ORIGINAL RESEARCH

Systematic facial nerve monitoring in middle ear

and mastoid surgeries: “Surgical dehiscence”

and “electrical dehiscence”

Yun-Hoon Choung, DDS, MD, PhD, Keehyun Park, MD, PhD,Min Jung Cho, MD, PhD, Pill-Hoon Choung, DDS, PhD,You Ree Shin, MD, PhD, and Hison Kahng, MD, Suwon and Seoul,Korea

OBJECTIVES: To evaluate and systemize intraoperative facialnerve monitoring (IOFNM) in middle ear and mastoid surgeries.STUDY DESIGN AND SETTING: A prospective study.METHODS: IOFNM was performed in 100 patients undergoingmiddle ear and mastoid surgeries. We checked “surgical dehis-cence” under microscopes, and also estimated the minimal thresh-old of electric current needed to change the electromyography offacial muscles using Nerve Integrity Monitor (NIM)-2 (Xomed,Minneapolis, MN, USA).RESULTS: Forty-three percent of cases showed “surgical de-hiscence” and responded to electric stimulation of 0.7 mA or less.“Electrical dehiscence” (� 0.7 mA) was presented in 73 (73.0%)cases, and 82.2% of these cases responded to 0.4 mA or less. Themean threshold of minimal electrical stimulation was 0.29 mA fortympanic segments and 0.41 mA for mastoid segments.CONCLUSIONS: We recommend an electrical stimulation of0.7 mA for the first screening and 0.4 mA for the second explo-ration in order to define the facial nerve using intraoperativeNIM-2 monitoring in middle ear and mastoid surgeries.© 2006 American Academy of Otolaryngology–Head and NeckSurgery Foundation. All rights reserved.

There are many causes of facial nerve (FN) palsy, butone of the preventable causes is iatrogenic injury.

Wiet1 reported the incidence of iatrogenic FN injury to be0.6% to 3.6% in all otologic cases. These figures increasedto 4% to 10% in revision cases. Nissen and Bui2 found theoverall incidence of facial palsy for 1024 consecutive mas-

toidectomies during a 10-year period to be 1.7%. The inci-dence of facial palsy as a postoperative complication re-ported in the literature is 1/1000 cases. However, in theclinical community, the incidence may be closer to 1/100cases.3

Delgado et al4 introduced electromyography as themethod for intraoperative FN monitoring in 1979. The ben-efits of routine monitoring of the FN were established by theNational Institutes of Health in the United States in 1991.5,6

Intraoperative FN monitoring has long been accepted as thestandard of care in surgeries for vestibular schwannomasand other cerebellopontine angle tumors.7-9 In spite of nat-ural FN dehiscence, unusual anatomy, and FN anomalies,the role of intraoperative FN monitoring in chronic earsurgeries is poorly defined.10,11

We tried to prospectively evaluate the role of intraoper-ative FN monitoring and systematize its application in mid-dle ear and mastoid surgeries.

MATERIALS AND METHODS

One hundred patients (male 55, female 45) who underwentmiddle ear and mastoid surgeries with intraoperative FNmonitoring in the Department of Otolaryngology, Ajou Uni-versity Hospital, Suwon, Korea between April 2003 andDecember 2004 were enrolled in this study. The study was

From the Department of Otolaryngology, Ajou University School ofMedicine, Suwon (Drs Y-H Choung, Park, Cho, Shin, and Kahng), and theDepartment of Oral and Maxillofacial Surgery, College of Dentistry, SeoulNational University (Dr P-H Choung).

Reprint requests: Yun-Hoon Choung, DDS, MD, PhD, Department ofOtolaryngology, Ajou University School of Medicine, San 5, Wonchon-dong, Yeongtong-gu, Suwon 443-721, Republic of Korea.

E-mail address: yhc@ajou.ac.kr.

