INTRAOPERATIVE MONITORING OF FACIAL NERVE

DR ROOHIA

Intraoperative Neurophysiological Monitoring

Intraoperative –during a surgical procedure

Neurophysiological –related to brain or nerve function

Monitoring –continual testing and interpreting results

What do we monitor?

Pathways Ascending (sensory) systems Descending (motor) systems

How do we monitor?

Pathways Stimulate one end Record at the other end Identify changes along the pathway

due to the surgery

Why do we monitor?

To reduce surgically related mobidityand mortality

•To protect the brain, nerves and spinal cord

When do we monitor?

Whenever the surgical team believes that the brain or nerve pathways are at risk due to:

The nature or extent of the disease The complexity of the surgical

approach

Neurophysiology in the Operating Room

Personnel: in the OR requires the services of professional personnel with specialized skills and experience

a new specialty field of intraoperative neurophysiologic monitoring has evolved, the American Society of Neurophysiological Monitoring (ASNM).

Instrumentation for cranial nerve electromyography

(EMG) monitoring several low-noise EMG amplifiers; a

multichannel display; and an audio monitor with a squelch circuit to mute the output during electrocautery.

Recording Electrodes: Type and Placement electrodes are the platinum or stainless

steel subdermal needles designed for EEG Intramuscular hook wire electrodes, which

are inserted with a hypodermic needle to avoid crosstalk from overlying muscles

For monitoring the facial nerve, it is desirable to place the electrodes in at least two different muscles supplied by the nerve

temporalis (supplied by V3m), orbicularis oculi and orbicularis oris (supplied by the seventh nerve), and trapezius (by the eleventh nerve

Broadband click stimuli from a small transducer on the chest are fed through plastic tubing into the ipsilateral ear through a foil-covered sponge insert that also serves as a recording electrode

An electrocautery ground pad is placed on the arm or shoulder as a signal ground

Stimulating Electrodes Both monopolar and bipolar stimulating electrodes have been used for intraoperative electrical stimulation

a spatial resolution of less than a millimeter is easily obtained even with monopolar stimulation

the threshold for bipolar stimulation depends strongly on the orientation of the two contacts with respect to the axis of the nerve.

Maintenance of a specific bipolar orientation is difficult in the close confines of the posterior fossa.

Prass and L?ders developed a malleable electrode, with the insulation continuous to a flush-tip that could be bent so that only the central portion of the tip contacted the desired tissue. They showed that this design minimized the spread of current to adjacent structure

Yingling and Gardi[11] developed a probe with a flexible platinum-iridium tip, insulated except for a 0.5-mm ball on the end

This electrode can be used to stimulate within dissection planes or even behind the tumor, out of the surgeon's view, without concern for inadvertently damaging unseen neural or vascular structures

Early identification of the facial nerve “around the corner” on the ventral surface of the tumor helps speed the procedure by allowing rapid removal of the remaining capsule

Stimulation Criteria monopolar constant-voltage stimulator

delivering pulses 0.2 ms in duration at a rate of 5 to 10 per second.

evokes a response from normal nerves at a threshold ranging between 0.05 and 0.2 V and averaging approximately 0.1 V.

Anesthesia The ABR and EMG responses that are used for

cranial nerve monitoring are essentially unaffected by any commonly used anesthetics

Contraindication to the use of muscle relaxants Both spontaneous and mechanically elicited EMG

activity tends to be suppressed by these agents No paralytic agents should be used during

surgery with cranial nerve monitoring, avoid injection near the stylomastoid foramen to

avoid anesthetizing the facial nerve

Cranial Nerve VII: Facial

Intratemporal –Facial Nerve

decompression –Mastoidectomy –Tympanoplasty –Cochlear implant –Translabapproach –Labrynthectomy

Intracranial CPA tumor resections MVD Vestibular nerve

section glomus jugulare

tumors and prepontine tumors, parotidectomy,

Other motor nerves—the third, fourth, motor portion of fifth, sixth, tenth, eleventh, and twelfth nerves.

Facial Nerve Monitoring

Direct visualization of facial contractures is the simplest form of facial nerve monitoring during surgeries

intraoperative EMG, recording of compound nerve action potentials (CNAPs) from the facial nerve,

intraoperative recording of nasal muscle F-wave, and video analysis to detect facial movements

Electromyography

intracranial electrical stimulation is used to identify and map the course of the nerves with evoked EMG activity and to determine the functional integrity of a nerve

spontaneous EMG activity is continuously monitored to detect changes related to mechanical, thermal, or electrical irritation of the nerves by intraoperative maneuvers such as retraction,tumor dissection, use of electrocautery or lasers, or ultrasonic aspiration.

