Question
Can I do ABR using bone conduction, and how would I set the parameters?
Answer
Yes, it is possible to measure ABR with bone conduction transducer. They can be easy to collect if you are using the appropriate parameters, but you need to keep in mind that your waveforms will not be as pretty as your upper level air-conduction ABRs. With the maximum output of the B-70 Bone Vibrator at about 50-55 dB nHL, you are likely measuring a response that is closer to threshold than for Air Conduction thresholds using 75 or 80 dB nHL. Keeping that in mind, the following parameters can be used to acquire reliable bone conduction ABR waveforms.
Electrode Montage
Ideally, you should make a 2 channel recording. The electrode placement should be high forehead for the non-inverting electrode linked (via a jumper cable) to the positive inputs for channel 1 and 2. The channel 1 and 2 inverting electrodes should be placed on the earlobe of each ear. You can use an electrode placed on the low forehead for ground.
Stimulus Parameters
You can use either click or tone burst stimuli. We recommend presenting the stimulus at a lower rate, approximately 7.1/sec. We also recommend an alternating stimulus to average out the stimulus artifact. The Bone Vibrator will emit a large amount of electromagnetic energy and it is not possible to physically separate the electrode recording site from the bone conduction stimulation site. The stimulus artifact will be so large that it will reduce the scale of the waveform making the response difficult to see, using an alternating stimulus should reduce the stimulus artifact. A contralateral recording can help identify the ear that is responding to the stimulus. Verifying a Wave I on the ipsilateral side ensures that you are recording a response from your test ear. If you cannot see a wave I on the ipsilateral ear, then you need to apply masking to rule out a response from the non-test ear.
Recording Parameters
The Bone Conduction ABR waveform characteristically has more low frequency energy than the air conduction response; therefore, you need to use 30-2000 filter settings. You also may want to increase the number of averages due to the lower level of stimulation. All other recording parameters can be set as you would for air conduction ABRs.
The Response
Because of the level of stimulation, your wave forms will have lower amplitudes and will probably be more rounded. Your bone conduction response at maximum output (50-55 dB nHL) should be similar to a response from air conduction stimulation at 50 dB nHL for a normal hearing person or person with a mild sensorineural hearing loss. If your patient has a conductive hearing loss, you should see that the latencies for the air conduction response are shifted out in time as compared with the bone conduction latencies.
Ellen Snodgrass graduated with an M.A. in Audiology from the University of Montana in 1985. She worked at the University of Kansas Medical Center on the Children's Rehabilitation Unit, then at Nicolet Biomedical Corporation as a research audiologist on Project Phoenix. She has also worked in the Neurology Department at the University of Wisconsin as a research Audiologist performing ABR and OAE testing for studies. Ellen took time off from Audiology to spend time with her children while they were young, and recently returned to Audiology as an Application Specialist at VIASYS Neurocare.
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