Учебники / Middle Ear Mechanics in Research and Otology Huber 2006

ing to previous trivial technical limitations. However, recording the full, physiologically relevant range of frequencies in a multi-channel setting is currently readily attained with commercially available EEG amplifiers, which have a wide dynamic range and a high sampling rate. Furthermore, over the past decades, source localization algorithms have been developed to improve spatial localization by determining the position and strength of electrical sources associated with surface recorded electrical potentials [4,5]. The afore-mentioned developments lend the opportunity to investigate deep brain structures in real-time.

While it is generally accepted that the auditory system is influenced by all three regions of the ear in terms of anatomical and functional divisions of the ear, little is known about the quasi-static pressure regulatory mechanisms in the middle ear. Because the middle ear is isolated from the direct communication with the environment, the functional prerequisite of pressure equilibrium is maintained intermittently through a pressure regulation mechanism. Similarly to the circumstances following dynamic pressure loads, the brainstem’s possible involvement in quasi-static pressure regulation has also been put forward by Eden et al [6]. Their experiments in primates have demonstrated anatomical and physiological connections between the tympanic plexus of the middle ear cavity, respiratory centres in the brainstem (nucleus of the solitary tract) and the muscles of the ET [6,7]. The extent of similar connections in humans, although likely is still not known.

It is known that the occurrence of acoustic stimuli received by the human ear gives rise to a erent neural activity, which is reflected by changes in AEPs. Similarly, a erent neural activity related to the brainstem in re- sponsetoquasi-staticpressurechangesofthemiddleearseemstobeapre- requisite, if a central control mechanism of MEP regulation exists. Multichannel EEG configurations, together with advanced methods of source

38analysis, may elucidate these plausible neural pathways and centres related to quasi-static pressure stimulus. Thus, this methodology seems useful to investigate the possible brainstem mechanisms for the dynamic as well as for the alleged quasi-static role in the human ear. New information on the di erences of these aspects could be of major significance in otological research. Hence, the objective of this study was to compare the di erences in waveform and location of deep brain sources following both dynamic and quasi-static load stimulation of the tympanic membrane on the same time scale.

2. Materials and Methods

Seven subjects (three females and four males) without any history of otological or neurological problems participated. They were aged between 32 and 44 (mean 37.7) years. Routine pure tone audiometry, tympanometry and otomicroscopy were in all cases normal. In addition, one 38-year old deaf subject was included as a control subject in order to di erentiate betweenauditoryandpressureresponsesthatmayarisefromaudiblepressure stimuli conducted in the experiment. His tympanometry and otomicroscopy were also normal. Informed and written consent was obtained from all participants, and the study was approved by our Ethical Committee (Ref No 2004/51).

For the acoustic stimulation, rarefaction click stimuli were produced by applying rectangular voltage pulses of 100 μs duration to insert earphones in an Eclipse EP25 (Interacoustics A/S, Assens , Denmark). The time interval between the onsets of two successive stimuli was fixed, yielding a constant stimulation rate of 10 Hz. Monaural stimulus conditions were tested with right stimulation. The monaural clicks were presented at a level of 80 dB HL in order to produce constant wave VI responses [8].

Forquasi-staticpressurestimulationtheexperimentswereconducted by stimulating the tympanic membrane with a novel computer controlled pressure triggering system for rapid 1 Hz synchronized pressure loads (+3 kPa). The pressure control system was connected to the ear via a tube system and an ear probe, and hence, pressure stimuli were transmitted to the ear canal and the TM.

The EEG was recorded from 64 surface electrodes using a standard EEG cap (Quick-Cap International, Neuroscan, El Paso, Texas, USA) following the extended international 10–20 system. The impedance at all 64 electrodes was below 3 kΩ. In addition, two electrodes were placed at the right upper brow and the left external canthus to monitor eye movements.

