Objectives: 1) To describe the auditory brain stem response (ABR) measurement system and optimized methods used for study of newborn hearing screening. 2) To determine how recording and infant factors related to the screening, using well-defined, specific ABR outcome measures.
Design: Seven thousand one hundred seventy-nine infants, 4478 from the neonatal intensive care unit (NICU) and the remaining from the well-baby nursery, were evaluated with an automated ABR protocol in each ear. Two channel recordings were obtained (vertex to mastoid or channel A and vertex to nape of neck or channel B) in response to click stimuli of 30 and 69 dB nHL in all infants as well as 50 dB nHL in infants who did not meet criteria for response at 30 dB. Criteria for response included F(SP) > or =3.1 and a tester-judgment of response. Criteria could be met in the first or repeat test with a maximum of 6144 accepted sweeps per test.
Results: More than 99% of infants could complete the ABR protocol. More than 90% of NICU and well-baby nursery infants "passed" given the strict criteria for response, whereas 86% of those with high risk factors met criterion for ABR response detection. The number of infants who did not meet ABR response criteria in one or both ears was systematically related to stimulus level with the largest group not meeting criteria at 30 dB followed by 50 and 69 dB nHL. Meeting criteria on the ABR was positively correlated with the amplitude of wave V, with low noise and low electrode impedance. Factors that predicted how many sweeps would be needed to reach criterion F(SP) included noise level of the test site, state of the baby (for example, quiet sleep versus crying), recording noise, electrode impedance and response latency. Channel A (vertex to mastoid) reached criterion more often than channel B (vertex to nape of neck) due to higher noise in channel B. Average total test time for 30 dB nHL screening in both ears was under 8 minutes. Well babies with risk factors took slightly longer to evaluate than other groups with this automated ABR procedure and have higher noise levels.
Conclusions: ABR implemented with an automated detection algorithm using a 30 dB nHL click stimulus is reliable technique for rapid assessment of auditory status in newborns. Factors other than hearing loss that influenced the test result include infant state, electrode location and impedance, testing site, and infant risk status. Even so, ABRs were reliably recorded in the vast majority of babies under circumstances in which most babies are found in the perinatal period.