The Effect of Tympanic Membrane Perforation on Real-Ear to Coupler Difference Acoustic Transform Function

Transcription

1 Med. J. Cairo Univ., Vol. 77, No. 1, March: 79-87, The Effect of Tympanic Membrane Perforation on Real-Ear to Coupler Difference Acoustic Transform Function MOHAMED TAREK GHANNOUM, M.D.*; MOHAMED EL-BADR EL-DABAE, M.D.**; SHEREEN MOHAMED EL-ABD, M.D.*; ABEIR OSMAN DABBOUS, M.D.* and HANAA AHMAD FADEL, M.Sc.** The of Audiology Unit, E.N.T. Department*, Faculty of Medicine, Cairo University and Audiology Unit**, Institute of Speech & Hearing Research, Embaba, Cairo. Abstract Real-Ear-to Coupler Difference (RECD) is the difference in decibels, as a function of frequency, between the SPL at a specified measurement point in the ear canal and the SPL in a 2cc coupler, for a specified input signal. Objective : We investigated the effects of tympanic membrane perforation on the RECD. Methods: RECD was obtained using an insertion gain analyzer for 24 adult patients with dry tympanic membrane perforation (group 1) (31 ears) and 20 healthy adult control subjects (group 2) (40 ears). Results: Cases showed moderate conductive hearing loss in their mean hearing threshold level. There was no statistically significant difference between both anterior and posterior locations as regards the degree of hearing loss or as regards the RECD. There was a statistically significant difference of RECD at frequencies 250Hz, 500Hz and 750Hz, between both groups. The RECD was 3 to 6dB lower in group 1 than in group 2. There was a statistically significant difference on comparing RECD at frequencies 250Hz, 500Hz, 750Hz and 1000Hz in right ears, and at frequencies: 250Hz and 500Hz and 750Hz in the left ears of both groups. In group 1, there was no statistically significant difference of RECD values at different frequencies between right and left sided perforations and there was no statistically significant difference of RECD values at different frequencies between both genders except at 1500Hz. There was no statistically significant correlation between the RECD and the age of the patients or their airbone gap. Conclusion: RECD in patients with tympanic membrane perforation is lower than normal at frequencies (250Hz- 750Hz). We recommend that this discrepancy should be compensated for if average normal RECD are used in the preselection of target and gain to avoid under-amplification at lower frequencies. However, the large inter-subject variation strongly suggests the need for individual RECD measurements whenever possible in fitting aids for patients having tympanic membrane perforations. Key Words: Real-ear-to coupler difference Tympanic membrane perforation. Introduction TYMPANIC membrane perforation (TMP) can result from disease (particularly infection), trauma, or medical care. Perforations can be temporary or persistent. Effect varies with size, location on the drum surface, and associated pathologic condition [1]. In determining which air-conduction hearing aid is most appropriate, as the hearing loss and magnitude of air-bone gap increases, the need to fit a BTE or body hearing aids should be considered the most appropriate fitting for chronic conductive hearing loss (CHL) when medical contra indication have been ruled out [2]. Regardless of the type of air-conduction hearing aids, the appropriate realear gain for patients with conductive or mixed hearing loss can be determined by using many of the prescriptive procedures. That is, prescribed real-ear gain to compensate for the magnitude of the air-bone gap [2-9]. The verification process for air-conduction hearing aids can either be real-ear gain using probe tube or functional gain measures, paired comparisons or subjective evaluations [10]. The real-ear-to-coupler difference (RECD) is the difference between the 2cc-coupler values and the individual auditory canal values measured with the identical sound pressure and conditions. RECD values are given in db and for every frequency [11]. The RECD is a powerful tool that can assist the clinician throughout the various stages of the amplification process [12]. The RECD may be used as an accurate and convenient means of deriving real-ear data. It may be used to estimate accurately the real-ear sound pressure level (SPL) produced by a hearing instrument from measurements using a 2cc coupler [13,14,15]. Given the difference in volume and impedance between the ear and the 79

