Otology Seminar
Intraoperative Facial Nerve Monitoring (FNM)
R3 蘇旺裕 2001/11/07
History
 1898/7/12 Dr. Fedor Krause, cochlear nerve section for tinnitus, “contractions of the right facial
region, especially of the orbicularis oculi, as well as of the branches supplying the nose and
mouth….”→slight FN paralysis, resolved within days
 Until late 1970s, observe face for movement related to intra-op events/electrical stimulation
Complete palsy (+) though facial movement (+), high level of stimulation itself was damaging
 1965 Jako, photoelectric devices (not very sensitive)
 1979 Delgado, facial electromyography (EMG)
 1987 Prichep et al, crossed auricular reflex (movement of pinnae in lower animals in response
to sounds, vial facial nerve, 12-16ms), require online digital filters since it’s small
 1988 Williams, “Bells against palsy”, suture small “jingle bells” at facial musculature
 1988 Schmid et al, compound nerve action potentials (CNAP)- intracranial stimulation &
record at stylomastoid foramen
 1985 Silverstein, redesigned the Jako stimulator (pressure sensor applied light pressure in the
corner of mouth), muscle movement detection devices (Silverstein stimulator/monitor S-8;
WR Medical Electronics@, USA)
 1986 Nicolet, Nadol and Prass nerve integrity monitor (NIM), electrical stimulation of FN
(Xomed-Treace@, $18,000)
 1989 Brackmann EMG system(more sensitive2), muscle electrical activity detection ($29,000)
instrumentation: stimulator+ amplifier+ oscilloscope+ audio monitor
EMG
Mechanical pressure sensor
Cheap, non-invasive, easy to install, More sensitive, more information, very
優
detects true m. contractions, easy to small bipolar electrodes, no FP other
interpret, artifact is easy to detect
CNs, could monitor other CN
n. crosstalk (CN5)
Sen to external electrical noise
缺
head movement
Difficult to interpret, Many influencing
factors, Baseline activity differs, Need
parotid surgery 不好用
expertise, Invasive, Costly machine
一起用 Sound only m. contract (mechnical), Stimulate at a distance (EMG)
False artifact don’t happen together
Indications
 Otoneurological surgery: AN surgery, vestibular neurectomy,
infratemporal fossa surgery, surgery of the petrous apex
 FN exploration & decompression, excision of FN tumors
 Reconstruction of congenital aural atresia
 Considered in CI, cholesteatoma with potential FN involvement
 Revision surgery of chronic ear disease
 Training residents
Setup
 personnel: from diverse backgrounds- neurology, neurophysiolgy, audiology, anesthesiology,
ASNM (American Society of Neurophysiological Monitoring)
 anesthetic considerations: EMG responses are essentially unaffected by any commonly used
anesthetic regimens
DO NOT use muscle relaxants: short-acting (eg. succinylcholine) may be used to facilitate
intubation, with verification it has cleared before manipulating facial n.
 Integrity of the device checked pre-op, absence of any artifact→indicative of incomplete
connection/setup or mechanical failure
 Artifact: OR- electrically hostile environment, eliminate 50/60Hz power line interference,
broad-band noise originating in other equipment (electrocautery, lasers, ultrasonic aspirators,
microscopes, anesthesia machines, electrified beds, patient warmers, compression stockings)
 Filter, grounding these items, plugging into a different AC outlet, rerouting cables away from
monitoring equipment, or disconnecting during crucial periods
 Electrodes: Surface-less specific, more prone to artifact, more time-consuming to apply
Needle- platinum, larger uninsulated surface, more sensitive (intra-muscular bipolar
hook wires, concentric needles, monopolar subdermal needles)
1. Monopolar- provide spatial resolution of < 1mm
2. Bipolar- more specificity and precision of localization (less current spread), bulky
 Insert into frontalis, orbicularis oculi & oris m., secure to skin with ligatures, drape with 3M
Steri-Drape
 Constant voltage: intracranial stimulation (Energy= V2/R= IR2), fluid exists→shunt of current,
if sucked, large current may damage n.
Constant current: preferred for transcutaneous stimulation
 establish threshold: monopolar, 0.2msec, 5~10/sec; constant voltage EMG threshold=
0.05-0.2V (ave=0.1), constant current EMG threshold= 0.05-0.2mA
use up to x3 threshold to sweep across the exposed surface to confirm absence facial n. fibers
low level→localization; 1V→R/O presence of nerve (reduce false- negative results)
 trace to most medial tumor-FN interface, then dissect from lateral to medial
Goals of using FN Monitoring (Herbert Silverstein)
 Early identification of FN by using electrical stimulators in soft tissue, tumor and bone
 Warning the surgeon of an unexpected facial stimulating (Monitoring ongoing EMG activity for
increased activity or changes- retraction, tumor dissection, electrocautery, lasers, ultrasonic
aspiration…)
 Reducing mechanical trauma to FN during rerouting or tumor dissection
 Identifying & mapping the course of the nerves
 Determining nerve function integrity & predict prognosis before end of surgery (>0.3mA→
evident post-op FN weakness; >6~8mA→FN damaged severely or transected)
Interpretations of FN Monitoring (1986 Prass & Luders)
 ↑Spontaneous EMG→ early indication of inadequate depth of anesthesia
 Mechanically evoked EMG→ related to tumor dissection, retraction, irrigation…
 Artifact: contact between surgical instruments, higher in freq (cracky vs. popping EMG sounds)
Activity
Spike
Burst
(phasic)
Featured MU discharge
Brief, synchronous
Robust MU discharge, single
discharge of multiple FN
axons (evident cause- andeffect relationship)
Causes
Direct n. contract or traction
Direct mechanical n. trauma, irrigation, thermal
changes, electrical currents, application of
materials to an exposed n.
