Auditory Brainstem Response: ​Threshold Estimation

Stimuli

• Two types of tone bursts are used for threshold estimation: clicks and tone bursts
  • Instantaneous onset of a click the energy introduced to the cochlea has the effect of synchronizing a broad range of auditory nerve fibers
    • Broadly stimulating the cochlea has its advantages in producing a clearly defined ABR with most, if not all, complementary waves present
    • Click is considered a broadband stimulus and therefore is not frequency specific
  • Pure tones are highly frequency specific but they  must be shaped in a particular manner so that they do not have instantaneous onsets like a click and their rise times are not too slow
  • Because neural synchrony remains a requirement to produce an ABR tone-burst stimuli must be short with relatively fast rise/fall times
  • Tone bursts are typically ramped using a linear or blackman envelope function with rise/fall times encompassing 2 cycles and a plateau of either 0 or 1 cycle
• ​If a protocol includes clicks and high frequency tone bursts, beginning with high frequency clicks at the outset could be viewed as redundant and wasteful
• Regardless of the stimuli used, most protocols attempt to optimize time efficiency
  • ​Using an ascending approach
  • Using 20 to 40 dB step sizes at suprathreshold levels with a descending approach
  • Not testing below some minimu required intensity level
  • Not using step sizes < 10dB (5 dB step sizes > 70 dB are recommended)
  • Not repeating runs at suprathreshold levels
  • Testing at a higher frequency first, followed by the lowest frequency and filling in the rest
  • ​Testing the assumed better ear first

Filter Settings

• Typical filter settings for threshold ABR in children are 30 to 3000 Hz or 30 to 1500Hz when compared with the typical 100 to 3000Hz filter setting used in a high intensity neurodiagnostic application

Averaging

• Use as many stimulus presentations (or sweeps) as needed to obtain a clear and repeatable wave V

Analysis Time

• Wave V latency will prolong with decreases in stimulus intensity level
• It is recommended that the analysis time window extend the standard 10 msec
• An analysis window of 15 msec should be sufficient for higher frequency tone bursts and clicks whereas an analysis window of 20 to 25 msec may be necessary for lower frequency tone burst stimuli

Estimating and Plotting Behavioral Thresholds

• Goal is to paint an audiometric picture, especially for young children and infants for whom conventional audiometry is inappropriate or cannot be completed
• ABR is recorded using a variety of stimuli; 500, 1000, 2000 and 4000Hz tone bursts and 100 clicks
• Another method developed by Bagatto and her colleagues and recommended for routine clinical practice is to plot ABR thresholds in dB nHL as behavioral, estimated hearing loss thresholds using the following correction factors
  • ​subtract 20dB at 500Hz
  • subtract 15dB at 1000Hz
  • subtract 10dB at 2000Hz
  • subtract 5dB at 4000Hz

Summary

• Threshold estimation using ABR remains the test of chioce for infants and young children before they are able to complete behavioral testing
• with experience, preparedness, patients, effective protocols and proper instrumentation, fairly accurate threshold estimations can be obtained using conventional evoked potential systems either with natural sleep or sedation

References

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