THRESHOLD PREDICTION USING THE ASSR AND THE TONE BURST CONFIGURATIONS

Transcription

1 THRESHOLD PREDICTION USING THE ASSR AND THE TONE BURST ABR IN DIFFERENT AUDIOMETRIC CONFIGURATIONS INTRODUCTION

2 INTRODUCTION Evoked potential testing is critical in the determination of audiologic thresholds in difficult to test populations. Tone burst Auditory Brainstem Response (tb ABR) and Auditory Steady StateState Response (ASSR) are the main electrophysiological approaches that help in estimating the thresholds and predicting the configuration of hearing loss without the patient s co operation. operation. (Kumaret et. al, 2008) INTRODUCTION Tone burst rstabr It comprises a transient pure tone signal of the desired audiometric frequency that has a defined rise time, plateau and fall time.

3 INTRODUCTION The tone burst stimulus is characterized by rapid onset and short duration resulting in a relatively broad spectrum in thefrequency domain. The frequency spectrum of the tone burst shows a main lobe of energy centred on the designated frequency, and side lobes of energy at lower and higher frequencies. (Johnson andbrown Brown, 2005) INTRODUCTION Acoustic spectra for 5-cycle 1000 Hz Cosine gated tdtones. Adapted from Linkwitz 1978

4 INTRODUCTION Gating function Sinusoids are gated with linear or non linear complex functions (e.g. Blackman function). Blackman gated tonal stimuli maintain a relatively narrow spectrum even with the useof a fast rise time through reducing the spectral splatter and thus producing excitation ti of a more narrowly defined cochlear region. (Gorga, 1989) INTRODUCTION Adapted from Stapells and Oates 1997 Acoustic spectra for 5-cycle 500 and 2000 Hz linear versus non-linear (Blackman) gated tones. Spectra for Blackman gated tones are indicated d by the darker lines while the lighter lines show spectra for linear gated tones.

5 INTRODUCTION Tone burst rstabr morphology Tone burst ABR waveform is usually broad and rounded and consists primarily of wave V and the negativity following wave V. The I to IV ABR waves are not typically seen except in response to high intensity intensity, high frequency tones. The tone burst ABR occurs at later latencies than clickevoked ABR, moreover, the responses to low frequency tones are later than those to higher frequency tones presented at the same intensity. (Stapells, 2000) INTRODUCTION Adapted fromvander Werff et al.2002 Auditory brainstem response waveforms from two subjects. The left panel shows a click ABR series from one subject whose threshold was determined to be 10 db nhl. The right panel shows the 500 Hz ABR from another subject, whose threshold was 55 db nhl.

6 INTRODUCTION Limitations of tone burst ABR Inappropriate interpretation of waveforms: As there are no automatic detection criteria for detecting the response to tonal stimuli, clinicians interpret the waveforms visually causing a bias in threshold interpretation. (Stapells, 2000) Output limitations: The short duration ABR stimuli need higher input voltages to the earphones in order to produce threshold level responses, however the transducer s s restriction, typically limits the stimulus output to 100 db nhl. (Vander Werff, 2002) INTRODUCTION Auditory Steady State Response ASSRs are continuous, periodic potentials that arise in response to regularly varying AM and/or FM sinusoidal auditory stimuli.

7 INTRODUCTION AM stimuliareveryfrequency specific; their acoustic spectra show energy at the CF plus two sidebands at frequencies equal to the CF plus/minus the frequency of modulation(mf). When FM is used in combination with AM, there is some spread of acoustic energy encompassing the amount of MF, consequently ASSR amplitude gets larger. (John, 2001) INTRODUCTION Adapted from Stapells et al Stimuli used to evoke ASSR. The stimulus waveforms are plotted in the time domain on the left and the spectra of the stimuli are shown on the right.

8 INTRODUCTION Auditory Steady State Response The response of ASSR has energy at the frequency of modulation (MF) and its harmonics, representing the synchronous discharge of auditory neurons in the brain stem, which is phase locked to the MF of the stimulus.(picton et al.,2003) INTRODUCTION FFT of the response at the MF allows for the amplitude and phase of these responses to be statistically detected. (Picton et al.,2003)

