Aided cortical testing – considerations when testing cochlear implant patients

An exciting application of the aided cortical test is for those patients who have been fitted with cochlear implants. This video will explore some important adjustments to the test technique, and elements of the waveform interpretation to be aware of when working with this group of patients.

You can read the full transcript below.

Aided cortical challenges in cochlear-implant patients

In addition to hearing aid users, patients who have been fitted with cochlear implants can also benefit from the aided cortical test.

Performing this test on cochlear implant patients comes with some unique challenges which need to be understood and considered.

The main challenge facing this cohort is the interference or radio frequency noise that can be generated by the device itself.

This can be detected by the recording electrodes, and lead to artefact in the waveform, which can make it difficult to accurately interpret whether a true response is present or not.

Studies on aided corticals in cochlear-implant patients

There are a number of published studies detailing the use of aided cortical responses in patients fitted with cochlear implants.

Kosaner et al tested 45 children who had been unilaterally implanted with MED-EL devices [1].

They found artefact in only 8 traces from 3 of the children tested.

Of the 108 adult participants tested by Távora-Vieira and colleagues and published in 2002, there were no reported artefacts from the unilaterally implanted MED-EL devices [2].

Another study by Távora-Vieira and colleagues, published in 2021, compared electronically evoked cortical potentials with acoustically evoked cortical potentials in unilaterally implanted adult patients using MED-EL devices [3].

Choi et al. reported a study of 20 participants in 2019 [4].

This adult cohort included unilateral and bilaterally implanted patients, who were all tested with just one implant switched on at a time.

These participants were fitted with a combination of Cochlear and Advanced Bionics devices, and although more artefacts were found in the AB devices, the artefacts produced by the Cochlear devices were larger in amplitude.

Identifying artefacts

It is essential that any clinician performing the aided cortical test with implanted patients is aware of the potential for artefacts being present in the waveform, knows how to identify these, and understands the impact that such artefacts can have upon the detection and interpretation of responses.

This set of waveforms was recorded from a young patient who had been fitted with a cochlear implant.

There is no apparent artefact in these traces and the morphology is very similar to that seen in hearing aid users.

We can see artefacts in this set of recordings.

Unfortunately, these artefacts can look quite similar to cortical responses.

Key warning signs that an artefact is present is the slightly triangular or squarish shape present in the waveform.

We can see the first positive inflection appears much earlier in latency than we would expect for a cortical response.

Here it is just after 0 milliseconds.

It is also likely to appear very quickly from the start of the recording.

Unfortunately, the Fmpi response detection algorithm is not able to differentiate between artefact and genuine response, and so in this case the Fmpi value was detected at over 95%, but it is not possible to be certain as to whether this was detected due to artefact, a genuine response being present, or a combination of both.

It is possible to examine any suspected artefacts by removing the display filter settings.

In this case we can see how the peak has become flat and square-shaped.

A true cortical response would not modify in this way.

Here we can see a cortical response without artefact.

When removing the high-pass filter setting in particular, there is very little change to the shape of the positive inflections.

When testing patients with cochlear implants, it is important to explore these elements of the waveform to help distinguish between genuine responses and artefacts.

Reducing artefacts

There have been a number of studies which have looked at different methods for reducing artefacts in cortical assessments.

These are summarized in a paper by Choi et al. from 2019 and largely focus on signal processing and analysis techniques [4].

There are, however, some steps that can be taken to minimize artefact in evoked potential recordings in the clinical test setup.

The main method for minimizing the likelihood of artefact in the waveform is to reduce the chance for electrical interference from the device to be detected by the recording electrodes.

The most successful way to achieve this is to simply increase the distance between the implant and the recording electrodes.

This can be done by opting for a three electrode montage with the reference electrode on the contralateral mastoid to the implant.

For those of you familiar with eABR testing, this is the same setup.

It is also possible to use a jumper cable with a 4-electrode cable collector, which effectively removes one of the electrodes from play and allows you to perform the testing with just 3 electrodes.

This is easily achievable when testing a unilaterally implanted patient, and is the methodology of choice in a majority of published research papers into aided cortical testing with cochlear implants.

For a bilaterally implanted patient, it is possible to explore testing with 4 electrodes and both implants switched on, but if artefact is present in the waveforms, then switching to unilateral assessment with three electrodes is the recommended process in these cases.

Which stimulus to use?

A further consideration when performing the aided cortical test on patients fitted with a cochlear implant surrounds which stimulus to use.

The ManU-IRU stimuli are a particularly appropriate choice for hearing aid users.

This stimulus set was designed with hearing aids in mind, hence calibration alignment with the ISTS.

Furthermore, these stimuli have been validated for use with hearing aids and data is available regarding the sensitivity of the test with these stimuli for hearing aid users from the Ladies in the Van study [5].

However, no such data currently exists regarding cochlear implants and the likelihood of being able to detect responses when performing the aided cortical test.

As such, caution is urged when interpreting the results of this test particularly for any absent responses.

For cochlear implant patients, until more data becomes available regarding the ManU-IRU stimuli, it may be more suitable to make use of the HD-Sounds.

The majority of research papers which have investigated cortical testing on implanted patients have used the /m/, /g/ and /t/ stimuli that were the predecessors to the HD-Sounds.

The unfiltered version of the HD-Sounds corresponds most closely to those original stimuli.

However, it is important to be aware of the poor frequency specificity of these sounds.

There are research papers which propose cochlear implant programming strategies based on how the electrodes correspond to these stimuli, and have explored methods for adjusting the T and C levels.

This is an exciting area for further research to build and develop the evidence base to support the clinical utility of the aided cortical test for cochlear implant patients.

References

[1] Kosaner, J., Van Dun, B., Yigit, O., Gultekin, M., & Bayguzina, S. (2018). Clinically recorded cortical auditory evoked potentials from paediatric cochlear implant users fitted with electrically elicited stapedius reflex thresholds. International journal of pediatric otorhinolaryngology, 108, 100–112.

[2] Távora-Vieira, D., Wedekind, A., Ffoulkes, E., Voola, M., & Marino, R. (2022). Cortical auditory evoked potential in cochlear implant users: An objective method to improve speech perception. PLoS ONE, 17(10), e0274643.

[3] Távora-Vieira, D., Mandruzzato, G., Polak, M., Truong, B., & Stutley, A. (2021). Comparative Analysis of Cortical Auditory Evoked Potential in Cochlear Implant Users. Ear and hearing, 42(6), 1755–1769.

[4] Choi, S. M. S., Wong, E. C. M., & McPherson, B. (2019). Aided cortical auditory evoked measures with cochlear implantees: the challenge of stimulus artefacts. Hearing, Balance and Communication, 17(3), 229–238.

[5] Visram, A. S., Stone, M. A., Purdy, S. C., Bell, S. L., Brooks, J., Bruce, I. A., Chesnaye, M. A., Dillon, H., Harte, J. M., Hudson, C. L., Laugesen, S., Morgan, R. E., O'Driscoll, M., Roberts, S. A., Roughley, A. J., Simpson, D., & Munro, K. J. (2023). Aided Cortical Auditory Evoked Potentials in Infants With Frequency-Specific Synthetic Speech Stimuli: Sensitivity, Repeatability, and Feasibility. Ear and hearing, 44(5), 1157–1172.