Oral Injuries Following Cerebellopontine Angle Surgery with or without Motor Evoked Potential Monitoring: three Case Reports and Literature Review

Oral Injuries Following Cerebellopontine Angle Surgery with or without Motor Evoked Potential Monitoring: three Case Reports and Literature Review | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Case Report Oral Injuries Following Cerebellopontine Angle Surgery with or without Motor Evoked Potential Monitoring: three Case Reports and Literature Review Yuanli Pi, Linlin Luo, Yu Li, Limei Luo, Mingxiang Xie, Tianyuan Luo This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4840493/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Background Oral injuries are occasional yet notable complications in neurosurgical procedures and are often associated with motor-evoked potential (MEP) monitoring; however, they are also influenced by factors such as prolonged neck flexion and inadequate oral protection. Case presentation This paper discusses three cases of oral injuries following pontocerebellar lesion resection surgeries, illustrating varying outcomes with different monitoring and intubation techniques. In one patient, orotracheal intubation with unilateral MEP monitoring led to fractured alveolar bones and dislodged teeth. Another patient, who was intubated nasally with bilateral MEP monitoring, experienced severe tongue biting, facial swelling, and subsequent airway obstruction requiring tracheotomy. A third patient, also nasally intubated but without MEP monitoring, developed a swollen and bleeding tongue postoperatively. Conclusion MEP monitoring is not the sole cause of oral injuries in neurosurgical procedures. Key factors contributing to these injuries, aside from the nonspecific stimulation of MEP, include prolonged surgical positioning, inappropriate anesthesia strategies, and patient-specific factors. The medical team should understand the underlying mechanisms of these complications, master systematic preventive strategies, and engage in effective collaboration to more efficiently reduce the incidence of these complications. Neurophysiological monitoring motor-evoked potential oral injury case report prevention Figures Figure 1 Figure 2 Figure 3 Figure 4 Background To increase the safety and efficacy of neurosurgical or spinal procedures, an increasing number of neurophysiological monitoring techniques are being implemented. Nonetheless, a specific complication that continues to warrant attention is oral injury. The reported incidence of oral injuries varies across different studies. For example, in a study of 15,000 cases conducted by MacDonald et al., the incidence rate was approximately 0.19% (29/15000)[ 1 ]. In contrast, Tamkus et al.’s retrospective study involving 17273 individuals reported an incidence rate of 0.63% (109/17273)[ 2 ]. Yata et al. reported a significantly higher incidence rate of 6.5% (12/186)[ 3 ]. Although the overall incidence rate is not high, oral injuries can lead to additional harm and potentially prolonged hospital stays. Severe cases may also present airway risks and necessitate oral surgery intervention. Owing to its unique characteristics, this phenomenon merits significant attention. Herein, we report three distinctive cases following cerebellopontine angle surgery to provide a comprehensive understanding of this type of complication. Case Presentation Case 1 A 31-year-old male patient, measuring 165 cm and weighing 55 kg (BMI 20.2), was diagnosed with a hemorrhagic right acoustic neuroma. Elective tumor resection surgery was planned. Anesthesia was initiated via the use of 20 mg of intravenous propofol, 20 µg of sufentanil, and 50 mg of rocuronium for smooth induction and endotracheal intubation. A standard dental pad secured the tube. The patient was positioned in the left lateral position during surgery, with anesthesia maintained through continuous infusion of remifentanil, propofol, and dexmedetomidine. During surgery, monitoring of somatosensory evoked potentials (SSEP), motor evoked potentials (MEP), and electromyography (EMG) was conducted; hence, muscle relaxants were not used throughout the entire procedure following anesthesia induction. MEP monitoring targeted the orbicularis oculi, orbicularis oris, chin, masseter, and sternocleidomastoid muscles. Biphasic stimulation used a positive electrode 2 cm from C3 and a negative electrode at the CZ, with parameters set to a 2 ms interval, 100–200 V intensity, and a 75 µs pulse width. The surgery lasted for 7 hours and 35 minutes. The intraoperative infusion volume was 2850 ml. The urine volume was 2000 ml, and the blood loss was 200 ml. Postoperatively, the patient presented with loose and displaced teeth (Fig. 1 A). An oral surgery consultation revealed displacement and gingival tear of teeth 42 − 31 with loose grades I-II. The initial diagnosis was traumatic dislocation of the lower anterior teeth, which was treated with ligature wire tethering and immobilization. This intervention aimed to prevent accidental swallowing or aspiration of loose teeth. Approximately two hours postoperatively, the patient was extubated and transferred back to the ward. The patient was discharged on the sixth postoperative day and was followed up for 6 months after surgery, and no residual dysfunction was reported. Case 2 A 32-year-old female, measuring 158 cm in height and weighing 70 kg (BMI 28.04), with a 9-year history of worsening dizziness and headache, was diagnosed with a pial cyst in the pontine and medullary anterior region and mild hydrocephalus. She was scheduled for neuroendoscopic, electrophysiological monitoring-guided, brainstem tumor resection via a right far lateral approach. Anesthesia was induced with midazolam, sufentanil, etomidate, and rocuronium. Nasal intubation was performed, with oral cavity protection via gauze packing. Throughout the operation, the patient remained in the left lateral position, with anesthesia maintained by continuous infusion of remifentanil, propofol, and dexmedetomidine. No muscle relaxants or inhaled anesthetics were used during the neurophysiological monitoring process. Monitoring included SSEP, MEP, and EMG, with MEPs focusing on the bilateral orbicularis oculi, orbicularis oris, mentalis, thyroarytenoid, and sternocleidomastoid muscles. The electrodes were positioned 2 cm lateral to C3 (positive) and C4 (negative) using biphasic stimulation with a 2 ms interval, 100–200 V intensity, and a 75 µs pulse width. The operation lasted 10 hours and 55 minutes. The operation lasted 10 hours and 55 minutes. The intraoperative infusion volume was 4450 ml. The urine volume was 2500 ml, and the blood loss volume was 200 ml. Postsurgery, tongue damage and swelling of the left jaw and face, high neck tension, tongue swelling and varying degrees of bite marks were observed (Fig. 1 B). On the sixth postoperative day, the patient's general condition stabilized, and removal of the tracheal tube was attempted; however, this led to severe respiratory distress, necessitating an emergency tracheotomy. The tracheotomy tube was successfully removed on day 41 after the patient fully regained consciousness and could independently maintain oxygen saturation. She was discharged on the 43rd day and reported no issues with eating or speaking. Case 3 The patient, a 54-year-old male, underwent a craniotomy to remove the left cerebellar mass. He had a height of 170 cm, a weight of 70 kg, and a BMI of 24.2. Anesthesia was induced using sufentanil, remazolam, etomidate, and cisatracurium, followed by nasal intubation. Intraoperative anesthesia was maintained with cisatracurium, remifentanil, propofol, and desflurane. No neurophysiological monitoring was conducted during the surgery. The patient was positioned on his right side for the 5-hour procedure, with a total anesthetic time of 7.5 hours. The intraoperative infusion volume was 3600 ml. The urine volume was 1500 ml, and the blood loss volume was 100 ml. Postoperatively, the patient presented with a swollen and bleeding tongue on the anterior part (Fig. 1 C). The tracheal tube was removed on the fifth postoperative day, and the patient was discharged on postoperative day 19 without any motor or taste disturbances. Discussion and conclusions These three cases highlight the complexity of preventing oral injuries in cerebellopontine angle surgeries. The first case underscores the critical importance of proper dental guard selection to mitigate tooth damage. In the second case, with nasotracheal intubation, despite precautions such as the use of gauze pads, severe facial swelling and tongue biting occurred, suggesting that existing preventive measures may be inadequate under certain conditions. The third patient, which lacked electrophysiological monitoring, still presented with significant oral complications, indicating that factors beyond MEP monitoring, such as surgical duration and patient positioning, also play a crucial role in the incidence of such injuries. This series of cases calls for a comprehensive review of current practices to minimize the risk of oral injuries in neurosurgical operations. Here, we conducted a literature review and summary to elucidate the underlying mechanisms of these complications and to develop comprehensive preventive measures. Literature review Search strategy The keywords "Transcranial Direct Current Stimulation", "Intraoperative Transcranial Electrical Stimulation", "Evoked Potential", "Intraoperative Neurophysiological Monitoring", "Tongue Bite", "Tongue Laceration", "Adverse reactions", "Tooth damage", "Oral mucosa injury", "Tooth displacement", and "Macroglossia" were used as keywords in PubMed and other foreign databases; corresponding Chinese keywords were also used in Chinese databases such as CNKI, Wanfang, and VIP for retrieval. Literature review findings After excluding duplicate cases, 37 articles were included (see Table S1 for a comprehensive review of published reports of tongue injuries in the literature), with 1 in Chinese and 36 in English, published between 1985 and 2024. Combined with the 3 cases reported this time, a total of 242 cases were included in the analysis (which included compound injuries, totaling 249 injured sites), 217 of which were associated with MEP monitoring, and an additional 25 patients without neurophysiological monitoring (Fig. 2 A). There were 230 sites (92.37%) with lip, tongue bite or macroglossia, 6 sites (2.41%) with oral mucosal injuries, 5 sites (2.01%) with incisor injuries, 2 sites (0.80%) with mandibular fractures, and 6 sites (2.41%) with maxillofacial edema (Fig. 2 B). Oral injuries produced following MEP monitoring were more severe than those caused by the absence of neurophysiological monitoring, which were characterized mainly by tongue edema and minor ulcers. However, more comprehensive data reporting in the future may be necessary to draw further conclusions. Most reported cases of oral injuries with documented surgery durations involved surgeries lasting more than 3 hours, with nearly half exceeding 6 hours, suggesting that a longer duration is a potential risk factor (Fig. 2 C). The cases reported included 69 cases (42.86%) in the supine position, 74 cases (45.96%) in the prone position, 8 cases (4.97%) in the prone + supine position, 9 cases (5.59%) in the lateral position, and 1 case (0.62%) in the sitting position (Fig. 2 D). This finding indicates that oral injuries can occur in any surgical position. Furthermore, analysis