Activity Evoked by Electrical Stimulation

Use of Stimulation to Identify and Map Nerves in Relation to Tumor

(1) to rule out the presence of a nerve in a region to be dissected (using suprathreshold stimulation levels, i.e., 1 V) and

(2) to map the precise locations of cranial nerves and determine their functional integrity, using stimulation levels at or just above threshold (i.e., 0.05 to 0.3 V).

Before electrical stimulation begins, correct functioning of the stimulating and recording system must be confirmed

In a retrosigmoid approach, the eleventh nerve usually can be stimulated at the jugular foramen as soon as the dura has been opened and the cerebellum retracted

In translabyrinthine procedures, the facial nerve can be stimulated within the mastoid bone in the course of the labyrinthine dissection (before the tumor is exposed), although the threshold will be higher (usually 0.6 to 1.0 V, although up to 2 V may be needed), depending on the thickness of the overlying bone

In smaller tumors (cerebellopontine angle component of 1 cm or less), the nerve usually can be located at its brainstem entry and an electrical response confirmed before dissection begins

With larger tumors, the location of the facial nerve may not be immediately apparent. In such cases, we start with 0.3 V and map the accessible region, and if no response is obtained, we repeat the search at 0.5 and 1.0 V. If no response is obtained at 1.0 V, it can be safely assumed that the facial nerve is not on the exposed surface, and dissection can proceed.

During dissection, the stimulator is used repeatedly to scan the operative field using suprathreshold stimulus

Once a response is obtained, stimulus intensity is reduced to 0.1 to 0.2 V, and the responsive region is narrowed

The spatial resolution of electrical mapping with monopolar stimulation is determined by stimulus intensity

At just suprathreshold levels, the spatial resolution is less than 1 mm, allowing the facial nerve to be easily distinguished from the adjacent vestibulocochlear complex.

the course of the facial nerve can be mapped from brainstem to internal auditory canal.

it frequently is compressed by the tumor in the cerebellopontine angle and may be seen as a broad, flat expanse of fibers splayed across the surface of the tumor

Assessment of Functional Status of Nerves after Tumor Removal

facial EMG responses elicited by low-threshold stimulation of the seventh nerve at the brainstem after total tumor resection constitute a good but not infallible predictor of postoperative function

a substantially elevated threshold or the inability to elicit a response with stimulation up to 1 V carries a significant likelihood of postoperative facial dysfunction, particularly in the short run

based on measurement of the compound muscle action potential (CMAP) amplitude

Taha and associates measured the ratio of CMAP amplitudes to stimulation at the brainstem proximally and at the internal auditory meatus distally after tumor excision and found that proximal-to-distal amplitude ratios greater than 2 : 3 were associated with excellent outcome.

Neff and colleagues evaluated a combination of threshold and amplitude and found that a minimum threshold of 0.05 mA together with a CMAP amplitude greater than 240 ?V predicts a good postoperative outcome 1 year postoperatively

The earlier the onset of recovery, the better its quality; however, ifevidence of recovery is lacking at 12 months, then it is unlikely to occur

Intraoperative Identification of the Nervus Intermedius to prevent confusing

this nerve with the facial nerve itself

produce a characteristic response in the orbicularis oris channel only: long latency, low amplitude, and higher in threshold than the facial nerve response

Spontaneous and Mechanically Evoked Activity

EMG responses to intracranial stimulation are the most specific indicators of cranial nerve localization and functional status

related to intraoperative events also are useful in preserving neural function

all patients exhibit some mechanically evoked facial EMG activity during tumor dissection, retraction, irrigation, or other intraoperative maneuvers

operative manipulations elicit EMG activity because of transmission of traction or pressure from the tumor to the nerve

EMG activity often is an indirect indicator of depth of anesthesia

Patterns of Mechanically Evoked Electromyographic Activity

Prass and L?ders distinguished two types : burst and train activity

phasic “burst” pattern, short, relatively synchronous bursts of motor unit potentials,

single discharge of multiple facial nerve axons, direct mechanical nerve trauma, free irrigation,

application of pledgets soaked with lactated Ringer's solution over the facial nerve, or electrocautery

less than 500 ?V in amplitude) is not of major concern

(greater than 500 ?V in amplitude

tonic or “train” activity, consisted of episodes of prolonged asynchronous grouped motor unit discharges that lasted up to several minutes.

facial nerve traction, usually in the lateral to medial direction

higher-frequency trains (50 to 100 Hz), dubbed “bomber potentials”

lower-frequency discharges (1 to 50 Hz), which were more irregular and had a sound resembling popping popcorn

A, Dense tonic (sustained) activity, often associated with nerve stretch and demonstrating a sinusoidal pattern.

