1. Auditory Brainstem Response
ABR

2. What is ABR??
It is short latency, small amplitude, far field electrical potentials responses of the auditory pathways that occur within 10—15 ms of an appropriate acoustic stimulus in normal subjects.

3. Why important? The stability of these potentials over subject state.
The relative ease with which they may be recorded.
Their sensitivity to dysfunctions of the peripheral and brainstem auditory systems

4. The ABR consists of a series of 5–7 waves.
Labeled: wave I to VII
The potentials comprising the ABR arise from the auditory nerve, as well as brainstem structures

5. Origin Wave I arise from the central end of the eighth nerve.
Wave II arise from the central end of the eighth nerve.
Waves I and II arise from structures ipsilateral to the side of stimulation.
Later waves may come from structures that receive ipsilateral, contralateral, or bilateral inputs

6. Basic ABR Measures

7. Amplitude
Amplitude typically is measured between a positive peak and the following negative trough
Peak-to-peak measures are favored because they avoid the difficulty of determining the baseline of the potential.

8. Latency
Absolute latency: The time difference between stimulus onset and the peak of the wave, such as wave I , Wave III and Wave V.
Interwave latencies (or interpeak intervals) are the differences between absolute latencies of two peaks, such as I–V, I–III, and III–V

10. Stimulus Parameters

11. Intensity It has effects on:
Latencies: increase as stimulus intensity decreases.
Amplitudes : decrease as the intensity decreases.
Waves : diminish and ultimately vanish, whereas Wave V often remains discernible down to levels approximating the behavioral thresholds for the same stimulus.

12. Stimulus intensities employed generally range between 40 and 120 dB pe SPL.
dBnHL: “decibels above normal hearing level” or dB HL (dB above the average hearing threshold of a group of normal young adults tested by the same laboratory under conditions identical to those used for recording BAEPs clinically)

13. Spectrum
It is influenced by the stimulus plateau and rise/fall durations.
The response is insensitive to the stimulus duration but quite dependent on the rise/fall times.
Response amplitudes decrease and latencies increase as rise time increases.

14. Clicks: are the most commonly used stimuli for eliciting the ABR
Clicks: are the most commonly used stimuli for eliciting the ABR. The abrupt onset and broad spectrum of a click result in synchronous excitation of a broad population of neurons. The click is usually the most effective stimulus and can provide high frequency information

15. Tone Burst: are frequency-specific, transient stimuli, so there is a spread of energy around the central frequency. The effective rise time may become progressively longer as the frequency decreases. This may reduce synchrony in the apical end of the cochlea, making it more difficult to measure.

16. Polarity
Negative pressure in front of the earspeaker diaphragm are referred to as rarefaction clicks
Positive pressure in front of the earspeaker diaphragm are referred to as condensation clicks

17. In certain pathologic conditions associated with severe, steep high-frequency hearing loss, BAEPs elicited by rarefaction clicks may differ in latency and, to a degree, in morphology from BAEPs evoked by condensation clicks

18. Many examiners prefer to use stimuli of alternating polarity, which help to minimize stimulus artifact and the CM, both of which can obscure Wave I. This approach can reduce or eliminate the need for electromagnetic shielding of the earphone.

19. Stimulus Rate
Stimulus rates employed vary widely from 5 to 200/s. depending on test applications. As the stimulus rate is increased, the latencies of all the waves are prolonged and the amplitudes of the early waves are decreased.

20. Masking
It is recommended that the contralateral (nonstimulated) ear be masked by white noise at 60 dB SPL to eliminate “crossover” responses, i.e., bone-conducted responses originating in this ear.
When??

21. Recording Parameters

22. BandPass
The recommended system bandpass for BAEP recording is to 2,500-3,000 Hz

23. Analysis time
An analysis time of ms from stimulus onset is suggested. An analysis time of no less than 15 ms is sometimes required to demonstrate extremely delayed responses in certain pathologic conditions. Analysis times of 15 ms are also essential for neonatal and intraoperative recordings.

24. Signal Averaging
It is suggested that about 1,000-4,000 individual trials be averaged until good waveform resolution has been achieved. Two or more responses must be obtained and superimposed to demonstrate replicability.

25. Electrode placement 1- Positive: on the scalp at the vertex
2-Negative: over the left and right mastoid processes
3- Ground electrode may be placed anywhere
on the body.

26. Patient State
BAEPs can be obtained during either wakefulness or sleep.
Sedation may occasionally be indicated with very young or tense patients,
Natural sleep OR hypnotics OR anaesthesia

27. Effect of Patient Age
ABR evaluations in premature infants and newborns require the use of age-adjusted norms and necessitate the use of a wider analysis window (e.g., 15–20 ms) than is typically used for adults (e.g., 10 ms).

28. Effect of Patient gender
Start in adolescence, males begin to develop longer Wave V latencies than females, which by adulthood amounts to an average intersex difference of approximately 0.2 ms.
Females display slightly larger Wave V amplitudes than do males.

29. Conductive hearing loss Effect
Conductive hearing losses cause prolong the latencies of all the waves of the ABR due to the effective lowering of the stimulus level.
Otoscopic examination, immittance testing are valuable.
It prolong the latencies of the waves without affecting interpeak latency value and cause essentially the same degree of latency shift at all stimulus levels

30. Cochlear hearing loss Effect
Sloping high frequency losses of moderate severity, and more, cause increased latencies.
The pure tone audiogram would be useful for accurate interpretation of the ABR evaluation..
Various corrections for Wave V latency have been suggested to take into account degree of peripheral loss

31. Analysis of the Results

32. Analysis
Response
Latency
Amplitude
Yes
Absolute
Absolute No
Interpeak
Ratio

33. How often are we missing it?
If somebody comes in with DKA, head CT is probably not the first thing we would do
4 cases cited under Pubmed