Bone-conduction auditory brainstem response (BCABR) is similar to air-conduction auditory brainstem responses, with the main difference being that the signal is transmitted via bone-conduction instead of air. The goal of bone ABR is to estimate cochlear function and to help identify the type of hearing loss present. Responses to air and bone-conduction ABRs are compared (for the same intensity and stimuli).
Techniques and results for bone-conduction auditory brainstem responses are presented in a review chapter by Stapells, as well as in a detailed assessment protocol by the British Columbia Early Hearing Program (BCEHP).
BC ABR For Infants.
When Is BCABR Needed?
Any infant showing elevated ABR thresholds to air-conduction stimuli should be tested using bone-conduction stimuli. Atresia, microtia, otitis media, and other outer/middle ear abnormalities, as well as infants with sensorineural hearing loss, will require the use of bone-conduction ABR testing. Infants who have a considerable amount of amniotic fluid in their middle ear space may need to be tested with BCABR. This fluid usually disappears by 48 hours after birth.
Problems With BCABR
It is very common for there to be a large amount of artifact while using bone-conduction ABR. This is especially true at high intensities (~50 dB nHL) and at earlier waves (I.e. Wave I). To avoid stimulus artifact, it is recommended that the bone oscillator be placed high on the temporal bone and that the inverting electrode is placed on the earlobe, mastoid, or nape of the neck. Using an alternating phase stimuli should be used to reduce artifacts. Since the output of most bone oscillators is around 45 to 55 dB nHL, it becomes difficult to distinguish between sensorineural or mixed hearing losses when the losses by bone exceed this number. This output limitation of the bone oscillator is a drawback.
With Bone ABR, the waves are typically more rounded that with traditional auditory brainstem response. The maximum output for bone is around 50 dB nHL and should look similar to the 50 dB HL response of air conduction for people with normal hearing or with mild SNHL. With conductive hearing losses, the latencies for air are shifted when compared to the latencies of bone conduction.
Mauldin & Jerger (1979) found that for adults, the Wave V latencies derived from bone-conduction ABR are approximately 0.5 ms longer than the same intensity level of air conduction. For infants, Wave V latencies for bone-conduction clicks are shorter than the air conduction clicks. These differences can be attributed to changes to the skull due to aging.
BCABR With Tone Bursts
As with air-conduction stimuli, thresholds for bone-conduction stimuli should be obtained using tone burst stimuli Stapells is one researcher who reported on the accuracy of using tone bursts with BC ABR to estimate cochlear hearing sensitivity. Stapells and Ruben, in 1989, demonstrated bone-conduction tone burst ABRs in infants with conductive hearing loss. Hatton, Janssen, and Stapells (2012) present bone-conduction tone burst ABR results in infants with normal bone-conduction thresholds or sensorineural hearing loss. BC ABR methods are described in the 2010 review chapter by Stapells.