of the limited body mass index (BMI) data available did not support our initial hypothesis that obese patients are at increased risk of oral injuries (Fig. 2 E). Among the 146 cases in which bite blocks were used (Fig. 2 F), dental pads were used in 5 cases (3.42%), soft bite blocks were used in 117 cases (80.17%), gauze packing was used in 6 cases (4.11%), a combination of hard pads and gauze packing was used in 5 cases (3.42%), a combination of soft bite blocks and gauze packing was used in 2 cases (1.37%), nothing was used in 10 cases (6.85%), and mouth gags were used in 1 patient for surgical maneuvers (0.68%). This finding indicates that even the use of soft bite blocks cannot completely prevent the occurrence of oral injuries during such surgeries. Among all the cases, 217 were accompanied by MEP monitoring. Of these, 25 used the C1 and C2 sites, 147 used the C3 and C4 sites or were more lateral to C3 and C4, and 45 did not specify the monitoring site. From the reported cases, MEP monitoring remains the primary determinant of oral injuries, especially when stimulating at the C3/C4 points. While oral injuries without MEP monitoring may be underreported due to milder severity and other factors, they should still be taken seriously in clinical practice. Additionally, the current reported cases generally lack accurate records of stimulation parameters, so this systematic analysis has not determined how higher stimulation voltages, currents, and stimulation modes may increase the likelihood of oral injuries. The prognosis of patients with oral injuries related to spinal surgery or neurosurgery is generally good. Among the 203 patients with a reported prognosis, 56 patients (27.59%) required surgery or specialist treatment, such as extensive tongue necrosis or dental damage, whereas the other 147 patients (72.41%) recovered spontaneously. Four of the 203 patients developed varying degrees of dysfunction, with the most serious adverse event outcomes. Two patients died, one from secondary sepsis and the other from severe airway obstruction[ 4 , 5 ]. Pathogenic mechanisms of oral injuries following neurosurgical surgery A systematic review of the literature and our case reports revealed that even without MEP monitoring, oral injuries can still occur. Although injuries are typically less severe in the absence of MEP, they mainly manifest as tongue edema with minor surface damage. This finding underscores the presence of factors independent of MEP stimulation that contribute to oral injuries. In light of this, we have summarized the following mechanisms underlying the occurrence of oral injury in neurosurgery and spine surgery. The first group of factors is the combination of prolonged surgery and special positioning. Venous return from the tongue, oral cavity, and craniofacial region is directed through the deep lingual, submandibular, and facial veins into the internal jugular vein. Excessive flexion or extension of the head and neck can lead to swelling due to restricted venous outflow (Fig. 3 A). Positions that have a significant effect on venous return to the head and neck are prone, lateral and prone–lateral, and sitting positions. To fulfill the need for adequate exposure of the surgical field, the head and neck tend to be hyperflexed, extended, or rotated, and the skin on the operative side is excessively taut, which often causes obstruction of venous return, leading to elevated intracranial pressure and potential venous stasis. Appropriate attention to minimize excessive flexion, extension, and rotation of the head and neck during positioning can help reduce these risks and promote safer surgical outcomes. Moreover, the effect of gravity on positioning is an important influencing factor (Fig. 3 B). All three patients in this report were in the lateral position, especially Patient 2, in whom the underside of the tongue was bitten while in the lateral decubitus position. Maintaining these positions for prolonged periods further exacerbates the impact on venous return. The tongue is prone to swelling due to gravitational downward displacement, restricted movement, and venous reflux disorders, which increase the likelihood of tongue-biting injuries[ 6 ]. Over time, sustained compression of the jugular veins and other venous pathways can lead to chronic venous congestion, which not only increases intracranial pressure but also exacerbates swelling in the operative and surrounding areas. In addition, prolonged compression of the lingual blood vessels and salivary glands by oral fillings, tracheal tubes, and other intraoral manipulations during surgery may also cause impaired blood return, lymphatic obstruction, and occlusion of Wharton's duct, and the associated secondary congestion and ischemia/reperfusion injuries may be the cause of postoperative salivary gland inflammation, swelling of the tongue, and airway obstruction[ 7 ]. Intraoperative injury to the lingual vasculature, such as the placement of tongue electrodes, may also present with postoperative swelling[ 8 ]. In addition, macroglossia associated with the posterior cranial fossa may be associated with dysfunction of somatic autonomic reflexes and/or impaired central regulation of the lingual vascular bed, and tracheal intubation may induce tongue swelling through the activation of somatic autonomic reflexes of the tongue and mouth, although clinical evidence is lacking[ 9 ]. The three cases reported here all had surgery times exceeding five hours, which is a significant contributing factor to complications. Surgeons should master patient positioning and expedite surgery as much as possible. Intraoperative monitoring of venous return is also necessary, utilizing techniques such as palpation for skin tension and ultrasound examinations. It is also extremely important to avoid prolonged pressure on the tongue. The second factor is the intense muscle contraction induced by MEP. MEP monitoring is a technique used in surgery to monitor the health of motor pathways in the brain and spinal cord. By applying electrical stimuli to specific areas of the motor cortex, MEP measures the muscle responses. This allows real-time feedback on motor function, aiding surgeons in avoiding damage to these pathways. During the process of monitoring, the stimulating electrode is often placed 2–2.5 cm in front of the scalp C1, C2, or C3, C4, corresponding to the central motor area representing the muscles of the upper limbs, lower limbs, and facial region. Generally, on the basis of cranial-specific anatomical landmarks, such as the highest point of the nose root and occipital protuberance, with the outer ear as a reference, the intersection of the two lines is the CZ, and the left and right sides are opened at 10% of the coronal line length to obtain C1 and C2. Similarly, the left and right sides are opened 20% of the coronal line length to obtain C3 and C4[ 10 – 12 ] (Fig. 4 A, 4 B). The monitored muscle depends on the surgical site involved. For surgeries that target the nerve conduction areas controlling the upper limbs, monitoring typically involves muscles such as the abductor pollicis brevis to assess hand and arm functions. When the focus is on areas controlling lower limb movement, the tibialis anterior muscle is usually monitored for leg and foot movement evaluation. In cases involving the facial nerve (Cranial Nerve VII) and the trigeminal nerve (Cranial Nerve V), monitoring includes muscles innervated by these nerves: for the facial nerve, muscles with facial expression, such as the orbicularis oris and orbicularis oculi, and for the trigeminal nerve, mastication muscles, such as the masseter and temporalis. This approach ensures that critical nerve functions are preserved and minimizes the risk of surgical damage; however, it also increases the risk of oral injury. The muscles controlling oral opening and closing, mastication, and tongue movement primarily include the masseter, temporalis, and medial and lateral pterygoids for chewing; the intrinsic and extrinsic muscles of the tongue for movement; and the orbicularis oris for lip closure. These muscles are innervated by various nerves: the masseter and temporalis via the mandibular branch of the trigeminal nerve, the muscles of the tongue via the hypoglossal nerve, and the orbicularis oris via the facial nerve. When subjected to strong stimulation, these muscles can contract forcefully, leading to a reduction in the oral cavity space, increased intraoral pressure, protrusion of the tongue, and increased biting force, ultimately resulting in oral injuries. Furthermore, transcranial MEP induction is a nonspecific stimulus that cannot be precisely and exclusively targeted to a single nerve's distribution area. When attempting to elicit responses from facial muscles, this broad stimulation may cause intense contractions across the entire craniofacial region, thereby increasing the risk of oral injury. For example, during brainstem area surgeries, to better monitor the functional status of nerves related to the brainstem, MEP electrodes are typically placed at C3’/C4’ (lateral to C3 and C4, see Fig. 4 B), which often directly triggers widespread muscle activity in the craniofacial area. This leads to contraction of the masticatory muscles, movement of the mandible, and contact between the upper and lower teeth, significantly increasing the occlusal force and resulting in injuries (Fig. 4 C). Incidence and associated risk factors for oral injuries Drawing from our reflection on the 3 cases in this study and the collation of information from relevant reported cases, we posit that the risk factors contributing to oral injuries may include the following. First, anesthesia-related factors such as the absence of a bite block, the use of a bite block that is too hard, excessively large or small, improperly placed or displaced, and inappropriate depth of anesthesia. The use of hard bite blocks, including standard dental pads, can lead to the accumulation of biting force, resulting in lip and tongue injuries and even alveolar bone fractures[ 13 ]; as experienced in our reported case 1, hard dental guards should be avoided. Bite blocks that are overly large or small, improperly affixed, and displaced for extended durations can compress the tongue, leading to ulcers, hematomas, and even ischemic necrosi[ 2 ]. In the second case reported here, where the patient was intubated nasally, protection was attempted via a soft dental pad made from gauze in the oral cavity. However, the possibility of an improper fit or displacement during surgery still fails to prevent injury to the tongue. This explains why, according to the literature search, some patients who used soft dental guards nevertheless sustained oral injuries. In addition, the choice of anesthetic drugs and the regulation of anesthesia depth also present a risk for oral injuries. Common anesthetics such as propofol and dexmedetomidine, as well as muscle relaxants, can influence neurophysiological monitoring signals to varying degrees. Similarly, when the concentration of inhaled anesthetics reaches 0.5%, it can lead to a significant decline in MEP amplitude, and complete suppression may occur as the concentration increases[ 14 ]. This could amplify the required stimulation intensity and increase the likelihood of oral injury. If the use of muscle relaxants and inhaled anesthetics is limited, the depth of anesthesia can hardly be guaranteed, and patients’ movement following electrical