Infants and young children present a special challenge to the audiologist when assessing hearing thresholds. Research has shown that untreated hearing loss during infancy and early childhood leads to delays in language acquisition and speech development. The Early Hearing Detection and Intervention guidelines are well established and familiar to audiologists who test children. These guidelines include ensuring a diagnosis of hearing impairment is made no later than three months of age and treatment must begin no later than six months of age. Auditory Brainstem Response (ABR) is the most reliable method for diagnosing hearing impairment in infants. However, not every audiologist receives the same level of ABR education and training.
ABR test procedures differ significantly based on the objective or purpose of the test. An ABR assessment for an adult is far different than one for an infant. Hearing threshold estimation is the most common reason for performing an ABR on an infant or very young child. A neurodiagnostic ABR is typically completed for an adult patient in the audiology clinic when retrocochlear pathology is suspected.
This article focuses on best practices to assist the audiologist when performing ABR on infants to estimate hearing thresholds.
Auditory brainstem response training - what has changed?
Advances in technology now allow audiologists to perform pediatric ABR in a variety of clinical environments, and during natural sleep without the need for sedation. Information and training on the advancements in ABR technology and procedures may not be readily available for some hearing professionals. However, pediatric audiologists should be sure to familiarize themselves with the most current best practice guidelines regarding pediatric electrophysiologic measures such as frequency specific ABR, auditory steady-state evoked potentials (ASSR) and with accurate prediction of behavioral hearing thresholds based on the ABR findings.
The protocol for performing a pediatric threshold seeking ABR is very specific and well documented in literature. Yet it is not only about the science. There are other considerations that are equally important to ensure accurate and complete test results.
The state of the patient is very important during testing. Even with advances in technology, such as weighted averaging, testing a restless infant will result in longer test times, noisy waveforms, and perhaps inconclusive or inaccurate results. Natural sleep is usually preferred over sedation. Steps taken by the hearing care provider prior to testing can help support natural sleep and a successful test session.
If the parent is instructed that their infant must sleep for the ABR test, they will likely arrive at the appointment with a sleeping child. Unfortunately, while prepping the electrode sites and applying the electrodes and insert earphones, the infant will awaken and will not be keen on falling back to sleep. Parents should be advised to not permit their infant to nap prior to the appointment, and to bring a bottle or plan to breast feed during the appointment. Feeding a tired, hungry baby just prior to testing will help promote sleep during the test.
Time is not on your side during an infant ABR evaluation. You may plan for a one or even two-hour session, but in actuality, the infant ABR test is over when the baby wakes up. With that in mind, be prepared to work smarter and with clear objectives:
Begin by prepping the electrode sites on the baby. A 3-electrode montage is usually sufficient for a traditional ABR, using high forehead (Fz) for the non-inverting electrode, and mastoid or earlobe for the inverting electrode (test ear) and the common electrode (non-test ear).
However, you may consider prepping and placing a 4th electrode just below Fz to prepare for testing both ears simultaneously with Auditory Steady State Response (ASSR) or a two channel collection, if needed. You may or may not use the 4th electrode, but once the infant is sleeping you will not have the opportunity to add that 4th electrode without waking the infant.
The primary objective of an infant ABR test is to estimate hearing thresholds within the 500 to 4000Hz range for each ear. Frequency-specific stimuli must be included in your protocol to achieve this objective. Additionally, the rate of stimulation must be considered to collect as much information as possible while the infant remains sleeping. Stimulus rates of 39.7/s can save time without negatively affecting waveform morphology. Consider starting with this faster stimulus rate, and lower it only if needed based on the quality of the waveforms.
Stimulus choices for the infant ABR test include broadband clicks, chirps and frequency specific tone bursts or modulated stimuli used with ASSR testing. The broadband click stimulus does not offer hearing threshold information at any specific frequency. Yet the waveform generated can provide confirmation of an intact auditory pathway with analysis of the absolute and interpeak latencies for Waves I, III and V and the interaural Wave V latency difference. Presentation of a click stimulus at a sufficient intensity above threshold may yield clear responses within 30 to 60 seconds. Comparing waveforms collected with rarefaction clicks to condensation clicks should be completed at this stage to rule out Auditory Neuropathy Spectrum Disorder (ANSD). Once the integrity of the auditory pathway is confirmed, testing continues with frequency specific stimuli. ASSR, if available, provides fast, objective data to estimate the infant audiogram while testing both ears simultaneously at multiple frequencies and intensities.
If ASSR is not available, each ear is tested individually at 500, 1000, 2000 and 4000Hz using multiple intensities. Traditional ABR training required replication of each waveform, however more recently, when clear responses are present, replication is only recommended at threshold.
Finally, the manner in which the responses are analyzed and categorized must not be overlooked. ASSR technology provides objective analysis based on signal to noise ratio and statistical probability of the presence of the response. Many ABR devices offer objective measures for the hearing professional to consider when identifying responses as ”clear response” or ”no clear response” however these practices may be underutilized. To learn more about evidence-based considerations for waveform interpretation criteria and clinical efficiency, click below to watch Dr. Guy Lightfoot’s lecture ”Techniques for threshold ABR estimation in newborns & beyond”.
What to consider when choosing an ABR device?
When choosing an ABR device, it is important to consider the type of patient and the objective of the ABR evaluation. Options to consider may include:
Are you looking for a reliable handheld ABR system? The Bio-logic® NavPRO ONE® is a modular ABR system offering flexibility to test air and bone conduction with traditional click and tone burst ABR and broadband and narrowband CHIRP stimuli. This modular system can include ASSR with adaptive testing protocols to not only test multiple frequencies simultaneously in both ears, but adapts to lower intensities as responses are identified achieving faster results. Add DP and/or TE OAE to supplement your ABR results providing the most comprehensive test results.
Here at Natus, we want to ensure that every audiologist has access to best practices for pediatric auditory brainstem response. We routinely receive questions regarding ABR usage, which protocols and methods achieve the best results and where to obtain ABR training. We have brought in a well-known and highly regarded expert to help clear things up.