Учебники / Middle Ear Mechanics in Research and Otology Huber 2006

months), the number of complications, and number of out-patient controls were recorded. The setting was a tertiary referral centre.

The insertions of the SVT included a tympanotomy performed under general anaesthesia exposing the bony annulus of TM, and a small groove around 0.7 mm long was drilled at its postero-inferior margin. The SVT was a silicone Per-Lee© 60o flange ventilation tube (Richards/Gyrus ENT, USA/UK) (Fig. 1). The tube was fitted in length and at its medial diameter, and hence inserted into the groove. Subsequently, the tympanomeatal flap wasreplaced,andthus,theSVTwaspositionedperipherallytotheTM.Finally, the ear canal was dressed in usual manner with gauze. Surgery often included additional procedures depending on the pathology, i.e. grafting of perforations or atrophic TM’s, tympanoplasty type II or III.

Fig. 1 Per-Lee© 60o flange silicone ventilation tube.

48

3. Results

Thestudygroupincluded58patientswith85SVTinsertions.In13patients, bilateral SVT’s were employed, whereas in 14 cases re-operations were reported; in two of these patients 3 insertions were performed. The mean age was 19 years (range 4 to 70). All patients had a follow-up period of at least one year after tube insertion. During the study period 3 patients were lost from follow-up, leaving 82 cases for analysis.

The mean life time was 58 months (range = 1.5 to 168; SD = 7; 95% confidence interval = 45 to 72; median 35 months). The duration of the SVT’s has been illustrated by a Kaplan-Meier survival plot (Fig. 2) At the

time of reviewing the clinical data 25 (31%) SVT’s were still in function, 37 (45%) had been removed surgically, and spontaneous extrusion was reported in 20 cases (24%).

Survival distribution function

1

0,9

0,8

0,7

0,6

0,5

0,4

0,3

0,2

0,1

0

Months

Fig. 2 Kaplan-Meier survival plot of the SVT life time.

The mean hearing gain was 17 dB (range –15 to 41 dB) measured as an average of puretoneaudiometryat 500,1000,2000 and4000Hz.The mean number of out-patient controls was 6 per year (range 1.4 to 36.6), i.e. once in every two months. During the course of these control visits, the number of complications were recorded and referred to as early complications, i.e. during the tube life time. The most common early complications were intermittent crusts, plugging, otorrhoea and granulations as shown in Table 1. Furthermore, the usage of ear drops was registered.

Table 1 Distribution of early complications.

After the extrusion or removal of the SVT, various complications were also recorded as late complications. These included persisting TM perforation in 9% of cases, decreased hearing compared to the pre-operative level in 9%, and cholesteatoma in 2%.

4. Discussion

4.1 SVT duration

The mean life time of the current method was 58 months, i.e. almost 5 years. In comparison, T-tubes inserted into the TM have been reported to maintain function between 32 and 38 months on average [3,4]. Recently, similar T-tubes with a subannular insertion technique have been reported with a mean duration of 24 months [6]. Our study showed a gap between the mean life time (58 months) and median (35 months). This indicates an early segregation with some SVTs being extruded relatively fast, whereas in other cases the tubes tend to remain in situ for a very long time (up to 14 years). At the time of review 31% of the SVT’s were still functional, whereas 45% had been removed, predominantly due to recurrent otorrhoea, plugging and granulations. In 24% of the cases the SVT was spontaneously extruded.

4.2 Hearing level

The improvement in hearing level was on average 17 dB, but ranged between –15 to 41. Negative values were obtained in cases with concomitant pathology that also a ected the hearing level; detailed analysis of the hearing gain related to these di erent other pathological conditions as well as related surgery awaits future studies. However, our current gain compares well to 15 dB reported for conventional T-tubes by Heerbeek et al. [3], and 13 dB reported for subannular T-tubes by Croutier et al. [6].

4.3 Early complications

Crusts were common findings appearing in 86% of all control visits and considered a minor and inevitable complication (Table 1). Various degrees of plugging were found in 49% of the visits explained by crusts, otorrhoea (39%), or granulations (33%), and hence, these conditions were often

50found together. Similar complications are also found in other tubes, and thus, otorrhoea has overall been reported in 18 to 33% of cases in T-tubes and subannular tubes [7]. One should bear in mind that the current group consisted of chronically diseased ears, where even T-tubes had been used earlieronmoreoccasions,andcomplicationswouldbeexpected.Eardrops were employed on many occasions (62%) for inflammation, but frequently also for the resolution of crusts.

The frequent out-patient control visits were important in order to maintain the function of the SVT and amounted to a mean of 6 per year. One extreme case with 36.6 controls per year was explained by a short complicated course of post-operative infection with 5 visits within 1.5 months, after which the SVT was removed. Further analysis of our data is

likelytodemonstratethatthemajorityoftheseearlyor“incourse”complications are related to cases with the shorter duration of the SVT; in some cases with long duration remarkably few complications were found, and these cases needed only 1 to 2 control visits per year.