stimulation might cause forced contraction of the masseter muscle, thereby increasing the likelihood of oral injury. Yata et al. also confirmed a significant correlation between tongue-biting injuries and intraoperative body movement[ 3 ]. Furthermore, patient factors also exert an impact. Tamkus et al.’s data analysis did not corroborate a correlation between age, sex, and the incidence of postoperative biting injuries[ 2 ]. However, malocclusion and missing teeth in patients are risk factors for intraoperative displacement of the bite block, which in turn increases the incidence of tongue-biting injuries. In addition, factors such as hypothermia, hypotension, hypoxemia, anemia, intracranial hypertension, electrolyte imbalance, and blood glucose abnormalities can decrease the MEP signal[ 15 ]. Moreover, individuals with a smaller oral cavity, enlarged tongue, or naturally stronger bite force are potentially at greater risk for oral injuries, yet systematic evidence confirming these correlations is currently insufficient. Additionally, angioedema is a common cause of macroglossia. A patient's previous history of allergies and the use of ACE inhibitors (ACE-Is) or angiotensin receptor blockers (ARBs) are also points of concern for triggering intraoperative angioedema, leading to or exacerbating tongue injury. Angioedema is typically associated with ACE inhibitors, with an incidence ranging from 0.1–0.7%. The associated pathophysiological mechanisms involve vasodilation and increased permeability and plasma extravasation, which are achieved through the inhibition of bradykinin and substance P degradation, both of which are vasodilatory agents that contribute to edema[ 13 ]. Although none of the three patients reported here had previously taken ACE inhibitors and had no history of allergies, patients using such medications may pose an additional risk factor for intraoperative oral injury. Therefore, we recommend that future related reports should include specific patient information, such as preoperative oral conditions, BMI, relevant medication usage, use of bite blocks, intraoperative position, and neurophysiological monitoring sites and parameters, to facilitate colleagues to summarize experience from it. In addition to the above factors concerning surgery and monitoring, Yata et al.’s experimental results indicate that the incidence of oral injuries caused by maximum stimulation intensity is noticeably greater than that caused by nonmaximum stimulation intensity, even if a statistically significant correlation between stimulation intensity and the incidence of tongue-biting injuries is absent[ 3 ]. High stimulation intensity may be a risk factor for oral injuries. Compared with monophasic stimulation, biphasic stimulation, which simultaneously activates both corticospinal tracts, is more likely to induce oral injuries[ 2 ]; the second case mentioned in the text employed bidirectional stimulation. In addition, the placement of the stimulation electrode is a crucial factor. Relative to C3/C4 stimulation electrodes, C1/C2 stimulation electrodes might limit direct activation of facial and axial muscles, possibly because the former are closer to the facial motor cortex, mandibular muscles, and trigeminal nerve[ 2 ]. In reported cases of oral injuries with MEP stimulation sites, stimulation at C3/C4 accounted for approximately 81.82% of the total cases. Hence, placing electrodes at C3/C4 may increase the risk of oral injuries. Stimulating even further to the outer edge of C3/C4 poses a greater risk, necessitating a balance between monitoring muscle position and stimulation intensity. Prevention of oral injuries Good preventive measures can significantly lower the rate of oral injuries during neurosurgical operations. On the basis of an analysis of the mechanisms and influencing factors of these injuries, addressing this issue requires the joint efforts of neurosurgeons, anesthesiologists, neurophysiological monitoring physicians, and nursing staff. From a surgical standpoint, it is crucial to balance optimal exposure of the surgical area with the effects of positioning on the patient's head, neck tension, and venous return. Excessive bending or stretching of the head and neck should be avoided. Monitoring the jugular vein status during surgery is essential. Any increase in tension or compromised venous return requires immediate adjustment of the patient's position. Additionally, intraoperatively, prolonged compression of local tissues or vessels must be avoided. Improving surgical efficiency to shorten the operation duration can help minimize complications. Furthermore, ensuring that patients maintain good oral hygiene before surgery can reduce the risk of infection following oral injuries. From the perspective of electrophysiological monitoring, the focus should be on meticulous MEP monitoring. The first step is to select the appropriate stimulation sites with precise localization, avoiding the use of C3/C4 stimulation points whenever C1/C2 can meet the requirements. However, it is crucial to recognize that monitoring certain surgical areas, such as those involving the cerebellopontine angle, as reported in our cases, necessitates the use of C3/C4 locations due to the risk of intraoperative damage to cranial nerves associated with facial movements. Furthermore, surgeries targeting regions related to the movements of the upper and lower limbs should ideally opt for C1/C2, which reduces the stimulation of the head and face motor cortex while obtaining satisfactory potential signals. Additionally, avoiding high-intensity and high-frequency electrical stimulation is recommended. It is best to start with low-intensity titration to achieve the minimum stimulation threshold for satisfactory signals. Simultaneously, enhancing the sensitivity of monitoring equipment and improving monitoring techniques also contribute to reducing the need for high-intensity stimulation. Employing a four-pole stimulation strategy (comprising two anodes and two cathodes) can significantly improve signal quality while allowing for lower stimulation intensities, effectively reducing the risk of nonspecific stimulation and ensuring the accuracy and safety of MEP monitoring during neurosurgical procedures[ 16 ]. From the perspective of an anesthesiologist, approaches can be initiated from two aspects: the anesthesia plan and the correct use of dental guards. First, anesthesiologists should formulate appropriate anesthesia plans on the basis of patient condition, drug characteristics, and surgical methods. While satisfying anesthesia depth and surgical safety, the need for neurophysiological monitoring should also be considered. The current recommended anesthesia regimen is anesthesia without muscle relaxants or total intravenous anesthesia[ 13 ]. The use of no muscle relaxants and the use of only intravenous anesthesia increases the use of propofol to prevent movement. The addition of dexmedetomidine (0.5 mcg/kg/h) reduces the need for propofol, stabilizing anesthesia and hemodynamics[ 17 ]. A subanaesthetic dose of ketamine can also be used in MEP monitoring to deepen anesthesia while causing gradual improvement in amplitudes without affecting latency[ 18 ]. A crucial aspect to note is the recommendation to monitor the depth of anesthesia in such surgeries, which ensures stable anesthesia levels, minimizing impacts on monitoring activities and preventing bodily movements, thereby mitigating the risk of oral injuries. Another strategy involves the judicious use of low-dose muscle relaxants to mitigate excessive biting force during stimulation. Research by Zhang, X. et al. indicated that a rocuronium infusion rate of 9 µg.kg^--1.min^--1 strikes a balance between the needs of neurophysiological monitoring and achieving adequate anesthesia depth[ 15 ]. Nonetheless, muscle relaxants may dampen or entirely obstruct MEP responses, potentially leading to increased stimulation intensity and subsequent injuries, necessitating close collaboration between anesthesiologists and monitoring physicians. Furthermore, the application of muscle relaxants should be guided by strict train-of-four (TOF) monitoring, accommodating the differential muscle sensitivity to relaxants and their dynamic effects over time. In patients with intraoperative somatic movements, the block can be given again if T1 is between 5% and 50% of baseline or if there are two detectable jerks, MEP baseline levels can be reached early with sugammadex[ 12 , 19 , 20 ]. However, whether these methods can reduce the incidence of oral injuries still lacks clinical data. In addition, the correct use of a bite block is crucial for oral protection. Using a soft bite block or gauze block can decrease the incidence of oral injuries[ 21 ]. In Tamkus’s report, the incidence of oral injuries associated with bite pads (4.42%) was significantly greater than that associated with the use of soft bite blocks (1.27%)[ 2 ]. The absence of bite blocks not only increases the incidence of oral injury but also increases the possibility of tracheal tube rupture and reintubation. Compared with placing a bite block only at the midline, placing bite blocks between the upper and lower molars on both sides is safe[ 22 ]. Salik et al. recommended placing a soft bite block in the midline of the tongue and between the molars on both sides[ 13 ]. During monitoring, the anesthesiologist needs to select the appropriate size of bite block, place and determine its position, and pull the lips out to prevent lip compression injury; avoiding pressure on the tongue is crucial. Proper fixation should be ensured to prevent damage to the tongue and teeth. During surgery, attention should be given to the displacement or dislodgment of the bite block[ 23 ]. In addition, some researchers suggest the use of tooth protection devices to provide better oral protection[ 24 ]. It is also recommended that the tracheal tube be secured to the operative side to minimize contralateral compression and that transnasal intubation be used to minimize the base-of-laryngeal contact area and pressure generation[ 4 ]. Regardless of the presence of MEP monitoring and whether intubation is nasal or oral, in high-risk patients, the use of a soft bite block could alleviate tongue pressure by increasing the oral space, thus preventing tongue swelling and biting injuries[ 25 ]. The future development of an oral protective pad designed to enlarge the oral cavity space suitably, prevent the tongue from interposing between the teeth, and furnish real-time feedback on alterations in bite force amidst stimulation, with the aim of diminishing interdental bite force, is anticipated to confer substantial benefits. Table 1 Prevention measures for Oral injuries Measures Anesthetic Measures 1. Monitor the depth of anesthesia, avoiding Intraoperative Body Movement; 2. Choose the right anesthesia plan, with TIVA recommended; 3. Select and correctly place a soft bite block, properly secure it, and check the position of the bite block during surgery; 4. For severe tongue bite injuries or facial edema, extubation requires careful deliberation. 5. Research and design new types of bite blocks. Patient Measures 1. Conduct thorough preoperative evaluations to identify those at high risk for oral injuries and customize anesthesia and oral care plans according to individual patient needs. 