2 80 The Effect of Tympanic Membrane Perforation on Real-Ear to Coupler coupler, RECD values are generally greater than or equal to 0dB (i.e., greater output in ear than coupler for same input signal level). As can be expected, RECD values can vary substantially across age groups [16]. Individual adult RECDs often conform closely to the average. However, children-even of similar age-frequently have very different RECD values [17]. The volume of the residual ear canal when aided together with the middle ear input impedance determines the RECD [18]. A negative RECD value may indicate an inadequate seal of the transducer to the ear (e.g., foam ear tip), a larger than average ear, a perforated ear drum or a myringotomy tube in place [19]. The aim of this study was to investigate the effects of tympanic membrane perforation on the real-ear to coupler difference (RECD). Subjects and Methods The study group (group 1) comprised 24 adult patients with dry tympanic membrane perforation; they were referred from E.N.T outpatient clinic to the audiology unit of Hearing and Speech Institute, prior to admission for myringoplasty. The control group (group 2) comprised 20 healthy adult subjects with no evidence of middle ear pathology, well matched to the cases as regards age and gender. The study lasted one year during which all subjects were submitted to the following: 1- Full history taking including history of present illness including the onset, course and duration of the perforation, the colour, the odour and the amount of the discharge, and history of ear diseases (hearing loss, tinnitus, vertigo, pain). 2- General examination. 3- Full E.N.T examination including examination of the external auditory canal and the tympanic membrane perforation for the site, the size of the perforation and presence of discharge. Only patients with dry perforation were included in this study. 4- Basic audiological evaluation using dual channel clinical audiometer (AC 40); calibrated according to the ISO standards; with TDH39 ear phones, in a sound treated room: Amplisilense. Testing included: A- Pure tone audiometry including: 1- Air conduction in frequency range of 250Hz-8000Hz at octave intervals. 2- Bone conduction in frequency range of 500Hz-4000Hz at octave intervals. B- Speech audiometry including speech reception threshold (SRT) using Arabic Spondee words [20] and word discrimination score (WD %) using Arabic phonetically balanced words [21]. C- Acoustic immittance using the immittancemeter: Interacoustic AZ 26 middle ear analyzer; calibrated according to the ISO standards. Testing included single component tympanometry and acoustic reflex threshold testing for intact tympanic membranes only in the frequency range of 500Hz-4000Hz. 5- RECD transform function, using the real-ear measurement system: Affinity HIT 440, AC 440 and REM440. RECD was obtained for each subject using the RECD protocol on an insertion gain analyzer (Affinity, REM440). The coupler measure was obtained for each subject by attaching Affinity 440 insert transducer directly to HA 2cc coupler and measuring the stimulus output level in db SPL as a function of frequency for normal level 70dB SPL. Next, the probe tube was inserted into the ear canal using a standard depth of 28mm for females and 3 1 mm for males. This ensured that the tip of the probe tube was within 5mm of the tympanic membrane which was ensured by red mark on the probe tube. Each patient sat about 1 meter from the loudspeaker. An ER-3A foam ear tip was attached to the REM 440 insert transducer and inserted into the ear canal entrance taking care not to move the probe tube. The SPL of the same stimulus was measured in real-ear and the difference in output between the real-ear and the coupler was automatically calculated by REM440 as RECD. Statistical methods: The data was quoted and entered using statistical package SPSS version 12. Data was summarized using mean, standard deviation and range for quantitative variables, and percent for qualitative ones. Comparisons between groups were done using chi Square test for qualitative variables and independent t test for normally distributed quantitative variables. Non parametric Mann-Whitney test was used for abnormally distributed qualitative variables. p value <0.05 will be considered statistically significant. Correlation was done to test the relation between quantitative variables. Results The study group (group 1) comprised 24 subjects with perforated tympanic membranes with a mean age of 37.6 years ± (ranging from 12

