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ABR measures

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

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ABR measures

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  1. ABR measures • Absolute latency • Inter-peak latency • Wave amplitudes • Threshold of wave V • Wave morphology at higher and lower click rates

  2. Late responses Auditory middle latency responses (AMLR): Latency: 15-60 ms Site of origin: Areas in brainstem and cortex Affected by state of individual Auditory late responses (ALR): Latency Latency: > 60 ms Site of origin: Areas in cortex Affected by state of individual Example: P300, MMN

  3. Behavioral 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

  4. 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.

  5. Growth of loudness

  6. SISI • 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.

  7. 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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