2. Maintain good oral hygiene before surgery. Surgical and Monitoring Measures 1. Avoid high-intensity, high-frequency, and bipolar stimulation and choose the appropriate electrode placement sites, C1 and C2 sites are recommended, especially in spinal surgery. 2. Pay attention to the impact of surgical positioning on postoperative oral complications, avoid excessive and long-term neck flexion and pressure; 3. Monitor venous return pressure under surgical positioning during the operation. 4. Minimize the duration of time spent in special positions as much as possible. Overall, we require collective efforts from the team (see Table 1 for prevention measures for oral injuries). Preoperative patient assessment is essential to identify high-risk individuals for potential oral injuries and to establish effective protocols for intraoperative oral protection. Additionally, it is crucial to establish an effective protocol for managing oral injuries when they occur. Patients with macroglossia and airway obstruction should be adequately evaluated prior to extubation, and airway patency can be ensured by imaging, visual laryngoscopy, and tracheal tube sleeve leakage testing to avoid postextubation airway obstruction. Notably, in patients with edema caused by prolonged pressure on the tongue, removal of the endotracheal tube without interfering with ventilation may be beneficial. Multidisciplinary collaboration is indispensable for ensuring proper management in such cases. Declarations Acknowledgements Not applicable. Authors ’ contributions Tianyuan Luo and Mingxiang Xie conceived and designed the study; Yuanli Pi and Yu Li collected and assembled the data; Linlin Luo and Limei Luo analyzed and interpreted the data; Yuanli Pi and Tianyuan Luo prepared the figures; Yuanli Pi and Tianyuan Luo wrote the manuscript. All authors read, critically reviewed, and approved the final manuscript. Funding The National Natural Science Foundation of China (No. 82060653 to TYL) and the Famous Clinical Doctor Program (No. 20211022 to TYL) of Zunyi Medical University. Data availability The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request. Ethics approval and consent to participate This work has been carried out in accordance with the Declaration of Helsinki (2000) of the World Medical Association. Ethical approval for this report was provided by the Ethical Committee of the Affiliated Hospital of Zunyi Medical University, Zunyi, China, on July 5, 2024 (No: KLL-2024-149). Written informed consent to publish this case and the associated images was obtained from the patients. Consent for publication Consent for publication has been obtained from the individuals involved. Competing interests The authors declare no competing interests. References MacDonald DB. Safety of intraoperative transcranial electrical stimulation motor evoked potential monitoring. J Clin Neurophysiol. 2002;19(5):416–29. Tamkus A, Rice K. The incidence of bite injuries associated with transcranial motor-evoked potential monitoring. Anesth Analg. 2012;115(3):663–7. Yata S, Ida M, Shimotsuji H, Nakagawa Y, Ueda N, Takatani T, Shigematsu H, Motoyama Y, Nakase H, Kirita T, et al. 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Bassi JS, Hsu F, Mnatsakanyan L, Rajan GR. Acute airway obstruction requiring nasotracheal intubation following hypoglossal neuromonitoring: a case report. BMC Anesthesiol. 2023;23(1):149. Ifeanyi I, Agnieszka A, Wilson C, Rwoof R, Fernando G, Jeffrey F. Macroglossia associated with brainstem injury. Neurocrit Care. 2014;20(1):106–10. Acharya JN, Abeer JH, Cheek J, Thirumala P, Tsuchida TN. American Clinical Neurophysiology Society Guideline 2: Guidelines for Standard Electrode Position Nomenclature. Neurodiagnostic J. 2016;56(4):245–52. Gordon EM, Chauvin RJ, Van AN, Rajesh A, Nielsen A, Newbold DJ, Lynch CJ, Seider NA, Krimmel SR, Scheidter KM, et al. A somato-cognitive action network alternates with effector regions in motor cortex. Nature. 2023;617(7960):351–9. Kawaguchi M, Iida H, Tanaka S, Fukuoka N, Hayashi H, Izumi S, Yoshitani K, Kakinohana M. Anesthesiologists MEPMGWGotSCotJSo: A practical guide for anesthetic management during intraoperative motor evoked potential monitoring. J Anesth. 2020;34(1):5–28. Salik I, Namkoong S, Lisov C, Lederman D, Abramowicz AE. Tongue injury associated with motor evoked potential monitoring: Causes, prevention and treatment options. J Clin Anesth. 2022;78:110617. Feng Z, Pei Y, Paerhati H, Hao Y, Jiang L. Risk Factors Analysis and Prevention Strategies of Intraoperative Neurophysiological Monitoring. Chin J Brain Dis Rehabil (Electronic Edition). 2021;11(04):232–6. Sloan TB, Heyer EJ. Anesthesia for intraoperative neurophysiologic monitoring of the spinal cord. J Clin Neurophysiol. 2002;19(5):430–43. Schwartz SL, Kale EB, Madden D, Husain AM. Quadripolar Transcranial Electrical Stimulation for Motor Evoked Potentials. J Clin Neurophysiol. 2022;39(1):92–7. Andleeb R, Agrawal S, Gupta P. Evaluation of the Effect of Continuous Infusion of Dexmedetomidine or a Subanesthetic Dose Ketamine on Transcranial Electrical Motor Evoked Potentials in Adult Patients Undergoing Elective Spine Surgery under Total Intravenous Anesthesia: A Randomized Controlled Exploratory Study. Asian Spine J. 2022;16(2):221–30. Phoowanakulchai S, Kawaguchi M. Updated review on the use of neuromuscular blockade during intraoperative motor-evoked potential monitoring in the modern anesthesia era. J Anesth. 2024;38(1):114–24. Gupta S, Siddiqui SA, Sinha U, Gupta G. Multimodal Intraoperative Neurophysiological Monitoring in Cranial and Spinal Tumour Surgeries: A Descriptive Observational Study. Cureus. 2023;15(11):e49411. Pavoni V, Gianesello L, De Scisciolo G, Provvedi E, Horton D, Barbagli R, Conti P, Conti R, Giunta F. Reversal of profound and deep residual rocuronium-induced neuromuscular blockade by sugammadex: a neurophysiological study. Minerva Anestesiol. 2012;78(5):542–9. Kostis WJ, Shetty M, Chowdhury YS, Kostis JB. ACE Inhibitor-Induced Angioedema: a Review. Curr Hypertens Rep. 2018;20(7):55. Kothbauer KF, Deletis V, Epstein FJ. Motor-Evoked Potential Monitoring for Intramedullary Spinal Cord Tumor Surgery: Correlation of Clinical and Neurophysiological Data in a Series of 100 Consecutive Procedures. NeuroSurg Focus. 1998;4(5):e1. Oshita K, Saeki N, Kubo T, Abekura H, Tanaka N, Kawamoto M. A novel mouthpiece prevents bite injuries caused by intraoperative transcranial electric motor-evoked potential monitoring. J Anesth. 2016;30(5):850–4. Tan WK, Liu EH, Thean HP. A clinical report about an unusual occurrence of post-anesthetic tongue swelling. J Prosthodont. 2001;10(2):105–7. Junghaenel S, Keller T, Mischkowski R, Hinkelbein J, Beutner D, Koerber F, Teschendorf P. Massive macroglossia after palatoplasty: case report and review of the literature. Eur J Pediatr. 2012;171(3):433–7. Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4840493","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Case Report","associatedPublications":[],"authors":[{"id":345697836,"identity":"05898d16-9928-411e-9d49-e1cf36210bbe","order_by":0,"name":"Yuanli Pi","email":"","orcid":"","institution":"Affiliated Hospital of Zunyi Medical University","correspondingAuthor":false,"prefix":"","firstName":"Yuanli","middleName":"","lastName":"Pi","suffix":""},{"id":345697837,"identity":"2d199943-933a-4121-a3ca-ae4fc387bc3d","order_by":1,"name":"Linlin Luo","email":"","orcid":"","institution":"Affiliated Hospital of Zunyi Medical University","correspondingAuthor":false,"prefix":"","firstName":"Linlin","middleName":"","lastName":"Luo","suffix":""},{"id":345697838,"identity":"cfd551b3-4c15-4484-a7a8-0ee00ee8b31a","order_by":2,"name":"Yu Li","email":"","orcid":"","institution":"Affiliated Hospital of Zunyi Medical University","correspondingAuthor":false,"prefix":"","firstName":"Yu","middleName":"","lastName":"Li","suffix":""},{"id":345697840,"identity":"a93d5b64-0e29-499a-b3ec-8015e372cd92","order_by":3,"name":"Limei Luo","email":"","orcid":"","institution":"Affiliated Hospital of Zunyi Medical University","correspondingAuthor":false,"prefix":"","firstName":"Limei","middleName":"","lastName":"Luo","suffix":""},{"id":345697842,"identity":"f4dcf02a-c832-4319-b733-a48943757551","order_by":4,"name":"Mingxiang Xie","email":"","orcid":"","institution":"Affiliated Hospital of Zunyi Medical University","correspondingAuthor":false,"prefix":"","firstName":"Mingxiang","middleName":"","lastName":"Xie","suffix":""},{"id":345697843,"identity":"9afb258e-83fe-4beb-bfd0-56ea2cd2b41f","order_by":5,"name":"Tianyuan Luo","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzklEQVRIiWNgGAWjYBACxmb+7x8kKmyY+dkbiNTC3N5gxmBxJo1dsucAkVrYew6YMVS2HeY3uJFApBbeGQlpD26wHZY2uPl44w2GGptoglokZyQcN5zBk24seTut2ILhWFpuAyEthjMSG6QlJKyT+W7nmEkwNhwmrMX+RjKD9B8D5vqGm2eI1MLYc4xNQiLBmVngBg+xWtp7mA0kDqQxS/YA/ZJAjF8Ym3kYH0j+A0Xl4Y03PtTYENaCDAwkEkhRDtFCqo5RMApGwSgYGQAAQolAAd53yScAAAAASUVORK5CYII=","orcid":"","institution":"Affiliated Hospital of Zunyi Medical University","correspondingAuthor":true,"prefix":"","firstName":"Tianyuan","middleName":"","lastName":"Luo","suffix":""}],"badges":[],"createdAt":"2024-08-01 08:25:45","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4840493/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4840493/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":64708826,"identity":"559b4ca1-1b37-495f-8459-c7c3ab63fb7d","added_by":"auto","created_at":"2024-09-18 01:42:49","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":132920,"visible":true,"origin":"","legend":"\u003cp\u003eThe Oral injures of the three cases. \u003cstrong\u003eA\u003c/strong\u003e. Case1: MEP monitoring resulted in the loosening of teeth 31-42 directly beneath the regular dental pad. \u003cstrong\u003eB\u003c/strong\u003e. Case2: Bilateral MEP monitoring resulted in tongue biting and facial swelling. \u003cstrong\u003eC\u003c/strong\u003e. Case3: The patient experienced tongue swelling and minor tongue surface biting in the absence of MEP monitoring.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4840493/v1/94edbe163bc555d92a5ab34e.jpg"},{"id":64708822,"identity":"947137e9-9afe-4338-b74d-2d43c5a07e42","added_by":"auto","created_at":"2024-09-18 01:42:49","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":315691,"visible":true,"origin":"","legend":"\u003cp\u003eChart summary of oral injury cases based on the reviewed literature. \u003cstrong\u003eA\u003c/strong\u003e. Distribution of the reported cases with or without MEP monitoring. \u003cstrong\u003eB\u003c/strong\u003e. Types and distribution of oral injuries. \u003cstrong\u003eC\u003c/strong\u003e. Partial reporting of cases with the length of surgery. \u003cstrong\u003eD\u003c/strong\u003e. Surgical positions reported in relevant cases. \u003cstrong\u003eE\u003c/strong\u003e. BMI status reported in cases. \u003cstrong\u003eF\u003c/strong\u003e. Bite block reported in related cases.\u003c/p\u003e","description":"","filename":"Figure2LiteratureBasedSummaryChartofOralInjuryCases.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4840493/v1/e8c0bafb3aa61a72d908dcc7.jpg"},{"id":64708825,"identity":"7386e0d7-c2de-4a0f-960e-fe95fcf64a74","added_by":"auto","created_at":"2024-09-18 01:42:49","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":4961445,"visible":true,"origin":"","legend":"\u003cp\u003eMechanism of tongue swelling with restricted venous return in the lateral position. \u003cstrong\u003eA\u003c/strong\u003e. Twisting of the neck in the lateral position. \u003cstrong\u003eB\u003c/strong\u003e. Increased exudation in the lateral position due to twisting of the neck and localized compression of blood vessels. \u003cstrong\u003eC\u003c/strong\u003e. Gradual swelling of the tongue with time under the effects of gravity and increased local exudation.