Hidden Hearing Loss (Cochlear Synaptopathy): Diagnostic Challenges and Emerging Tests

Hidden hearing loss (HHL), often attributed to cochlear synaptopathy, is characterized by difficulty understanding speech in noisy environments despite clinically normal pure-tone thresholds. Moderate noise exposure or aging may destroy synapses between inner hair cells (IHCs) and auditory nerve fibers (ANFs) without hair-cell death, leading to permanent deafferentation [1,2,3]. This neural loss degrades suprathreshold neural coding (e.g., reduced ABR wave I amplitude, impaired envelope-following responses [EFR]) and may manifest as speech-in-noise deficits, tinnitus, or hyperacusis [4,5,6]. However, diagnosing HHL is challenging: standard audiometry is insensitive to synaptopathy, and no consensus diagnostic criteria exist [7,8,9]. Emerging objective measures—reductions in ABR wave I, anomalies in EFR/FFR (frequency-following response), attenuated middle-ear muscle reflexes (MEMR), and altered electrocochleography (ECochG) SP/AP ratios—show promise as biomarkers of synaptopathy [10,11,12,13]. This review covers the neurobiological basis of cochlear synaptopathy, findings from animal and human work, the status of electrophysiological diagnostics (ABR, EFR/FFR, MEMR, ECochG), clinical implications for audiologists and neuroscience researchers, and future directions including standardized test batteries and possible therapies.