9 INTRODUCTION Frequency enc of modulation ASSRcanbeobtainedforalargerangeof MFs from 4 to 450 Hz. 40 Hz ASSR is used to obtain good estimations of behavioural thresholds but is affected by sleep or sedation due to cortical origins resulting in elevated and less reliable thresholds from infants. Modulation rates above 70 Hz allow for responses to be consistently recorded during sleep and at low SPL because they originate from the brainstem. (Cone Wesson 2002) INTRODUCTION Advantages of ASSR The potential advantages of ASSR come from the objective nature of response determination and the continuous character of the stimuli causing potentially better frequency specificity and the ability to obtain higher output levels. These are primary reasons for the current upsurge These are primary reasons for the current upsurge in clinical interest in the ASSR. (Stapells et al., 2004)

10 INTRODUCTION Limitation of ASSR Artifactual ASSRs to high intensity low frequency stimuli in individuals with profound hearing loss are occasionally recorded. However in relaxed subjects and on using high frequency stimuli no such responses were reported. Those artifactual responses appear to be physiologic but of non auditory origin, most probably vestibular in origin. So far there is no method to differentiate the nonauditory from the auditory responses. (Swanepoel,2004) INTRODUCTION Since tone burst ABR and ASSR are the two commonly used approaches that help us estimate the thresholds at any given frequency and predict the configuration of hearing loss, having their own merits and demerits, further studies are done to compare the threshold estimation using both techniques.

11 AIM OF THE WORK AIMOF THEWORK Check the efficacy of estimating hearing thresholds by tone burst ABR and ASSR. Assess whether ASSR could compete with tone burst ABR in the assessment of adults with normal hearing and those with different configurations of SNHL.

12 MATERIALS AND METHODS MATERIALS This study wasconducted d on 50 ears of 50 adult subjects attending the Audiology Unit, E.N.T. Department, Alexandria University Hospital. The better hearing ear was selected for assessment. The subjects were divided into 5groupsof: I. 10 ears with normal hearing II. 10 ears with flat SNHL III. 10 ears with high frequency sloping SNHL IV. 10 ears with highfrequency steeping SNHL V. 10 ears with low frequency SNHL

13 METHODS All subjects in the study were subjected to the following procedures: I. Clinical examination: history taking and otological examination. II. Basic audiological evaluation: 1. Pure tone audiometry Was obtained at frequencies of 250, 500, 1000, 2000, 4000 and 8000 Hz. 2. Speech audiometry 3. Acoustic immittance measures METHODS III. Electrophysiological l i l tests: t 1. Toneburst auditory brainstem response 2. Auditory steady state response In both tests one channel recording was obtained using ipsilateral electrode montage. The active electrode was placed on the high forehead, the reference electrode on the mastoid of the stimulated ear and the ground electrode on the contralateral mastoid. The recording time was measured for each test.

14 METHODS METHODS Stimulus parameters: 500, 1000, 2000 and 4000 Hz tone burst stimuli Behaviourally calibrated in db nhl Blackman gated with cycles Repetition rate of 41.9 stimuli/sec Alternating polarity 100 db nhl maximum stimulation i levell Tone burst ABR parameters Recording parameters: Hz BP filter 100,000 gain Artifact rejection of 31 µv 12.5 msec time window 1024 sweeps averaged and replicated at least once. Threshold was defined as the lowest stimulus level at which wave V was visually detected

15 wave V latency in msec latency in msec Wave V l LIF of wave V in Normal group Intensity in dbnhl in msec Wave V latency LIF of wave V in steeping HL group Intensity in dbnhl LIF of wave V in flat HL group wave V la atency in msec LIF of wave V in sloping HL group Intensity in dbnhl Intensity in dbnhl Intensity in dbnhl ec Wave V latency in mse LIF of wave V in low frequency HL group METHODS Stimulus parameters: Stimuli with CF at 500, 1000, 2000 and 4000 Hz presented monaurally and individually. Calibrated in db HL 100% AM and 10% FM Modulated at rates of 74, 81, 88 and 95 Hz, respectively. 120 db HL maximum stimulation level. ASSR Parameters Recording parameters: Hz BP filter 64 samples were statistically analysed by phase coherence. If (p < 0.03) a response was considered to be present Threshold was defined as the lowest stimulus level at which a statistically significant response could be obtained (phase locked)

16 METHODS METHODS Statistical Procedures Pearson product moment correlations between pure tone threshold and evoked potential threshold across the 4 frequencies and between tone burst ABR threshold and ASSR threshold were assessed. Difference score values were obtained by subtracting the pure tone thresholds from the tb ABR or ASSR thresholds at each frequency for each group.

