Next, this case introduces you to some normal tone-burst evoked responses. The stimulus was a brief 500 Hz tone burst.

Note that there are numbers and letters to the left of the waveform, for example, 15-50LB.  15 means it was the 15th waveform obtained.  50 is the intensity -50 dB nHL.  L stands for left, B is for the “B” channel which is the left ipsilateral channel.  A is the right ipsilateral channel.  (A and B are a little more complicated than that, but for this series we are only showing the ipsilateral traces, so this generalization will hold true.)

The norms this group uses is that hearing threshold is estimated at 20 dB lower than the 500 Hz evoked potential response. This child was not tested at an intensity lower than 30 dB nHL, as it predicts that hearing is at or lower than 10 dB HL, which is a normal finding. Note that the traces replicate. Wave V is the only wave of importance; its shoulder is marked. Note that the latency of wave V is prolonged relative to what you see for a click ABR when testing with 500 Hz stimuli. The signal itself takes several milliseconds to turn on – that will prolong the response latency. Additionally, the 500 Hz signal must travel towards the cochlea’s apex to evoke the response; the traveling wave delay accounts for further prolongation.

Notice the normal, expected prolongation of the waves as intensity decreases. This also helps increase confidence that it is a physiologic response. If V had appeared earlier, that would not make sense — you may be reading myogenic noise as a response.

Ideally control runs would be obtained to compare the responses to, but the child was sedated, and the runs replicated, so the audiologist did not spend the time doing that.

Notice how well the lowest intensity responses replicate. That’s a great indicator that the response is reliable.

The right ear had only one run of 40 tested. The audiologist went right to 30 after the 40 appeared present. Had she not seen a replicable wave at 30 she would have repeated the 40 run.

The marking of wave V latency is not as important. Latencies can vary between patients, and the tone burst latencies are not typically reported, though it is important to recognize the general latency expected of a wave V for 500 Hz (e.g. later than 8 ms in general, and prolonging with intensity decrease.) With threshold testing, what you are looking for is presence/absence of a response. The absolute or latency is no lot critical they way it is for neuro-audiological click-evoked testing. Latency is used to ensure that the response is likely a response (e.g. if you saw a peak at 6 ms, that is too early to be wave V if the stimulus is 500 Hz), and latency differences between ears suggests hearing asymmetry.

Occasionally you will see waves I and III for a 500 Hz response, but it is not typical.