Otolaryngology–Head and Neck Surgery (2006) 135, 872-876

0194-5998/$32.00 © 2006 American Academy of Otolaryngology–Head and Neck Surgery Foundation. All rights reserved.doi:10.1016/j.otohns.2006.04.008

approved by the Institutional Review Boards of the AjouUniversity School of Medicine (Suwon, Korea). We usedNerve Integrity Monitoring (NIM)-2 (Xomed, Minneapolis,MN, USA) for intraoperative FN monitoring.

One of the authors defined FN dehiscence with inspec-tion and palpation under a microscope (“surgical dehis-cence”). The same person checked the location and lengthof the dehiscent FNs. The FN was divided into three parts:the geniculate ganglion, tympanic segment, and mastoidsegment. The tympanic segment was subdivided into threeparts, with the central part being around the stapes. Themastoid segment was also subdivided into three equal parts.

Two sterile needle electrodes were placed in the ipsilat-eral orbicularis oculi and orbicularis oris muscles, and aconstant, unipolar, current stimulus was used. The imped-ance of each electrode was less than 5 K�, and the imbal-ance between the two electrodes was less than 0.5 K�.Muscle relaxants were avoided during the operations tofacilitate the monitoring of electromyographic (EMG) ac-tivity. We stimulated the FNs with a constant, unipolarcurrent with a frequency of 4 pulses/second for 100 �s. Weestimated the minimum threshold of electric currents tomake the FN show the first response of EMG change onNIM-2. We observed the FN dehiscence by surgical micro-scope and defined “surgical dehiscence.” In this study, alldehiscent FNs responded to stimulation of 0.7 mA or less.Stimulation greater than 0.7 mA resulted in nonspecificresponses of FNs irrespective of dehiscence. Therefore, wedefined the response to electrical stimulation within 0.7 mAas “electrical dehiscence.”

RESULTS

The mean age of the patients was 40.4 (range: 4-65) yearsold. Ninety-five cases were for primary surgeries (95.0%),

and 5 were for revision surgeries (5.0%). The cause ofsurgery in 71 cases (71.0%) was middle ear cholesteatomas,and in 29 cases (29.0%) was noncholesteatomatous otitismedia.

Surgical Dehiscence of the FNSurgical dehiscence under a microscope was found in 43(43.0%) cases (Fig 1A). Thirty (69.8%) of the 43 cases weredehiscent in the middle portion of the tympanic segment.Nine (20.9%) were dehiscent in the mastoid segment, and 5(11.6%) were dehiscent in the geniculate ganglion (Table 1).

In 71 cases with middle ear cholesteatomas, 38 cases(53.5%) showed FN dehiscence. However, in 29 cases withchronic otitis media, 5 (17.2%) showed FN dehiscence.

All 43 cases that showed surgical dehiscence respondedto electrical stimulation of 0.7 mA or less. Fifteen cases(34.9%) of all surgically dehiscent FNs responded to min-imal electrical stimulation between 0.25 and 0.3 mA, fol-lowed by 14 cases (32.6%) responding to 0.15 to 0.2 mA, 5cases (11.6%) to 0.35 to 0.4 mA, 4 cases (9.3%) to 0.05 to0.1 mA, 3 cases (7.0%) to 0.65 to 0.7 mA, and 2 cases(4.7%) to 0.45 to 0.5 mA (Fig 1B). Three cases that re-sponded to electrical stimulation of 0.7 mA showed thick-

Figure 1 Surgical dehiscence of the facial nerves was noted in 43 (43.0%) of 100 cases (A). All surgically dehiscent facial nervesresponded to 0.7 mA or less of electrical stimulation (B).

Table 1

Surgical dehiscence of the facial nerves according

to location

Location Number (%)

Geniculate ganglion 5 (11.6)Tympanic segment 39 (90.7)Proximal 21 (48.8)Middle 30 (69.8)Distal 16 (37.2)

Mastoid segment 9 (20.9)

873Choung et al Systematic facial nerve monitoring in middle ear . . .

ened nerve sheaths and surrounding granulations due tochronic inflammation.

