Facial Nerve Monitoring
Injury to the facial nerve is one of the most feared and potentially devastating complications of otologic and neurotologic surgery. The facial nerve is at risk in otologic and neurotologic surgeries as a result of its tortuous course in the temporal bone and the cerebellopontine angle. To reduce risk of damage to the facial nerve, intraoperative nerve monitoring systems have been developed.
Indications for Facial Nerve Monitoring
Cerebellopontine Angle Tumors
The decision to use facial nerve monitoring is surgeon and case specific; its use has been described in nearly all otologic and neurotologic procedures.1,2 Facial nerve monitoring has been consistently recommended in resection of cerobellopontine angle tumors and tumors of the internal auditory canal.3 Here, it has achieved widespread use due to the limited anatomic exposure, the altered anatomy of the facial nerve caused by tumor compression and displacement, and increased vulnerability associated with the absence of epineurium in this region. Not uncommonly, the appearance of the facial nerve may also be transformed; the nerve′s fibers may become splayed into a thin ribbon of tissue rather than its normal cylindrical shape. Dissection can become dangerous in these situations, especially as tumor size increases. In these instances, electrophysiologic facial nerve identification can occur before visual identification.
Middle Ear Surgery
The decision to use facial nerve monitoring when operating in the middle ear is surgeon dependent. Some have advocated the use of facial nerve monitoring in routine as well as complex middle ear cases.1 Routine use was shown to be more cost-effective than the cost associated with selective use of facial monitoring combined with the management of complications of facial nerve injury in cases of nonuse.4 Other surgeons elect to use facial nerve monitoring only in situations of chronic or recurrent middle ear disease. Patients with chronic ear disease, cholesteatoma, and patients with prior surgery may have obscured/absent anatomic landmarks. In these cases, facial nerve monitoring may allow the surgeon to dissect with greater safety in the midst of cholesteatoma, granulation tissue, and fibrosis.1,5 Facial nerve monitoring may also be useful to detect a dehiscent facial nerve before it can be visualized in the surgical field.1,6–8
Pediatric Otology
The facial nerve continues to develop through age 4.9 The mastoid tip also continues to develop and enlarge during this time, and as it does the facial nerve moves from a relatively lateral position to a more medial, adult position. Pediatric patients that require otologic surgery may have congenital abnormalities of their inner, middle, or external ear, predisposing them to have aberrant facial nerve anatomy.10 In cases of congenital aural atresia, the facial nerve has been shown to have greater anatomic variability. Specifically, it tends to lie more anterior and superior in position in its vertical segment.11 Facial nerve monitoring in these cases may help confirm the location of the facial nerve and prevent injury.
Types of Facial Nerve Monitoring
History
The first example in the literature of facial nerve monitoring was published by Krause in 1898, at which time electrical stimulation was described. During a cochlear nerve section, Dr. Krause noted that “unipolar faradic irritation of the nerve trunk… resulted in contractions of the right facial region, especially of the orbicularis oculi, as well as the branches supplying the nose and mouth.”12 The receptive portion of facial nerve monitoring still consisted of an assistant observing the face under the drapes. Facial nerve monitoring has progressed since this original description, evolving into a highly technological science that often employs auxiliary personnel to execute. In the past, devices that sensed facial muscle contraction were employed. The most commonly employed facial nerve monitoring systems at present are electromyography (EMG) systems; however, other systems such as video monitoring and direct monitoring of nerve impulses are available.
Electromyography
EMG uses facial muscle response as an indicator of facial nerve continuity and activity.13 EMG can be used intraoperatively to alert a surgeon when the facial nerve is nearby. It can also be used to survey the operative field before dissecting in an area. A nerve stimulator can identify the nerve and map the location of the nerve in the surgical field. At the conclusion of dissection, the nerve stimulator can be used to confirm nerve continuity and give prognostic information regarding the functional status of the nerve. In addition, important information can be captured without nerve stimulation. For example, neurotonic discharges arising during a dissection can serve as warnings of impending facial nerve injury.
