**4. DPOAEs and MD**

De Kleine et al. [29] found that, in patients affected by MD, DPOAEs have smaller amplitude than the unaffected ears in relation to the mechanical alterations which were hydropsinduced. An early report on MD by Eggermont and Schmidt [30] stated that in the early phase of MD, a minimal variation on the outer hairy cells function, caused by hydrops, can determine the typical auditory threshold fluctuation; this phenomenon can be indirectly observed as a reduction in the DPOAE amplitude particularly in the low DPOAE frequencies. In cases of advanced MD, the severe damage or the loss of inner ear, outer hair cells, is responsible for DPOAEs absence. Unfortunately, these claims have not been verified in subsequent reports and one of the criticisms Eggermont received [31] was that DPOAEs have very low signal-tonoise ratios (S/N) at the lower frequencies (i.e., 0.5 kHz and 0.75 kHz).

DPOAEs have also been considered as an objective monitor system, for any middle ear functional changes induced by the administration of the glycerol test [32], during the hydrops phase of MD. The osmotic effect and its influence on the intracranial pressure determine a reduction of the labyrinth hypertension, because of the movements of fluids outside the inner ear. This effect can be monitored through: (i) a tonal/vocal audiometry. Effects include a hearing threshold improvement of 10 dB HL in least two frequencies between 500 Hz and 2000 Hz, or an improvement of the verbal intelligibility score of at least 10%; (ii) through electrocochleography (EchoG). Effects include a decrease of the summating potential amplitude [33]; (iii) through OAEs, in particular with an improvement of the DPOAEs amplitude [19]. Overall, the data in the literature [19, 20, 29, 32] suggest that DPOAEs can monitor successfully how glycerol recovers the hearing threshold, compromised by the presence of hydrops.

DPOAEs are very sensitive to changes in the intracranial pressure [34]. Although Rotter et al. [35] found that DPOAEs are not as accurate as the transtympanic electrocochleography, other authors [27, 29, 30] support the role of DPOAEs as a reliable method allowing the detection of endolymphatic hydrops and the cochlear damage in MD [19, 33].

Theoretically, an MD case presenting multiple lesions in the inner ear should condition the OAE responses. In this context, one expects that the OAE responses should be attenuated at low or at high frequencies. There are a few reports in the literature showing the exact opposite [36]. In a study by van Huffelen et al. [36], MD cases were classified into 4 groups. When hearing thresholds were above 60 dB HL, no detectable OAEs were recorded. For those cases, presenting hearing thresholds within 30–60 dB HL, abnormally high DPOAE responses were observed. The authors suggested that these emissions are generated by cochlear sites, sustaining the residual hearing of the patient. Some MD case studies have also been reported presenting anomalous DPOAEs. Hall [37] describes a case diagnosed with a unilateral MD, where the left ear DPOAE responses were shown to be quite robust, despite the fact that the audiogram showed a low-frequency sensorineural hearing impairment. The same author shows that data analyses from groups of MD patients follow the patterns reported widely in the literature (i.e., attenuation of OAEs caused by OHC dysfunction).
