Transtympanic electrocochleography in auditory neuropathy

Auditory neuropathy is a disorder identified by the absence or the severe impairment of auditory brainstem responses (ABRs) with the preservation of otoacoustic emissions. It is generally accepted that the lesion should be localized at the level of the inner hair cells, the auditory nerve fibers or the synapse in between. The presence of otoacoustic emissions indicates that the outer hair cells are spared and possibly they are functional. We performed an audiologic evaluation in three patients showing distortion product otoacoustic emissions and absent ABRs. Since these findings could be attributed both to a selective damage involving the peripheral afferent component and/or to a de-synchronization of brainstem neural generators, patients underwent the recording of transtympanic electrocochleography (ECochG). The results were: 1) The first patient (aged 1) with hyperbilirubinemia at birth showed only the cochlear microphonic in the electrocochleographic recording; 2) The second patient (aged 17) had a severe impairment of speech discrimination out of proportion of the auditory threshold. The ECochG performed in this subject consisted in the summating potential followed by a neural activity highly desynchronized which was identifiable at stimulation intensities lower than the hearing threshold; 3) The third patient (aged 5) showed a moderate hearing loss at low frequencies and a severe impairment in speech discrimination associated with bilateral optic nerve atrophy. A compound action potential with normal amplitude and latency was identifiable in the ECochG recording till to a stimulation intensity corresponding to the PTA threshold. On the basis of ECochG recordings the lesion should be localized at the level of the cochlear afferent component only in the first two subjects. The presence of a CAP response at high to moderate stimulus intensities together with an absent ABR point to a superimposed brainstem lesion in the third patient. These results suggest that electrocochleography can be useful in the assessment of the auditory neuropathy since only the CAP detection in ECochG recordings is a reliable estimate of the auditory peripheral function in the presence of a de-synchronized ABR.