Herbert Silverstein, M.D.,and David W. White, M.D.

Continuous Electrical Stimulation

as a Helpful Adjunct

During lntraoperative

Facial Nerve Monitoring

Since 1985 routine intraoperative monitoring of fa-cial nerve function has been used by the senior authorduring 900 consecutive otologic procedures and has ad-ded another dimension of safety in protecting the facialnerve during surgery. 1-3 In principle, facial movement isdetected by an ultrasensitive strain gauge sensor which isplaced in the corner of the patient's mouth. Electricalstimulation of the facial nerve, which results in contrac-tion of facial muscles, activates the sensor, and sets off a

warning horn. In the model WR-S8, the monitoring deviceis ultrasensitive and will record minute facial contractionsbefore movement is visualized or can be palpated. Electri-cal stimulation of the facial nerve can be performed with a

remote controlled probe tip. During the past year, contin-uous current has been applied to our instruments and airdrill to allow them to be used as an electrical probe duringsurgery. This was accomplished with a device, an adaptorfor continuous stimulation (SACS), that connects instru-ments to the WR-S8. The use of electrified instrumentsreduces surgical time, since the instruments take the placeof the WR-S8 probe tip. In this report, the SACS will be

described. The technique and results in using electrifiedinstruments during surgery will be presented.

METHOD

The new WR-S8 Monitor/Stimulator (WR MedicalElectronics, Stillwater, MN) uses adjustable currentpulses to stimulate the facial nerve. Currents used are

from 0.05 to 10 mA with a pulse width of 0.2 msec at a

frequency of 5 pulses/sec. The current is deliveredthrough a remote-controlled pencil-shaped probe intowhich various size probe tips can be inserted. A new,

solid-state circuit detects minute muscle movementsthrough a clothespin-shaped strain-gauge sensor that isplaced in the corner of the patient's mouth on the side ofthe operation. The sensitivity of the sensor can be ad-justed. The current is read from a backlit digital display,which can be seen in a darkened operating room. Astimulus verification indicator monitors all cable connec-

tions and verifies that proper current is being delivered at

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Skull Base Surgery, Volume 1, Number 2, April 1991 Ear Research Foundation, Sarasota, Florida Presented at the North American Skull BaseSociety, February 18, 1990, Los Angeles, California Reprint requests: Dr. Silverstein, Ear Research Foundation, 1921 Floyd St., Sarasota, FL 34239Copyright ©) 1991 by Thieme Medical Publishers, Inc., 381 Park Avenue South, New York, NY 10016. All rights reserved.

SKULL BASE SURGERYNOLUME 1, NUMBER 2 APRIL 1991

the probe-tip or instrument. The instrument has recharge-able batteries and is packaged in a shatterproof case(Fig. 1).

Adaptor for Continuous Stimulation

The device consists of three parts. A coiled, soft,flexible wire is used to transmit current from the WR-S8simultaneously to two metal instruments. Banana-typeplugs are attached to the ends ofthe wire. One banana plugis inserted into the AUX positive output of the WR-S8 andthe other end is inserted into an alligator clip device that isattached to the end of the microsurgical instruments or anair drill (Fig. 2). The banana plug has a quick disconnectand can be moved rapidly from one instrument to another.The coiled wire allows easy manipulation of the instru-ments. On instructions from the surgeon, the assistantadjusts the current with the remote control buttons on theWR-S8 probe. The use of insulated instruments (StorzInstrument Co., St. Louis, MO, or WR Medical Elec-tronics, Stillwater, MN) helps prevent electrical shortingof current during stimulations. Since there may be a short-ing of current away from the facial nerve when usingnoninsulated instruments, care must be taken not to allowthe instrument handle to touch skin, bone, or soft tissue atthe same time the facial nerve is being stimulated.

Figure 2. Adaptor for constant stimulation (SACS)attached to air drill and microsurgical sickle knife.

128 Figure 1. WR-S8 Monitor/Stimulator with mouth sensor and remote control stimulator probe.

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CONTINUOUS ELECTRICAL STIMULATION-SI LVERSTEIN, WHITE

SURGICAL TECHNIQUE Skull Base Procedures

Enlarging Bony External AuditoryCanal and Mastoidectomy

The external auditory canal is widely enlarged intympanoplasty, transcanal eighth nerve section, and sin-gular neurectomy, delineating the vertical facial nervefrom an anterior approach. The SACS is used on the drillto warn the surgeon that the facial nerve is in close prox-imity to the diamond burr. To indicate a functional systemat the onset of drilling, the burr is placed against bone ofthe external canal and the current is increased (3 to 5 mA)in order to stimulate the facial nerve. The current is thenreduced to 1 mA. In a previous study3 it was found that 1mA of current indicates approximately 1 mm of bonecovering the facial nerve. When there is an indication offacial nerve stimulation, the current is progressively re-duced and more bone is removed from around the facialnerve until it is exposed. The SACS is used in a similarfashion when locating the vertical facial nerve in mastoidsurgery and locating the petrosal portion of the facialnerve in the translabyrinthine approach.