A linked-ears reference was used. EEG signals were sampled at 20,000 Hz 39 for a high resolution, and band-pass filtered between 150 and 3500 Hz (SynAmps2, Neuroscan, El Paso, Texas, USA). The event related potentials (ERPs) were gathered separately and sampled from 2 ms before and 15 ms after the onset of the stimulus.

In order to investigate which latencies provided a major contribution to source activity, the Global Field Power (GFP) measure was used. The objective of the GFP measure is to quantify the immediate global activity across the spatial potential field sampled over the scalp. The result of this analysis is a waveform that represents the temporal changes in GFP. The method of Multiple Signal Classification (MUSIC) fits dipoles to potential dipolar locations in the regions of interest. The underlying process is a

singularvaluedecompositionassignedtoseparatethesignalspace[5].The commercially available MUSIC algorithm (ANT ASA, Enschede, Netherlands) was used here to confirm the anatomic localization of the dipole in the brainstem.

3. Results

Table 1a depicts the normal ABR responses of the subjects in accordance to the International Federation of Clinical Neurophysiology (IFCN) guidelines [9], while Figure 1 illustrates a common pattern of neuro-electrical activity detected in all seven subjects, but with some inter-individual differences. In order to compare the most robust acoustic signal (i.e signal with highest signal to noise ratio) on the same time scale as that of pressure evoked response we considered the localization of the acoustic brainstem wave VI response. The monaural stimuli produced a dipole fit found in the contra-lateral hemisphere. In the average over subjects, the mean distance in x-direction between dipoles corresponding to monaural right stimuli is only 1.2 cm. Figure 2 also shows that wave VI was likely located in the lateral lemniscus.

Table 1a Distributions of latencies for ABR responses: interpeak intervals of (I–III), (III–V) and (I–V) for all subjects (N=7).

40Table 1b Distributions of PREP peaks N1 to P3 showing average latency for all subjects.

For a relative examination with AEPs the study demonstrates the early pressure related event potentials (PREP) following the quasi-static pressure loads of the tympanic membrane. Figure 3 shows the common PREP pattern detected in all the normal subjects; a similar pattern was found in the deaf subject. Table 1b depicts the normal PREP responses including the individual variation of the subjects for comparative purposes following the methodology outlined in the International Federation of Clinical Neurophysiologyguidelinesforevokedbrainresponses[9].Althoughthese waveforms are given in the same time scale as the auditory brainstem potentials, they di er significantly (p>0.05) from those based on the standard features. Changes were seen in the latency response times and the amplitude responses were approximately twice in magnitude.

AsinthecaseofAEP,sourcelocalizationwasalsoadoptedonarealistic head model for PREPs to show the location of these early neural generators on a similar time scale. The dipole model showed a residual variance lower than 5%, and therefore it could be reliably applied to the individual data.Figure4showsthattheearliestdipolaractivitieswereobservedinthe medulla followed by the activity that was generated by the cerebellum.

42

Fig. 3 The early PEPs from –2 to 15 ms, which includes activity of an individual healthy subject from N1 to P3 in a butterfly plot arrangement (this superimposes all 64 channels together) (upper part), while the lower part shows the mean global field power.

Fig.4ThetwoearliestsourcesfromtheN1/P1complexforthesameindividualsubject as in Fig. 1 in the brainstem and cerebellum (left) superimposed on MNI standard MRI head model.

4. Discussion

Accurate non-invasive techniques for determining the neuro-electrical activity in human brainstem structures are needed for functional localization of electrophysiological signals. The present study attempted to address this issue by investigating the spatio-temporal activity in human brain-

stem structures through the use of a multi-channel EEG method. The ap- 43 proach focused on the source analysis of monaurally evoked auditory late brainstem potentials and compared it to quasi-static pressure related event potentials. The results showed a significant di erence in both latency and amplitude of the two waveforms, while the source analysis indicated that

the subsequent localization of activation for the acoustic and quasi-static pressure stimuli were di erent. The pump delivering the quasi-static pressure stimuli inherently also resulted in a small acoustic stimulus due to the valve, which could influence the PREP’s. However, our deaf subject showed PREP responses similar to the normal group. Hence, this also indicated a distinct pattern related to pressure stimuli, since acoustic responses were not possible in this case.