\u003c/p\u003e","description":"","filename":"Figure3Mechanismoftongueswelling.png","url":"https://assets-eu.researchsquare.com/files/rs-4840493/v1/a5adeb2a4a0673fe03fbc256.png"},{"id":64708824,"identity":"b873fccb-39d5-4760-9927-95a11287085b","added_by":"auto","created_at":"2024-09-18 01:42:49","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":110537,"visible":true,"origin":"","legend":"\u003cp\u003eElectrode placement and stimulation in MEP monitoring. \u003cstrong\u003eA\u003c/strong\u003e. MEP stimulation site on the surface of the skull. \u003cstrong\u003eB\u003c/strong\u003e. MEP stimulation area on the basis of the latest cortical localization maps of the primary motor cortex; the placement of stimulation electrodes in the motor area of the face and jaw is more likely to cause oral injuries. \u003cstrong\u003eC.\u003c/strong\u003e Illustration of facial and jaw muscle contraction with unilateral MEP electrode stimulation.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4840493/v1/a619bd513a84f00599b2d9e2.jpg"},{"id":77970457,"identity":"7f6b2414-0b11-413d-84d8-20335c9c3e77","added_by":"auto","created_at":"2025-03-07 10:47:58","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5799717,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4840493/v1/418192fb-1a62-48c6-a290-2d99f155e3d1.pdf"},{"id":64708823,"identity":"3c0f6b67-25c9-4800-91e5-71e9dbca0986","added_by":"auto","created_at":"2024-09-18 01:42:49","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":75889,"visible":true,"origin":"","legend":"","description":"","filename":"TableS1.docx","url":"https://assets-eu.researchsquare.com/files/rs-4840493/v1/4afbf2cc90bd396476917ae0.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Oral Injuries Following Cerebellopontine Angle Surgery with or without Motor Evoked Potential Monitoring: three Case Reports and Literature Review","fulltext":[{"header":"Background","content":"\u003cp\u003eTo increase the safety and efficacy of neurosurgical or spinal procedures, an increasing number of neurophysiological monitoring techniques are being implemented. Nonetheless, a specific complication that continues to warrant attention is oral injury. The reported incidence of oral injuries varies across different studies. For example, in a study of 15,000 cases conducted by MacDonald et al., the incidence rate was approximately 0.19% (29/15000)[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. In contrast, Tamkus et al.\u0026rsquo;s retrospective study involving 17273 individuals reported an incidence rate of 0.63% (109/17273)[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Yata et al. reported a significantly higher incidence rate of 6.5% (12/186)[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Although the overall incidence rate is not high, oral injuries can lead to additional harm and potentially prolonged hospital stays. Severe cases may also present airway risks and necessitate oral surgery intervention. Owing to its unique characteristics, this phenomenon merits significant attention. Herein, we report three distinctive cases following cerebellopontine angle surgery to provide a comprehensive understanding of this type of complication.\u003c/p\u003e"},{"header":"Case Presentation","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eCase 1\u003c/h2\u003e \u003cp\u003eA 31-year-old male patient, measuring 165 cm and weighing 55 kg (BMI 20.2), was diagnosed with a hemorrhagic right acoustic neuroma. Elective tumor resection surgery was planned. Anesthesia was initiated via the use of 20 mg of intravenous propofol, 20 \u0026micro;g of sufentanil, and 50 mg of rocuronium for smooth induction and endotracheal intubation. A standard dental pad secured the tube. The patient was positioned in the left lateral position during surgery, with anesthesia maintained through continuous infusion of remifentanil, propofol, and dexmedetomidine. During surgery, monitoring of somatosensory evoked potentials (SSEP), motor evoked potentials (MEP), and electromyography (EMG) was conducted; hence, muscle relaxants were not used throughout the entire procedure following anesthesia induction. MEP monitoring targeted the orbicularis oculi, orbicularis oris, chin, masseter, and sternocleidomastoid muscles. Biphasic stimulation used a positive electrode 2 cm from C3 and a negative electrode at the CZ, with parameters set to a 2 ms interval, 100\u0026ndash;200 V intensity, and a 75 \u0026micro;s pulse width. The surgery lasted for 7 hours and 35 minutes. The intraoperative infusion volume was 2850 ml. The urine volume was 2000 ml, and the blood loss was 200 ml. Postoperatively, the patient presented with loose and displaced teeth (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA). An oral surgery consultation revealed displacement and gingival tear of teeth 42\u0026thinsp;\u0026minus;\u0026thinsp;31 with loose grades I-II. The initial diagnosis was traumatic dislocation of the lower anterior teeth, which was treated with ligature wire tethering and immobilization. This intervention aimed to prevent accidental swallowing or aspiration of loose teeth. Approximately two hours postoperatively, the patient was extubated and transferred back to the ward. The patient was discharged on the sixth postoperative day and was followed up for 6 months after surgery, and no residual dysfunction was reported.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eCase 2\u003c/h3\u003e\n\u003cp\u003eA 32-year-old female, measuring 158 cm in height and weighing 70 kg (BMI 28.04), with a 9-year history of worsening dizziness and headache, was diagnosed with a pial cyst in the pontine and medullary anterior region and mild hydrocephalus. She was scheduled for neuroendoscopic, electrophysiological monitoring-guided, brainstem tumor resection via a right far lateral approach. Anesthesia was induced with midazolam, sufentanil, etomidate, and rocuronium. Nasal intubation was performed, with oral cavity protection via gauze packing. Throughout the operation, the patient remained in the left lateral position, with anesthesia maintained by continuous infusion of remifentanil, propofol, and dexmedetomidine. No muscle relaxants or inhaled anesthetics were used during the neurophysiological monitoring process. Monitoring included SSEP, MEP, and EMG, with MEPs focusing on the bilateral orbicularis oculi, orbicularis oris, mentalis, thyroarytenoid, and sternocleidomastoid muscles. The electrodes were positioned 2 cm lateral to C3 (positive) and C4 (negative) using biphasic stimulation with a 2 ms interval, 100\u0026ndash;200 V intensity, and a 75 \u0026micro;s pulse width. The operation lasted 10 hours and 55 minutes. The operation lasted 10 hours and 55 minutes. The intraoperative infusion volume was 4450 ml. The urine volume was 2500 ml, and the blood loss volume was 200 ml. Postsurgery, tongue damage and swelling of the left jaw and face, high neck tension, tongue swelling and varying degrees of bite marks were observed (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB). On the sixth postoperative day, the patient's general condition stabilized, and removal of the tracheal tube was attempted; however, this led to severe respiratory distress, necessitating an emergency tracheotomy. The tracheotomy tube was successfully removed on day 41 after the patient fully regained consciousness and could independently maintain oxygen saturation. She was discharged on the 43rd day and reported no issues with eating or speaking.\u003c/p\u003e\n\u003ch3\u003eCase 3\u003c/h3\u003e\n\u003cp\u003eThe patient, a 54-year-old male, underwent a craniotomy to remove the left cerebellar mass. He had a height of 170 cm, a weight of 70 kg, and a BMI of 24.2. Anesthesia was induced using sufentanil, remazolam, etomidate, and cisatracurium, followed by nasal intubation. Intraoperative anesthesia was maintained with cisatracurium, remifentanil, propofol, and desflurane. No neurophysiological monitoring was conducted during the surgery. The patient was positioned on his right side for the 5-hour procedure, with a total anesthetic time of 7.5 hours. The intraoperative infusion volume was 3600 ml. The urine volume was 1500 ml, and the blood loss volume was 100 ml. Postoperatively, the patient presented with a swollen and bleeding tongue on the anterior part (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC). The tracheal tube was removed on the fifth postoperative day, and the patient was discharged on postoperative day 19 without any motor or taste disturbances.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion and conclusions","content":"\u003cp\u003eThese three cases highlight the complexity of preventing oral injuries in cerebellopontine angle surgeries. The first case underscores the critical importance of proper dental guard selection to mitigate tooth damage. In the second case, with nasotracheal intubation, despite precautions such as the use of gauze pads, severe facial swelling and tongue biting occurred, suggesting that existing preventive measures may be inadequate under certain conditions. The third patient, which lacked electrophysiological monitoring, still presented with significant oral complications, indicating that factors beyond MEP monitoring, such as surgical duration and patient positioning, also play a crucial role in the incidence of such injuries. This series of cases calls for a comprehensive review of current practices to minimize the risk of oral injuries in neurosurgical operations. Here, we conducted a literature review and summary to elucidate the underlying mechanisms of these complications and to develop comprehensive preventive measures.\u003c/p\u003e"},{"header":"Literature review","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eSearch strategy\u003c/h2\u003e \u003cp\u003eThe keywords \"Transcranial Direct Current Stimulation\", \"Intraoperative Transcranial Electrical Stimulation\", \"Evoked Potential\", \"Intraoperative Neurophysiological Monitoring\", \"Tongue Bite\", \"Tongue Laceration\", \"Adverse reactions\", \"Tooth damage\", \"Oral mucosa injury\", \"Tooth displacement\", and \"Macroglossia\" were used as keywords in PubMed and other foreign databases; corresponding Chinese keywords were also used in Chinese databases such as CNKI, Wanfang, and VIP for retrieval.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eLiterature review findings\u003c/h2\u003e \u003cp\u003eAfter excluding duplicate cases, 37 articles were included (see Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e for a comprehensive review of published reports of tongue injuries in the literature), with 1 in Chinese and 36 in English, published between 1985 and 2024. Combined with the 3 cases reported this time, a total of 242 cases were included in the analysis (which included compound injuries, totaling 249 injured sites), 217 of which were associated with MEP monitoring, and an additional 25 patients without neurophysiological monitoring (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThere were 230 sites (92.37%) with lip, tongue bite or macroglossia, 6 sites (2.41%) with oral mucosal injuries, 5 sites (2.01%) with incisor injuries, 2 sites (0.80%) with mandibular fractures, and 6 sites (2.41%) with maxillofacial edema (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB). Oral injuries produced following MEP monitoring were more severe than those caused by the absence of neurophysiological monitoring, which were characterized mainly by tongue edema and minor ulcers. However, more comprehensive data reporting in the future may be necessary to draw further conclusions. Most reported cases of oral injuries with documented surgery durations involved surgeries lasting more than 3 hours, with nearly half exceeding 6 hours, suggesting that a longer duration is a potential risk factor (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eC).