Comparison of Transtympanic and Extratympanic Electrocochleography

doi:10.4236/ijohns.2013.25035

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International Journal of Otolaryngology and Head & Neck Surgery, 2013, 2, 160-164

http://dx.doi.org/10.4236/ijohns.2013.25035 Published Online September 2013 (http://www.scirp.org/journal/ijohns)

Comparison of Transtympanic and Extratympanic

Electrocochleography

Alan K. Dobbs1, Wesley W. O. Krueger2, Sheryl Bishop1

1University of Texas Medical Branch, Galveston, USA,

2Ear Institute of Texas, San Antonio, USA

Email: [email protected]

Received April 2, 2013; revised May 10, 2013; accepted June 12, 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

Electrocochleography (ECoG) has been an important tool in the diagnosis of Meniere’s disease or endolymphatic hy-

drops. There are two methods employed, transtympanic and extratympanic. Many have regarded the results of these

methods as being equally reliable. The purpose of this study is to determine any differences in sensitivity between the

two methods. In this study patients with known endolymphatic hydrops or Meniere’s disease underwent ECoG testing

with both the extratympanic method and the transtympanic method on the same day in the same ear. The results show a

significant difference between the two methods, with the transtympanic wave values being smaller and therefore more

sensitive than the extratympanic method. In addition, transtympanic ECoG resulted in better waveform morphology and

better correlation with the audiometric findings in endolymphatic hydrops and Meniere’s disease. The results emphasize

the superior role of transtympanic ECoG over extratympanic ECoG as a valuable component in the confirmation of

Meniere’s disease or endolymphatic hydrops along with history and audiometric findings characteristic of the disease.

Implications of the study promote the use of transtympanic ECoG rather than extratympanic ECoG in patients with

symptoms suggestive of Meniere’s disease or endolymphatic hydrops.

Keywords: Meniere’s Disease; Endolymphatic Hydrops; Transtympanic Electrocochleography; Extratympanic

Electrocochleog raphy; ECoG

1. Introduction

Electrocochleography (ECoG) has been an important tool

in the diagnosis and monitoring of Meniere’s disease/

endolymphatic hydrops and includes the components of

the summating potential (SP) and action potential (AP)

generated by the cochlea and auditory nerve respectively.

The generated waveform potentials of each can then be

evaluated and compared. The SP is elevated relative to

the AP and therefore produces the more pronounced ele-

vated SP/AP ratio found in endolymphatic hydrops or

Meniere’s disease [1].

Electrocochleography is a near-field cochlear evoked-

potential measurement. The recording needle electrode

may be placed on the cochlear promontory transtympani-

cally (TT) or placed within the medial external auditory

canal/TM extratympanically (ET). The nearer in prox-

imity to the recording electrode is placed in the cochlea,

the more robust the evoked potentials will be. There are

two techniques for obtaining the waveforms: transtym-

panic (TT) and extratympanic (ET). The proper place-

ment has been considered vital in obtaining reproducible

and well-formed waveforms with the best location being

the round window niche [2]. However, Krueger and

Wagner demonstrated that the same waveforms may be

obtained by placing the needle at the promontory, lateral,

or medial niches [3].

The trans-tympanic method is a much more precise

method and involves placing an electrode through the

tympanic membrane (TM) to the promontory [4].

The ET method involves placing an electrode in the

ear canal or on the surface of the TM. The ET method is

less invasive, however the TT method has typically been

quite safe and well received by patients [1]. Each method

involves the delivery of a stimulus as either a tone burst

or a click. The electrode array consists of three electrodes.

The primary/non-inverting recording electrode (+) is in

the canal, the inverting electrode is on the forehead or

tragus, and a ground electrode is utilized [4].

Once the stimulus is delivered, a waveform is created.

