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 5 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) One of the two children (aged 1) with hyperbilirubinemia at birth showed only the cochlear microphonic in the electrocochleographic recording; the second child (aged 4) had also a high amplitude summating potential; 2) The third 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) Of the two patients with only a moderately elevated PA threshold in eectrocochleography, the first child (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. The second child had a profound hearing loss at the behavioral evaluation in spite of the peripheral threshold estimated by EcochG recordings. 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.