\u003c/p\u003e \u003cp\u003eThe cases reported included 69 cases (42.86%) in the supine position, 74 cases (45.96%) in the prone position, 8 cases (4.97%) in the prone\u0026thinsp;+\u0026thinsp;supine position, 9 cases (5.59%) in the lateral position, and 1 case (0.62%) in the sitting position (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eD). This finding indicates that oral injuries can occur in any surgical position. Furthermore, analysis of the limited body mass index (BMI) data available did not support our initial hypothesis that obese patients are at increased risk of oral injuries (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eE).\u003c/p\u003e \u003cp\u003eAmong the 146 cases in which bite blocks were used (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eF), dental pads were used in 5 cases (3.42%), soft bite blocks were used in 117 cases (80.17%), gauze packing was used in 6 cases (4.11%), a combination of hard pads and gauze packing was used in 5 cases (3.42%), a combination of soft bite blocks and gauze packing was used in 2 cases (1.37%), nothing was used in 10 cases (6.85%), and mouth gags were used in 1 patient for surgical maneuvers (0.68%). This finding indicates that even the use of soft bite blocks cannot completely prevent the occurrence of oral injuries during such surgeries.\u003c/p\u003e \u003cp\u003eAmong all the cases, 217 were accompanied by MEP monitoring. Of these, 25 used the C1 and C2 sites, 147 used the C3 and C4 sites or were more lateral to C3 and C4, and 45 did not specify the monitoring site. From the reported cases, MEP monitoring remains the primary determinant of oral injuries, especially when stimulating at the C3/C4 points. While oral injuries without MEP monitoring may be underreported due to milder severity and other factors, they should still be taken seriously in clinical practice. Additionally, the current reported cases generally lack accurate records of stimulation parameters, so this systematic analysis has not determined how higher stimulation voltages, currents, and stimulation modes may increase the likelihood of oral injuries.\u003c/p\u003e \u003cp\u003eThe prognosis of patients with oral injuries related to spinal surgery or neurosurgery is generally good. Among the 203 patients with a reported prognosis, 56 patients (27.59%) required surgery or specialist treatment, such as extensive tongue necrosis or dental damage, whereas the other 147 patients (72.41%) recovered spontaneously. Four of the 203 patients developed varying degrees of dysfunction, with the most serious adverse event outcomes. Two patients died, one from secondary sepsis and the other from severe airway obstruction[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003ePathogenic mechanisms of oral injuries following neurosurgical surgery\u003c/h2\u003e \u003cp\u003eA systematic review of the literature and our case reports revealed that even without MEP monitoring, oral injuries can still occur. Although injuries are typically less severe in the absence of MEP, they mainly manifest as tongue edema with minor surface damage. This finding underscores the presence of factors independent of MEP stimulation that contribute to oral injuries. In light of this, we have summarized the following mechanisms underlying the occurrence of oral injury in neurosurgery and spine surgery.\u003c/p\u003e \u003cp\u003eThe first group of factors is the combination of prolonged surgery and special positioning. Venous return from the tongue, oral cavity, and craniofacial region is directed through the deep lingual, submandibular, and facial veins into the internal jugular vein. Excessive flexion or extension of the head and neck can lead to swelling due to restricted venous outflow (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eA). Positions that have a significant effect on venous return to the head and neck are prone, lateral and prone\u0026ndash;lateral, and sitting positions. To fulfill the need for adequate exposure of the surgical field, the head and neck tend to be hyperflexed, extended, or rotated, and the skin on the operative side is excessively taut, which often causes obstruction of venous return, leading to elevated intracranial pressure and potential venous stasis. Appropriate attention to minimize excessive flexion, extension, and rotation of the head and neck during positioning can help reduce these risks and promote safer surgical outcomes. Moreover, the effect of gravity on positioning is an important influencing factor (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eB). All three patients in this report were in the lateral position, especially Patient 2, in whom the underside of the tongue was bitten while in the lateral decubitus position. Maintaining these positions for prolonged periods further exacerbates the impact on venous return. The tongue is prone to swelling due to gravitational downward displacement, restricted movement, and venous reflux disorders, which increase the likelihood of tongue-biting injuries[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Over time, sustained compression of the jugular veins and other venous pathways can lead to chronic venous congestion, which not only increases intracranial pressure but also exacerbates swelling in the operative and surrounding areas. In addition, prolonged compression of the lingual blood vessels and salivary glands by oral fillings, tracheal tubes, and other intraoral manipulations during surgery may also cause impaired blood return, lymphatic obstruction, and occlusion of Wharton's duct, and the associated secondary congestion and ischemia/reperfusion injuries may be the cause of postoperative salivary gland inflammation, swelling of the tongue, and airway obstruction[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Intraoperative injury to the lingual vasculature, such as the placement of tongue electrodes, may also present with postoperative swelling[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. In addition, macroglossia associated with the posterior cranial fossa may be associated with dysfunction of somatic autonomic reflexes and/or impaired central regulation of the lingual vascular bed, and tracheal intubation may induce tongue swelling through the activation of somatic autonomic reflexes of the tongue and mouth, although clinical evidence is lacking[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. The three cases reported here all had surgery times exceeding five hours, which is a significant contributing factor to complications. Surgeons should master patient positioning and expedite surgery as much as possible. Intraoperative monitoring of venous return is also necessary, utilizing techniques such as palpation for skin tension and ultrasound examinations. It is also extremely important to avoid prolonged pressure on the tongue.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe second factor is the intense muscle contraction induced by MEP. MEP monitoring is a technique used in surgery to monitor the health of motor pathways in the brain and spinal cord. By applying electrical stimuli to specific areas of the motor cortex, MEP measures the muscle responses. This allows real-time feedback on motor function, aiding surgeons in avoiding damage to these pathways. During the process of monitoring, the stimulating electrode is often placed 2\u0026ndash;2.5 cm in front of the scalp C1, C2, or C3, C4, corresponding to the central motor area representing the muscles of the upper limbs, lower limbs, and facial region. Generally, on the basis of cranial-specific anatomical landmarks, such as the highest point of the nose root and occipital protuberance, with the outer ear as a reference, the intersection of the two lines is the CZ, and the left and right sides are opened at 10% of the coronal line length to obtain C1 and C2. Similarly, the left and right sides are opened 20% of the coronal line length to obtain C3 and C4[\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB). The monitored muscle depends on the surgical site involved. For surgeries that target the nerve conduction areas controlling the upper limbs, monitoring typically involves muscles such as the abductor pollicis brevis to assess hand and arm functions. When the focus is on areas controlling lower limb movement, the tibialis anterior muscle is usually monitored for leg and foot movement evaluation. In cases involving the facial nerve (Cranial Nerve VII) and the trigeminal nerve (Cranial Nerve V), monitoring includes muscles innervated by these nerves: for the facial nerve, muscles with facial expression, such as the orbicularis oris and orbicularis oculi, and for the trigeminal nerve, mastication muscles, such as the masseter and temporalis. This approach ensures that critical nerve functions are preserved and minimizes the risk of surgical damage; however, it also increases the risk of oral injury. The muscles controlling oral opening and closing, mastication, and tongue movement primarily include the masseter, temporalis, and medial and lateral pterygoids for chewing; the intrinsic and extrinsic muscles of the tongue for movement; and the orbicularis oris for lip closure. These muscles are innervated by various nerves: the masseter and temporalis via the mandibular branch of the trigeminal nerve, the muscles of the tongue via the hypoglossal nerve, and the orbicularis oris via the facial nerve. When subjected to strong stimulation, these muscles can contract forcefully, leading to a reduction in the oral cavity space, increased intraoral pressure, protrusion of the tongue, and increased biting force, ultimately resulting in oral injuries. Furthermore, transcranial MEP induction is a nonspecific stimulus that cannot be precisely and exclusively targeted to a single nerve's distribution area. When attempting to elicit responses from facial muscles, this broad stimulation may cause intense contractions across the entire craniofacial region, thereby increasing the risk of oral injury. For example, during brainstem area surgeries, to better monitor the functional status of nerves related to the brainstem, MEP electrodes are typically placed at C3\u0026rsquo;/C4\u0026rsquo; (lateral to C3 and C4, see Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB), which often directly triggers widespread muscle activity in the craniofacial area. This leads to contraction of the masticatory muscles, movement of the mandible, and contact between the upper and lower teeth, significantly increasing the occlusal force and resulting in injuries (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eC).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eIncidence and associated risk factors for oral injuries\u003c/h2\u003e \u003cp\u003eDrawing from our reflection on the 3 cases in this study and the collation of information from relevant reported cases, we posit that the risk factors contributing to oral injuries may include the following.