Bite injuries caused by transcranial electrical stimulation motor-evoked potentials’ monitoring: incidence, associated factors, and clinical course


Bite injuries caused by transcranial electrical stimulation motor-evoked potentials’ monitoring: incidence, associated factors, and clinical course

Sachiko Yata1 · Mitsuru Ida2 · Hiroko Shimotsuji1 · Yosuke Nakagawa1 · Nobuhiro Ueda1 · Tsunenori Takatani3 · Hideki Shigematsu4 · Yasushi Motoyama5 · Hiroyuki Nakase5 · Tadaaki Kirita1 · Masahiko Kawaguchi2


Purpose The incidence of bite injuries associated with transcranial electrical stimulation motor-evoked potentials monitor-ing reportedly ranges from 0.13 to 0.19%. However, in clinical practice, bite injuries appear to occur more frequently than previously reported. Our aim was to identify the incidence of and perioperative risk factors associated with bite injuries caused by transcranial electrical stimulation motor-evoked potential monitoring.

Methods Patients who underwent elective surgery with transcranial electrical stimulation motor- evoked potential monitor-ing at a single tertiary hospital in Japan between June 2017 and December 2017 were included in this study. All patients were assessed by oral surgeons preoperatively and postoperatively. The associated factors with bite injuries were explored by the univariate analysis.

Results 12 of 186 patients experienced 13 bite injuries, including three lip, six oral mucosa, and four tongue injuries. No patient required suture repair. 11 of 12 patients had uneventful postoperative courses and were cured within 12 postoperative days. One patient with a tongue ulcer and a hematoma had difficulty in oral intake and persistent dysgeusia. Patient severe movement during transcranial electrical stimulation motor-evoked potential monitoring was associated with bite injuries (p = 0.03).

Conclusions The incidence of bite injuries assessed by oral surgeons was 6.5% in patients with transcranial electrical stimu-lation motor-evoked potential monitoring, and the patients with severe movement during the monitoring tended to incur bite injuries. In rare cases, transcranial electrical stimulation motor-evoked potential monitoring may cause difficulty in oral intake and dysgeusia.

Keywords Bite injury · Motor-evoked potentials · Transcranial electrical stimulation · Intraoperative movement


Intraoperative motor-evoked potential (MEP) monitoring has been used effectively to detect and prevent motor dysfunc-tion [1, 2]. There are two different MEP types based on stim-ulating techniques: transcranial electrical stimulation (TES) and direct brain motor cortex stimulation. In general, higher voltage and higher current are required in TES, because the cerebral cortex is stimulated through the scalp and skull. TES using high voltage or high current can cause bite inju-ries because of masseter muscle contractions [3]. Bite inju-ries have been reported by anesthesiologists, surgeons, and technicians with the occurrences of 0.13–0.69% [3–5]. We have also recognized the adverse effects of bite injuries and need to provide uniform medical care in patients undergoing

TES–MEP monitoring. Therefore, since June 2017, our oral surgeons began to assess all patients > 12 years old undergoing elective surgery with TES–MEP monitoring preoperatively. Our study purpose was to evaluate the inci-dence of and factors associated with bite injuries caused by TES–MEP monitoring.

Materials and methods

This was a retrospective observational study approved by the Nara Medical University Institutional Review Board, Kashi-hara, Nara, Japan. Patients who underwent TES–MEP moni-toring during elective surgeries at Nara Medical University between June 2017 and December 2017 were included in this study. Patients who were intubated via tracheo- cuta-neous fistula and those with edentulous jaw disease were excluded from statistical analysis. We collected the patient’s demographic data, intraoperative data, and incidence of bite injuries associated with TES–MEP monitoring.

Perioperative management of patients undergoing TES–MEP monitoring

In routine preoperative practice, oral surgeons assess a patient’s lip, oral cavity, and teeth, and in patients for whom the oral surgeon has recognized the need for a mouthpiece, such as because of lost teeth, they make a mouthpiece.

