While basic research on the mechanisms and physiology of hearing loss has seen important advances over the years, todays’ clinical hearing diagnostics is still based on the pure-tone audiogram (“raise your hand when you hear a beep”); adopted worldwide since the 1920s. Although the audiogram is a robust marker for damage to the outer hair-cells in the inner-ear, recent evidence has shown that ageing, ototoxicity and noise exposure can damage the auditory-nerve fiber synapses while leaving the hair cells and audiogram intact. The audiogram is hence insensitive to detecting a new and widespread form of sensorineural hearing loss that reduces the population of auditory nerve fibers (i.e. cochlear synaptopathy, CS) and that degrades the processing of fast temporal sound fluctuations necessary for robust speech perception. In our prior work, we developed an auditory EEG-based test for the non-invasive diagnosis and quantification of CS in humans (i.e. the CochSyn test). As part of this project, we performed experiments with human test subjects to refine the test and to bring it closer to clinical practice, the hearing industries and standardisation.
To this end, we performed a robustness and test-retest analysis to demonstrate that the test can be performed in less than 10 minutes as part of a clinical hearing screening procedure. We used computer model simulations to predict the sensitivity and specificity of the CochSyn-test and we applied it to a large cohort with diverse sensorineural hearing loss pathologies to build a unique normative dataset that assesses its prevalence and severity. We recruited 89 participants with clinically normal audiograms (up to 4 kHz) between 18 and 70 years of age and report a 6% reduction in the CochSyn-test amplitude per age decade, corresponding to a predicted 15%-increase in cochlear synaptopathy. This suggests that by the age of 45, 50% of the auditory nerve population is lost while the clinical audiogram still indicates normal hearing. This finding is important because it shows that permanent hearing damage occurs before standard clinical indicators indicate damage, and we will broadly communicate these findings to hearing professionals and government agencies of hearing loss prevention. Because WHO reports that early-diagnosis and treatment of hearing loss is a cost-effective measure to reduce its global societal and economic burden, we believe our findings can help improve clinical hearing diagnostics and hearing screening programs. In further experiments, we studied the interplay between the CochSyn-test and other aspects of sensorineural hearing damage (hair cell and ototoxic damage) and studied how a range of early-markers of hearing damage predict speech intelligibility. Taken together, we collected a unique normative dataset that can be used as a reference to diagnose and estimate the degree of CS in individuals who perform the test. We protected the intellectual property of the test, performed an initial market study, and used our dataset to apply for a valorisation trajectory (EIC Transition project) in which we will perform medical device trials to apply for a CE mark to bring our matured proof-of-concept ideas to the patient and industry.
