Cochlear Microphonic and Summating Potential Responses from Click-Evoked Auditory Brain Stem Responses in High-Risk and Normal Infants.BACKGROUND
 Examination of cochlear and neural potentials is necessary to assess sensory and neural status in infants, especially those cared for in neonatal intensive care units (NICU) who have high rates of hyperbilirubinemia and thus are at risk for auditory neuropathy (AN).


 The purpose of this study was to determine whether recording parameters commonly used in click-evoked auditory brain stem response (ABR) are useful for recording cochlear microphonic (CM) and Wave I in infants at risk for AN.  Specifically, we analyzed CM, summating potential (SP), and Waves I, III, and V. The overall aim was to compare latencies and amplitudes of evoked responses in infants cared for in NICUs with infants in a well-baby nursery (WBN), both of which passed newborn hearing screening.


 RESEARCH DESIGN
 This is a prospective study in which infants who passed ABR newborn hearing screening were grouped based on their birth history (WBN and NICU).  All infants had normal hearing status when tested with diagnostic ABR at about one month of age, corrected for prematurity.


 Thirty infants (53 ears) from the WBN [mean corrected age at test = 5.0 weeks (wks.)] and thirty-two infants (59 ears) from the NICU (mean corrected age at test = 5.7 wks.) with normal hearing were included in this study.  In addition, two infants were included as comparative case studies, one that was diagnosed with AN and another case that was diagnosed with bilateral sensorineural hearing loss (SNHL).


 DATA COLLECTION AND ANALYSIS
 Diagnostic ABR, including click and tone-burst air- and bone-conduction stimuli were recorded.  Peak Waves I, III, and V; SP; and CM latency and amplitude (peak to trough) were measured to determine if there were differences in ABR and electrocochleography (ECochG) variables between WBN and NICU infants.


 No significant group differences were found between WBN and NICU groups for ABR waveforms, CM, or SP, including amplitude and latency values.  The majority (75%) of the NICU group had hyperbilirubinemia, but overall, they did not show evidence of effects in their ECochG or ABR responses when tested at about one-month corrected age.  These data may serve as a normative sample for NICU and well infant ECochG and ABR latencies at one-month corrected age.  Two infant case studies, one diagnosed with AN and another with SNHL demonstrated the complexity of using ECochG and otoacoustic emissions to assess the risk of AN in individual cases.


 CONCLUSIONS
 CM and SPs can be readily measured using standard click stimuli in both well and NICU infants.  Normative ranges for latency and amplitude are useful for interpreting ECochG and ABR components.  Inclusion of ECochG and ABR tests in a test battery that also includes otoacoustic emission and acoustic reflex tests may provide a more refined assessment of the risks of AN and SNHL in infants.
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