Auditory Brainstem Response: Cochlear Deafferentation Indicator

A new study has established the first sex-specific normative ranges for a key auditory brainstem response (ABR) measurement, providing clinicians with a potential tool to diagnose cochlear deafferentation in patients with normal hearing tests. The research, led by Sean D. Kampel and colleagues at the VA National Center for Rehabilitative Auditory Research, found that 51% of a high-risk Veteran sample fell below the new normal ranges for an 8-kHz toneburst [PMID: 42635501].

Why ABR Wave I Amplitude Matters for Hidden Hearing Loss

Cochlear synaptopathy, often called “hidden hearing loss,” describes damage to the connections between inner hair cells and the auditory nerve. This damage can occur from noise exposure or aging without raising thresholds on a standard audiogram. The consequence is not a loss of quiet sounds, but a degraded neural signal to the brain, potentially leading to problems like tinnitus, difficulty understanding speech in noise, and decreased sound tolerance (hyperacusis or misophonia).

In animal studies, a reduced amplitude of Wave I in the Auditory Brainstem Response (ABR) is a direct indicator of this synaptopathy. The ABR measures the brain’s electrical response to sound, and Wave I specifically reflects the synchronous firing of the auditory nerve. Until now, a lack of established human normative data has prevented clinicians from using this measure to diagnose individuals. For more on how ABR functions as a cochlear health indicator, see our article on Auditory Brainstem Response: Cochlear Health Indicator.

Building a Baseline: The Low-Risk Sample

To define what is normal, the team first tested 169 young, non-Veteran adults. This low-risk group had clinically normal audiograms, minimal history of noise exposure, and no self-reported auditory complaints like tinnitus, hyperacusis, or speech-in-noise difficulties. They served as the healthy control population.

Researchers measured each participant’s ABR Wave I amplitude in response to tonebursts at 2, 4, and 8 kHz. They also measured distortion product otoacoustic emissions (DPOAEs), which assess the health of the outer hair cells. The goal was to create statistical models that accounted for known variables. They generated normative ranges adjusted for both sex and DPOAE levels, and separately, for sex alone.

Testing the Tool: The High-Risk Veteran Sample

The diagnostic power of these new ranges was tested against a high-risk group of 91 military Veterans. All Veterans had normal audiograms but reported at least one persistent auditory complaint—tinnitus, decreased sound tolerance, or speech-in-noise problems. This group represents the exact patient population for whom a standard hearing test is unhelpful, yet who struggle with real-world auditory perception.

The comparison yielded clear results. The sex-specific normative ranges successfully differentiated the low-risk and high-risk groups. The most striking finding was for the 8 kHz toneburst presented at 105 dB peSPL: 51% of the Veteran sample had a Wave I amplitude that fell below the established normal range. This suggests a high prevalence of cochlear deafferentation in this population.

An unexpected finding was that adjusting the ABR norms for DPOAE levels—a method intended to isolate nerve health from outer hair cell function—did not improve the tool’s effectiveness. The study authors concluded this adjustment “may not be necessary and could be problematic,” simplifying the potential clinical application. The core issue appears to be the neural signal, independent of outer hair cell activity.

Clinical Implications: From Lab Finding to Patient Care

This work translates a laboratory measure into a practical clinical tool. For the first time, an audiologist can perform an ABR test on a patient with a normal audiogram but complaints of tinnitus, sound sensitivity, or listening difficulties, and compare the Wave I amplitude to a sex-specific normative chart. A result below the normal range provides objective evidence of cochlear deafferentation.

This objective diagnosis is vital. It validates the patient’s experience, moving their symptoms from a subjective report to a measurable physiological condition. It can directly inform management strategies, such as auditory training or sound therapy, and underscore the importance of hearing protection to prevent further neural loss. Understanding this “hidden” pathology is especially relevant for patients with conditions like misophonia, where the perceptual problem may originate in aberrant central nervous system processing of a degraded peripheral signal.

Furthermore, this biomarker can be used to track cochlear nerve health over time or to assess the protective effects of interventions. It also provides a clearer physiological target for research into treatments aimed at repairing or regenerating synaptic connections. For researchers investigating tinnitus and speech-in-noise performance in normal hearing, ABR Wave I amplitude offers a concrete variable to correlate with perceptual deficits.

A New Diagnostic Pathway for Auditory Complaints

The study by Kampel, McMillan, Heassler, and their team marks a significant step forward in auditory diagnostics. By establishing and validating simple, sex-specific norms for ABR Wave I amplitude, they have provided a missing link between patient-reported symptoms and their underlying biological cause. This evidence-based tool promises to improve the assessment and management of the millions of individuals who hear “normally” on a pure-tone test but struggle in their daily auditory lives.

The full research paper, “Normative Ranges for Auditory Brainstem Response Wave I Amplitude: A Potential Diagnostic Indicator of Cochlear Deafferentation,” is available online ahead of print in the American Journal of Audiology [DOI: 10.1044/2026_AJA-25-00296].