Neurodiagnostic

Latency Measures

  • 1 ‐ Interpeak latencies (IPL): The most powerful index. I ‐ V IPL is usually 4.0 msec (sd 0.2 msec). I ‐ III IPL is slightly above 2.0 msec & III‐ V IPL is slightly below 2.0 msec (sd 0.1 to 0.2 msec).
    • IPLs are not affected by conductive or cochlear hearing loss, and therefore no latency correction factor is needed to offset the loss.
  • 2 Absolute latencies: If wave I is absent or ambiguous, one has to rely on absolute latency of wave V by comparing it with norms. The normative data should be stimulus type, intensity level, rate ‐ specific. First subtract the correction factor for the loss at 4000 Hz.
    • If wave V latency is > 2 sd from the norms, diagnose retrocochlear lesion.
  • 3 Interaural latency difference (ILD): it compares absolute wave Vlatencies between both ears, assuming normal value for one ear. Again hearing loss difference between ears should be offset before comparison.
    • If ILD is greater than 0.4 msec, diagnose retro‐cochlear lesion.

Waveform Morphology Assessment

  • 1‐ Total absence: 25 – 50% of tumor cases showed absent waveforms, depending on tumor size
  • 2‐ Partial absence: Absent wave V or waves III & V in presence of wave I is indicative of low BS or CPA lesion.

Amplitude Ratio

  • It was found that in 50% of acoustic neuorma Amplitudes of V/I is less than one (normally should be more than one, since wave V amplitude is always bigger than wave I amplitude).

Protocol: High Intensity

  • The ABR is typically performed at a high intensity (75 dB nHL for infants and 80 to 90 dB nHL for adults)
  • With sometimes a faster repetition rate
  • Right: click presented at 80 dBnHL at 13.1, 21.1, 55.1 and 90.7 clicks/second
  • Notice that the latency of Wave V shifts out to the right (is delayed) with faster rates

Repetition Rate Effects

  • The use of high RR stresses the auditory system and may uncover a hidden lesion.
  • If you lose the response at a faster rate, is it because you are taxing the auditory system and the neurological components are not functioning properly? A normal ABR should be attainable with a rate of 90.1 in a normal ear.
  • Normally 0.1 msec shift of wave V latency is allowed for every 10 pulse increase with sd of 0.2 msec.
  • For example, pulse increase from 10 to 50/sec should yield no more than 0.6 msec delay

Wave I

  • Use a high intensity to interpret results.
  • Ensure that behavioral thresholds at 3‐4 KHz are at least 15 dB less than stimulus intensity
  • BP‐filter 100‐2/3000 Hz to obtain clear definitions of waves.
  • Enhance wave I by:
  • Rarefaction clicks
  • ear lobule reference &
  • ipsilateral recording.

References

Auditory electrophysiology: Atcherson, S. and Stoody, T. (2012). Auditory Electrophysiology; A Clinical Guide. New York: Thieme.