Cochlear synaptopathy: new findings in animal and human research

In animal models, prolonged exposure (2 h) to high-level noise causes an irreparable damage to the synapses between the inner hair cells and auditory nerve fibers within the cochlea. Nevertheless, this injury does not necessarily alter the hearing threshold. Similar findings have been observed as part of typical aging in animals. This type of cochlear synaptopathy, popularly called "hidden hearing loss," has been a significant issue in neuroscience research and clinical audiology scientists. The results obtained in different investigations are inconclusive in their diagnosis and suggest new strategies for both prognosis and treatment of cochlear synaptopathy. Here we review the major physiological findings regarding cochlear synaptopathy in animals and humans and discuss mathematical models. We also analyze the potential impact of these results on clinical practice and therapeutic options.

Evaluating Auditory Pathway by Electrical Auditory Middle Latency Response and Postoperative Hearing Rehabilitation

Objective: To establish an effective detection method to evaluate auditory pathway in patients by electrical evoked middle latency response (EMLR) before artificial cochlear implantation, and to analyze the relationship between postoperative hearing rehabilitation and auditory cortex functions. Methods: Twenty-three patients with artificial cochlear implant were recruited. EMLR was measured after adjusting the depth of anesthesia. The electrical auditory brainstem response (EABR) mode with monopolar stimulation and two-phase alternating current square waves was selected. The parameters of EMLR waveforms were recorded by the EABR measurement system. Nerve response telemetry (NRT) was examined by measuring threshold level (T value) and comfortable level (C value) 1 month after power-on, and hearing and speech development was followed up 12 months later.Results: The detection rate of EMLR was 95.65%. The waveforms of EMLR were comparable to those of auditory middle latency response (AMLR), showing decreased latency and interval but similar amplitude. The induction rate of NRT was 69.23%, which was much lower than that of EMLR. The EMLR thresholds were significantly correlated to the T and C values, and were comparable to the T values numerically. The Spearman's r value between EMLR waveforms and CAP scores after using the cochlear implant for 12 months was 0.673 (P < 0.01). Conclusion: An effective detection method to measure EMLR before artificial cochlear implant was established. The thresholds of EMLR were lower than those of NRT. The method can be useful for objective evaluation of auditory cortex functions and postoperative hearing rehabilitation.

Audiometry in young children

The author of this article reviews various techniques in the auditory assessment of infants and young children. The success of these tests depends on the overall functioning of the child, and not on chronological age alone. Any significant deviation from the normal auditory behaviour should raise suspicion of possible auditory impairment. Diagnostic audiology involves more than mere testing of the peripheral auditory mechanism in isolation. It necessitates investigation of possible neurological impairment, mental retardation, emotional disturbance, and hearing loss. Early diagnosis is facilitiated by the battery of auditory tests comprising behavioural, impedance audiometry and auditory brain-stem response audiometry. Interpretation of test findings must be carefully assessed in the light of other pertinent information.