Arch. Otorhinolaryngol. 222, 153--160 (1979) Amhh,~"-s of Oto-~Laryngo iogy �9 Springer-Verlag 1979

Simultaneous Recording of Fast and Slow Auditory Evoked Potentials (Slow-Fast-Simultaneous-ERA, SFS-ERA)

E. Lehnhardt and R.-D. Battmer

Hals-Nasen-Ohrenklinik der Medizinischen Hochschule Hannover (Direktor: Prof. Dr. Dr. E. Lehnhardt), Karl-Wiechert-Allee 9, D-3000 Hannover, Federal Republic of Germany

Summary. In order to reduce measurement time we have used part of the inter- stimulus interval of the slow cortical responses to record fast brain stem re- sponses. For this purpose we reconstructed a programmable audiometer and preamplifier which are controlled by a computer. These, with 1/4 ms-click and tone-burst stimulation simultaneously gained fast and slow responses, not only give a frequency-related view of the threshold and the kind of hearing loss - because of amplitude and latency - , but also in some cases - because of differ- ences between both responses - give ideas of the possible location of the heating impairment through the auditory pathways. The audiological value of this new method is discussed and explained by giving examples of two interesting cases, one central-neural hearing loss and one questionable aggravation.

Key words: Auditory evoked potential (AEP) - Electric response audiometry (ERA) -- Brain-stem electric response (BSER) -- Tone-burst -- Click

Up to now approaches to record auditory evoked potentials (AEP) simultaneously from different levels of the auditory pathways originated from physiological point of view, i.e., the peculiarities of the different responses were of interest (Gestring et al., 1973; Keidel and Finkenzeller, 1975). Therefore, the authors could commit to stimu- late exclusively with clicks and thus renounce the possibility of frequency-specific evocation of the slow cortical responses.

For audiometric purposes, however, tone bursts must be used where a fre- quency-related response is to be expected, namely in the cortex, and clicks should be used only as far as there is no frequency specificity - or a restricted one - i.e., in the upper brain stem. The technical effort to fulfil this purpose is of course comparatively higher.

As advantages of the simultaneous recording are to be stated: 1. The possibility to obtain AEPs from the upper brain stem and the cerebral

cortex synchronously and thus with same degree of vigilance, 2. in cases of central-neural hearing defects to draw topical-diagnostic conclu-

sions from the simultaneous recording of the cortical and brain stem potentials,.

0302-9530/79/0222/0153/$ 01.60

154 E. Lehnhardt and R.-D. Battmer

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Fig. 1. a Basic Scheme of Measurement. It is started with 500 cps-tone-bursts in high intensity alternating with ~-ms clicks in the relation of 1:50 (1). The measuring is continued with 1,000 cps-tone-bursts of equal intensities and further on alternating with Y4-ms clicks (la). Measurements in the frequencies 500 and 1,000 cps (2 x 50 sweeps) therefore timely correspond to the measurement of totally 5,000 i/4 ms-clicks. The same operation is repeated with lower intensity levels (2 and 2a, resp. 3 and 3a); it may, however, be modified in various manners, in case the responses fail or the ration of the fast to the slow responses is disturbed by frequent artefacts. b Oscillographic reproduction of the stimulus sequence. Time marks 200 ms. Each tone burst is followed by 50 clicks

3. for the exclusively frequency-related questions to be able to limit the registra- tion of the slow potentials to the low and medium frequency range, since the fast

A E P is to be related to the high-frequency range. The simultaneous recording anticipates that both potentials are recorded from

the same electrode position, i.e., right forehead against left mastoid; as ground serves the forehead electrode leftside. A fourth electrode is posit ioned at the right mastoid and is only switched on for examination of the second ear.

In the basic test arrangement we apply 500 - resp. 1000 - cps-tone bursts (durat ion 400 ms, rise and decay time 25 ms) simultaneously alternating with 1/4 ms- clicks. Each tone burst is always followed by 50 clicks with a repetition rate of approximately 20 cps and an interstimulus interval of 1,000 ms. Since the filter re- quires a transient t ime of also approximate ly 1,000 ms for the change-over, a dura- tion of 5,000 ms results for a single sweep (Fig. l a and b). With 50 sweeps, for the

Simultaneous Recording of Fast and Slow Auditory Evoked Potentials 155

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single frequency 50 tone bursts and 2,500 clicks are given, for two frequencies (500 and 1,000 cps) of equal intensities 2 x 50 tone bursts and 5,000 clicks.

