ABR: Are nHL to eHL correction factors different for narrow band CE-Chirps® when compared to tone bursts?

I want to ask a question about the article from Ferm et al., 2013 titled: “Comparison of ABR response amplitude, test time, and estimation of hearing threshold using frequency specific chirp and tone pip stimuli in newborns”

During the IA Academy presentation on youtube, she is telling that, which also exists in your CE-Chirp® overview presentation; Because the response is larger, it is seen to lower levels, so the nHL – eHL correction is 5dB less than for tone pips (Ferm et al. 2013, Ferm and Lightfoot 2015). Is this information also valid for Tone Burst Stimulus vs NB CE-Chirp®?

Answer: In the CE-Chirp® webinar presented by Inga Ferm, which describes two key articles (Ferm et al. 2013, Ferm and Lightfoot 2015), Inga is focussing on the narrow-band (NB) family of CE-Chirp® stimuli. So the direct answer to your question would be yes, the info is valid for NB CE-Chirp® stimuli.

But please note that when Ferm uses the phrase “tonepip” then we can take that to mean toneburst i.e. a toneburst of short duration (2-1-2 cycles) suitable for ABR testing. It is a small point of confusion since in ABR testing the words “tonepip” and “toneburst” are often used interchangeably. Her meaning is clear from the context of the study. I mention this as your question asked if Ferm’s tonepip versus NB CE-Chirp®  comments are also valid for toneburst versus NB CE-Chirp® – I think you’re talking about the same thing. 

I shall use “toneburst” in the below.

What Ferm does is compare the performance of tonebursts and NB CE-Chirps® by measuring the ABR first using the traditional tonebursts (Ferm et al. 2013 = 4 kHz and 1kHz; Ferm and Lightfoot 2015 = 0.5 kHz and 2 kHz), and then repeating the procedure using the NB CE-Chirps® centred at the same frequencies. The studies compare ABR amplitude, Fmp, estimated threshold and residual noises between the respective stimuli. 

One of the key findings is that the NB CE-Chirp® results at each frequency gives a greater amplitude than the equivalent toneburst stimuli (hence lower test time/fewer inconclusive findings and higher Fmp and response confidences). The higher amplitude NB CE-Chirp® ABRs also meant that the measured threshold was lower than for the tonebursts i.e. the results were closer to the true behavioural threshold, meaning the nHL-eHL correction factor can be smaller.

As an example, imagine a patient with a true behavioural threshold of 30 dB HL at 4 kHz. We measure their ABR using tonebursts and the ABR might disappear into the noise floor at 40 dB nHL, say (not uncommon). We would then apply the nHL-eHL correction factor of 10dB, to give us a threshold of 30 dB eHL (which is a good estimation of the true threshold). If we now measure the ABR again using NB CE-chirps we would expect the ABR to disappear at a lower level, like 35 dB nHL, because of the advantages of greater response amplitude offered by the NB CE-Chirp® signal. So now since we are closer to the true threshold the correction factor should be lowered accordingly i.e. 5 dB - we apply this new correction factor to again arrive at 30 dB eHL.

But please note it is very important that the correction factor is only applied when we reach a threshold. Otherwise we could run into difficulties.

Imagine for example that someone had a normal hearing threshold, 0 dB HL. We might measure their ABR using toneburst and when we reach down to say 20 dB nHL we would probably stop further testing in many cases (as we would already have enough information to show hearing is in the normal range). If you then apply the 10 dB nHL-eHL correction factor for toneburst you would arrive at an eHL value of 10, which is not a very good estimation of their true hearing threshold (i.e. 10 dB above the true threshold). 

If you followed the same testing logic with NB CE-Chirps® and also stopped at 20 dB nHL and then applied the chirp correction factor you would arrive at an eHL value of 15, which is now a greater error than the toneburst estimation. Of course it would not be a clinically significant issue in this example since both estimations are still within the normal range, but it is worth bearing this aspect in mind. Also, remember you have not “lost” anything in terms of accuracy with the CE-Chirp® by stopping at 20 dB nHL since we chose to stop early. And, despite the greater eHL error that you could get if you chose to do this, you would still be at an advantage with in the sense that you would have got to that stage in the procedure more quickly than with the toneburst. 

You can also download a spreadsheet which automatically converts ABR threshold in nHL to the corresponding eHL value according to whether CE-Chirps® or tonebursts were used.  click here to download the sheet. 


References and caveats
Ferm, I. et al. (2013) Comparison of ABR response amplitude, test time, and estimation of hearing threshold using frequency specific chirp and tone pip stimuli in newborns. IJA 52: 419–423

Ferm, I. and Lightfoot, G. (2015) Further comparisons of ABR response amplitudes, test time, and estimation of hearing threshold using frequency-specific chirp and tone pip stimuli in newborns: Findings at 0.5 and 2 kHz. IJA 54: 745–750 

Igna Ferm explains more about the CE-Chirp® family in this webinar


November 2017
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