Chronic Otitis Media

The SACS is used with a sickle knife or other instru-ment when dissecting tissue in an area where the facialnerve could be dehiscent (such as removing granulationtissue from the epitympanum). Other instances where theSACS is helpful are during removal of thick mucosa orgranulation tissue overlying the horizontal facial nerve,dissecting granulations from around the stapes, or elevat-ing a graft over the horizontal facial nerve in revisionsurgery. The current is usually kept low (0.1 to 0.2 mA) sothat if the facial monitor horn sounds, it warns of a possi-ble dehiscent facial nerve.

Acoustic Neuroma Dissection

While dissecting tumor tissue from the facial nerve,the current is usually placed at 0.05 mA. With the tip ofthe sickle knife, the exact location of the lateral border ofthe facial nerve can be delineated in tumor tissue. Differ-entiating the facial nerve from the vestibular and cochlearnerves can also be accomplished. After debulking thetumor, the course ofthe facial nerve on the anterior capsulein the cerebellopontine angle can be determined by stimu-lating through the capsule using higher currents (1to 3 mA).

The stimulator can be used effectively for locatingthe cranial nerves (CN) XI and XII. Stimulation of the CNXII will activate the mouth sensor when the tongue muscletouches the inner blade of the sensor. The CN XI stimula-tion causes the shoulder to move. A sensor for the larynx isbeing developed now. Electrifying the scissors has helpedin locating the facial nerve distal to the stylomastoidcanal.

RESU LTS

The SACS has been used in 150 otologic and neuro-otologic cases. It has been found to be safe, easy to use,and decreases surgical time. No patient has developed aknown facial weakness from continuous electrical stimu-lation during surgery. Lowest currents are always used toproduce a facial contraction. Continuous stimulation ofthe facial nerve at high currents is avoided because of thepossibility of overstimulation causing a neuropraxia. Sur-gical time has been shortened when removing bone overthe facial nerve in chronic ear and acoustic neuromasurgery.

During soft tissue dissection in chronic otitis media,the immediate warning that a dehiscent facial nerve waspresent prevented possible trauma to the facial nerve in sixcases. In three revision mastoid-tympanoplasty cases, thehorizontal facial nerve was found completely dehiscent asthe graft was being elevated over the horizontal fallopiancanal. In two of these cases, bony covering of the facialnerve was documented at the initial procedure. SACS hashelped differentiate granulation tissue and thick mucosafrom a dehiscent facial nerve. It has also been helpfulwhen dissecting the facial nerve from an acoustic neuromaby continuously showing the surgeon the plane of dis-section.

We have observed that the digastric muscle, whenexposed at surgery, appears to be the first muscle tocontract when lowest electrical currents are used to stimu-late the facial nerve.

PRECAUTIONS

Lowest currents should always be used to produce afacial contraction. Although we have not encountered thisproblem, continuous use of high currents may cause neu-ropraxia and postoperative facial weakness. Using toohigh a current can also result in spread of current fromadjacent tissue or nerve (that is, vestibular nerve) to thefacial nerve, giving a false impression of its location. Ifnecessary, bipolar forceps can be used for positive identi-

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SKULL BASE SURGERYNOLUME 1, NUMBER 2 APRIL 1991

fication of the facial nerve and, when attached to WR-S8,will produce a facial contraction when both tips are simul-taneously touching the facial nerve. When using noninsu-lated instruments, while the facial nerve is being stimu-lated, care must be taken not to cause an electrical short byallowing the instrument handle to touch soft tissue orbone. Touching bone produces less of a shorting effectthan soft tissue.

Because the cheek sensor is ultrasensitive, move-ment of the head during drilling may activate the WR-S8warning horn. If this happens, the sensitivity should bereduced until the warning horn is quiet. To reduce falsewarnings, we have taped the patient's head to the surgicaltable, taped the sensor wire to the patient's cheek, andplaced a cardboard surgical mask on the sensor to preventthe drapes from pulling on the sensor. The SACS cannot beused with electrically driven drills.