Whereasthepreviousneuro-tracerresultshavebeenbasedonanimalstud- ies and have shown respiratory centers in the medullary parts of the brainstem (nucleus of the solitary tract) [6], a feedback mechanism controlling the MEP pressure is most likely to be present in humans also, and a few studies have been corroborating this hypothesis: the presence of mechano- receptor-like corpuscles in the human TM [10], the pressure sensivity of the human TM including neural adaptation and its decreased sensitivity in pathological ears [11], and the e ects of local anaesthetics of the human TM resulting in a decreased ET function [12]. Hence, our current results confirm the a erent components of these results, but additionally suggest a role of the cerebellum, which might be explained by its motoric control of the Eustachain tube function.

However it should be noted that in practical applications, attempts to localize deep brain activity are confounded by numerous sources for errors due to insu cient spatial sampling, too low number of electrodes, inaccuratemodels,aswell asnoise.In ordertoovercome theselimitations, EEG was recorded from 64 surface electrodes using a standard EEG cap. We used the realistic head model to avoid localization errors caused by the spherical approximation, which is known to deteriorate with deeper sources. Additionally, the good signal to noise ratio which was maintained following the pressure stimulation makes these deep sources as robust as their cortical counterparts [13]. Therefore this technique may also be used in future experiments along this line and may prove useful in more detailed descriptions of components in MEP feedback control.

Acknowledgments

Oda Petersens Fund and ENT specialist Poul Traun-Petersens Fund con-

44tributed financially to our study. Jackie Eriksen, Medical Equipment ApS, Denmark, kindly provided the NeuroScan Amplifyiers used.

References

1.Jewett D.L., Romano M.N., Williston J.S., Human auditory evoked potentials: possible brain stem components detected on the scalp. Science 167 (924) (1970) pp. 1517–1518

2.Chiappa K.H., Evoked potentials in clinical neurology, 3rd ed. New York: Raven Press; 1997

3.Biacabe B., Chevallier J.M., Avan P., Bonfils P., Functional anatomy of auditory brainstem nuclei: application to the anatomical basis of brainstem auditory evoked potentials ANL 28 (2001) pp. 85–94

4.Scherg M., Fundamentals of dipole source potential analysis. In: F. Grandori, M. Hoke, G. L. Romani (Eds.): Auditory Evoked Magnetic Fields and Electric Potentials. Karger, Basel, vol. 6. (1990) pp. 40–69

5.Mosher J.C., Lewis P.S. and Leahy R.M., Multiple dipole modeling and localization from spatio-temporal MEG data. IEEE Trans. Biomed. Eng 39 (1992) pp. 541–557

6.Eden AR, Gannon PJ. Neural control of middle ear aeration. Arch Otolaryngol Head Neck Surg 113 (1987) pp. 133–137

7.Eden AR, Laitman JT, Gannon PJ. Mechanisms of middle ear aeration: Anatomic and physiologic evidence in primates. Laryngoscope 100 (1990) pp. 67–75

8.Maurer K., Uncertainties of topodiagnosis of auditory nerve and brain-stem auditory evoked potentials due to rarefaction and condensation stimuli. Electroencepalogr. Clin Neuro-physiol; 62 (1985) pp. 135–140

9.Nuwer M., Amino M., Goodin D., Matsuoka S., Maugiere F., Starr A. and Vibert J.-F., IFCN recommended standards for brainstem auditory evoked potentials.

