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Electroacoustic Limitations in Fitting Children with Severe to Profound Hearing Loss Shane Moodie, M.Cl.Sc. Clinical Supervisor in Audiology School of Communication Sciences and Disorders, University of Western Ontario [email protected] Children with severe to profound hearing loss often present as our most difficult population in which to provide appropriate amplification Limiting Factors would include: a) Overall degree and configuration (slope) of the hearing loss b) Changes in the ear acoustics related to growth/aging c) Electroacoustic performance (limitations) of hearing instruments We are fairly knowledgeable about the affects of degree of loss and ear acoustics on amplification requirements. (DSL) However, we are not always certain of the abilities of hearing instruments to provide audibility of amplified long-term average speech spectrum (LTASS) for a given hearing loss. We should know that a definition of hearing in sound pressure level (SPL) is dependent on both: a) The minimal response levels provided by the child in HL b) The unique acoustic transform provided by the child’s ear (and Ear Mould) Frequency (Hz) 250 500 1000 2000 4000 6000 8000 -10 0 10 Hearing Threshold Level (dB) 20 30 40 50 60 70 80 90 100 110 120 If at 8 months of age the measured thresholds in HL are these. And at 8 months of age the ear and ear mould acoustics are represented by this RECD If you know this And you have measured this You can predict this Dial Reading HL + RETSPL+ Acoustic Transform = Threshold in SPL They will define these thresholds and auditory area in SPL Frequency (Hz) 250 500 1000 2000 4000 6000 8000 -10 0 10 Hearing Threshold Level (dB) 20 30 You can predict this for average And you assume this is average If you know this 40 50 Dial Reading HL + RETSPL+ Acoustic Transform = Threshold in SPL 60 70 80 90 100 110 120 They will also provide this predicted HL audiogram And generate these targets for coupler gain for speech Target coupler response =REAGT – mic - RECD Ear acoustics are an important developmental factor They will change as the child grows and can have important implications on fitting. And independent of a change in hearing levels, will require that the hearing instrument gain and output be modified. That is, as the ear grows we have to increase the output into the ear! Frequency (Hz) 250 500 1000 2000 4000 6000 8000 -10 0 10 Hearing Threshold Level (dB) 20 30 40 50 60 70 80 90 100 110 120 The predicted HL audiogram An average adult RECD ? Adult 20 dB 8 months 2 cc coupler targets for amplified LTASS Consequently, with changes in ear acoustics The questions is not … can we meet the hearing needs of the 6 month old ? But rather … Can we meet the hearing needs of the child as they grow? In today’s world, many children with Permanent Childhood Hearing Impairment (PCHI) will be fitted by 6 months of age. They will wear their “first” hearing instruments for up to three years. Therefore, we need to account for potential growth in the ear canal and ear mould in the hearing instrument fit. With this in mind, if we want to determine the fitting range of a hearing instrument… the question is not … If I have a three year old with given ear acoustics …. what Hearing Instrument works for this hearing loss? BUT rather …. If I have a three year old with given ear acoustics… What loss works for this hearing instrument? Case Example outline: 1) Assume an average 3 year old RECD 2) Set hearing instrument to maximum at all frequencies 3) Set hearing instrument to maximum in mid to high frequency area and reduce lows as much as possible (Slope) 4) Determine what loss can be fitted (audibility of LTASS at all relevant audiometric frequencies) 4 Hearing Instruments: 1) Digital Power – WDRC 2) Digital Power – Output compression 3) Analogue Programmable Power – Output Compression 4) Mini Digital Power - WDRC 3 year old average RECD Bagatto et al. 2002 4 HI responses Frequency (Hz) 250 500 1000 2000 4000 6000 8000 -10 0 10 Hearing Threshold Level (dB) 20 HL Corrected audiogram (flat) for optimal match to targets 30 40 50 60 70 80 90 100 110 120 Limits for amplification Receiver Resonance Frequency (Hz) 250 500 1000 2000 4000 6000 8000 -10 0 10 Hearing Threshold Level (dB) 20 30 40 50 60 70 80 90 100 110 120 HL Corrected audiogram (sloping) for optimal match to targets Frequency (Hz) 250 500 1000 2000 4000 6000 8000 -10 0 10 Hearing Threshold Level (dB) 20 30 40 50 60 70 80 90 100 110 120 HL fitting range RECD = 0 Speech Input 140 130 120 110 Ear Canal SPL (dB) 100 90 80 70 60 50 40 30 20 10 0 100 1000 Frequency (Hz) 10000 Adult 8 months What the HI can provide for speech like input versus what we want Best Fit? HI gain for 50 dB SPL swept pure tone 53 50 53 59 57 47 Max Gain for Speech Gain for Speech is not the same as gain for Pure tone! Whether gain or output is measured, if we are going to predict audibility for speech, it has to be measured with speech! In the case of severe to profound hearing loss… Unless we can maintain the acoustical advantage of an infant’s ear…. It does not appear that we can optimize speech perception in the growing child beyond 2000 Hz. 15 Undamped Response, 36 mm tubing 10 5 0 -5 -10 -15 100 1000 1900 2800 3700 4600 5500 6400 7300 CFA#2 CFA#4 REVERSE HORN Ear mould modifications will only provide minimal benefit. 15 Damped Response, 36 mm tubing 10 5 0 ` -5 -10 -15 100 1000 1900 2800 3700 4600 5500 6400 7300 CFA#2 (damped) CFA#4 REVERSE HORN Summary 1) We are limited in our ability to provide high frequency amplification for children with severe to profound hearing loss, primarily due to the receiver resonance in the hearing instrument. Summary 2) High power for hearing instruments appears to mean higher output for low frequency sounds. Summary 3) Our ability to provide audibility of amplified high frequency speech sounds will decrease with changes in the ear and ear mould acoustics. Summary 4) Given compression, if we are to predict audibility for speech, we need to use speech input. Summary 5) We can not expect to enhance the high frequency response of a hearing instrument significantly through the use of an ear mould. Summary 6) This has important implications for counseling and planning treatment? - predicting the HL audiogram - understanding the limitations of hearing instruments - what you can provide to the infant, is not what you can provide to the 3 year old. - with severe to profound hearing loss, responsiveness will change with time.