The SP is produced from the cochlear hair cells and the

stria vascularis. The AP is created from the auditory

A. K. DOBBS ET AL. 161

nerve. The SP is present throu ghout the stimulus wh ereas

the AP is apparent immediately after the delivery of the

stimulus. Using a longer interval tone burst helps to dif-

ferentiate the SP from the AP [4].

Figure 1 shows an ET ECoG wavefor m on top and TT

ECoG waveform below. The baseline (BSL), AP, and SP

are clearly marked. The morphology of the waveforms

will assist in establishing the SP and AP amplitudes for

calculation of the ratio. This is seen in Figure 1, where

the “shoulder area” used to determine the SP is more im-

mediately apparent in the TT technique compared to the

ET technique. Typically, SP and AP measurements taken

from the transtympanic technique are greater in ampli-

tude than the extratympanic technique.The measurement

of the magnitude of the variation of SP in comparison to

AP is most useful clinically. The SP/AP ratio translates

into cochlear pressure. An increase in the SP/AP ratio is

indicative of endolymphatic hydrops or Meniere’s dis-

ease, which characteristically involves an increase in co-

chlear pressure [5 ] .

There has been much debate over which method, TT

or ET, produces the most reliable results. Lustig, et al.,

suggests that the use of extra-tympanic methods produce

amplitudes that are diminished, which can be a drawback

in assessing a patient with moderate-to-severe hearing

loss. One study involving a comparison of the two meth-

ods demonstrated a difference in amplitude, but no dif-

ference in ratio values [6]. Matsuura, et al., tested ten

subjects with normal hearing and compared the results

with subjects having Meniere’s disease. The method was

extra-tympanic using iontop heretic anesthesia on the TM

with the electrode in contact with the TM. The ionto-

pheretic anesthesia appeared to have no effect on the test

results, which revealed an increased SP/AP ratio in pa-

tients with Meniere’s disease [7].

Another valuable use for TT ECoG was demonstrated

in a study in which TT ECoG was utilized intra-opera-

tively with patients who had the cerebellopontine angle

tumors [8]. The study involved patients undergoing sur-

geries via middle fossa or a retrosigmoid approach with

simultaneous TT ECoG monitoring. The study showed

that even the slightest variations in auditory function

were reflected on TT ECoG, thus having the potential to

avert intra-operative auditory damage.

Another study examined TT against ET with 19 heal-

thy subjects who had normal audiograms prior to testing.

A TT electrode was placed in the ears bilaterally of each

subject and ECoG was performed followed immediately

by ET ECoG. Three subjects were excluded since a re-

producible SP could not be obtained. In all cases, the TT

amplitudes were larger than ET amplitudes. Testing was

repeated for each method to assess reproducibility.

However, one TT test and nine ET tests were unable to

be calculated for the repeat test because the SP waveform

Figure 1. Transtympanic & extratympanic ECOG wave-

forms.

could not be identified. Of those that could be repro-

duced and the SP waveform identified, the SP/AP ratios

were equivalent in the TT and ET tests. Though the

variability was not statistically significant in the study,

the author suggests that if extratympanic testing is to be

performed in a clinical setting, that it should be repeated.

If the results are not reproducible, then the TT method

should be employed [2].

Due to the variability in Roland’s study, further com-

parison should be undertaken. The purpose of this study

is to revisit the comparison between TT and ET methods

of electrocochleography. Characteristics of the waveform

and variability were ex amined. The importance o f further

investigation of various methods of ECoG cannot be un-

derstated. An accurate ECoG measurement can mean the

difference between diagnosing and ruling out hydrops,

which has subsequent treatment consequences for the pa-

tient. For example, Meniere’s disease is an absolute dis-

qualification for a commercial driver’s license and should

be considered a significant factor in assessing fitness for

duty in other hazardous occupations [9].