EEG & CN 91 (1994) pp. 12–17

10. Nagai T, Tono T. Encapsulated nerve corpuscles in the human tympanic membrane. Arch Otorhinolaryngol 246 (1989) pp. 169–172

11. Rockley TJ, Hawke WM. The middle ear as a baroreceptor. Acta Otolaryngol. 112(5) (1992) pp. 816–823

12. Nagai T, Nagai M, Nagata Y, Morimitsu T. The e ects of anesthesia of the tympanicmembraneontheEustachiantubefunction.ArchOtorhinolaryngol246(1989) pp. 210–212

13. Whittingstall K., Stroink G., Gates L., Connolly J.F. and Finley A., E ects of dipole position, orientation and noise on the accuracy of EEG source localization. Biomed Eng Online 6 (2003) pp. 2–14

45

SUBANNULAR VENTILATION TUBES IN TREATMENT OF CHRONIC TUBAL DYSFUNCTION – RESULTS IN 85 CONSECUTIVE CASES

Martin Glümer Jensen, Henrik Jacobsen, Michael Lyhne Gaihede, Jørn Rosborg

Department of Otolaryngology, Head and Neck Surgery,

Aalborg University Hospital, Hobrovej 18–22, DK 9000 Aalborg, Denmark

Martin Glümer Jensen: dmgj@rn.dk, Phone +45 9932 2911, Fax +45 9932 2938 Henrik Jacobsen: heja@rn.dk, Phone +45 9932 2911, Fax +45 9932 2938 Michael Lyhne Gaihede: mlg@rn.dk, Phone +45 9932 2911, Fax +45 9932 2938 Jørn Rosborg: jornrosborg@yahoo.dk, Phone +45 9932 2911, Fax +45 9932 2938

Corresponding author: Martin Glümer Jensen

Key words: Long term ventilation, subannular ventilation tube, middle ear pressure, chronic Eustachian tube dysfunction

Purpose: The appearance of a negative middle ear pressure is well-known during the course of secretory otitis media, occasionally leading to complications like tympanic membrane retraction, atelectasis or cholesteatoma. The usual treatment is insertion

46of ventilation tubes into the tympanic membrane (TM), a procedure that often has to be performed several times, perhaps with insertion of long-term T-tubes. However, repeated tube insertions increase the risk of persisting TM perforations, and for T- tubes the risk may amount to 33%. Hence, there is an obvious need for methods of long-term ventilation with lower risk of persisting perforation. The objective of the present study was to describe the clinical course including the life-time of subannular ventilation tubes (SVT) for long-term middle ear ventilation.

Material and methods: A retrospective study of patient records was performed in a consecutive series of 85 patients, who had an SVT inserted during the period from 1979 to 2004. A tympanotomy was performed and a small groove was drilled in the

bony annulus of the floor of the external ear canal. A SVT (Per-Lee© 60˚-angle) was fitted and inserted into the groove, and the tympanomeatal flap repositioned. Hence,

the tube was placed peripherally to the tympanic membrane itself. The life-time was determinedfortheSVTaswellastheimprovementinhearinglevelsandcomplications during the course of treatment.

Results:Thegroupincluded85SVT’sin58patients.Themeanlife-timewas58months (range 1.5 to 168). The mean hearing improvement was 17 dB (PTA)(range –15 to 41). The average number of out-patient visits was 6 per year, and the rate of subsequent TM perforations was 9% after extrusion or removal of the SVT.

Conclusions:ThedurationoftheSVTwasconsiderablylongerthanbothconventional ventilation tubes (mean 9 months) and long-term T-tubes (mean 20 months). Maintaining its function required regular out-patient visits with removal of crusts, treatment of occasional episodes of plugging, otorrhoea, and granulation. The rate of persisting perforation after extrusion or removal was smaller than for T-tubes.

The study included a consecutive series of patients treated with SVT in our department from 1979 to 2004. All patients showed chronic tubal dysfunctionina varietyofconditions:chronic secretoryotitismedia,severeretraction or atelectasis; in some cases cholesteatoma was included. The patient recordswereretrospectivelyreviewedforinformationaboutthelifetimeof the SVT, which was determined by the time di erence in months between insertion of the tube and the first date of control, where the tube was found non-functioning. Further, hearing levels before and after surgery (3 to 12