\u003c/p\u003e \u003cp\u003eFirst, anesthesia-related factors such as the absence of a bite block, the use of a bite block that is too hard, excessively large or small, improperly placed or displaced, and inappropriate depth of anesthesia. The use of hard bite blocks, including standard dental pads, can lead to the accumulation of biting force, resulting in lip and tongue injuries and even alveolar bone fractures[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]; as experienced in our reported case 1, hard dental guards should be avoided. Bite blocks that are overly large or small, improperly affixed, and displaced for extended durations can compress the tongue, leading to ulcers, hematomas, and even ischemic necrosi[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. In the second case reported here, where the patient was intubated nasally, protection was attempted via a soft dental pad made from gauze in the oral cavity. However, the possibility of an improper fit or displacement during surgery still fails to prevent injury to the tongue. This explains why, according to the literature search, some patients who used soft dental guards nevertheless sustained oral injuries. In addition, the choice of anesthetic drugs and the regulation of anesthesia depth also present a risk for oral injuries. Common anesthetics such as propofol and dexmedetomidine, as well as muscle relaxants, can influence neurophysiological monitoring signals to varying degrees. Similarly, when the concentration of inhaled anesthetics reaches 0.5%, it can lead to a significant decline in MEP amplitude, and complete suppression may occur as the concentration increases[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. This could amplify the required stimulation intensity and increase the likelihood of oral injury. If the use of muscle relaxants and inhaled anesthetics is limited, the depth of anesthesia can hardly be guaranteed, and patients\u0026rsquo; movement following electrical stimulation might cause forced contraction of the masseter muscle, thereby increasing the likelihood of oral injury. Yata et al. also confirmed a significant correlation between tongue-biting injuries and intraoperative body movement[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFurthermore, patient factors also exert an impact. Tamkus et al.\u0026rsquo;s data analysis did not corroborate a correlation between age, sex, and the incidence of postoperative biting injuries[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. However, malocclusion and missing teeth in patients are risk factors for intraoperative displacement of the bite block, which in turn increases the incidence of tongue-biting injuries. In addition, factors such as hypothermia, hypotension, hypoxemia, anemia, intracranial hypertension, electrolyte imbalance, and blood glucose abnormalities can decrease the MEP signal[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Moreover, individuals with a smaller oral cavity, enlarged tongue, or naturally stronger bite force are potentially at greater risk for oral injuries, yet systematic evidence confirming these correlations is currently insufficient. Additionally, angioedema is a common cause of macroglossia. A patient's previous history of allergies and the use of ACE inhibitors (ACE-Is) or angiotensin receptor blockers (ARBs) are also points of concern for triggering intraoperative angioedema, leading to or exacerbating tongue injury. Angioedema is typically associated with ACE inhibitors, with an incidence ranging from 0.1\u0026ndash;0.7%. The associated pathophysiological mechanisms involve vasodilation and increased permeability and plasma extravasation, which are achieved through the inhibition of bradykinin and substance P degradation, both of which are vasodilatory agents that contribute to edema[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Although none of the three patients reported here had previously taken ACE inhibitors and had no history of allergies, patients using such medications may pose an additional risk factor for intraoperative oral injury. Therefore, we recommend that future related reports should include specific patient information, such as preoperative oral conditions, BMI, relevant medication usage, use of bite blocks, intraoperative position, and neurophysiological monitoring sites and parameters, to facilitate colleagues to summarize experience from it.\u003c/p\u003e \u003cp\u003eIn addition to the above factors concerning surgery and monitoring, Yata et al.\u0026rsquo;s experimental results indicate that the incidence of oral injuries caused by maximum stimulation intensity is noticeably greater than that caused by nonmaximum stimulation intensity, even if a statistically significant correlation between stimulation intensity and the incidence of tongue-biting injuries is absent[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. High stimulation intensity may be a risk factor for oral injuries. Compared with monophasic stimulation, biphasic stimulation, which simultaneously activates both corticospinal tracts, is more likely to induce oral injuries[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]; the second case mentioned in the text employed bidirectional stimulation. In addition, the placement of the stimulation electrode is a crucial factor. Relative to C3/C4 stimulation electrodes, C1/C2 stimulation electrodes might limit direct activation of facial and axial muscles, possibly because the former are closer to the facial motor cortex, mandibular muscles, and trigeminal nerve[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. In reported cases of oral injuries with MEP stimulation sites, stimulation at C3/C4 accounted for approximately 81.82% of the total cases. Hence, placing electrodes at C3/C4 may increase the risk of oral injuries. Stimulating even further to the outer edge of C3/C4 poses a greater risk, necessitating a balance between monitoring muscle position and stimulation intensity.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003ePrevention of oral injuries\u003c/h2\u003e \u003cp\u003eGood preventive measures can significantly lower the rate of oral injuries during neurosurgical operations. On the basis of an analysis of the mechanisms and influencing factors of these injuries, addressing this issue requires the joint efforts of neurosurgeons, anesthesiologists, neurophysiological monitoring physicians, and nursing staff.\u003c/p\u003e \u003cp\u003eFrom a surgical standpoint, it is crucial to balance optimal exposure of the surgical area with the effects of positioning on the patient's head, neck tension, and venous return. Excessive bending or stretching of the head and neck should be avoided. Monitoring the jugular vein status during surgery is essential. Any increase in tension or compromised venous return requires immediate adjustment of the patient's position. Additionally, intraoperatively, prolonged compression of local tissues or vessels must be avoided. Improving surgical efficiency to shorten the operation duration can help minimize complications. Furthermore, ensuring that patients maintain good oral hygiene before surgery can reduce the risk of infection following oral injuries.\u003c/p\u003e \u003cp\u003eFrom the perspective of electrophysiological monitoring, the focus should be on meticulous MEP monitoring. The first step is to select the appropriate stimulation sites with precise localization, avoiding the use of C3/C4 stimulation points whenever C1/C2 can meet the requirements. However, it is crucial to recognize that monitoring certain surgical areas, such as those involving the cerebellopontine angle, as reported in our cases, necessitates the use of C3/C4 locations due to the risk of intraoperative damage to cranial nerves associated with facial movements. Furthermore, surgeries targeting regions related to the movements of the upper and lower limbs should ideally opt for C1/C2, which reduces the stimulation of the head and face motor cortex while obtaining satisfactory potential signals. Additionally, avoiding high-intensity and high-frequency electrical stimulation is recommended. It is best to start with low-intensity titration to achieve the minimum stimulation threshold for satisfactory signals. Simultaneously, enhancing the sensitivity of monitoring equipment and improving monitoring techniques also contribute to reducing the need for high-intensity stimulation. Employing a four-pole stimulation strategy (comprising two anodes and two cathodes) can significantly improve signal quality while allowing for lower stimulation intensities, effectively reducing the risk of nonspecific stimulation and ensuring the accuracy and safety of MEP monitoring during neurosurgical procedures[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFrom the perspective of an anesthesiologist, approaches can be initiated from two aspects: the anesthesia plan and the correct use of dental guards. First, anesthesiologists should formulate appropriate anesthesia plans on the basis of patient condition, drug characteristics, and surgical methods. While satisfying anesthesia depth and surgical safety, the need for neurophysiological monitoring should also be considered. The current recommended anesthesia regimen is anesthesia without muscle relaxants or total intravenous anesthesia[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. The use of no muscle relaxants and the use of only intravenous anesthesia increases the use of propofol to prevent movement. The addition of dexmedetomidine (0.5 mcg/kg/h) reduces the need for propofol, stabilizing anesthesia and hemodynamics[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. A subanaesthetic dose of ketamine can also be used in MEP monitoring to deepen anesthesia while causing gradual improvement in amplitudes without affecting latency[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. A crucial aspect to note is the recommendation to monitor the depth of anesthesia in such surgeries, which ensures stable anesthesia levels, minimizing impacts on monitoring activities and preventing bodily movements, thereby mitigating the risk of oral injuries. Another strategy involves the judicious use of low-dose muscle relaxants to mitigate excessive biting force during stimulation. Research by Zhang, X. et al. indicated that a rocuronium infusion rate of 9 \u0026micro;g.kg^--1.min^--1 strikes a balance between the needs of neurophysiological monitoring and achieving adequate anesthesia depth[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Nonetheless, muscle relaxants may dampen or entirely obstruct MEP responses, potentially leading to increased stimulation intensity and subsequent injuries, necessitating close collaboration between anesthesiologists and monitoring physicians. Furthermore, the application of muscle relaxants should be guided by strict train-of-four (TOF) monitoring, accommodating the differential muscle sensitivity to relaxants and their dynamic effects over time. In patients with intraoperative somatic movements, the block can be given again if T1 is between 5% and 50% of baseline or if there are two detectable jerks, MEP baseline levels can be reached early with sugammadex[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. However, whether these methods can reduce the