To record or monitor ECochG, preoperative plans should be made for placing a reference electrode deep in the external ear canal (or through the tympanic membrane onto the promontory) along with other monitoring leads before the patient is prepped for surgery. Intraoperative ECochGs are made using an FZ to promontory recording electrode array. The FZ site is located in the midline approximately halfway between the bridge of the nose and vertex. A well-formed response is characterized by the presence of all major wave components such as the ECochG summating potential (SP), the action potential (AP), and often ABR waves I, II, III, and V [3]. Intraoperatively, the ECochG components of interest are almost always the SP and the AP waves. Both the latency and amplitude of each component are regularly calculated and monitored. The latency is typically defined as the time in milliseconds between the click stimulus presentation and the peak of the wave. Amplitude values of SP and AP are calculated from a common baseline. The major advantage of intraoperative ECochG is that it has improved surgical technique. Intraoperative ECochG instantly confirms to the surgeon which vessel or which maneuver caused a loss of response and this immediate feedback has resulted in significantly improved surgical technique for hearing preservation.

Parotid resection will also necessitate the need for facial nerve monitoring (see Chap. 7, “Electromyography”). The absence of clinically detectable facial nerve deficits does not rule out the presence of subclinical nerve damage. The facial nerve was embedded in fibrous tissues of the tumor; therefore, continuous EMG monitoring of facial muscles during parotidectomy was performed to reduce the incidence of facial paresis or paralysis. To assist with the identification of the facial nerve and the mapping of its branches and to protect against inadvertent surgical damage, subdermal needle electrodes were placed to record EMG from the frontalis, orbicularis oculi, orbicularis oris, and mentalis muscles. This provided continuous intraoperative nerve monitoring of the four peripheral branches of the facial nerve: frontal, zygomatic, buccal, and marginal mandibular using free run and triggered EMG. The EMG activity was monitored continuously to ensure anatomical integrity of the facial nerve pathways and to correctly identify the distal branches prior to dissection. Spontaneous EMG activity during surgical manipulation was promptly communicated back to the surgeon to ensure that it was only transient and not sustained, which would be consistent with nerve injury.

There is a growing body of evidence that supports the value of cranial nerve monitoring in a variety of surgical environments. Intraoperative facial nerve monitoring is an important adjunct to enhance neural preservation, particularly when tumor, infection, trauma, or anatomic variation places the nerve at increased risk. Contemporary intraoperative facial nerve monitoring is based on facial muscle electromyographic activity . Recording evoked electromyographic activity from the muscle rather than the nerve itself takes advantage of the amplification that occurs at the neuromuscular junction. This form of monitoring is used to provide the surgeon with information regarding the location and extent of the facial nerve contour, surgical trauma, and nerve function.

Surgically evoked facial EMG activity may be classified as either nonrepetitive or repetitive depending on whether single or repetitive discharges occur with a given response. Nonrepetitive activity or phasic bursts may be observed with electrical, mechanical, or thermal stimulation [4]. Surgical manipulation of the facial nerve may result in elicitation of nerve fiber action potentials. An important feature of nonrepetitive evoked EMG activity is the apparent lack of temporal delay between the stimulus and the observed response. Repetitive firing of facial nerve fibers or train activity may be elicited by nerve traction and particularly lateral, nerve compression. Facial nerve EMG activity such as this evoked during surgery has been referred to as an injury potential or neurotonic discharge. There is typically a significant temporal delay, up to a minute, between the provocative or stimulus event and the onset of repetitive activity. Because of this, the identity of the initiating event may be unclear.

Electrical stimulation of the nerve may be an important addition to the surgeon’s ability to assess the anatomic location of the facial nerve. Monopolar stimulation is best used to map the general vicinity of the nerve with regard to the tumor mass. After localization of the facial nerve has been accomplished, microtrauma during dissection may result in significant nerve injury. Surgical manipulation often provokes a mechanically evoked facial nerve response that results from rapid neural deformities producing ionic depolarization. Traction responses tend to occur as multiple asynchronous potentials in contrast to a synchronized potential seen with brief direct mechanical contact. Drilling adjacent to the nerve using high-speed pneumatic drills may elicit mechanical and thermal evoked potentials. Even if bone overlies the nerve, vibration can be transmitted through the bone, which could affect nerve firing. Thermal changes can also evoke train potentials. Cold irrigation fluid causes asynchronous potentials to occur, which will subside as the irrigation fluid temperature increases. With drilling, heat buildup may also occur, which could cause aberrant nerve firing. Care should be taken to irrigate the drill to avoid heat buildup during boney dissection. Other techniques to help reduce intraoperative artifacts are listed in Table 29.2. It is imperative to have an adequate baseline with minimal electrical activity.

