Auditory Brainstem Response: Cochlear Health Indicator

Sean D. Kampel and colleagues at the VA National Center for Rehabilitative Auditory Research have established normative ranges for a specific auditory brainstem response (ABR) test. Their study, published in the *American Journal of Audiology*, suggests this measure could allow clinicians to identify cochlear deafferentation in patients with normal hearing tests but auditory complaints.

## The Challenge of Diagnosing Hidden Hearing Loss

A normal audiogram does not guarantee a fully healthy auditory system. Researchers have identified cochlear synaptopathy, a loss of connections between inner hair cells and the auditory nerve, as a possible contributor to auditory complaints like tinnitus, misophonia, hyperacusis, and difficulty hearing in noisy environments—despite passing a standard hearing test. This synaptopathy, also called cochlear deafferentation, has been difficult to diagnose in individual patients.

The ABR Wave I amplitude, which reflects the synchronous firing of the auditory nerve, is sensitive to synaptopathy in animal studies. Until now, clinicians lacked established norms to determine if a patient’s Wave I amplitude is unusually low for their sex and hearing status.

## Establishing Norms for a Low-Risk Population

To create a benchmark, the research team first defined a low-risk group. This sample included 169 young, non-Veteran adults with normal audiograms, minimal noise exposure history, and no self-reported auditory complaints. They measured each participant’s ABR Wave I amplitude in response to tonebursts at 2, 4, and 8 kilohertz. They also measured distortion product otoacoustic emissions (DPOAEs), which indicate outer hair cell health.

Because sex affects ABR Wave I amplitude, with males typically having larger amplitudes than females, the researchers built separate normative ranges for men and women. They generated two sets of norms: one adjusted statistically for both sex and DPOAE levels, and another adjusted for sex only.

## Comparing Against a High-Risk Veteran Sample

The researchers then tested these norms against a high-risk sample: 91 military Veterans with normal audiograms but at least one auditory complaint, such as tinnitus, decreased sound tolerance, or speech-in-noise difficulty. This group’s ABR results were compared to the low-risk normative ranges.

The results were clear. While the DPOAE-adjusted norms distinguished the groups, the simpler sex-only adjusted norms worked just as well. In fact, adjusting for outer hair cell function might complicate interpretation unnecessarily. The sex-only norms effectively separated the low-risk and high-risk samples.

For an 8-kHz toneburst at a high intensity (105 dB p.e. SPL), a striking 51% of the high-risk Veteran sample fell below the established normative range. This indicates a high prevalence of significant cochlear deafferentation in this population of patients with normal audiograms.

## Practical Implications for Clinical Diagnosis

This work provides a practical tool for clinicians. “In patients with normal audiograms, sex-specific ABR Wave I amplitude normative ranges can be used by clinicians to identify patients with high degrees of cochlear deafferentation,” the authors conclude in their paper (source: PMID: 42635501).

For patients struggling with conditions like tinnitus or misophonia but who receive no objective confirmation from a standard hearing test, this could be validating. An objective, physiological finding of cochlear deafferentation can shift the clinical conversation from subjective complaint to measurable condition. This may guide more targeted management strategies and help explain reports of speech-in-noise difficulty despite normal hearing.

## Future Directions and Cautions

The study has limitations. The high-risk group consisted solely of Veterans, a population with a high prevalence of noise exposure. More research is needed to confirm the utility of these norms in broader civilian clinics. Furthermore, the test does not diagnose the specific *type* of auditory complaint, but rather provides evidence of a potential underlying neurological deficit common to several conditions.

The finding that DPOAE adjustment may not be needed simplifies the protocol, but it also suggests that the Wave I amplitude is capturing neural deficits somewhat independently of outer hair cell function. This strengthens its role as a direct indicator of the auditory nerve’s health.

Ultimately, this research moves the field from theoretical understanding to clinical application. By providing a clear, evidence-based method to identify cochlear deafferentation, it offers a new diagnostic pathway for patients whose hearing struggles have remained invisible on paper.