscles are retracted to expose the thyroid gland. The RLNs are identified visually to avoid injuring them, but this is not always straightforward. A hand-held nerve stimulator can also be used, in combination with a finger placed behind the larynx to detect contraction of the vocal cord muscles and the arytenoid cartilages on nerve stimulation.
2.2.1 IONM is used as an adjunct to conventional thyroid surgery under general anaesthesia. It requires placement of electrodes close to the vocal cords. This can be achieved either by the use of a specially adapted endotracheal tube with surface or integral electrodes, which are positioned close to the vocal cords, or by the placement of electrodes into the vocal muscles on each side of the thyroid gland, when using a standard endotracheal tube. Following tracheal intubation, non-paralysing anaesthesia is used for the rest of the procedure as muscle relaxants can interfere with the nerve monitoring process.
2.2.2 The electrodes are connected to the neuromonitoring device, which uses sound and graphics on the monitor screen to alert the surgeon when a surgical instrument comes close to either RLN during surgery. A hand-held probe can also be used to confirm the location of either nerve at any time during the operation. Postoperative laryngoscopy is used to assess RLN function.
Sections 2.3 and 2.4 describe efficacy and safety outcomes which were available in the published literature and which the Committee considered as part of the evidence about this procedure. For more details, refer to the Sources of evidence.
2.3.1 Four non-randomised studies of 16,448, 684, 639 and 136 patients (29,998, 1043, 1000 and 190 nerves) reported permanent rates of vocal cord paralysis ranging from 0% to 2% in the IONM groups, compared with 0% to 1% in the control groups (visual RLN identification or no RLN identification). No statistically significant differences were seen between procedures undertaken with or without IONM. Three case series of 328, 288 and 171 patients reported rates of permanent vocal cord paralysis using IONM in 3% (15/502), 1% (6/429) and 1% (2/271) of RLNs, respectively.
2.3.2 Four non-randomised studies of 684, 639, 165 and 136 patients (1043, 1000, 236 and 190 nerves) reported rates of transient vocal cord paralysis ranging from 3% to 5% in the IONM groups, compared with 3% to 4% in the control groups (none were statistically significant). Another non-randomised study reported that vocal cord immobility was detected at 3-month follow-up in 6% (6/104) of patients when IONM was used and 5% (5/100) of patients when IONM was not used (p = 0.55). The three case series of 328, 288 and 171 patients reported rates of transient RLN palsy as 9% (43/502), 9% (37/429) and 5% (13/271), respectively.
2.3.3 The non-randomised study of 639 patients (1000 nerves at risk), which compared IONM with visual identification of the RLN, reported that IONM indicated no nerve damage in 10 out of 21 vocal cords that were paralysed as a result of surgery. Conversely, IONM indicated nerve damage in 27 out of 480 patients who were found to have normal postoperative vocal cord function.
2.3.4 The Specialist Advisers considered key efficacy outcomes to be reduction in nerve damage and subsequent vocal cord palsy. Two of the nine Advisers stated that this procedure is useful for teaching. One Adviser commented that there are significantly different opinions between surgeons as to whether this technology improves outcomes or whether it gives false reassurance to inexperienced surgeons.
2.4.1 No adverse events resulting from IONM were reported in the studies.
2.4.2 The Specialist Advisers considered the main safety concerns to be false-negative or false-positive readings leading to the misidentification of the nerve. In particular, a false-negative reading may lead to RLN damage. One Adviser suggested that there is also a potential for false signals if the electrodes are placed incorrectly.