Electrical Dehiscence of the FNSince all surgically dehiscent FNs responded to electricalstimulation of 0.7 mA or less, we defined the FNs that re-sponded to electrical stimulation of 0.7 mA or less as “elec-trical dehiscence.” Actually, if the stimulation was over 0.7mA, nonspecific responses were occasionally observed, re-gardless of the location of FNs. Electrical dehiscence wasfound in 73 cases (73.0%) (Fig 2A). Most of them re-sponded between 0.2 and 0.4 mA (Fig 3B). Twenty-three(31.5%) of all electrically dehiscent FNs responded to min-imal electrical stimulation between 0.25 and 0.3 mA, fol-lowed by 20 (27.4%) FNs responding to 0.15 to 0.2 mA, 13(17.8%) to 0.35 to 0.4 mA, 6 (8.2%) to 0.65 to 0.7 mA, 5(6.8%) to 0.45 to 0.5 mA, 4 (5.5%) to 0.05 to 0.1 mA, and2 (2.7%) to 0.55 to 0.6 mA (Fig 2B).

The electrically dehiscent FNs were noted in 30 (52.6%)cases among 57 surgically nondehiscent FNs. About 73%

of these electrically dehiscent but surgically nondehiscentFNs responded to electrical stimulation of 0.4 mA or less(Fig 3).

Figure 4 showed a cumulative distribution of electricaldehiscence according to minimal stimulation thresholds.The cumulative frequency of electrical dehiscence increasedto 0.4 mA (82.2% of the electrical stimulation) along theprimary curve. Beyond 0.4 mA, the cumulative frequencyformed a plateau.

The minimal electrical threshold according to the loca-tion of FNs was, on average, 0.29 mA � 0.12 mA in thetympanic segment and 0.41 mA � 0.24 mA in the mastoidsegment (Table 2). The frequency of electrical dehiscence,according to cause of surgery, was 58 (81.7%) of 71 caseswith cholesteatomas and 15 (51.7%) of 29 cases with non-cholesteatomatous otitis media.

Figure 2 Electrical dehiscence of the facial nerves was noted in 73 (73.0%) of 100 cases (A). Distribution of the minimum electricalstimulation for facial nerves (B).

Figure 3 Distribution of the minimum electrical stimulation inelectrically dehiscent but surgically nondehiscent facial nerves.

Figure 4 Cumulative distribution of electrical dehiscence ac-cording to minimum stimulation thresholds. Electrical stimulationsof 0.7 mA for the first screening and 0.4 mA for the secondexploration are appropriate.

874 Otolaryngology–Head and Neck Surgery, Vol 135, No 6, December 2006

DISCUSSION

Iatrogenic FN injury is one of the most severe complicationsamong temporal bone surgeries. FN injury can be increasedwhen the normal anatomic landmarks of the temporal boneare altered.12,13 Previous surgery, granulation tissue, andcholesteatomas can distort the normal anatomy and compli-cate the surgery.13,14 It is very difficult for the surgeon topredict before surgery variations of the FN, such as con-genital bony dehiscence or abnormal course of the facialcanal. Therefore, intraoperative FN monitoring should berequired for temporal surgeries including congenital auralatresia, cerebellopontine angle (CPA) tumor surgeries, co-chlear implantation, the infratemporal fossa approach, andmiddle ear and mastoid surgeries. However, the role ofintraoperative FN monitoring in middle ear and mastoidsurgeries has not been well established.10,11

There are some differences in rates of FN dehiscenceaccording to the kind of detection method used and dis-ease present. Li et al15 found FN dehiscence of 11.4% bymicroscope use during surgeries. Baxter16 reported a dehis-cence rate of 55% based on temporal bone histopathol-ogy. Sheehy et al17 reported a surgical dehiscence of 44%including congenital dehiscence of 15% and cholesteatoma-derived dehiscence of 17%. Harvey et al18 found that sig-nificant facial nerve dehiscence was present in 6% of mas-toid surgeries. In our study, surgical dehiscence was 43.0%,which is a little higher than those in any previous report.This different result was thought to be because of the higherportion (71%) of cholesteatomas in this study as well as theprospective design.