In all patients, anesthesia was induced by administering propofol, fentanyl or remifentanil, and rocuronium, and was maintained with propofol and fentanyl or remifentanil. If trauma occurs during tracheal intubation, it was recorded in the anesthesia chart. Propofol was titrated to achieve a target bispectral index from 40 to 60 or for a patient state index from 40 to 55. No rocuronium was repeated after induction, and sugammadex was administered to recover a train-of-four (T4/T1) ratio > 0.8 at the first TES–MEP monitoring. To prevent bite injuries, anesthesiologists have routinely used soft bite blocks, usually rolled gauze (Fig. 1), but the position of the soft bite blocks was at the discretion of the anesthesiologists.

For TES–MEP recording, TES was performed using an MS-120B (Nihon Koden, Tokyo, Japan) that delivered constant-current stimulus or an SEN-4100 (Nihon Koden, Tokyo, Japan) that delivered constant-voltage stimulus. Both stimuli were provided under the following common condi-tions except for stimulus duration time; train -of-five pulses with an interstimulus interval of 2 ms or by tetanic stimu-lation of peripheral nerves if needed [ 6], stimulus rate of 500 Hz, recording time of 100–200 ms, high-cut filter from 1.5 to 3 kHz, and low-cut filter from 1 to 10 Hz. The stimu-lus duration times under constant-current stimulus and con-stant-voltage stimulus were 0.2 ms and 50 µs, respectively.

Fig. 1  Soft bite block. Soft bite block is made by folding three-sterile gauze in half and winding like a cylinder

Tetanic stimulation to the peripheral nerves with an intensity of 50 mA for 5 s was started 6 s before TES (interstimulus interval of 1 s). The outputs were delivered to the scalp by screw electrodes or disk electrodes applied to C3 and C4 (International 10–20 System). The stimulus intensity of transcranial stimulation was determined at the beginning of MEP monitoring and was set as the suprathreshold or supramaximal level in TES–MEP.

The compound muscle action potentials were recorded using a Neuromaster MEE1232 intraoperative TES of the MEP measurement system (Nihon Koden, Tokyo, Japan). Different types of surgery had different recorded muscles. In craniotomy, cerebral aneurysm clipping, and carotid endarterectomy, compound muscle action potentials were recorded from the skin over the abductor pollicis brevis mus-cle, tibialis anterior, gastrocnemius, and abductor hallucis. In transsphenoidal surgery, compound muscle action poten-tials were recorded from the skin over the abductor pollicis brevis and abductor hallucis. In cervical surgery, compound muscle action potentials were recorded from the skin over the biceps, deltoid, abductor pollicis brevis, quadriceps mus-cles, tibialis anterior, gastrocnemius, and abductor hallucis. In thoracic and lumbar surgery, compound muscle action potentials were recorded from the skin over the deltoid, abductor pollicis brevis, quadriceps muscles, hamstring, tibialis anterior, gastrocnemius, and abductor halluces.

Intramonitoring patient movement was evaluated by the surgeon using the movement score (1: no movement, 2: mild movement—microscopic surgery is possible, 3: moderate

movement—microscopic surgery is impossible, but macro-surgery is possible, and 4: severe movement—no surgery is possible) [7]. All data related to TES–MEP were recorded by technicians.

Oral surgeons assessed the patients within 24 h after sur-gery at first, and injuries to the lips, oral cavity, and teeth that did not occur during tracheal intubation were regarded as bite injuries associated with TES–MEP monitoring. All patients received a second assessment performed at a perio-perative management center before discharge. In case bite injuries remained, the patient was followed up by oral sur-geons until the bite injuries healed.

Explanatory variables

The patients’ demographics and intraoperative data were collected from medical records. Patient’s demographics included age, sex, height, weight, and types of surgery (craniotomy, transsphenoidal surgery, cerebral aneurysm coiling, carotid endarterectomy, and spine surgery). Intra-operative data included patients’ position (supine, prone, and lateral position), position of the endotracheal tube (middle and lateral side), position of the bite block (middle and lat-eral side), presence of mouthpiece, durations of anesthesia and surgery, stimulation with voltage or current, stimulus intensity of TES–MEP (low: 0–250 V or 0–100 mA, high: 250–500 V or 100–200 mA), presence of tetanic stimula-tion, stimulation per case, and patient movement during TES–MEP monitoring.

