INTRODUCTION
Resecting tumors in an infratentorial lesion requires a high surgical precision, because in that operable area many lower cranial nerves (LCN; CN IX–XII) nuclei exist [
1,
2,
3]. In that case, the LCN play an important role in swallowing and speech function and deficits of these nerves lead to dysphagia, dysarthria, and hoarseness. Therefore, preserving the function of LCN during infratentorial surgery is one of the most important goals to neurosurgeons performing these types of surgical procedures. Additionally, the intraoperative monitoring (IOM) can help localize nerves and prevent nerve injury during surgery [
1,
2,
3].
For the IOM of LCN, in the same as other cranial nerves, a spontaneous electromyography (s-EMG), triggered electromyography (t-EMG) and transcranial motor evoked potential (TcMEP) could be used. Among them, the TcMEP is the only the indirect real-time technique to monitor the nerve function without approaching to the nerves. And there are many reports about monitoring CN IX, CN X, and CN XI by TcMEP as seen in a literature review [
1,
2,
3,
4,
5,
6,
7,
8,
9,
10]. However, the techniques to monitor the function of CN XII are restricted in s-EMG and t-EMG, and the use of TcMEP of CN XII has been neglected until now.
Here, we report two cases of IOM of the CN XII using hypoglossal TcMEP with s-EMG in infratentorial surgery, and the result of a postoperative swallowing function. These cases offer the potential result that the IOM of hypoglossal TcMEP could be used as an indicator for detecting injury of the hypoglossal nerve and predict postoperative complications.
DISCUSSION
During infratentorial lesion surgery such as the cerebellum, brainstem, or floor of fourth ventricle, the LCN are placed at high risk of injury [
1,
2,
3]. It is noted that the deficits of the LCN can result in dysarthria, dysphagia and a decreased quality of life. Thus, IOM of LCN is important to utilize to prevent the postoperative complications in infratentorial surgery, because the IOM can give immediate feedback when the nerve is in danger of injury during surgical manipulation [
3].
To the best of our knowledge, there has been one study investigating the predictive value of TcMEP from the tongue muscles for a noted postoperative functional outcome [
6]. The study reported a non-significant trend between the final-to-baseline MEP ratio of the MEP width and tongue deviation. However, the ratio is considered impractical as an ongoing warning criteria, for use during a surgery. Generally, the disappearance, amplitude reduction, acute threshold elevation, and morphology simplification have been used for characterizing a muscle MEP warning criteria [
6,
10]. In addition, the patients were simply classified by either being present or absent of dysphagia by asking a question, and not evaluated objectively. Therefore, it is still unclear whether hypoglossal MEP can be used to predict postoperative functional deficits such as dysphagia and dysarthria.
For the IOM of cranial nerves, sEMG, tEMG and TcMEP could be utilized in this case. Although many studies have demonstrated that the deterioration of cranial nerves can be monitored using TcMEP [
7,
8,
9,
10], most of those case studies are restricted in the review of facial nerves [
8,
10]. And the reports about using TcMEP to monitor the deterioration of LCN have been rare and limited in a review of the glossopharyngeal (CN IX) and vagus (CN X) nerves [
7,
9,
10]. Fukuda et al. [
9] analyzed an associated correlation between the pharyngeal MEP (PhMEP) and postoperative swallowing function in 21 patients. Similarly, Ito et al. [
10] demonstrated the usefulness of the TcMEP from the vocalis muscles for detecting the deterioration of the vagus nerve during surgery. However, performing the TcMEP of the CN IX and CN X has some difficulties with the electrode placements [
9,
10]. For the IOM of these nerves, the recording electrodes are inserted in the pharyngeal wall, soft palate, or vocal cord and the appropriate approach to these areas requires complicated and specialized skills to be utilized for optimum outcomes [
9,
10]. Using an endotracheal tube with surface electrodes instead of a needle electrode also can be used for monitoring CN X. Although it should be noted that this method can usually work to detect not the pharyngeal branch of CN X, but the recurrent laryngeal nerve instead. Thus, this device is noted as having restrictions to predict any applicable postoperative swallowing complications. Additionally, this device is more expensive than the cost of using a conventional needle electrode.
There are some advantages of using the TcMEP for monitoring the deficits of CN XII, rather than using s-EMGs and/or t-EMGs. Conventionally, the hypoglossal nerve was monitored by s-EMGs and t-EMGs [
2,
3,
4]. But, these methods cannot give continuous real-time monitoring in the early steps of surgery, because the CMAP monitoring needs to have proximity of the nerve in the surgical field, to accomplish that goal [
10]. Thus, these methods are unusable if the target nerve is inaccessible in surgical manipulation. In contrast, the TcMEP is an indirect monitoring method in this case, and the closeness of monitoring to the nerve is unnecessary. Furthermore, the intact response of muscle from the EMG does not always mean that there is preservation of the intact quality of the nerve, because the distal part from the nerve damage can produce signals to the target muscles. On the other hand, the TcMEP can provide the information of the functional integrity, from the corticobulbar tract to the hypoglossal nerve [
10] and is in the position to better reflect functional deficits. Thus, the TcMEP is recommended to detect the functional deterioration of the hypoglossal nerve.
The present report has some limitations as noted in the study. This is the first case reviewed concerning the usefulness of the amplitude of MEP from CN XII, and before this time, a warning criterion of hypoglossal MEP deficits has not been established in a prior literature review. Therefore, further studies of intraoperative hypoglossal MEP monitoring in larger populations are needed to establish a verifiable reliability and a warning criterion for the hypoglossal MEP monitoring.
In conclusion, the disappearance of the hypoglossal MEP recorded from the tongue could be a predictor of postoperative hypoglossal nerve deficit and subsequent dysphagia. Therefore, future research should explore the development and test more structured monitoring methods in large number of patients.