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. 2006 Apr;26(2):78-95.

Cochlear microphonic potential recorded by transtympanic electrocochleography in normally-hearing and hearing-impaired ears

R Santarelli  1 , P Scimemi, E Dal Monte, E Arslan
Affiliations

Cochlear microphonic potential recorded by transtympanic electrocochleography in normally-hearing and hearing-impaired ears

R Santarelli et al. Acta Otorhinolaryngol Ital. 2006 Apr.

Abstract
in English, Italian

The cochlear microphonic is a receptor potential believed to be generated primarily by outer hair cells. Its detection in surface recordings has been considered a distinctive sign of outer hair cell integrity in patients with auditory neuropathy. This report focuses on the results of an analysis performed on cochlear microphonic recorded by transtympanic electrocochleography in response to clicks in 502 subjects with normal hearing threshold or various degrees of hearing impairment, and in 20 patients with auditory neuropathy. Cochlear microphonics recorded in normally-hearing and hearing-impaired ears showed amplitudes decreasing by the elevation of compound action potential Cochlear microphonic responses were clearly detected in ears with profound hearing loss. After separating recordings according to the presence or absence of central nervous system pathology (CNS+ and CNS-, respectively), cochlear microphonic amplitude was significantly higher in CNS+ than in CNS- subjects with normally-hearing ears and at 70 dB nHL compound action potential threshold. Cochlear microphonic responses were detected in all auditory neuropathy patients, with similar amplitudes and thresholds to those calculated for normally-hearing CNS- subjects. Cochlear microphonic duration was significantly higher in auditory neuropathy and normally-hearing CNS+ patients compared to CNS- subjects. Our results show that: 1. cochlear microphonic detection is not a distinctive feature of auditory neuropathy; 2. CNS+ subjects showed enhancement in cochlear microphonic amplitude and duration, possibly due to efferent system dysfunction; 3. long-lasting, high frequency cochlear microphonics with amplitudes comparable to those obtained from CNS- ears were found in auditory neuropathy patients. This could result from a variable combination of afferent compartment lesion, efferent system dysfacilitation and loss of outer hair cells.

Il microfonico cocleare (CM) è un potenziale recettoriale che si ritiene venga generato principalmente dalle cellule ciliate esterne (OHCs). Il suo rilevamento mediante registrazione di superficie è stato considerato un segnale distintivo dell’integrità delle OHCs nei pazienti affetti da neuropatia uditiva (AN). In questo lavoro vengono presentati i risultati relativi ad una analisi del CM registrato mediante elettrococleografia transtimpanica in risposta a click effettuata in 502 pazienti con soglia uditiva normale o con diversi gradi di deficit uditivo ed in 20 pazienti con AN. In tutti gli orecchi con soglia normale o con ipoacusia è presente il microfonico cocleare la cui ampiezza diminuisce all’aumentare della soglia del potenziale d’azione (CAP). Potenziali di grande ampiezza sono stati evidenziati chiaramente anche negli orecchi di pazienti che presentavano una ipoacusia profonda. Suddividendo i soggetti esaminati in base alla presenza (CNS+) o all’assenza (CNS-) di patologia del Sistema Nervoso Centrale, si è notato che l’ampiezza del CM è significativamente maggiore nei soggetti CNS+ rispetto ai soggetti CNS- sia nei soggetti con un udito normale che in quelli con una soglia del potenziale d’azione a 70 dB nHL. Le risposte CM sono state rilevate in tutti i pazienti con neuropatia uditiva con soglie e ampiezze simili a quelle presenti nei soggetti CNS- con udito normale. La durata del CM è risultata significativamente maggiore nei pazienti normoacusici CNS+ e in quelli affetti da neuropatia uditiva rispetto ai pazienti CNS- con udito normale. I nostri risultati dimostrano che: 1. il rilevamento del CM non è una caratteristica peculiare della AN; 2. i soggetti CNS+ presentano un incremento di ampiezza e di durata del CM, probabilmente dovuto ad una disfunzione del sistema efferente; 3. CM ad alta frequenza, di lunga durata e di ampiezza comparabile a quella ottenuta negli orecchi dei pazienti CNS-, sono stati riscontrati nei pazienti con AN. Questo potrebbe essere dovuto alla combinazione variabile di fattori quali una lesione nel compartimento afferente, la disfacilitazione del sistema efferente e la perdita di OHCs.

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Figures

Fig. 1
Fig. 1
Electrocochleography recordings obtained from three representative ears showing normal CAP threshold, elevated CAP threshold and the absence of neural response at maximum stimulation intensity (120 dB pe SPL). In this and following figures, CAP and CM traces, obtained by classic procedure of averaging recordings to condensation and rarefaction clicks, are reported for decreasing stimulation intensities.
Fig. 2
Fig. 2
Electrocochleography recordings from two representative normally–hearing ears, obtained from one CNS- (left) or CNS+ (right) subject, respectively. Note the high amplitude, low threshold and “long-ringing” appearance of cochlear microphonic recorded in CNS+ patient.
Fig. 3
Fig. 3
CM amplitudes obtained at various CM (left) or CAP (right) thresholds. Individual CM amplitudes calculated at 120 dB peSPL are reported at each CM or CAP threshold together with corresponding mean values. Interrupted line represents exponential curve fitting individual data.
Fig. 4
Fig. 4
Percentage of DPOAE presence at various CAP thresholds. Open squares: all ears; closed squares: percent occurrence of DPOAE obtained excluding AN ears. N and nw: total number of ears exhibiting DPOAEs with or without the inclusion of AN patients, respectively.
Fig. 5
Fig. 5
Cochlear microphonics recorded from all patients with auditory neuropathy at maximum stimulation intensity (120 dB peSPL). In each subject, upper trace refers to right ear. Marked differences in CM amplitude and duration as well as in oscillation frequency are evident.
Fig. 6
Fig. 6
Upper graph shows CAP thresholds, CM thresholds and amplitudes calculated for normally–hearing CNS- and CNS+ subjects and AN patients. Values are expressed as percent variation of CNS- values. Grand averages of CM curves recorded at maximum stimulation intensity (120 dB peSPL) obtained from CNS-, CNS+ and AN subjects illustrated below. Beginning of each trace corresponds to first deflection observed in CM in recordings obtained in each ear.
Fig. 7
Fig. 7
Electrocochleographic responses evoked by condensation and rarefaction clicks recorded from two normally–hearing ears in two CNS- subjects at maximum stimulation intensity (120 dB peSPL). The law–amplitude oscillatory activity following negative deflection, corresponding to neural response, shows an opposite phase from condensation to rarefaction stimuli.
Fig. 8
Fig. 8
Cochlear microphonics (left), instantaneous frequency (middle) and FFT spectrum–calculated on full time window right obtained from four representative ears from CNS-, CNS+ or AN groups. NB: longer CM duration in both CNS+ and AN ears compared to CNS- curve as evaluated by the instantaneous frequency graph. NB: also high frequency of CM third row, which is clearly evident both in instantaneous frequency graph and FFT spectrum.
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