3 Mohamed T. Ghannoum, et al. 81 to 65 years). Ten of them were males (41.7%) and 14 were females (58.3%). Seven of them (29.17%) had bilateral perforation (14 ears) and 17 (70.83%) had a unilateral perforation (17 ears), resulting in a total of 31 perforated tympanic membranes: 12 right sided perforations (38.7%) and 19 left sided perforations (61.3%). Regarding the site of the perforation, 16 ears (51.60%) had anterior perforation and 15 ears (48.40%) had posterior perforation. All cases had medium sized perforation as was clinically seen on ototoscopic examination. They were compared to a control group (group 2) which comprised 20 normal subjects with intact tympanic membranes. Eleven of them (55%) were males and 9 (45%) were females, with a mean age of 29.3 years ± (ranging from 18 to 40 years). Table (1) shows the distribution of cases and controls. There was no statistically significant difference between both groups as regards the gender p= There was no statistically significant difference between both groups as regards the age: p=0.189; t= Table (1): The distribution of cases and controls. Intensity (dbhl) No. of No. of No. of No. of subjects ears right ears left ears Group 1 24 cases 31 perforated TM Group 2 20 controls 40 intact TM Fig. (1) shows the mean values of air and bone conduction thresholds and Table (2) shows the mean air-bone gap, in dbhl at different frequencies in group (1). Air Bone 250Hz 500Hz 1000Hz 2000Hz 4000Hz 8000Hz Frequency Fig. (1): Mean values of air and bone conduction in group 1. Table (2): Mean and SD of Air-bone gap in dbhl at different frequencies in group (1). Mean SD Min Max db db Regarding the location of perforation, whether anterior or posterior, there was no statistically significant difference between both locations as regards the degree of hearing loss (data not shown). Figs. (2,3) show examples of RECD in one of our controls and in one of our cases respectively. RECD (Coupler difference) 1-70dB 2-70dB Ear 3-70dB Coupler khz Fig. (2): Example of RECD in one of our controls. (Right ear) db: Real ear- to coupler difference db: Real ear measurement db: Coupler measurement. RECD (Coupler difference) 1-70dB 2-70dB Ear 3-70dB Coupler khz Fig. (3): Example of RECD in one of our cases. (Right ear) db: Real ear-to coupler difference db: Real ear measurement db: Coupler measurement.

4 82 The Effect of Tympanic Membrane Perforation on Real-Ear to Coupler Table (3) shows that there was a statistically significant difference between group (1) and group (2) as regards RECD in frequencies 250Hz, 500Hz and 750Hz. Table (3): Comparison of RECD between group (1) and group (2). Ears of Group (1) n=31 Ears of Group (2) n= * * * Table (4) shows that there was statistically significant difference in RECD values at frequen- cies 250Hz, 500Hz, 750Hz and 1000Hz between ears with right perforated TM & right control ears. Table (4): Comparison of the right ear RECD values at different frequencies in the two groups under the study. Right ear of Group (1) n=12 Right ear of Group (2) n= * * * * Table (5) shows that there was statistically significant difference in RECD values at frequen- cies 250Hz, 500Hz and 750Hz between ears with left perforated TM and left control ears. Table (5): Comparison of the left ear RECD values at different frequencies in the two groups under the study. Left ear of Group (1) n=19 Left ear of Group (2) n= * * *

5 Mohamed T. Ghannoum, et al. 83 Table (6) shows that there was no statistically significant difference of RECD values at different frequencies between right and left sided perforation in group (1). Table (6): Comparison of RECD values at different frequencies between right and left-sided perforation in group (1). Right (n=12) Left (n=19) Table (7) shows that there was no statistically significant difference of RECD values between anterior and posterior perforation at different frequencies in group (1). Table (7): Comparison of RECD values at different frequencies according to the site of the perforation whether anterior or posterior in group (1). Anterior (n=16) Posterior (n=15) Table (8) shows that there was no statistically significant difference of RECD values at different frequencies between male and female patients in group (1) except at 1500Hz. Table (8): Comparison of RECD values at different frequencies according to the gender of the patient group (1). Male Female *