B, Lower tonic activity, called popcorn activity

. C, Phasic (transient) burst activity typically associated with direct contact with the nerve. Such events are not of major significance unless they involve large-amplitude trains and occur during critical stages of dissection.

D, Burst activity superimposed on ongoing small-amplitude train;

Limitations of Electromyographic Monitoring relatively low specificity EMG channels can easily pick up artifacts, A, Upper trace: Artifact produced by contact

of different metallic instruments in surgical field. Lower trace: Single electromyographic spike with a low-amplitude EMG background and no exponential decay

. B, Upper trace: Regular sinusoidal artifact produced during drilling of the internal auditory canal (IAC). Lower trace: Irregular EMG activity while drilling IAC.

C, Upper two traces: Regular artifact with two time scales, 200 msec/cm and 5 msec/cm. Lower two traces: EMG activity on the same two time scales. At 200 msec/cm, it may be difficult to differentiate between true EMG and artifact. However, with the faster 5 msec/cm time base, trace 2 shows that the artifact waveform is regular and synchronized, whereas trace 4 reveals the irregularities that characterize true EMG activity.

Microvascular Decompression

trigeminal neuralgia and hemifacial spasm

This method is based on the finding of abnormal muscle response in patients with hemifacial spasm

recording electrodes are inserted into the mentalis muscle (innervated by the marginal mandibular branch of the seventh nerve) and the orbicularis oculi (temporal branch). Subdermal needle electrodes also are inserted adjacent to the marginal mandibular and temporal branches of the seventh nerve for stimulation

Parotid Surgery

typical four-channel montage includes closely spaced bipolar electrode pairs in the frontalis muscle (temporal branch of the seventh nerve), lower orbicularis oculi (zygomatic), upper orbicularis oris (buccal), and mentalis (marginal mandibular

single channel indicates activation of a single branch of the nerve distal to the pes

activity in two or three channels generally indicates a location within the pes,

and a response on all channels indicates activation of the main trunk of the nerve between the pes and the stylomastoid foramen

Other Motor Nerve Monitoring

Third, Fourth, and Sixth Nerve Monitoring: Extraocular Muscles

cavernous sinus tumors, prepontine tumors, and petrous apex lesions with significant anterior or medial extension

inferior rectus/inferior oblique (third nerve), superior oblique (fourth nerve), and lateral rectus (sixth nerve). The illustration shows monopolar needle electrodes;

Monitoring of Lower Cranial Nerves: Ninth, Tenth, Eleventh, and Twelfth Nerves

Larger acoustic neuromas tumors of the

posterolateral cranial base, such as glomus jugulare tumors, meningiomas, or schwannomas of the ninth, tenth, or eleventh nerve

The glossopharyngeal (ninth nerve) nerve primarily mediates sensation to the upper pharynx and taste to the posterior third of the tongue

stylopharyngeus, which is not easily accessible for insertion of EMG recording electrodes

electrodes in the posterior part of the soft palate ipsilateral to the tumor will pick up volume-conducted activity from the stylopharyngeus

Placement of paired needle electrodes in the soft palate for monitoring the ninth cranial nerve and in the tongue for monitoring the twelfth cranial nerve

Electromyography (EMG) endotracheal tube. Two pairs of wires contact vocalis muscles bilaterally to record EMG activity resulting from activation of the recurrent laryngeal nerve, a component of the tenth cranial nerve

B, Laryngeal surface electrode. The electrode is inserted into the postcricoid space after intubation with a standard endotracheal tube to record from the posterior cricoarytenoid muscles (tenth cranial nerve

Needle electrode pairs are placed into the soft palate (ninth nerve), false vocal cord (tenth nerve), trapezius (eleventh nerve), and tongue (twelfth nerve). The electrodes placed in the false cord

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