Statistical analysis

Demographic data and intraoperative data are presented as the mean (standard deviation) or number. We divided the patients into two groups, with or without bite injuries and explored related factors by performing Fisher’s exact test, the Mann–Whitney U test, or the unpaired t test. Although patient movement was rated according to a four-grade eval-uation, univariate analysis was performed by classifying movement as severe movement that greatly hindered surgery and other group. All data were analyzed using SPSS 22.0 (IBM Inc., Armonk, NY, USA), and statistical significance (2-tailed) was accepted for p values < 0.05.


194 patients were included. Two tracheal tubes were intu-bated via tracheo-cutaneous fistula, two patients had eden-tulous jaw disease, and four patients had no data about the number of stimuli; therefore, the data of 186 patients were analyzed. Patients’ demographics and intraoperative data are shown in Table 1. All patients adhered to anesthetic

management protocols, and no patient suffered trauma dur-ing tracheal intubation. There were 13 reported bite injuries in 12 (6.5%) patients. The 13 bite injuries included three lip injuries, six buccal mucosa injuries, and four tongue injuries. No patient required suture repair. One patient experienced both lip injury and a tongue injury during the same surgery. Univariate analysis revealed a significant association of bite injuries with patient movement (p = 0.03).

8 of 12 patients who suffered from bite injuries had completely recovered by the second assessment performed between the 3rd and 12th postoperative days. Three of the remaining four patients continued to be followed up after the second assessment. They had no complications affecting their intake and recovered by the 10th or 11th postoperative days (third assessment) . However, one patient had a tongue ulcer, a hematoma, and a tongue laceration that hindered his oral intake. We described his clinical course (Fig. 2).

Case report

  1. 76-year-old man diagnosed as having glioma with dysar-thria underwent brain tumor resection under general anes-thesia with TES–MEP monitoring involving post-tetanic stimulation. His endotracheal tube was placed on the right side, and three soft bite blocks were placed on the same and opposite sides to the endotracheal tube and between upper incisors and lower incisors. He had no history of ingestion and swallow dysfunction. He was transported to the inten-sive care unit after uneventful anesthesia and surgery. On the first postoperative day, a tongue ulcer, a hematoma, and a tongue laceration on the same side as the endotracheal tube and soft bite block were found during a routine evaluation by the oral surgeon. His ulcer was in the lateral portion of his tongue. On the other hand, his laceration was in the posterior portion of his tongue. He was not able to eat regular food because of tongue pain. There were no effective treatments without dexamethasone oral ointment, and he was followed up while adjusting food form. On the 15th postoperative day, he was able to eat regular food and follow-up was discontin-ued. However, his dysgeusia persisted.


In this study involving patients who underwent surgery with TES–MEP monitoring, postoperative bite injuries occurred in 6.5% of the patients, and one patient experienced pro-longed difficulty in oral intake because of pain and persistent dysgeusia. Furthermore, the patients with severe movement during TES–MEP monitoring were more likely to incur bite injuries.

A previous review article evaluating the occurrence of complications caused by TES–MEP monitoring estimated

Table 1  Comparison of patients’ data between the patients with and without bite injuries

Bite injury (+) (n = 12) Bite injury (−) p value
(n = 174)
Age (year) 60.4 (20.0) 61.5 (16.7) 0.75
Male/female 5/7 85/89 0.76
Body mass index (kg/m2) 23.6 (3.6) 22.6 (3.3) 0.32
Surgical procedure 0.57
Craniotomy 4 65
Transsphenoidal surgery 0 8
Cerebral aneurysm coiling 1 12
Carotid endarterectomy 0 4
Spine 7 85
Length of anesthesia (min) 329 (115) 344 (119) 0.67
Length of surgery (min) 234 (115) 253 (111) 0.55
Patients’ position 0.79
Supine 4 74
Prone 7 84
Lateral position 1 16
Location of endotracheal tube
Middle/lateral 2/10 61/113 0.22
Location of bite block
Middle/lateral 2/10 60/114 0.34
Mouthpiece 0 13 1
Stimulation side
Both/right or left 0/12 2/172 1
Tetanic stimulation 2 49 0.51
Stimulus method
Current/voltage 7/5 101/74 0.36
Stimulus intensity
Low/high 0/12 20/154 0.26
Stimulations per case 56.8 (27) 63.7 (29) 0.43
Patient movement
None/mild/moderate 1/9/0 0/152/19 0.03
Severe 2 3