6 84 The Effect of Tympanic Membrane Perforation on Real-Ear to Coupler Table (9): The Pearson correlation coefficient ( r) between RECD values and the age of the patients in group (1). 250 HZ 500 Hz 750 Hz 1000 Hz 1500 Hz 2000 Hz 3000 Hz 4000 Hz 6000 Hz r p-value Table (10): The Pearson correlation coefficient ( r) between RECD values and air-bone gaps at different frequencies in group (1). 500 Hz 1000 Hz 2000 Hz 4000 Hz r p-value Table (9) shows that there was no statistically significant correlation between RECD and age of the patients in group (1). Table (10) shows the Pearson correlation coefficient (r) between RECD values and air-bone gaps at different frequencies in group (1). There was no statistically significant correlation between RECD and air-bone gap in group (1). Discussion Different RECD values result from a variety of factors such as ear mold acoustics, ear canal length and geometry, middle ear impedance and head and body diffraction effects. In addition, significantly different RECD values may be obtained when different methodological procedures (e.g. different signal delivery configurations or acoustic coupling) are used [22]. RECD differ also in middle ear pathology [23,24]. In this study, RECD was done on 24 subjects with dry tympanic membrane perforation (group 1). Seven of them had bilateral perforation, and 17 had a unilateral perforation, resulting in 12 right sided ears (38.7%), and 19 left sided ears (61.3%). The study group was compared to 20 normal control subjects (group 2). There was no statistically significant difference between both groups as regards the gender distribution or age. The mean hearing threshold level of group (1) showed moderate conductive hearing loss. The greatest loss was in the low frequencies (250Hz & 500Hz), followed by the high frequencies (4KHz & 8KHz) then the mid frequencies (1KHz & 2KHz). Our mean airbone gaps in group (1) were: 31.61, 24.19; 17.9 & 24.68dBHL at frequencies 500, 1000, 2000 & 4000Hz respectively. Mehta et al., [25] and Voss et al., [26] also demonstrated that the tympanic membrane perforation cause the frequency dependant hearing loss that is greatest at low frequencies. Regarding the location of perforation, whether anterior or posterior, there was no statistically significant difference between both locations as regards the degree of hearing loss. This was in accordance with Mehta et al., [25] and Voss et al., [26] who stated that there were no differences in sound pressure when similar sized perforations at different locations in the same ear were compared. Thus there is no evidence that perforation location affects either the magnitude or angle of the sound pressures acting on the oval or round windows. They stated that this experimental result was consistent with a theoretical argument that the pressures near the oval and round windows should not be affected by perforation location. The wavelengths of audible sound are much greater than the dimensions of the tympanic membrane, and thus, spatial variations in the middle ear air space pressure that result from perforation location are expected to be minimal. (At 1 00Hz and 10,000Hz the wavelengths of sound are 3,400mm and 34mm, respectively. Perforations at different locations are much closer together than these wavelengths, as the largest dimension across the tympanic membrane is only 1 0mm). On the other hand, Durko et al., [27] showed that hearing loss in perforations involving posteriorinferior quadrant was found to be up to 30dB while in the rest of central perforations an average of 20dB conductive hearing loss was found. Berger et al., [28] in his study over 120 cases also found that of all locations, perforations involving the posterior-inferior quadrant of the ear drum were associated with largest air-bone gap. Nepal et al., [29] also found that out of his studied 100 cases, 64 cases involving posterior-inferior quadrant, 50.0% (32 cases) had moderate-41-53db conductive hearing loss and in the remaining cases, 39.0% (25cases) had mild-26-40db and 11.0% (7cases) had minmal-16-25db hearing loss. When the perforation overlies the round window in posteriorinferior quadrant the hydraulic advantage produced by tympanic membrane on oval window disappears, so that sound reaches both the windows more or less at the same time with equal force and at nearly equal time. The resultant cancellation of the vibratory movement of the cochlear fluid column produces the maximum hearing loss observed in even