One aim of this study is to demonstrate that results

obtained from TT ECoG and ET ECoG will often yield

significantly different results. Demonstration of varying

results will help guide future practice in diagnosis of Me-

niere’s or hydrops by directing clinicians to the most

accurate ECoG test available. The hypothesis is that TT

ECoG is superior to ET ECoG due to the ability of TT

A. K. DOBBS ET AL.

162

ECoG to produce waveforms of lesser amplitude and a

smaller amount of variability, and could therefore aid in

the diagnosis of endolymphatic hydropsor Meniere’s dis-

ease to a more accurate level.

The American Academy of Head and Neck Surgeons

(AAO-HNS) created guidelines for the diagnosis of Me-

niere’s disease. The guidelines state that, for a definitive

diagnosis of Meniere’s disease, there should be at least

two episodes of vertigo lasting 20 minutes of more, au-

diometrically documented hearing loss on at least one

occasion, tinnitus or aural fullness in the affected ear, and

that other causes be excluded [10]. Despite the guidelines,

some practices use only ET ECoG and make a diagnosis

of hydrops or Meniere’s disease based only on these re-

sults. If the ET ECoG lacks the sensitivity to aid in the

diagnosis a better method should be considered. The aim

of this study is to determine if there is a significant dif-

ference in ET and TT ECoG methods. The hypothesis is

that TT ECoG will demonstrate a higher sensitivity than

ET ECoG.

2. Methods

The current study is a secondary data analysis of medical

records from 51 private practice neurotology clinic pa-

tients in San Antonio, Texas. Patients were selected based

on a history compatible with endolymphatic hydrops or

Meniere’s disease and either a unilateral or bilateral hear-

ing loss. After having an audiogram, all subjects con-

sented for evaluation by both TT ECoG and ET ECG as

part of their clinical assessment. The study was approved

by the IRB at the University of Texas Medical Branch.

Hearing data was collected and recorded in the patient’s

medical record between May of 2004 and February of

2008. A Tiptrode was used for the extratympanic method

of testing and the Viking Nicolet needle electrode was

used for the transtympanic method. Patients who had

conductive hearing loss or prior otologic surgery were

excluded from this evaluation. Each patient met the crite-

ria for Meniere’s diseaseas set forth by the American

Academy of Otolaryngology—Head Neck Surgeons

(AAO-HNS), which includes vertigo, hearing loss, and

tinnitus or aural fullness.

Protocol

Patients had an audiogram, followed by ET ECoG and

subsequent TT ECoG. All testing occurred on the same

day for each patient to avoid variability in day-to-day

hearing performance. The audiogram was performed in

the sound booth with headphones and a bone conducting

transducer. First, the air and bone conduction audiogram

was performed by delivering tones through ear inserts

followed by conducting transducer.

ET ECoG evaluations were performed first using the

ET Tiptrode placed on the tympanic membrane of the af-

fected ear. This was followed immediately by TT ECoG

testing. The recording needle was placed using otomi-

croscopy and secured in place with a foam insert to pre-

vent migration. Topical phenol was placed on the TM

being tested and the Nicolet TT needle electrode pierced

the TM and was placed on the promontory, near the

round window niche. ECoG responses were evoked by

click stimulus.

ET and TT ECoGs were individually recorded for each

ear. As a reference, patients were rated according to the

American Speech Hearing Associations’ classifications

based on the audiogram and the results were associated

with the ECoG results from the respective patients. The

audiograms for each patient were suggestive of Me-

niere’s disease having a low tone hearing loss.

Medical records for all 51 patients were initially iden-

tified by chart number alone, and then recoded into a

randomly generated four-digit identification number and

password protected. ET waveforms were obtainable for

47 (n = 47) of the 51 tests. TT waveforms were obtain-

able for 46 of the 51 tests. Comparisons were made only

on those waveforms that were reproducible.

3. Plan of Analyses

Specific Aim: To explore the differences between TT

ECoG and ET ECoG hearing assessment and correlate

these waveforms to the hearing loss (e.g. to determine

which technique would be better correlated with Me-

niere’s disease or endolymphatic hydrops and the associ-

ated hearing loss).

