Abstract
Background:
Electrocochleography (ECochG) is the measurement of stimulus-related cochlear potentials
and the compound action potential (AP). Its primary clinical application is with the
assessment of inner ear disorders. There are few studies examining the variability
of ECochG measures.
Purpose:
The objective of the study was to examine the effect of test (i.e., initial versus
retest), electrode (i.e., extratympanic versus tympanic), and stimulus rate (i.e.,
7.7 versus 77.7/sec) on ECochG indices (i.e., summating potential [SP] amplitude,
AP latency, AP amplitude, SP/AP amplitude ratio, and SP/AP area ratio).
Research Design:
Correlational and three-factor repeated measures designs were employed.
Study Sample:
Eighteen normal-hearing young adults participated.
Data Collection and Analysis:
ECochG responses were obtained with 90 dB nHL click stimuli for an initial test and
retest at two stimulus rates with a commercially available extratympanic (TIPtrode™)
and tympanic (Lilly TM-Wick) electrode. Separate repeated measures linear mixed-model
analysis of variance examined the effect of test, electrode, and rate for all ECochG
indices. Test–retest variability was also examined with correlation analyses; an examination
of mean test–retest differences and their 95% confidence intervals (CI); and construction
of Bland-Altman plots.
Results:
The presence of SP and AP responses varied across experimental conditions. Electrode
and rate were statistically significant predictors (p < 0.05) of SP and AP responses: SP and AP responses were more likely to be present
with the tympanic electrode and at the slow rate. Statistically significant correlations
(p < 0.05) were found between initial tests and retests with all ECochG indices with
both electrodes with the exception of SP amplitude with the TIPtrode™ electrode. There
were no significant main effects of test (initial versus retest) or interactions of
test and electrode or rate for any of the ECochG indices (p > 0.05). The 95% CI of the mean test–retest differences contained 0 confirming that
the effect of test was not statistically significant. There was a statistically significant
main effect of electrode (p < 0.05) on three ECochG measures. The Lilly TM-Wick electrode produced larger SP
amplitudes, AP amplitudes, and SP/AP area ratios than TIPtrode™ electrodes. A statistically
significant main effect of rate (p < 0.05) was identified for all ECochG measures. The effect of rate on AP latency
and amplitude was expected. Increasing the stimulus rate prolonged the AP latency
and decreased AP amplitude. SP amplitude was larger for the faster rate.
Conclusions:
There was no difference between electrodes with regard to test–retest measures. However,
considering the higher likelihood of ECochG SP and AP responses and larger SP amplitude,
SP/AP amplitude ratio, and SP/AP area ratio indices, the tympanic electrode placement
is recommended for clinical practice. The addition of a fast stimulus rate may be
considered for enhanced SP amplitude, SP/AP amplitude ratio, and SP/AP area ratio
albeit with the consideration of the loss of SP and AP responses in some individuals.
Key Words
action potential - electrocochleography - electrode - stimulus rate - summating potential
- test–retest variability