incidence of oral injuries still lacks clinical data. In addition, the correct use of a bite block is crucial for oral protection. Using a soft bite block or gauze block can decrease the incidence of oral injuries[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. In Tamkus\u0026rsquo;s report, the incidence of oral injuries associated with bite pads (4.42%) was significantly greater than that associated with the use of soft bite blocks (1.27%)[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The absence of bite blocks not only increases the incidence of oral injury but also increases the possibility of tracheal tube rupture and reintubation. Compared with placing a bite block only at the midline, placing bite blocks between the upper and lower molars on both sides is safe[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Salik et al. recommended placing a soft bite block in the midline of the tongue and between the molars on both sides[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. During monitoring, the anesthesiologist needs to select the appropriate size of bite block, place and determine its position, and pull the lips out to prevent lip compression injury; avoiding pressure on the tongue is crucial. Proper fixation should be ensured to prevent damage to the tongue and teeth. During surgery, attention should be given to the displacement or dislodgment of the bite block[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. In addition, some researchers suggest the use of tooth protection devices to provide better oral protection[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. It is also recommended that the tracheal tube be secured to the operative side to minimize contralateral compression and that transnasal intubation be used to minimize the base-of-laryngeal contact area and pressure generation[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Regardless of the presence of MEP monitoring and whether intubation is nasal or oral, in high-risk patients, the use of a soft bite block could alleviate tongue pressure by increasing the oral space, thus preventing tongue swelling and biting injuries[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. The future development of an oral protective pad designed to enlarge the oral cavity space suitably, prevent the tongue from interposing between the teeth, and furnish real-time feedback on alterations in bite force amidst stimulation, with the aim of diminishing interdental bite force, is anticipated to confer substantial benefits.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePrevention measures for Oral injuries\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMeasures\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAnesthetic Measures\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1. Monitor the depth of anesthesia, avoiding Intraoperative Body Movement;\u003c/p\u003e \u003cp\u003e2. Choose the right anesthesia plan, with TIVA recommended;\u003c/p\u003e \u003cp\u003e3. Select and correctly place a soft bite block, properly secure it, and check the position of the bite block during surgery;\u003c/p\u003e \u003cp\u003e4. For severe tongue bite injuries or facial edema, extubation requires careful deliberation.\u003c/p\u003e \u003cp\u003e5. Research and design new types of bite blocks.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePatient Measures\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1. Conduct thorough preoperative evaluations to identify those at high risk for oral injuries and customize anesthesia and oral care plans according to individual patient needs.\u003c/p\u003e \u003cp\u003e2. Maintain good oral hygiene before surgery.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSurgical and Monitoring Measures\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1. Avoid high-intensity, high-frequency, and bipolar stimulation and choose the appropriate electrode placement sites, C1 and C2 sites are recommended, especially in spinal surgery.\u003c/p\u003e \u003cp\u003e2. Pay attention to the impact of surgical positioning on postoperative oral complications, avoid excessive and long-term neck flexion and pressure;\u003c/p\u003e \u003cp\u003e3. Monitor venous return pressure under surgical positioning during the operation.\u003c/p\u003e \u003cp\u003e4. Minimize the duration of time spent in special positions as much as possible.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eOverall, we require collective efforts from the team (see Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e for prevention measures for oral injuries). Preoperative patient assessment is essential to identify high-risk individuals for potential oral injuries and to establish effective protocols for intraoperative oral protection. Additionally, it is crucial to establish an effective protocol for managing oral injuries when they occur. Patients with macroglossia and airway obstruction should be adequately evaluated prior to extubation, and airway patency can be ensured by imaging, visual laryngoscopy, and tracheal tube sleeve leakage testing to avoid postextubation airway obstruction. Notably, in patients with edema caused by prolonged pressure on the tongue, removal of the endotracheal tube without interfering with ventilation may be beneficial. Multidisciplinary collaboration is indispensable for ensuring proper management in such cases.\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u003c/strong\u003e\u003cstrong\u003e\u0026rsquo;\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTianyuan Luo and Mingxiang Xie conceived and designed the study; Yuanli Pi and Yu Li collected and assembled the data; Linlin Luo and Limei Luo analyzed and interpreted the data; Yuanli Pi and Tianyuan Luo prepared the figures; Yuanli Pi and Tianyuan Luo wrote the manuscript. All authors read, critically reviewed, and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe National Natural Science Foundation of China (No. 82060653 to TYL) and the Famous Clinical Doctor Program (No. 20211022 to TYL) of Zunyi Medical University.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work has been carried out in accordance with the Declaration of Helsinki (2000) of the World Medical Association. Ethical approval for this report was provided by the Ethical Committee of the Affiliated Hospital of Zunyi Medical University, Zunyi, China, on July 5, 2024 (No: KLL-2024-149). Written informed consent to publish this case and the associated images was obtained from the patients.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConsent for publication has been obtained from the individuals involved.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eMacDonald DB. Safety of intraoperative transcranial electrical stimulation motor evoked potential monitoring. J Clin Neurophysiol. 2002;19(5):416\u0026ndash;29.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTamkus A, Rice K. 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Anesthesiologists MEPMGWGotSCotJSo: A practical guide for anesthetic management during intraoperative motor evoked potential monitoring. J Anesth. 2020;34(1):5\u0026ndash;28.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSalik I, Namkoong S, Lisov C, Lederman D, Abramowicz AE. Tongue injury associated with motor evoked potential monitoring: Causes, prevention and treatment options. J Clin Anesth. 2022;78:110617.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFeng Z, Pei Y, Paerhati H, Hao Y, Jiang L. Risk Factors Analysis and Prevention Strategies of Intraoperative Neurophysiological Monitoring. Chin J Brain Dis Rehabil (Electronic Edition). 2021;11(04):232\u0026ndash;6.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSloan TB, Heyer EJ. Anesthesia for intraoperative neurophysiologic monitoring of the spinal cord. 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Motor-Evoked Potential Monitoring for Intramedullary Spinal Cord Tumor Surgery: Correlation of Clinical and Neurophysiological Data in a Series of 100 Consecutive Procedures. NeuroSurg Focus. 1998;4(5):e1.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eOshita K, Saeki N, Kubo T, Abekura H, Tanaka N, Kawamoto M. A novel mouthpiece prevents bite injuries caused by intraoperative transcranial electric motor-evoked potential monitoring. J Anesth. 2016;30(5):850\u0026ndash;4.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTan WK, Liu EH, Thean HP. A clinical report about an unusual occurrence of post-anesthetic tongue swelling. J Prosthodont. 2001;10(2):105\u0026ndash;7.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJunghaenel S, Keller T, Mischkowski R, Hinkelbein J, Beutner D, Koerber F, Teschendorf P. Massive macroglossia after palatoplasty: case report and review of the literature. Eur J Pediatr. 2012;171(3):433\u0026ndash;7.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Neurophysiological monitoring, motor-evoked potential, oral injury, case report, prevention","lastPublishedDoi":"10.21203/rs.3.rs-4840493/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4840493/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eBackground \u003c/strong\u003eOral injuries are occasional yet notable complications in neurosurgical procedures and are often associated with motor-evoked potential (MEP) monitoring; however, they are also influenced by factors such as prolonged neck flexion and inadequate oral protection.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCase presentation\u003c/strong\u003e This paper discusses three cases of oral injuries following pontocerebellar lesion resection surgeries, illustrating varying outcomes with different monitoring and intubation techniques. In one patient, orotracheal intubation with unilateral MEP monitoring led to fractured alveolar bones and dislodged teeth. Another patient, who was intubated nasally with bilateral MEP monitoring, experienced severe tongue biting, facial swelling, and subsequent airway obstruction requiring tracheotomy. A third patient, also nasally intubated but without MEP monitoring, developed a swollen and bleeding tongue postoperatively.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConclusion \u003c/strong\u003eMEP monitoring is not the sole cause of oral injuries in neurosurgical procedures. Key factors contributing to these injuries, aside from the nonspecific stimulation of MEP, include prolonged surgical positioning, inappropriate anesthesia strategies, and patient-specific factors. The medical team should understand the underlying mechanisms of these complications, master systematic preventive strategies, and engage in effective collaboration to more efficiently reduce the incidence of these complications.\u003c/p\u003e","manuscriptTitle":"Oral Injuries Following Cerebellopontine Angle Surgery with or without Motor Evoked Potential Monitoring: three Case Reports and Literature Review","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-18 01:42:45","doi":"10.21203/rs.3.rs-4840493/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"31da5552-c5f6-43c3-a5de-1d0c3b0259e9","owner":[],"postedDate":"September 18th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-03-07T10:39:41+00:00","versionOfRecord":[],"versionCreatedAt":"2024-09-18 01:42:45","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4840493","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4840493","identity":"rs-4840493","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}