7 Mohamed T. Ghannoum, et al. 85 small perforations overlying posterior-inferior quadrant. In frequencies below 2000Hz, the hearing losses were found to be more by 10-15dB on average in comparison to Hz, which is statistically significant. Comparing the RECD of both groups, there was a statistically significant difference of RECD values at frequencies 250Hz, 500Hz and 750Hz, between both groups. The RECD was 3 to 6dB lower in group 1 than in group 2. Our result was in accordance with Liu and Lin, [30] who reported that the RECD was up to 8dB smaller in the frequency range from 500Hz to 1000Hz. Fikret-Pasa and Revit, [31] also demonstrated a lower RECD response in the lower frequencies from the subject having a perforated tympanic membrane compared with other normal subjects in their study. Martin et al., [23] who studied RECD in patent Grommets, also reported that there was a statistically significant decrease in the RECD transform function at lower frequencies. Our findings were in agreement with Martin et al., [24] who reported that the RECD transform value of the patients with perforated tympanic membrane was 9-12dB lower than the corresponding subjects with intact tympanic membrane at audiometric frequencies below 1.5KHz. Martin et al., [23,24] gave 2 possible explanations for lower RECD in tympanic membrane perforation. Firstly they stated that this difference is probably due to the perforation acting as a vent and allowing low frequency acoustic energy to escape into the middle ear cavity. Secondly, the impedance of the middle ear cavity is decreased because the air is no longer compressed and expanded during ear drum motion. The stiffness of the middle ear cavity inversely affects the transmission of low frequency sounds through the cavity. Therefore reduced stiffness will cause more low frequency sounds to be transmitted into the middle ear cavity. These 2 theories can explain our results in the present study that revealed a significantly decreased RECD at lower frequencies in patients with ear drum perforation compared to the controls. There was a statistically significant difference as regards the RECD values at frequencies: 250Hz, 500Hz, 750Hz and 1 000Hz between right ears with perforated tympanic membranes and right control ears. Also there was a statistically significant difference as regards the RECD values at frequencies: 250Hz, 500Hz and 750Hz between left ears with perforated tympanic membranes and left control ears. There was no statistically significant difference of RECD values at different frequencies between right and left sided perforation in group (1). This was in accordance with Munro and Bullfield, [32] who reported that the mean difference between right and left RECD was close to 0dB for all ear mold configurations and there was no statistically significant difference on repeated measures analysis of variance. This also agreed with Munro and Hatton, [15] who used the RECD from right ear of 21 adult participants to derive the realear response of the left ear. The mean difference between the measured and RECD-derived response in the left ear was close to 0dB indicating that, on the average, using the RECD from the opposite ear was valid. Regarding the location of the perforation in group 1, whether anterior or posterior, there was no statistically significant difference between both locations as regards the RECD. This was in agreement with Martin et al., [24] who found no obvious relationship between the location of the perforation and the RECD. Comparing RECD values at different frequencies according to the gender of the patients in the study group (group 1); there was no statistically significant difference between males and females values at any tested frequency except at 1500Hz only. This might be related to anatomical variations between both genders and ear canal acoustics, but needs further investigations to explain it. When correlating the RECD to the age of the patients in group (1), there was no statistically significant correlation between the RECD and age. Previous studies by Berger et al., [28] ; Mehta et al., [25] and Voss et al., [26] have demonstrated that hearing loss increases as the size of the perforation increases and hence the air-bone gap increases as the size of the perforation increases. And since we were not able to measure the exact size of the perforation, we took the air-bone gap as indicator of the perforation size. There was no statistically significant correlation between the RECD values and the air-bone gap in group (1). However, all our cases had more or less medium sized perforation as was clinically seen on ototoscopic examination. Liu and Lin, [30] showed that the effect of perforation size on RECD reduction was not significant. On the contrary Martin et al., [24] reported that there was negative correlation between RECD and the air-bone gap at frequencies 500Hz, 1 000Hz

