The prevalence of thyroid cancer has significantly increased over the past few decades [1]. Increased accessibility to ultrasound and fine-needle aspiration cytology has led to early detection of thyroid cancer, including papillary thyroid microcarcinoma. Thus, hemithyroidectomy is being increasingly performed [2]. Owing to the development of surgical techniques, most patients with thyroid cancer have a good prognosis. Therefore, patients expect positive changes to their quality of life, especially their voice, after surgery.

Post-thyroidectomy voice disorder (PTVD) occurs in 25%-84% of thyroidectomies [3]. According to Lee, et al., the most common cause of voice change is damage to the recurrent laryngeal nerve (RLN) and external branch of the superior laryngeal nerve (EBSLN) [4]. RLN damage can cause hoarse voice, voice fatigue, and breathing voices, whereas EBSLN damage makes it difficult to produce high-frequency tones [5,6]. Other causes of voice impairment include cricothyroid muscle injury, strap muscle injury, endotracheal intubation-related trauma, vascular congestion, and local hematoma [7-10]. PTVD after thyroid surgery is well known; however, limited data are available on PTVD after hemithyroidectomy, which has been increasingly performed nowadays.

No international consensus has been reached on the diagnostic criteria for PTVD, and a few criteria have been used in previous studies (Fig. 1). When voice handicap index (VHI) score >12 is used to define PTVD, the incidence of PTVD increases to 24% 1 week after surgery followed by falling to less than 8% after 3 months [11,12]. According to grade, roughness, breathiness, asthenia, strain (GRBAS) score >0, it is highest at 6% 1 month after surgery, followed by a decreasing trend [12]. Pitch maximum (Pmax) <80% of its preoperative value is used as a diagnostic criterion and is taken as a standard of Pmax. It is highest at 11% 1 month after surgery followed by a decreasing trend [3].

In recent years, several efforts have been made to prevent voice impairment after thyroid surgery. In South Korea, an electromyographic (EMG) endotracheal tube may be used as an intraoperative neuromonitoring (IONM) device if the National Health Insurance standards are met, which include the following: 1) recurrent thyroid cancer in the central neck region, 2) unilateral vocal cord palsy before thyroid surgery, 3) thyroid cancer with definitive central neck lymph node metastasis, 4) thyroid cancer with definitive or suspicious extra-thyroid extension, and 5) high-risk thyroid surgery and parathyroid surgery, such as surgeries performed for Graves’ disease or significant thyroid enlargement. IONM can help identify the anatomical disposition of the nerves near the thyroid, which helps prevent damage to the RLN and EBSLN [5]. However, owing to the high cost ($250 in South Korea), considerable rate of false ‘Loss of signal’ (10%-15%) as well as limited indications, most surgeons perform thyroid surgery without IONM.

Therefore, other types of IONM are needed. Recent studies have shown that electrodes attached to the skin can complement the EMG endotracheal tube [13], and skin electrode IONM in thyroid surgery is increasingly used.

Voice changes may occur even after hemithyroidectomy; however, no methods are available to predict it. Therefore, in this study, we tried to determine whether PTVD could be predicted using the parameters obtained by skin IONM and subjective and objective voice test results of patients. Moreover, the incidence and clinical progress of PTVD after hemithyroidectomy were investigated.

PTVD is known to have various causes, such as nerve damage, muscle strap and constraint, esophageal adhesion, and vocal cord damage during anesthesia, which can occur even in the absence of obvious nerve damage. The literature to date has included data on the occurrence of voice disorders after thyroidectomy, and limited data are available on voice disorders after lobectomy, which has been increasingly performed. A meta-analysis showed that IONM during thyroid surgery significantly reduces damage to the RLN [14]. In this study, we investigated the incidence and progression of voice disorders after lobectomy and evaluated whether the occurrence of PTVD could be predicted through IONM using skin electrodes.

When the skin IONM parameters were measured, the average value of R2 was higher than that of R1. Unlike R1, R2 is a value measured after RLN exposure; therefore, it showed a higher signal value even with a lower-intensity stimulus. Thus, we used the difference in V values that were measured at the same location and using the same-intensity stimulus.

Extensive data are available on IONM parameter values using endotracheal tubes, and standard values are being established; however, data on IONM parameter values using skin electrodes are limited. Prior research has demonstrated that the median amplitude of vagus nerve stimulation using the attached electrodes typically measures around 200 μV, which is less than that of the EMG tube [15]. Our current study yielded comparable results (Table 2), with the reduced intensity attributed to signal attenuation during the transmission of signals from the vocal cord muscles to the skin.