17 METHODS Two way ANOVA compared the frequency effect, group effect and frequency by groupinteractionon the tb ABR and the ASSR difference score results separately. Student t test was used for direct comparison of tb ABR versus ASSR difference score values across frequencies in different groups. ASSR and ABR recording times were compared by t test. RESULTS

18 RESULTS Mean and SD of pure tone, tb ABR and ASSR thresholds RESULTS PTA, ASSR and 500 Hz tb ABR of a normal hearing subject

19 RESULTS PTA, ASSR and 2000 Hz tb ABR of a subject with flat HL RESULTS PTA, ASSR and 4000 Hz tb ABR of a subject with sloping HL

20 RESULTS PTA, ASSR and 1000 Hz tb ABR of a subject with steeping HL RESULTS PTA, ASSR and 500 Hz tb ABR of a subject with ihlow frequency HL

21 Correlation between tb ABR/ASSR and pure tone thresholds Pure tone threshold y = x r = TB ABR 500 HZ Pure tone threshold 120 y = x r = ASSR 500 HZ Pure tone thresh hold y = x r = Pure tone thresh hold y = x r = TB ABR 1000 HZ ASSR 1000 HZ Pure tone th hreshold y = x r = Pure tone th hreshold y = x r = TB ABR 2000 HZ ASSR 2000 HZ Pure to one threshold y = x r = Pure to one threshold y = 1.045x r = TB ABR ASSR 4000 HZ 4000 HZ RESULTS Correlation between tb ABR and ASSR thresholds

22 RESULTS Tb ABR to behavioural mean difference score ranged from 15 to 24 db in normal hearing subjects and from 10 to 17 db in hearing impaired subjects. Two way ANOVA showed significant difference between subject groups (normal versus HL)and significance variation by test frequency (500 Hz versus 1000 Hz), but there was no statistically significant interaction between group and frequency. RESULTS ASSR to behavioural mean difference score ranged from 19.5 to 25 db in normal hearing subjects and from 13 to 25 db in hearing impaired subjects. Two way ANOVA showed significant difference between subject groups (normal versus HL)but was non significant for test frequency and there was no statistically significant interaction between these factors.

23 RESULTS Comparison between tb ABR and ASSR difference scores at 500Hz 30 TB ABR ASSR 25 Mean Differ rence Score (d db) Normal Flat HL Sloping high Steeping high Low frequency frequency HL frequency HL HL RESULTS Comparison between tb ABR and ASSR difference scores at 1000Hz nce Score (db B) * TB ABR ASSR Mean Differe Normal Flat HL Sloping high frequency HL Steeping high frequency HL Low frequency HL

24 RESULTS Comparison between tb ABR and ASSR difference scores at 2000Hz 25 TB ABR ASSR Mean Differ rence Score (d db) Normal Flat HL Sloping high frequency HL Steeping high frequency HL Low frequency HL RESULTS Comparison between tb ABR and ASSR difference scores at 4000Hz * 25 TB ABR ASSR Mean Differe ence Score (db B) Normal Flat HL Sloping high frequency HL Steeping high frequency HL Low frequency HL

25 RESULTS Comparison between recording times of tb ABR and ASSR 35 TB ABR ASSR 30 Tim me in minutes Normal Flat HL Sloping high Steeping high Low frequency frequency HL frequency HL HL CONCLUSION

26 CONCLUSION Both tone burst ABR and ASSR were found to correlate well with behavioural thresholds for all carrier frequencies and audiometric configurations. Both testst hd had poorer predictions of normal hearing audiograms. So to overcome the discrepancy in test results between normal and hearing loss regions, linear equations are found to be more appropriate than a constant correction factor. CONCLUSION Both tests had higher correlations with the higher frequencies compared to the lower frequencies. ASSR however, was superior to tone burst ABR in its sensitivity to the 500 Hz lower frequency stimulus. Both tests could efficiently predict the configuration of the audiogram, however, ASSR was found to be less affected by the steepness of the audiogram.

27 CONCLUSION High correlations between the ASSR and the pure tone audiogram and between the ASSR and the tone burst ABR suggest that ASSR may present a reasonable alternative to tone burst ABR for frequency specific audiometry. ASSR had overall shorter recording time than the tone burst ABR and thus is practical for clinical use. ASSR had the additional advantage of automated objective response detection and higherh output limits allowing for measuring residual hearing in severe to profound individuals. RECOMMENDATIONS

28 RECOMMENDATIONS It is recommended to apply ASSR measurement in preference to tone burst ABR when collecting data on peripheral hearing loss whenever an objective measure with realistic recording time is needed for difficult to test t t t populations. lti