Principles of Analysis and Interpretation
- Generally, most analysis and interpretation techniques take place following the completed, averaged recording
- Auditory steady-state response (ASSR) is analyzed spectrally through a time to frequency conversion process
- Important questions
- Is a response present, or is it just artifact or noise?
- Is the response reliable and repeatable?
- If the response is present, how does it compare with others obtained from individuals with normal hearing and auditory processes?
- How confident am I that the recorded response will help me with differential diagnosis or threshold estimation?
- Are there internal or external factors that explain the outcome of the recording?
- Still depends on a final, subjective judgment from the clinician
- Example: the ABR is commonly recorded with 256 points in a 10-msec time window, each individual point is 0.039msec in duration
- 10 msec / 256 points = 0.039 msec OR 0.010s / 256 * 1000 = 0.039msec
- Example: the ABR is commonly recorded with 256 points in a 10-msec time window, each individual point is 0.039msec in duration
- Appreciate that any part of a recorded waveform may be selected and compared with normative data collected from a larger group of individuals
- normative data typically yield the central tendency of the group (mean) as well as the variability of the group (standard deviation)
- if a patient has a measurement that exceeds 2SD above or below the mean, there is a greater likelihood that there is an abnormality
Analyses in the Time Domain
- analyses and interpretation techniques that follow the collection of the averaged response
- ABR
- Marking of primary waves I, III, V although other waves may be marked such as II and IV
- Marking wave V only and tracking its presence to decreasing intensity levels
- Looking for signs of ringing cochlear microphonic in an absent or grossly malformed ABR waveform
- as is in the case of auditory neuropathy spectrum disorder (ANSD)
- LATENCIES
- Absolute and relative latencies for waves I, III, V, I-III, III-V and I-V are deemed most important; interaural latency difference for wave V between ears
- Wave V usually appears around 5.6msec following a high intensity click
- Absolute and relative latencies for waves I, III, V, I-III, III-V and I-V are deemed most important; interaural latency difference for wave V between ears
- AMPLITUDE
- Absolute amplitudes are too variable to be used clinically; however, wave V/I ratio and wave V can be used for threshold estimation and stacked ABR
- Absolute amplitudes are too variable to be used clinically; however, wave V/I ratio and wave V can be used for threshold estimation and stacked ABR
- ABR
Marking or Labeling Waves
- Each individual sample's latency and amplitude value will be influenced by the overall signal to noise ration achieved during the averaging process
- Because of signal to noise issues that effect every recording, no two recordings will ever appear exactly alike
- thus the repetition of every recording is a practice that is strongly encouraged to increase confidence that a response is present or absent
- having two or more recordings can help with marking the waveform
- thus the repetition of every recording is a practice that is strongly encouraged to increase confidence that a response is present or absent
Latency-Intensity functions
- graph to observe shifts in peak latency with decreases in stimulus intensity