Is it necessary to obtain normative data with your ABR system for tone bursts prior to evaluating tone burst responses from your patients? If so, how many samples are considered sufficient?
Correction factors and normative data for evoked potential systems differ by manufacturer. You should contact the manufacturer to determine how the normative data was obtained and to seek their recommendation for your specific system.
Latency and threshold normative data:
Normative data can be referred to as ''latency intensity'' information; such as we are accustomed to seeing for click ABR's. This type of data is not readily available for tone bursts currently, and hence each clinic would benefit from developing their own norms. Usually 10 normal hearing subjects are tested and the latencies of their wave V tone bursts are recorded along with their lowest intensity measurable for each tone burst frequency.
For example, a 500 Hz tone burst may have an average wave V latency of 12 ms and may be recorded down to 20 dB presentation level (dial setting) for normals with 500 Hz audiometric thresholds at 0 dB HL. This means that when predicting an audiogram, 20 dB would be subtracted from the intensity of the lowest measured 500 Hz tone burst, to approximate the hearing threshold. Hence, a 45 dB 500 Hz tone burst ''threshold'' would be roughly equivalent to a 25 dB audiometric threshold.
Normalized decibel levels for tone bursts dB nHL values:
Tone bursts differ in perceived loudness from pure tone stimuli. A tone burst will often require a correction factor to equate its loudness to that of a pure tone. Some evoked potential equipment provides correction factors for you and others don't. If your system is not calibrated in dB nHL you will need to manually correct for this difference.
This is accomplished by taking 10 normal hearing subjects and performing an audiometric test using tone bursts as the stimuli. You can simply present a tone burst stimulus from your EP system and have the subjects signal when they hear the tone burst, using a standard behavioral Hughson-Westlake threshold procedure. The lowest intensity they can hear at each frequency identifies the correction factor.
For example, when presenting a 500 Hz tone burst to a subject, it may require the dial to be set at 20 dB before the subject can just barely hear the signal. Assuming the subject has an audiometric threshold of 0 dB HL, at 500 Hz, this means they cannot hear any stimulus when the tone burst dial reads below 20 dB HL. Hence, 20 dB would be the correction factor for that frequency. This would be done for each frequency and for each transducer used. Importantly, transducers EACH require their own calibration and verification. They cannot be switched randomly!
The correction factors are subtracted from the dial setting, to determine the approximate dB HL threshold.
For example: If the average audiometric threshold of 10 subjects tested for a 500 Hz tone burst was 20 dB HL, then a patient with a 500 Hz wave V tone burst recorded at 60 dB dial setting, would have an actual reported threshold corresponding to approximately 40 dB nHL intensity (60 dB - 20 dB correction factor).
Michelle Petrak, Ph.D.
Dr. Petrak is a clinical audiologist and product manager for evoked potential technologies at ICS Medical Corporation in Schaumburg, IL.
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