It is important to note that what is being measured is compound muscle action potentials, or the electrical activity being generated in a muscle, which reflects the health of the nerve innervating the muscle being monitored. EMG electrodes do not measure the compound nerve action potential (CNAP), which is the electrical activity generated by the nerve itself.
Setup
There are many systems commercially available. Once the patient has been properly positioned, nerve electrodes are inserted into the facial muscles in the authors’ typical setup ( Fig. 18.1 ). The authors use two electrodes each in the orbicularis oculi, orbicularis oris, and frontalis muscles. Guo et al performed a prospective study examining placement of electrodes to give optimal compound muscle action potential output. They recommend that the orbicularis oculi electrodes be placed at the orbital rim and the upper eyelid with needles separated by 1.5 cm, and that the orbicularis oris electrodes be placed 5 mm lateral to the oral commisure and 2 cm away at the lower lip.14 Ground electrodes for the intramuscular electrodes and for the nerve stimulator are placed into the trapezius muscle. The electrodes are secured with clear adhesive strips, and care is taken to not run wires over the eyes. All wires should be draped over the patient such that they exit the table away from the operative site. The electrode wires are connected to a circuit box that directly interfaces with a monitor. A sterile stimulation probe is included in the instrument setup and is connected to the circuit box once the surgical field is set up. At this point, the patient is sterilely prepped and draped. Electrodes are prepped if the face is to be included in the surgical field. The authors prefer to employ a neurophysiologist to monitor EMG tracings, although some surgeons use a loudspeaker system that emits audible responses to electrical or mechanical stimulation of the facial nerve.
Anesthesia
Both nondepolarizing and depolarizing long-acting neuro-muscular blockers commonly used to induce and maintain paralysis during anesthesia will impair the proper functioning of EMG nerve monitoring. Therefore, it is important to avoid use of muscle relaxants during cases in which EMG nerve monitoring is to be used, with the exception of succinylcholine at induction. Succinylcholine is short-acting with a half-life of 3.5 minutes, allowing for complete emergence from neuromuscular blockade within 15 minutes.15 This 15-minute period after induction is generally inconsequential. Some have described the use of partial neuromuscular blockade monitored by twitches.16,17 The authors, however, do not find partial paralysis to be necessary or assistive in otologic and neurotologic cases, and some researchers have even found that partial paralysis decreases spontaneous and mechanically evoked EMG activity.18 Of note, EMG nerve monitoring is unaffected by inhalational or intravenous anesthetic agents, so adequate anesthesia may be obtained without the use of neuromuscular blockers.18 Local anesthetic is commonly used at the incision site and within the ear canal; however, care must be taken to avoid overinjection near the stylomastoid foramen or into the middle ear to prevent infiltration of local anesthetic along the facial nerve.
Operative Use
At the start of the operation, it is prudent to test the integrity of the EMG circuit. Baseline EMG data should be recorded before commencement of the procedure. If the surgical procedure is amenable, the function of the circuit should be verified before dissection near the facial nerve by stimulating another motor nerve. In the case of skull base approaches to a cerebellopontine angle tumor, this can be achieved by stimulating cranial nerve XI at the jugular foramen. Any dysfunction in the EMG system should be addressed before proceeding to tumor dissection. In middle ear cases, a control motor nerve is not available within the field.
A stimulation probe may be used to confirm the location of the facial nerve in the operative field. Probes may be either monopolar or bipolar. Bipolar probes offer a theoretical advantage of having the ability to stimulate a smaller area of the nerve; however, their use requires a precise orientation between the probe and the nerve. This may be precluded by anatomic constraints. Monopolar probes are more commonly used.15 Such probes can be used to map the path of the nerve and to distinguish the facial nerve from surrounding tissue such as arachnoid, tumor, cholesteatoma, granulation tissue, or fibrosis. When confirming the location of the nerve, the probe should be set to the lowest intensity to identify a threshold and allow for a neuromuscular response without delivering a potentially damaging amount of energy. Two stimulation probe systems are available: those that deliver a constant current and those that deliver a constant voltage. Despite the theoretical differences between the two systems, neither has achieved clinical advantage19; both systems are currently used.
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