Tongue bite injury after use of transcranial electric stimulation motor-evoked potential monitoring.

Sir,

Monitoring of motor-evoked potential (MEP) during spine surgery is considered a safe technique but potential hazards include bite injuries, possibility of hazardous stimulator output, patient movement induced injury, seizures, cardiac dysrhythmias, and epidural electrode-related complications.[1] Tongue and lip bite injuries are the most common reported complications during MEP monitoring with an incidence ranging from 0.2% to 0.63%.[12] Previous case reports have reported bite injuries including minor tongue lacerations, broken teeth, and even bitten endotracheal tube.[1]

Thirty-seven-year-old, ASA physical status I, male with BMI 24.3 Kg/m2, was posted for L3-4 laminectomy and excision of L3 intradural tumor in the prone position with transcranial electric stimulation (TES) MEP monitoring. After induction of anesthesia and intubation, the endotracheal tube was taped to the right side and a soft bite block (rolled up gauze) was inserted in the midline. Surgery and anesthesia were uneventful and lasted two hours. Tracheal extubation was performed in the OR but the patient was mildly sedated. Postoperative examination of the oral cavity revealed a tongue hematoma on the left side [Figure 1]. He was reassured about the self-resolving nature of the hematoma and was advised oral care regimen.

Figure 1

Tongue hematoma

Risk factors for tongue injury during TES MEP monitoring include C3-4 focused stimulation that directly activates the temporalis muscle[3] and prone position (as in our patient) as it predisposes to tongue swelling.[4] The mechanism of tongue injury may involve both corticobulbar activation with pulse-trains and direct muscle or trigeminal nerve stimulation, because jaw-clenching also occurs with single pulses. Use of C3/4 TES might produce stronger biting than C1/2 TES because the electrodes are closer to facial motor cortex, jaw muscles, and trigeminal nerves.[1]

Placement of a bite block is standard practice during MEP monitoring, but it does not necessarily prevent injury to oral structures as is evident in our case. Tongue injury due to bite block dislodgement and movement of the tongue between the teeth has been reported earlier.[3] There is no consensus on the type and number of bite blocks to reduce these injuries. Most reviewers suggest the use of soft bite blocks[15] as rigid bite blocks may cause pressure injury to the tongue and lingual nerve and dental trauma. The use of three soft bite blocks (one in between the molars on each side and one in the centre)[5] and dental guards on the mandibular and maxillary dental lines with a soft bite block in between have been suggested.[6] Frequent intraoperative checking of the position of the bite block and the tongue has been recommended but this may be difficult if the patient is in the prone position, as in our case.

All anesthesiologists need to be aware of and discuss the risk of injury to structures of the oral cavity when using TES MEP monitoring with the patient. Careful assessment for tracheal extubation is warranted in cases at high risk for injury, as an injured and swollen tongue may cause airway obstruction and need for re intubation. The use of less frequent and low voltage stimulation, correct placement of bite blocks, and continued vigilance to their position can go a long way in preventing patient bite injuries during TES MEP monitoring.