While previous research has examined the pre- and postoperative amplitude ratios of R1 to R2 and V1 to V2 [15], no prior studies have explored the connection between intraoperative nerve monitoring parameters and voice outcomes. In this study, we hypothesized that a larger difference in amplitude was associated with more severe nerve damage and worse voice change. Therefore, patients were divided into two groups using the median change in the V value as the cut-off value.

Contrary to expectations, no statistically significant difference was observed in the subjective parameters between the two groups. Until postoperative 1 month, when the subjective parameters showed deterioration (Fig. 2), only F0 significantly decreased in the group with larger ΔV. At postoperative 3 months, F0, jitter, and NHR worsened in the group with larger ΔV, and no significant difference was observed between the groups after that period. To evaluate the differences caused by sex when measuring F0, Pmax, and Pmin, we conducted a subgroup analysis in the male and female groups; however, no significant differences were observed. From these results, if the difference between V1 and V2 is large, we can predict that the voice analysis results might be worse than their counterparts within postoperative 3 months followed by recovery after postoperative 6 months.

Voice alterations following thyroid surgery are a well-acknowledged phenomenon, extending beyond instances of RLN injury [16]. A comparative investigation involving pre- and postoperative voice metrics, similar to our own study, revealed that post-operative voice impairment is most pronounced immediately following the surgical procedure. Subsequently, there is a noteworthy improvement at the three-month mark, although it remains inferior to pre-operative levels. However, it tends to fully return to pre-operative status within one year [17].

After thyroidectomy, adhesions form between the larynx and the adjacent strap muscles and subcutaneous fat, constricting laryngeal mobility and precipitating voice alterations [18]. Notably, this study ascertained a significant restoration of voice quality at the three-month juncture in comparison to the one-month post-operative assessment, a pattern that aligns with our present investigation.

Furthermore, it is noteworthy that voice impairment can manifest even in the absence of overt RLN damage, attributable to adhesions with the surrounding tissues of the larynx. Such impairments tend to exhibit discernible recovery trends approximately three months post-surgery. The extent of the difference in amplitude between pre- and postoperative vagus nerve stimulation values can impact voice quality. Nevertheless, even if the voice initially deteriorates, it often tends to recover over time.

A limitation of our study is that when using a skin electrode, the measured value may be different for each person depending on the location of attachment and skin thickness; therefore, our research results cannot be generalized. In this study, every operation was performed by a single senior surgeon. Therefore, the inter-tester errors were minimized.

In our study, only 82 patients were examined; therefore, we attempted to predict voice changes using median cut-off values. Further studies with a larger number of patients should be conducted to establish standardized values to predict voice outcomes using skin electrode IONM.

We used the V values in this study instead of the R values, which have a more direct effect on the voice owing to different experimental settings. In future studies, we might measure R values with the same power before and after hemithyroidectomy to obtain meaningful results in predicting voice changes using delta R values. A subsequent study may also consider conducting a comparison of both latency and amplitude in order to arrive at more inclusive findings. Furthermore, the investigation could extend to evaluating the parameters of the superior laryngeal nerve, which has the potential to influence high-pitched voice production, thereby contributing to a more comprehensive assessment of voice prognosis.

In conclusion, early diagnosis of thyroid cancer has become possible because of the early implementation of ultrasound and fine-needle aspiration cytology, and hemithyroidectomy is being increasingly performed. This study was conducted to predict the incidence and course of the voice disorders that occur after hemithyroidectomy.

Several studies have attempted to predict the incidence of PTVD after thyroidectomy using various scales. The incidence tended to increase until 1 month after surgery and was reported to be up to 24%. Subsequently, it showed a tendency to gradually decrease, and after 3 months, it was usually normalized. This study showed a trend consistent with previous studies and revealed that voice outcomes deteriorated until 3 months after surgery and then gradually improved.

We attempted to predict post-hemithyroidectomy voice changes using IONM parameters with skin electrodes. IONM using a skin electrode can be used during thyroid surgery in cases where the EMG endotracheal tube cannot be used because of limited indications. As shown in this study, it may be possible to predict the occurrence of PTVD in patients based on the ΔV value measured during surgery. A greater difference in the amplitude of the IONM parameter was associated with the likelihood of PTVD occurrence. Further studies are required to include more patient groups with more detailed subgroup analyses to reveal the association between various voice parameters and PTVD, such that the operators are able to predict PTVD more accurately and ultimately improve voice outcomes after thyroidectomy.