4.4 Late complications

Persisting perforation of the TM after removal or extrusion of the SVT was found in 9% of the cases. In comparison, Croutier found 8% perforations usingT-tubesinsertedsubannularly[6].However,forT-tubesinsertedinto the TM, perforation rates are significantly higher between 16 and 33 % [3– 5,7]. Decreased hearing compared to pre-operative level was reported in 9%ofthecasesduetolesssuccessfullateresultsofossicularreconstruction. Cholesteatoma was found in 2 cases (2%).

In summary, the treatment of chronic tubal dysfunction can be repeated conventional ventilation tubes or long-term T-tubes. Our method o ers an alternative with longer duration and low perforation rates. Alternatively, some surgeons may favour grafting the TM with cartilage, and hence, enhancing the sti ness of the TM resulting in a greater pressure resistance, but the acoustic properties can be hampered by this method [8]. Furthermore, increasing the TM sti ness may prevent retractions, but negative pressure and transudation in the middle ear may still occur without ventilation. In addition, the current method could be performed with other concomitant otosurgery depending on pathology. The patients includedinthisgroupwereallundergeneralanaesthesiaduetoeitheryoung age or the extent of pathology, but in fact, insertion of the SVT could be done under local anaesthesia as well.

5. Conclusions

middle ear with the longest mean lifetime of 58 months reported in the literature. The occurrence of early complications during the course of the SVT life time was inevitable in these chronically diseased ears, thus necessitating frequent controls in order to maintain the tube functionality. The hearing improvement of 17 dB and perforation rate of 9% was comparable to other studies using a subannular approach. The procedure was safe andcouldbeperformedincombinationwithotherotosurgicalprocedures. Overall, we find this technique to be a worthy alternative in the treatment of chronic tubal dysfunction, though it should obviously be remembered that a TM perforation is not necessarily a problem, unless accompanied by larger hearing deficits or repeated infections.

References

1.Sadé J., Ar A., Middle ear and auditory tube: Middle ear clearance, gas exchange, and pressure regulation. Otolaryngol Head Neck Surg 116 (1997) pp. 499–524.

2.Avraham S., Luntz M. and Sadé J., The influence of ventilating tubes on the surgical treatment of atelectatic ears. Eur Arch Otorhinolaryngol. 248 (1991) pp. 259–261

3.Van Heerbeek N., De Saar G. M. A. C. and Mulder J. J. S., Long-term ventilation tubes: results of 726 insertions. Clin Otolaryngol 27 (2002) pp. 378–383

4.Strachan D., Hope G. and Hussain M., Long-term follow-up of children inserted with T-tubes as a primary procedure for otitis media with e usion. Clin Otolaryngol Allied Sci. 21 (1996) pp. 537–541

5.Mangat K. S., Morrison G. A. J. and Ganniwalla T. M., T-tubes: a retrospective review of 1274 insertions over a 4-year period. International Journal of Pediatric otorhinolaryngology 25 (1993) pp.119–125.

6.Cloutier J.F., Arcand P., Martinez J., Abela A., Quintal M.C. and Guerguerian A.J., Subannular Ventilation Tubes: Retrospective Study. The Journal of Otolaryngology, 34 (5) (2005) pp. 312–316

7.Kay D. J., Nelson M. and Rosenfeld R. M., Meta-analysis of tympanostomy tube sequelae. Otolaryngol Head Neck Surg, 124 (2001) pp. 374–380.

8.Murbe D., Zahnert T., Bornitz M. and Hüttenbrink K. B., Acoustic properties of di erent cartilage reconstruction techniques of the tympanic membrane. Laryngoscope 112 (2002) pp. 1769–1776

52

ON THE WAY TO DIFFERENTIALLY DIAGNOSING MIDDLE-EAR

AND INNER-EAR DISORDERS

Diana urcanu, Ernst Dalhoff, Hans-Peter Zenner, Anthony W. Gummer

University Tübingen, Department Otolaryngology, Tübingen Hearing Research Centre Section Physiological Acoustics and Communication, Elfriede-Aulhorn-Str. 5, 72076 Tübingen, Germany, Phone: +49 7071 29 88191, Fax: +49 7071 29 4174 Email addresses: diana.turcanu@uni-tuebingen.de, ernst.dalhoff@web.de, anthony.gummer@uni-tuebingen.de

Homepage: http://www.uni-tuebingen.de/cochlea

Laser Doppler vibrometry has proved to be very useful in diagnosing ossicular chain disorders by measuring umbo velocity. Although it has been supposed that these measurements contain information about both the middle ear and the inner ear, it has not been possible to separate the information, nor to provide information about the inner ear.