This basic scheme can be modified arbitrarily via the computer, as soon as the relation is disturbed by artefacts, respectively by the artefact suppression. If, e.g., already 2 "x 50 cortical responses had been recorded, however not yet 5,000 brain stem potentials, then either the rest of the measuring is switched exclusively to click stimulation for recording fast responses (in special cases with 1-ms clicks too), or the conditions for a further measuring of cortical AEPs are fed in. Also, if necessary, it is changed over to exclusive tone burst stimulation, namely if the slow responses must be recorded in the high frequency range as well.

The basic circuit diagram is shown in Figure 2. The stimulus generator (Madsen ERO 74) as well as the fdter in the preamplifier (Madsen ERP 74) are controlled by the computer 1. The display of the AEPs takes place synchronously for cortical and brain stem responses (Fig. 3), the print-out is arranged for both separately and

1 For planning and carrying out the reconstruction of the devices necessary for this purpose we are especially indebted to Mr. W. Weiss

156

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Fig. 3. Simultaneous reproduction of the fast and slow potentials on the display of the computer. Leftside of the picture always the actual recording: Upper time marks at 100-180 ms, the lower at 6-10 ms, number of samples always 256; on the rightside the two preceded measurings. The sample- duration for the fast potentials is 20.48 ms, that for the slow potentials 512 ms. The fdter boundaries are at 150--4,500 cps, resp. 0.25--15 cps

automatically depending on frequency and intensity level. Concerning the audiomet- ric problems combined intensity-latency-diagrams ensue like in Figure 4.

Up to now in the simultaneous recording we have restricted our efforts to the examination of adults or already cooperating children. The fast potentials have been recordable also without sedation, probably because of a well functioning suppression of artefacts.

As long as frequency-related values are obtained, which approximately corre- spond to the normal auditory threshold or - in case of aggravants - are lying distinctly better than the alleged threshold, there will suffice to test with 500 cps and 1,000 cps and to project the fast responses to the high-frequency range (Fig. 5). Then the recording will be reduced to approximately one fourth of the usual dura- tion-time.

A specific constellation ensues in cases of central-neural hearing disorder, for example such as caused by tumor pressure on the upper brain stem.

In these cases, from the regularly preserved stapedius reflex or the normal coch- leogram the proper function of the peripheral and lower sections of the auditory pathways can be deduced. The impairment of the P6-response then proves the hear- ing defect to be localized up to the inferior coUiculus; if otherwise P6 is still recorda- ble up to the normal auditory threshold, the lesion would have to be searched for farther cortically. Simultaneous recording of fast and slow potentials, in these cases has predominantly a topo-diagnostic value (Fig. 6a and b).

Simultaneous Recording of Fast and Slow Auditory Evoked Potentials 157

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158 E. Lehnhardt and R.-D. Battmer

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Fig. 5. AEPs in an exclusively frequency-related problem as stated in the case of questionable aggrava- tion: At 500 and 1,000 cps we succeeded in recording the slow potentials up to 45 dB HL; also the fast responses - to be projected to the high frequency range - could be recorded up to 30 dB p.e. HL (peak equivalent hearing level). The averaged potentials thus confirmed approximately the subjective statements (Josip, I., 17. 3. 37)

Fig. 6. Central-neural hearing disorder rightside. Meningioma of the corpus pineale with pressure to the colliculi on the rightside - confirmed by operation, a AEPs with topic-diagnostic question: On the rightside also the fast potentials are involved. Threshold at 40 dB p.e. HL; leftside threshold at 10 dB p.e. HL. Latencies marked by prolonged on both sides. Cortical responses leftside approx, normal, on the rightside hardly recordable (circles for uncertain responses), b Audiogram of the same patient: Central-neural hearing impairment with maximal tone decay and with a normal stapedius reflex. Dichotic discrimination on the rightside entirely abolished, Still nearly normal tone threshold on the rightside origins from the left cortex (slow responses on the leftside recordable up to 30 dB HL) (Roland, B., 26. 3. 46)

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References

Gestring, G. F., Burian, K., Innitzer, J.: Corhputer sweep-splitting for simultaneous ECOG and ERA display. IIIrd Symposium of the International Electric Response Audiometry Study Group, 10.-12. September 1973, Bordeaux. ERA 29. Issue 1973

Keidel, W. D., Finkenzeller, P.: Simultaneous computation of ECG, early response and ERA. IV. Biennial Symposium der International ERA Study Group in London, 28.-30. July 1975. ERA 39. Issue 1975

Received July 24, 1978