DISCUSSION

The new, ultrasensitive WR-S8 Monitor/Stimulatoris a great improvement over the original WR-S7 model. Itcan record the slightest facial movement before it is de-tected by palpation and the degree of sensitivity can beadjusted on the WR-S8 unit. Other new features of theWR-S8 model, such as a backlit digital readout, remote-control probe, and rechargeable batteries make it easy toset up and use without the need of specialized personnel.The SACS allows the surgeon to use both the instrumentsand the burr as probe tips while performing surgicalmaneuvers near the facial nerve. This saves time becausethe surgeon does not need to stop surgery to help find thefacial nerve. During drilling, when near the facial nerve,continuous electrical stimulation has helped in finding thefacial nerve quickly. The facial nerve can be located inbone before it is seen. During soft tissue work, an insu-lated sickle knife with SACS set at low current has beenused to dissect tissue or tumor away from the facial nerve.When elevating a graft flap in revision surgery, the use ofSACS has helped demonstrate dehiscence in the nerve andhas helped prevent injury to the nerve.

We use the WR-S8 for all otologic and neuro-otologic cases. By using it routinely, the operating roompersonnel and surgeon become accustomed to the device,and it is always available. When doing acoustic neuroma

surgery, we also use the nerve integrity monitor (NIM) torecord electromyographic responses. However, electricalstimulation at lowest current (0.05 mA) will activate boththe WR-S8 and NIM simultaneously. Stretching the facialnerve, which does not cause a facial contraction, willactivate the NIM and not the WR-S8. The digastric muscleappears to be the most sensitive muscle responding tofacial nerve stimulation. Recently, we have placed theelectromyographic needle in the digastric muscle and haveobserved the muscle contracting to confirm stimulation ofthe facial nerve.

Intraoperative facial nerve monitoring is an excitingnew development in otology but does not take the place ofeither careful dissection around the facial nerve or a thor-ough knowledge of the facial nerve anatomy in the tempo-ral bone. The routine use of facial nerve monitoring withthe SACS has helped prevent iatrogenic injuries and hasimproved our ability to save the facial nerve during oto-logic and neuro-otologic surgery.

SUMMARY

An adaptor for continuous stimulation of the facialnerve (SACS) to be used with the WR-S8 Monitor/Stimulator during otologic surgery has been developed.This device allows the surgeon to use the electrified airdrill and microsurgical instruments instead of a probe tipduring dissection. The SACS saves surgical time, is easyand convenient to use, and is safe. Routine monitoring offacial nerve function using SACS has helped preventiatrogenic facial nerve injuries and has improved ourability to save the facial nerve during otologic and neuro-otologic surgery.

REFERENCES

1. Silverstein H: Microsurgical instruments and nerve stimulator-monitor for retrolabyrinthine vestibular neurectomy. OtolaryngolHead Neck Surg 94:409-411, 1986

2. Silverstein H, Smouha E, Jones RO: Routine intraoperative facialnerve monitoring during otologic surgery. Am J Otol 9:269-275,1988

3. Silverstein H, Smouha E, Jones R: Routine identification of thefacial nerve using electrical stimulation during otological andneurotological surgery. Laryngoscope 96:726-730, 1988

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CONTINUOUS ELECTRICAL STIMULATION-SILVERSTEIN, WHITE

REVIEWER'S COMMENTS

Dr. Silverstein is to be commended for his continued efforts in the advancement of intraopera-tive facial nerve monitoring. The surgeon employing facial nerve monitoring must have anunderstanding of the fundamental principles of neurophysiology, as well as the practical pitfallsthat can be encountered clinically. Prior investigators have shown that false-negative electricstimulation can be reduced by using constant voltage stimulation (Moller A, Jannetta P: Preserva-tion of facial function during removal of acoustic neuromas: Use of monopolar constant voltagestimulation and EMG. J Neurosurg 61:757-760, 1984) or flush-tip insulation (Prass R, Luders H:Constant current-constant voltage stimulation. J Neurosurg 62:622-623, 1985) which reducethe effects of loss of stimulating current into adjacent soft tissue blood or spinal fluid. These sameprinciples must be considered when using surgical instruments that are connected to a stimulusgenerator. The surgeon should be aware that due to noninsulation of the "electrified" drill or the"electrified" sickle knife with a large noninsulated segment, current shunting with possible false-negative errors are possible. By the nature of "stimulus dissection instruments," however, somedegree of current shunting must be anticipated. We have tried to reduce this shunting by using afine insulation coating to within 1 mm of the instrument's cutting edge.(Kartush J: Electron-eurography and intraoperative facial monitoring in contemporary neurotology. Otolaryngol HeadNeck Surg 101:496-503, 1989).

Jack M. Kartush, M.D.

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