ABR measures

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Description
ABR measures. Absolute latency Inter-peak latency Wave amplitudes Threshold of wave V Wave morphology at higher and lower click rates. Late responses. Auditory middle latency responses (AMLR): Latency: 15-60 ms Site of origin: Areas in brainstem and cortex
Transcript
ABR measures
  • Absolute latency
  • Inter-peak latency
  • Wave amplitudes
  • Threshold of wave V
  • Wave morphology at higher and lower click rates
  • Late responsesAuditory middle latency responses (AMLR): Latency: 15-60 ms Site of origin: Areas in brainstem and cortex Affected by state of individualAuditory late responses (ALR): Latency Latency: > 60 ms Site of origin: Areas in cortex Affected by state of individual Example: P300, MMNBehavioral tests of lesion
  • Used to differentiate cochlear/retrocochlear site of lesion.
  • Mostly replaced by OAE and electrophysiological tests.
  • Examples of behavioral tests:
  • ABLB (Alternate binaural loudness balance)
  • SISI (Short increment sensitivity index)
  • Tone decay test
  • ABLB
  • Based on phenomenon of recruitment.
  • In normal hearing ears and hearing loss without recruitment, loudness grows in both ears in the same way. Equal loudness at equal sensation levels.
  • Recruitment: Abnormally rapid growth of loudness in the poorer ear. Seen in cochlear hearing loss.
  • Decruitment: Loudness grows very slowly in the poorer ear. Even very intense sounds may not be very loud. Seen in auditory nerve lesions.
  • Growth of loudnessSISI
  • Aim: To detect a small increment ( 1 dB) superimposed on a 20 dB SLtone.
  • Rationale: Because of abnormal loudness growth, individuals with cochlear hearing loss obtain high scores (are able to detect the increment very well).
  • Normal hearing listeners and retrocochlear hearing loss obtain poor SISI scores.
  • Tone decay
  • For sustained tones, threshold increases as the duration of the tone increases.
  • Steps:
  • Present tone at 20 dB SL
  • As soon as patient signals that the tone is heard, start stopwatch.
  • When patient stops hearing, then increase level by 5 dB without interrupting it.
  • Procedure continued till a) tone is heard for 60 s, b) tone is 30 dB above starting level and patient cannot hear for 60 s at that level, or c) audiometric limits have been reached.
  • Interpretation
  • Type I: Tone is heard for 60 s. Seen mostly in normal-hearing listeners, conductive hearing loss.
  • Type II: As level is raised, tone decay reduces (tone is heard for longer and longer durations). Strong indicator of cochlear hearing loss.
  • Type III: Even with increasing levels, tone decay still observed. Indicator of auditory nerve lesions.
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