The Effect of Parkinson's Disease on Otoacoustic Emissions and Efferent Suppression of Transient Evoked Otoacoustic Emissions

Purpose Several studies have demonstrated increased auditory thresholds in patients with Parkinson's disease (PD) based on subjective tonal audiometry. However, the pathophysiological mechanisms underlying auditory dysfunction in PD remain elusive. The primary aim of this study was to investigate cochlear and olivocochlear function in PD using objective measurements and to assess the effect of dopaminergic medication on auditory function. Method Eighteen patients with PD and 18 gender- and age-matched healthy controls (HCs) were included. Patients with PD participated in medication on and off conditions. Linear mixed models were used to determine the effect of PD on tonal audiometry, transient evoked and distortion product otoacoustic emissions (OAEs), and efferent suppression (ES). Results Tonal audiometry revealed normal auditory thresholds in patients with PD for their age across all frequencies. OAE signal amplitudes demonstrated a significant interaction effect between group (PD vs. HC) and frequency, indicating decreased OAEs at low frequencies and increased OAEs at high frequencies in patients with PD. No significant differences were found between patients with PD and HCs regarding ES. In addition, no significant effect of medication status was found on auditory measurements in patients with PD. Conclusions Altered OAEs support the hypothesis of cochlear alterations in PD. No evidence was found for the involvement of the medial olivocochlear system. Altogether, OAEs may provide an objective early indicator of auditory alterations in PD and should complement subjective tonal audiometry when assessing and monitoring auditory function in PD.

Hyperactivity in the medial olivocochlear efferent system is a common feature of tinnitus and hyperacusis in humans

Tinnitus and hyperacusis are common, burdensome sources of morbidity with a high rate of co-occurrence. Knudson et al. ( J Neurophysiol 112: 3197–3208, 2014) demonstrated that efferent suppression of cochlear activity by the medial olivocochlear system is enhanced in individuals with tinnitus and/or hyperacusis. Their findings stress that atypical activity in the efferent auditory pathway may represent a shared substrate, as well as a potential therapeutic target, in tinnitus and hyperacusis.

Overexpression of SK2 Channels Enhances Efferent Suppression of Cochlear Responses Without Enhancing Noise Resistance

Cochlear hair cells express SK2, a small-conductance Ca2+-activated K+ channel thought to act in concert with Ca2+-permeable nicotinic acetylcholine receptors (nAChRs) α9 and α10 in mediating suppressive effects of the olivocochlear efferent innervation. To probe the in vivo role of SK2 channels in hearing, we examined gene expression, cochlear function, efferent suppression, and noise vulnerability in mice overexpressing SK2 channels. Cochlear thresholds, as measured by auditory brain stem responses and otoacoustic emissions, were normal in overexpressers as was overall cochlear morphology and the size, number, and distribution of efferent terminals on outer hair cells. Cochlear expression levels of SK2 channels were elevated eightfold without striking changes in other SK channels or in the α9/α10 nAChRs. Shock-evoked efferent suppression of cochlear responses was significantly enhanced in overexpresser mice as seen previously in α9 overexpresser mice; however, in contrast to α9 overexpressers, SK2 overexpressers were not protected from acoustic injury. Results suggest that efferent-mediated cochlear protection is mediated by other downstream effects of ACh-mediated Ca2+ entry different from those involving SK2-mediated hyperpolarization and the associated reduction in outer hair cell electromotility.

Patients with Auditory Neuropathy/Dys-synchrony Lack Efferent Suppression of Transient Evoked Otoacoustic Emissions

Function of the olivocochlear reflex, measured by suppression of transient evoked otoacoustic emissions, is assessed in nine patients with bilateral auditory neuropathy/dys-synchrony and compared to matched control subjects with normal auditory function. TEOAEs were acquired using 65 dB peak sound pressure linear clicks with and without the presence of broad-band noise presented binaurally, ipsilaterally, or contralaterally in a forward masking paradigm. Efferent suppression differed significantly between subject groups (p < .0001). Mean suppression was less than 0.22 dB across all suppressor noise conditions for the auditory neuropathy/dys-synchrony subjects. TEOAE suppression averaged 4.47 (binaural), 2.41 (ipsilateral), and 1.52 (contralateral) dB in the control subjects. Suppression characteristics across the three suppressor conditions were also assessed in one patient with unilateral auditory neuropathy/dys-synchrony. The results suggest that poor efferent responses are related to compromised afferent input to the OCR pathway and support the use of efferent suppression of otoacoustic emissions as a differential measure of auditory function in patients with auditory neuropathy/dys-synchrony.