The most common site of dehiscence was the tympanicsegment (90.7%) (Table 1), which was similar to Baxter’sresults (85%).16 This result suggests that the most riskyportion is the tympanic segment of the FN, and more cau-tion is needed during manipulation of the FN around thestapes.

In this study, most (76.7%) FNs with surgical dehis-cence responded to electrical stimulation of 0.3 mA or less(Fig 1B). This result is consistent with the known fact thatthe response threshold of healthy nerves is about 0.1 to0.5 mA.19 Three cases that responded to electrical stimula-tion of 0.7 mA showed thickened nerve sheaths and sur-rounding granulations due to chronic inflammation. That is,chronic inflammation can result in histopathologic change oftissues around a FN and can increase the minimal threshold ofelectrical stimulation in intraoperative FN monitoring.

In addition, the number of FNs with electrical dehiscencewas 73 (73.0%) out of 100 cases. This was 30 more than thenumber with surgical dehiscence. That is, 30 FNs showedboth surgical nondehiscence and electrical dehiscence. Thisresult may have been due to partial micro-dehiscence, whichcannot be inspected by microscope, or a very thin bonycovering that can easily conduct with low electrical current.In these cases, the possibility of FN injury is increased withheat created by surgical drills and tough manipulations ofoperating instruments. Therefore, electrical dehiscence pro-vides for a greater margin of safety to preserve FNs thansurgical dehiscence in intraoperative FN monitoring in mid-dle ear and mastoid surgeries.

Cumulative distribution of electrical dehiscence accord-ing to minimal stimulation thresholds (Fig 4) showed that0.7 mA is a very useful criterion for the first stimulatingthreshold, and 0.4 mA is a useful criterion for the detailedsecond exploration of an FN in intraoperative FN monitor-ing. This means that we can detect the exposed FNs almostcompletely under electrical stimulation of 0.7 mA (firstthreshold), even with thickened nerve sheaths. We can alsoeasily find normal FNs under 0.4 mA (second threshold),decreasing the possibility of a nonspecific response. Theaverage minimal electrical threshold was 0.29 mA for thetympanic segment and 0.41 mA for the mastoid segment(Table 2). This difference is thought to be from the differentthicknesses of myelin sheath around the FNs.

To explore a more detailed location of FNs in intraop-erative FN monitoring, 0.3 mA is appropriate for the tym-panic segment and 0.4 mA is appropriate for the mastoidsegment. Of course, for more precise exploration of FNs, weneed to decrease the threshold of electrical stimulation tounder 0.3 mA.

Intraoperative FN monitoring is considered to have agreat role, not only in vestibular schwannoma surgeries, butalso in middle ear and mastoid surgeries. A systematic appli-cation is required in clinical fields. Therefore, we needobjective clinical studies to find the optimal threshold forFN stimulation according to type of operation, location ofFNs, and thickness of facial canals.

CONCLUSIONS

Electrical dehiscence, based on responses of electricalstimulation, is safer to use than surgical dehiscence,

Table 2

Minimum threshold of electrical stimulation for facial nerves during middle ear and mastoid surgeries

Second genu(n � 5)

Tympanic segment(n � 68)

Mastoid segment(n � 15)

Minimal threshold of electrical stimulation (mA) 0.31 � 0.17 0.29 � 0.12 0.41 � 0.24

875Choung et al Systematic facial nerve monitoring in middle ear . . .

based on surgical observations in middle ear and mastoidsurgeries.

Electrical stimulations of 0.7 mA for the first screeningand 0.4 mA for the second exploration are considered ap-propriate to define an FN using intraoperative NIM-2 mon-itoring in middle ear and mastoid surgeries.

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