Why Perform Bone Conduction ABR?

In this video, you will learn why it is important to perform bone conduction auditory brainstem response (ABR) testing, and how best to set up for bone conduction testing, with some tips and suggestions for recording the most accurate results. You can read the full transcript below.

Why perform bone conduction ABR?

It can be helpful to consider ABR testing in the same way as pure tone audiometry or behavioral audiometry. When testing newborn infants, the goal is usually to establish hearing thresholds, and testing is generally started using air conduction, either headphones or insert earphones.

When performing ABR testing using air conduction, the stimulus presented has to travel along the full auditory pathway. This means traveling along the ear canal, through the tympanic membrane, through the middle ear ossicles, the malleus, incus and stapes, and into the cochlea via the oval window.

If normal hearing is established at this point, then bone conduction testing is not required. However, if a hearing loss or raised hearing levels are found, then there are some questions that need answering.

Based only on air conduction testing, it is not possible to establish where the source of the hearing loss is. At this point, all that is known is that there is something causing a response to be recordable only when the stimulus is made louder. Identifying what that is requires further testing.

Using pure tone audiometry or auditory brain stem response testing, it is possible to combine the components of the auditory pathway into two separate areas. The middle ear and the inner ear.

There is also the outer ear to be aware of. But typically, any abnormalities relating to the outer ear are identified by otoscopic inspection rather than audiometry or ABR testing.

Tympanometry is a popular and valuable test of middle ear function, and in particular, wideband tympanometry can help identify specific problems relating to the middle ear in detail. However, abnormal tympanometry results combined with raised air conduction ABR thresholds are not sufficient to confirm that the hearing loss is being caused by the middle ear problem.

It is possible and not uncommon, particularly in young infants, where ABR testing is most often used to have a combination of pathologies. An issue relating to the inner ear can be easily hidden by the presence of middle ear dysfunction, such as middle ear effusion. Making an interpretation and diagnosis based on the results of tympanometry and air conduction alone presents a risk of missing an underlying or sensorineural hearing loss.

By performing bone conduction ABR testing, it is possible to bypass the middle ear and isolate the inner ear and record threshold information about the underlying sensory neural function.

How to perform bone conduction ABR

Performing bone conduction ABR testing can seem daunting at first. However, it is actually very similar to performing air conduction ABR testing. The main difference is obviously using a different transducer.

It is essential to ensure your bone conductor is connected to the Eclipse hardware correctly and calibrated annually to the appropriate standards. There are two bone conductors available for testing using the Eclipse: B-71 and B-81. The latter allows for testing at louder intensities.

When testing adults or older children via bone conduction, it is typical to leave the bone conductor attached to the headband and simply place this over the head as you would in normal pure tone audiometry. However, in babies or very young infants, the headband is usually too large and applies an uncomfortable amount of pressure that can cause the patient to become unsettled.

A simple solution is to remove the bone conductor box itself from the headband, which is easy to do, and then apply finger pressure to the box to hold it in place against the mastoid.

There are some important considerations to be aware of when performing bone conduction testing that can help you to record the best results.

Positioning the bone conductor on a bony portion of the skull will help to deliver the stimulus to the cochlea accurately. If it's placed on a spongier part of the head, the stimulus will not be transmitted as easily, and your results may be affected. The mastoid is the ideal placement as it is close to the cochlea.

Similarly, it is important to try and avoid placement on top of any hair. This can also affect the delivery of the stimulus. It can help to try sweeping some of the hair out of the way before placing the bone conductor in position.

Ensure that the bone conductor does not touch the electrode or the pinna, as this can introduce stimulus artifact. It can help to use a separate finger to hold the pin up out of the way. Ideally, you want at least a finger's width between the electrode and the bone conductor.

Once you are in a good position, make sure you keep an eye on your patient and your bone conductor placement. It can be easy for your hand to slip or the baby to move without you realizing, particularly if you are focused on the results collection in the software.

Always monitor your bone conduction placement and try to maintain the same positioning throughout testing to ensure accurate results.