Facial nerve (FN ) EMG monitoring , though almost a care standard for these types of surgical procedures, has some critical limitations. The method is applicable only to exposed portions of the nerve and is hard to utilize if the nerve is covered by tumor. Also, the activation of the facial nerve by direct stimulation is not uniform. The nerve structure may be altered with spreading of nerve fascicles and flattening or expansion of the normal shape of the nerve due to tumor compression. The motor action potential proximal and distal to the tumor is not reliable, and the prognostic value of the compound muscle action potential amplitude at the end of surgery is of limited predictive value. Transcranial electric stimulation for eliciting motor evoked potentials of the facial nerve (FMEPs ) and vagus (vagal MEPs) along with other cranial nerves, (collectively termed “corticobulbar MEPs”) have been introduced as an adjunct to EMG monitoring [5]. The use of FMEPs allows the activation of motor pathways proximal to the site of the lesion and enables checking the condition of the facial nerve before its visualization. Despite positive reports of their use, FMEPs and corticobulbar MEPs have not yet become a routine tool in neurosurgery or otolaryngology. This may be due to difficulties in application and interpretation of signals. There is considerable artifact and interference associated with CN MEPs. Artifact suppression is improved using insulated head holder pins and by short interstimulus intervals less than 2 ms. Stimulating electrode position can influence waveforms and response latencies. Some authors report placement of the stimulating electrodes at C3 or C4 and others 1 cm anterior near the motor strip at M3 or M4 [6]. Also, a correlation is observed between the extent of tumor size and the quality of the MEP response. MEP latency is shorter with small tumors but increases significantly with increasing tumor size [7]. This could be due to edema or stretch of the nerve by the tumor. MEP amplitude is decreased with increasing tumor sizes corresponding to a reduction of active nerve fibers. Using MEPs, facial nerve status can be assessed even before the surgery has started. FN function can also be predicted by the use of an amplitude ratio that measures waveform amplitudes at the end versus the start of surgery. Patients having good FN function, House/Brackman (HB) 1 or 2, had a ratio of 85 % or greater. The HB facial nerve grading system is a commonly used, standardized, and reliable method to assess facial function. Grade 1 is normal, whereas Grade II is slight weakness only noticeable on close inspection. Grade III is gross and obvious facial dysfunction; it is not disfiguring but weakness is noted. Grade IV is more severe, with disfiguring asymmetry with incomplete eye closure and mouth asymmetry. Grade V is severe dysfunction with barely perceptive movement of the affected side, while Grade VI is complete paralysis. With HB3 function, the amplitude ratio was found to be in the 60–70 % range, and with a more severe FN deficit, HB4, the ratio is around 30–35 %. With severe FN injury corresponding to HB 5 or 6, the FMEP is below 15 % or lost completely. FMEPs are identified by their shape and latency. Physiologically, an appearance of a centrally mediated FMEP before 12-ms latency is not realistic. In the case of space-occupying lesion, longer latencies are expected. Contralateral function of FMEP is also important. Recording of the healthy side serves as a technical control at the start and during all stages of the operation. By contralateral stimulation, the neurophysiologist may verify whether stimulation itself is deficient, whether air accumulation prevents reliable motor cortex activation, or whether the problem is related to the surgical site.

FMEPs and EMG fulfill different tasks in neuromonitoring. FMEP provides information on overall facial nerve integrity and correlates well with functional status. EMG and CMAP are best used for focal nerve identification and mapping. Either method does not affect the other but compliments their respective information. FMEP helps to identify nerve integrity and overcomes the specific limitations of stimulation intensity and reliability when using EMG. FMEP latency and amplitude provide relevant information on the functional status of the FN before the start and throughout surgery. A preserved FMEP end-to-start ratio above 55 % predicts preserved eye closure.

After the tumor is removed, nerve integrity can be assessed by stimulating it at an area that is proximal to the dissection. Nerves that require elevated stimulus current to reach a firing threshold will demonstrate some degree of postoperative weakness [8, 9]. The presence or absence of a facial evoked response must be interpreted within the context of the surgical procedure. Electrical silence may occur from an absence of facial nerve stimulation, a severe insult which precludes depolarization, transection of the nerve, or technical problems with the equipment. Nerves that have been significantly manipulated during the dissection may demonstrate fatigue. In these instances, stimulus current may need to be increased. Ultrasonic aspiration of the tumor creates low-level artifacts or triggers the autonomic muting circuitry. The use of monopolar cautery may charge the stimulator probe, which can result in a false-positive response. Severe neuropraxic nerve injury can prevent depolarization, just as axonal injuries do. Most surgeons will not resect nerves solely on the basis of the lack of an ability to stimulate. A complete list of problems that could be present with monitoring and possible solutions is found in Table 29.3.