Here,weproposeamethodtoprovideinformationabouttheinnerear,separating it from middle-ear components, in records of vibration measurements on the human umbo. For this purpose, distortion product otoacoustic emissions (DPOAEs) were measured on the human eardrum using a very sensitive laser Doppler vibrometer. Besides providing valuable information about cochlear status, especially at low level stimulation,velocity-DPOAEsallowedustoobjectivelyestimatethehearingthreshold.

Thresholds were predicted with a standard deviation of only 8 dB. Moreover, the 53 DPOAEs can provide information about the input and output impedances of the eardrum, opening perspectives for characterising and diagnosing middleand inner-

ear function.

We propose that vibration measurements of DPOAEs on the human eardrum could become a new tool for separating middle-ear from inner-ear disorders and for objective assessment of hearing threshold.

1. Introduction

Laser Doppler vibrometry is a non-contacting optical measurement technique that allows measurement of instantaneous velocity of a vibrating object. The technique is valuable for the evaluation of eardrum vibration in response to sound. From its first use on the cat eardrum [1], it has proved capableofdetectingmiddle-eardisorders.Subsequentexperimentsonoth- er animals, human temporal bones and finally living humans certified this ability and showed a high sensitivity in diagnosing ossicular chain disorders.

Since its first employment in living humans [2], e orts were made to diagnose also inner-ear disorders and subsequently to perform a di erential diagnosis between middle and inner-ear disorders [3]. However, in the meantime no statistical di erence was observed between normal subjects and subjects with sensorineural hearing loss [4,5]. Remembering that cochlear amplification occurs at sound pressure levels (SPL) of less than 60 dB [6], the reason for this negative finding might be the lack of sensitivity oftheavailableLDVs–theyarelimitedtomeasurementsabove60dBSPL. Also, there is a paucity of algorithms to separate middle-ear from innerear components.

In this study we propose a method for characterising and diagnosing middle and inner-ear function in records of vibration measurements on the human umbo. For this purpose, we developed a very sensitive LDV which allowed measurements of the distortion product otoacoustic emissions (DPOAEs) as vibration of the human umbo. Using features of these velocity-DPOAEs, we were able to evaluate cochlear status, objectively estimate the hearing threshold and provide perspectives for separating middle and inner-ear components.

54 2. Materials and Methods

The study was performed in 20 ears from 20 volunteer subjects (age 20–30 years),selectedtohaveanormaleardrumandDPOAEspresentatthecubic di erence frequency, 2f1–f2. Classic audiometric tests indicated “normal” hearing in all subjects.

Using the new LDV, the experiment concentrated on measuring DPOAEs,especially2f1–f2 DPOAEinput-output(I/O)functions.Forcom- parison purposes, DPOAE I/O functions were also measured acoustically.

The vibration-measurement set-up consisted of a custom-built LDV capable of measuring picometer-sized umbo vibrations in the presence of extraneous movements of the human body (e.g. swallowing, heart beat). The light source was a laser diode emitting in the near infrared (λ =

810 nm) and the interferometer was a conventional heterodyne type. The object beam (0.43 mW) was coupled into the imaging path of an ENT-op- erational microscope and focused to a 1/e-diameter of 23 µm in the object plane of the microscope.

Stimulus generation and data acquisition were performed using cus- tomsoftware,runningonacomputerequippedwitha16-bitAD/DAcard. Acoustic stimuli were delivered free-field by separate channels using DT 48 earphones, coupled with acoustically damped plastic tubes to the tip of a modified standard aural speculum, fitted into the subject’s ear canal during the experiments. Sound pressure in the ear canal was controlled with a probe-tube microphone ER-7C, whose tube was advanced 2–4 mm from the eardrum. A small piece of reflecting material (0.5x0.5 mm2, 40–50 μg) was placed on the umbo of the subject using a suction needle, in order to enhance signal-to-noise ratio (SNR). The SNR improvement due to the reflector was up to 30 dB. Some experiments were also conducted without introducing a reflector. Control experiments with direct focussing on the unloaded eardrum gave no indication of artefacts in the frequency spectra or I/O functions up to at least 7 kHz.

Vibration measurements were performed in open-field. Microphone and laser signals were low-pass filtered, sampled by the A/D converter and analysed with a 2048-point Fast Fourier Transform, resulting in a 25-Hz frequency resolution. Raw data were saved for o -line processing.

The stimulus parameters were: f2 = 4–9.5 kHz, f2/f1 = 1.2, L2 = 25–65 dB SPL, in 10 dB steps, and L1 = 0.4 L2 + 39 dB. For a single DPOAE measurement, 200–500 data blocks were required.