Hypothesis: TT ECoG offers greater sensitivity to ET

ECoG as demonstrated by TT ECoG waveforms of sig-

nificantly lesser amplitude.

Analyses included paired t-tests to compare between

methods on measures of amplitude and variability.

4. Results

SP/AP ratios were obtained from ECoG testing. Paired

t-tests were conducted comparing transtympanicand ex-

tratympanic responses in each ear. SP/AP ratios for the

transtympanic method revealed mean values significantly

(p < 0.000) smaller compared to SP/AP ratios obtained

with the extratympanic method in both ears (right: t =

4.744, df = 21, m = 0.3509, sd = 0.0375 versus m =

0.05368, sd = 0.1255, respectively; left: t = 5.767, df =

21, m = 0.3145, sd = 0.1278 versus 0.04764, sd = 0.0906,

respectively) indicating greater sensitivity for transtym-

panic assessment, see Figure 2.

5. Discussion

Diagnostic sensitivity is enhanced with the use of the

A. K. DOBBS ET AL. 163

Figure 1. ECoG values by comparison.

transtympanic method according to the results of our

study. The implication is that a diagnosis of Meniere’s

disease or hydrops could be disregarded utilizing the ex-

tratympanic method. This study utilized techniques per-

formed by one physician so there was less chance of va-

riation in technique.

In a retrospective review, Pou, Hirsch, Durant, Gold,

& Kamerer, demonstrated a specificity for endolym-

phatic hydrops of 94% with extratympanic ECOG [11].

The sensitivity was 57% however. The study by Pou, et

al. demonstrated the usefulness of ECoG in the diagnosis

of hydrops while underscoring the need for more sensi-

tive testing. The use of transtympanic ECoG would be

expected to increase the sensitivity of diagnosing Me-

niere’s disease or endolymphatic hydrops according to

the results of our study. A prospective study by Ghosh,

Gupta, and Mann, also illustrated a greater sensitivity

and specificity with TT ECoG over ET ECoG [12]. Fur-

thermore, ECoG appears to be more sensitive detecting

endolymphatic hydrops than alternate tests, such as Co-

chlear Hydrops Analysis Masking Procedure (CHAMP)

[13].

Hornibrook, Kalin, Lin, O’Beirn, & Gourley, deter-

mined that TT ECoG correlates more closely to Gibson

scores than the American Academy of Head Neck Sur-

geons Committee on Hearing and Equilibrium (AAO-

HNS CHE) guidelines for diagnoses of Meniere’s dis-

ease [14]. Such a finding emphasizes the need to use

more than one diagnostic tool when making the diagnosis

of Meniere’s or endolymphatic hydrops. The study also

highlights the need to preferably perform ECoG during

symptomatic periods in order to gain meaningful test re-

sults. ECoG testing during asymptomatic phases are less

likely to yield meaningful results.

The study by Roland et al compared TT ECoG and ET

ECoG in patients who were free of otologic disease [2].

The study found no difference in TT ECoG and ET

ECoG in patients who were free of otologic disease. The

findings validate that ECoG is valueless in patients who

do not demonstrate symptoms of Meniere’s or endolym-

phatic hydrops.

In conclusion, TT ECoG appears to be a more sensi-

tive test for the detection of Meniere’s disease or endo-

lymphatic hydrops. This is likely to be more apparent

during episodes of exacerbations of Meniere’s or hydro-

pic symptoms. It would be desirable to utilize TT ECoG

when available according to the results obtained in this

study. Further research with TT ECOG from a multitude

of testing locations would be worthwhile to reinforce the

conclusions of this study. It is important to note that the

results of ECoG studies should correlate with the history

and audiometric results as per the guidelines in place by

the American Academy of Head and Neck Surgeons

(AAO-HNS). One review found that only 39.8% of pa-

pers on Meniere’s disease actually adhered to the AAO-

HNS guidel ines [15].

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