Mean (standard deviation), number

the incidence of bite injuries to be 0.19% (29 reported bite injuries in approximately 15,000 cases, including unpub-lished cases) [3 ]. However, it is possible that authors have underestimated the occurrence of bite injuries, because only remarkable cases might have been reported. One retrospec-tive review of incident reports of TES–MEP-associated bite injuries showed that the incidence was 0.63% (109 patients in 17,273 surgical procedures) and included tongue injuries (79. 3%), lip injuries (19.8%), and broken incisors (0.1%) [4]. However, these outcomes were reported by technolo-gists. Another retrospective study found that 26 of 18,862 patients incurred bite injuries reported by anesthesiologists or surgeons [5]. Our incidence was higher than those of the previous studies, which may be because bite injuries were assessed accurately by oral surgeons. Furthermore, the previ-ous studies did not provide the descriptions of the follow-up

process in cases with bite injuries. Our study revealed that 11 (91.6%) patients with bite injuries had an uneventful post-operative course and were cured by the 12th postoperative day. On the other hand, one patient experienced difficulty in oral intake and persistent dysgeusia.

Bite blocks to prevent bite injuries can damage the lip, oral cavity, and teeth, which may have been caused by improperly sized bite blocks or displacement of them. In the case presented here, the long period of tongue compression by bite blocks may have contributed to his tongue ulcer with hematoma. New devices different from the conventional bite blocks have been developed, and their effectiveness must be verified in the future [8, 9].

Two pathways may be involved in mandibular movement during TES–MEP monitoring: one is via stimulus to corti-cobulbar pathway, and the other is via the direct stimulus

Right Left

Dorsum of tongue inferior lingual surface inferior lingual surface


POD 10

POD 13

Fig. 2  Clinical course of the representative case of bite injury. No photographs show tongue ulcers with hematoma and tongue lacera-tion on the first postoperative day. The row indicates postoperative days, and the columns indicate the sites of injuries. His tongue ulcer

with hematoma and tongue laceration are on the same side as the endotracheal tube and soft bite block. They recovered over time with-out any treatment

of the masseter muscles, temporal muscle, and trigeminal nerve caused by extracranial leakage current. The severity of a patient’s movement during TES–MEP monitoring may reflect higher stimulus intensity that is related to occlusal movement. However, in our study, the stimulus intensity was not statistically significantly associated with bite injury, which is attributed to the fact that higher stimulus intensity was adopted in many cases. Furthermore, it was difficult to analyze the stimulus intensity as a continuous variable, because there were two types of stimulus methods: current stimulus and voltage stimulus. The use of neuromuscular blockade has been suggested as a strategy to reduce patient movement. However, neuromuscular blockades have not been used in our clinical practice, because MEP amplitude varies with changing transmission across the neuromus-cular junction, and different muscle groups have different sensitivities to neuromuscular blockades. To avoid patient’s movement, it is preferable to adapt optimal stimulus inten-sity with the consideration of accuracy and complications. In case patient moves severely, it may be useful to administer neuromuscular blockades with constant concentration and use post-tetanic stimulation.

A strength of our study was that all patients who under-went TES–MEP monitoring were assessed by oral surgeons perioperatively, and no patients were lost to follow-up. How-ever, because of the retrospective observational nature of

  1. single-center study, we acknowledge the inherent limita-tions regarding generalizability, and there might have been

important unmeasured factors associated with bite injuries, such as bite alignment. Furthermore, this study did not include enough cases to perform multiple regression analy-sis and the future study including large number patients is required.

In conclusion, we found that bite injuries associated with TES–MEP monitoring were more common than those in the previous reports. Most bite injuries cured within several days and were rarely accompanied by complications, such as dysgeusia. Patient movement during TES–MEP monitor-ing was an important factor associated with adverse events. Although the advantages of TES–MEP monitoring to detect and prevent motor dysfunction have often been emphasized, we should recognize the risks associated with TES–MEP monitoring. In the future, further study with large number of patients and unmeasured associated factors is required to identify the risk factors and to prevent bite injuries.

Compliance with ethical standards 

Conflict of interest The authors have no conflicts of interest to declare.


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