Clinical significance
No necessary relationship
to n. injury.
( Integrity of the nerve↓
→ injury occurred)
Nerve traction, irrigation
nerve ischemia, injury
Train Prolonged asynchronous
(tonic) varies in frequency, amplitude ∵latency exists→ less evident cause-and-effect potentials (esp. bomber)
& interval regularity
↑post-op FN paralysis
50-100Hz- “bomber”; 1-50Hz- “popping”
Pulse Rhythmic, synchronous MU Direct electrical stimulation, synchronous with
(motor unit) discharges
the timed intervals of the applied current
Clinical outcome
 Leonetti et al, unmonitored- 48% Gr V or VI palsy (11/23) at discharge
monitored- 80% Gr I or II palsy (12/15) greatly↓Gr V/VI palsy
 Silverstein 1985-1996, 1200 chronic ear & stapes procedures: 0% immediate or permanent FN
palsy recorded, predicting dehiscence in bony canal during middle-ear & mastoid surgery
Nearly 100% of the graduates of Ear Research Foundation (Florida, USA) use FNM routinely in
otologic surgery
 Harner-“I don’t think I could convince anybody at our institution (the Mayo Clinic) with
experience to give up monitoring under any circumstances”
11
 Charles D. Yingling (UCSF)- refuses to proceed with an acoustic neuroma operation unless
cranial nerve monitoring capability is available
 1991 NIH Consensus Conference on acoustic neuroma: “the benefits of routine monitoring of
the FN are established”
Conclusions
 No a replacement for surgical knowledge or technical skill (US$200/ hour)
 Beware of false positive/ negative & its limitations (not unfailable)
 Modify & advance surgical techniques
 Can’t rely on the instrument to “find the nerve”, visualizing the course of FN by skeletonizing it
in its bony canal remains the optimal method of avoiding injury
Future directions
 Computer-based system with simultaneous capacity for EMG & ABR recording
 Rapid data collection with online digital filtering
 Better artifact rejection
 Automated control of stimulation and recording parameters
 User- friendly interfaces and displays of current data as well as trends during the operation
 Relationship between intra-op recordings & ultimate clinical outcome
References
1.
Newton J. Coken and Herman A. Jenkins: Intraoperative facial nerve monitoring. WB Saunders 2001.
2.
Erez Bendet, Seth I. Rosenberg, Thomas O. Willcox, et al. Intraoperative facial nerve monitoring: a comparison
between electromyography and mechanical-pressure monitoring techniques. Am J Otol. 20:793-99, 1999.
3.
Michael J. Olds, P. Todd Rowan, Jon E. Isaacson, and Herbert Silverstein. Facial nerve monitoring among
graduates of the ear research foundation. Am J Otol. 18:507-11, 1997.
4.
Fabrizio Salvinelli & Antonio De la Cruz: Otoneurosurgery and lateral skull base surgery. WB Saunders 1996.
5.
Richard L. Prass. Iatrogenic facial nerve injury. Otolaryngol Clin N Am. 29(2):265-75, 1996.
6.
Aage R. Moller: Intraoperative neurophysiolgic monitoring. Harwood Academic Publishers, 1995.
7.
Myles L. Pensak, Jay Paul Willging, and Robert W. Keith. Intraoperative facial nerve monitoring in chronic ear
surgery: a resident training experience. Am J Otol. 15(1):108-10, 1994.
8.
Robert K. Jackler. Indications for cranial nerve monitoring during otologic and neurotolgic surgery. Am J Otol.
15(5):611-13, 1994.
9.
Charles D. Yingling and John N. Gardi. Intraoperative monitoring of facial and cochlear nerves during acoustic
neuroma surgery. Otolaryngol Clin N Am. 25(2):413-48, 1992.
10. Herbert Silverstein, Seth Rosenberg. Intraoperative facial nerve monitoring. Otolaryngol Clin N Am.
24(3):709-25, 1991.
11. Herbert Silverstein, Eric Smouha, and Raleigh Hones. Routine identification of the facial nerve using electrical
stimulation during otological and neurotological surgery. Laryngoscope. 98 (July):726-30, 1988.
12. Carlo Zini, and Angelo Grandolfi. Facial-nerve and vocal-cord monitoring during otoneurosurgical operations.
Arch Otolaryngol Head Neck Surg 113:1291-93, 1987.|