8 86 The Effect of Tympanic Membrane Perforation on Real-Ear to Coupler and 4000Hz with a larger air-bone gap resulting in a more negative RECD. Moryl et al., [33] showed reduced real-ear unaided response in 1 patient with a large tympanic membrane perforation. Liu and Lin [30] did not find any significant correlation between the ear canal volume in ears with perforated tympanic membrane and depth of trough in the RECD (the difference in db in the most negative and most positive RECD in the frequency range of 250Hz and 1500Hz. They suggested that ear canal volume did not appear as clinically useful predictor for RECD at lower frequencies in patients with ear drum perforation. Martin et al., [24] did not investigate the relationship between the ear canal volume and RECD because it was not possible to obtain a seal on tympanometry in most subjects. Conclusion: RECD in patients with tympanic membrane perforation is approximately 3-6dB lower than in normal at low frequencies (250Hz-750Hz). RECD is not related to the site (anterior or posterior) or side (right or left) of the perforation. RECD is not correlated to the air-bone gap of the audiogram. RECD is not related to age or gender of the patients. Recommendation: We recommend that this discrepancy should be compensated for if average normal RECD are used in the pre-selection of target and gain to avoid under-amplification at lower frequencies. However, the large inter-subject variation strongly suggests the need for individual RECD measurements whenever possible in fitting aids for patients having tympanic membrane perforations. References 1- HOWARD M.L.: Middle ear, Tympanic membrane perforations; at www: emedicine.com/ent/topic 206 htm GOEBEL J., VALENTE M., VALENTE M., ENTRIETTO J., LAYTON K.M. and WALLACE M.S.: Fitting Strategies for patients with conductive or mixed hearing loss. In: Strategies for selecting and verifying hearing aid fittings. Valente M. (editor); second edition, Thieme publishing, New York, Chapter 3: p , LYBERGER S.F.: Basic Manual for fitting Radio Ear Hearing Aids. Pittsburgh, PA: Radio Ear Corp, LYBARGER S.F.: Simplified fitting system for hearing aids. Cononsberg PA: Radio Ear Corp, BERGER K.W., HAGBERG E.N. and RANE R.L.: Prescription of Hearing Aids: Rationale, Procedures, and Results. 5th ed. Kent, OH: Herald Publishing, BYRNE D. and DILLON H.: The National Acoustic Laboratory (NAL) new procedure for selecting the gain and frequency response of a hearing aid. Ear Hear, 7: , COX R.: Using ULCL measures to find frequency/gain and SSPL90. Hear Instrum, 34: 17-22, MCCANDLESS G. and LYREGAARD P.: Prescription of gain/output (POGO) for hearing aids. Hear Instrum, 34: 16-21, LIBBY E.R.: The 1/3-2/3 insertion gain hearing aid selection guide. Hear Instrum, 37: 27-28, VALENTE M.: Strategies for selecting and verifying hearing aid fitting. Fitting stratigies for patients with conductive or mixed hearing loss. 2nd edition, New York Theime, Chapter 10: p , BOHNERT A. and BRANTZEN P.: RECD and clinical verification in children. May 2004; 11 (5): 50-52; Experiences when fitting children with a digital directional Hearing Aid. Feb., 11 (2): 50-55, PUMFORD J. and SINCLIAR S.: Real-Ear Measurement: Basic Terminology and Procedures, at www: audiologyonline.com, MOODIE K.S., SEEWALD R.C. and SINCLAIR S.T.: Procedure for predicting real-ear hearing aid performance in young children. Am. J. Audiol., 3 (1): 23-31, SEEWALD R.C., MOODIE K.S., SINCLAIR S.T. and SCOLLIE S.: Predictive validity for a procedure for paediatric hearing instrument fitting. Am. J. Audiol., 8 (2): , MUNRO K.J. and HATTON N.: Customized acoustic transform functions and their accuracy at predicting realear hearing aid performance. Ear Hearing, 21: 59-69, FEIGIN J., KOPUN J., STELMACHOWICZ P. and GOR- GA M.: Probe-tube microphone measures of ear canal sound pressure levels in infants and children. Ear and Hearing, 10 (4): , BAGATTO M.P., SCOLLIE S.D., SEEWALD R.C., MOODIE K.S. and HOOVER B.M.: Real-ear-to-coupler difference (RECD) predictions as a function of age for two coupling procedures. J. Am. Acad. Audiol., 13 (8): , DILLON H.: Electroacoustic Performance and Measurement. In: Hearing Aids; Dillon H. (editor); first edition, Thieme Publishing, Chap. 4, p , MARTIN H., MUNRO K. and LANGER D.: Real-ear to coupler differences in children with grommets. British Journal of Audiology, 31 (1): 63-69, SOLIMAN S.: Development of SSW test. Paper presented at 8th, Ain-Shams Annual Congress, SOLIMAN S.: Speech discrimination audiometry using Arabic phonetically balanced words. Ain Shamas Med. J., 27: 27-30, WESTWOOD G.F. and BAMFORD J.M.: Probe-tube microphone measures with very young infants: Real ear

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