For the analysis, the whole time signal of one DPOAE measurement was divided into three equal length parts. Each part was first separately averaged in the time domain and finally the three spectra were RMS-aver- aged.

Sound-field measurements of DPOAE I/O functions were made 55 closed-field with a clinical measurement system (DP 2000, Starkey, Eden Prairie, MN, USA), using the same stimulus parameters.

Pure-tone threshold was measured in each subject immediately after the DPOAE measurements using Békésy audiometry. The threshold was measured for the f2 frequencies where vibration-DPOAEs were obtained. The frequency was swept in 100-Hz steps and the sound pressure level was changed in 1-dB steps, with a speed of 5 dB/s.

3. Results

3.1 DPOAEs measured as vibration on the human eardrum

DPOAEs were measured as vibration of the human umbo. They are called velocity-DPOAEs. In all subjects, we were able to obtain velocity-DPOAEs at the cubic distortion level 2f1–f2. The number of velocity-DPOAEs we could measure in one subject varied between 5 and 24 (mean: 15 DPOAEs per subject). The time required to measure one velocity-DPOAE was 1–3 minutes. The amplitudes for the velocity-DPOAEs across subjects varied between 1 and 8 pm; the noise level for the vibration measurements was better than 1 pm.

3.2Hearing threshold estimation using DPOAEs measured as vibration on the human eardrum

A linear dependency was found between the amplitude of DPOAE-velocity (vDP)andthelogarithmofthevelocityamplitudeatf2 (v2).Thiscorresponds tothesamebehaviourobtainedfortheacousticallymeasuredI/Ofunctions (in closed sound field) by Boege and Janssen [7] and confirmed by Gorga et al. [8] and by our sound field measurements (136 acoustically measured I/O functions).

Extrapolation of the linear regression line fitted to the I/O function gives the estimated DPOAE threshold (EDPT). For the vibration measurements, the EDPT represents the value of v2 for which vDP is zero and

is denoted as vEDPT. Using the measured relation between L2 and v2 on the eardrum, we calculated the sound pressure which would be necessary to

yield vEDPT, and called it pEDPT. By comparison, the EDPT derived from the acoustically measured I/O functions represents the value of L2 for which

theleveloftheDPOAE(LDP)iszeroandisdenotedasLEDPT [7].Theimportance of EDPT lies in its ability to estimate objectively the hearing thresh-

old at f2.

56 In order to be included in the analysis, each velocity-DPOAE I/O functionwasrequiredtosatisfycertaincriteria:i)fortheregressionline:r2 ≥ 0.8 and slope ≥ 0.2 µPa/dB, ii) standard deviation of the EDPT < 10 dB and iii) SNR for the velocity-DPOAE > 4 dB; if the SNR ≤ 4 dB, the data were accepted if the average value of S/N and S/σs (σs = unbiased estimate of the standard deviation of S derived from the three equal length parts of onemeasurement–seeMaterialandMethods)was>1.58(i.e.>4dB).For the acoustically measured DPOAE I/O functions, the criteria were similar excepting for the SNR, which was required to be > 6 dB.

Thirty velocity-DPOAE I/O functions fulfilled the criteria and were accepted for the threshold estimation. Békésy threshold was compared with pEDPT using linear regression analysis with a fixed slope of 1.18 dB/dB

(originally reported by Boege and Janssen [7]). The standard deviation from the regression line was only 8.6 dB SPL. For comparison, the same analysis was performed for an identical data set selected from the acousti- cally-measuredDPOAEI/Ofunctions;thisyieldedastandarddeviationof 16.7 dB SPL – this is much larger than the one obtained from the vibration of the umbo.

From the 136 acoustically-measured DPOAE I/O functions, 116 were accepted for the threshold estimation and yielded a standard deviation of 11.2 dB SPL, comparable to the standard deviation reported by Boege and Janssen [7] for a large number of I/O functions (N = 4236).

4. Discussion

The results show that measurement of eardrum vibration near auditory threshold is possible in human subjects up to high frequencies. This allowed us to evaluate the mechanical status of the cochlea, especially at low sound pressure levels, and to perform an objective estimation of the hearing threshold. The standard deviation of the auditory thresholds derived from the vibration measurements was 8 dB smaller than that derived from acoustical measurements. The success of this estimator is probably due to smaller sound-field calibration errors for an open ear-canal condition for the vibration measurements, compared with those for the closed ear-canal condition used for conventional acoustic DPOAEs.

Measurement of velocity-DPOAEs on the eardrum also opens perspectives for better characterising and diagnosing middle and innerear function. Since the output and input impedances of the umbo can now be obtained in vivo, manipulating middle-ear impedances could provide information for di erentially diagnosing middle-ear and cochlear disorders.

57