Question
Do you have any normative data on bone conduction tone burst ABR by frequency (500, 2,4k Hz) for diagnosing hearing loss in newborns? I need to build our own norms, but would like to see what others have found. I would like collection parameters also.
Answer
Before you administer bone-conduction ABR stimuli, you will need to establish dBnHL for the stimuli you plan to use. This process will need to be done with 10 to 20 older children and/or adults. The transduction characteristics of infant heads are different from older children and adults. However, if you couple the bone-conduction transducer with 400 to 450 g of force, you will be appropriate. Several reports suggest the use of a headband and a spring scale. We (and others) have found that carefully positioning the metal headband seems to work well. You can use a folded washcloth to cover the band ending opposite to the bone vibrator to cushion the head. Others use a headband or handheld placement. The handheld placement is difficult because the variability in pressure throughout the measure may obscure results. You can make measures of click stimuli or toneburst stimuli. A set of parameters that you might try is as follows:
Bone Conduction Clicks: B-70 oscillator, click duration of 0.1ms, ramping (windowing) is transient, maximum intensity will be between 40 to 55 dB nHL, alternating polarity, rate of 11.1 per second, Filters from 30 to 3000 Hz, time window of 15 ms, number of sweeps is dependent on signal to noise ratio. For frequency stimuli a 2-1-2 toneburst (2 cycle rise, 1 cycle plateau, 2 cycle fall) with rarefaction has worked for us. We have had success in measuring cochlear reserve in infants with middle ear pathology and aural atresia, etc. We have found it difficult to establish clinically effective masking levels. If you see wave I, you know which ear is being stimulated. Another technique for determining the test ear is to compare the wave V latency for the ipsilateral and contralateral traces. The wave V amplitudes should be larger in the ipsilateral channel relative to the contralateral channel. If you are interested, an alternative approach is with SAL (sensorineural acuity level). With SAL, the efficiency of a bone-conducted noise in masking an air-conducted stimulus is used to estimate sensory-neural reserve. We have not used SAL at our institution.
Roanne K. Karzon, Ph.D. CCC-A has been an audiologist for 27 years. She is the Manager of Audiology at St. Louis Children's Hospital and resides in St. Louis, Missouri. She can be reached at roannekk@bjc.org