Auditory neuropathy: an update

To describe the round window electrocochleography (RWECochG) and electric auditory brainstem responses (EABR) in ears affected by auditory neuropathy (AN), and to determine if these electrophysiological tests can predict the outcome following cochlear implant surgery.

A longitudinal study of all pediatric cochlear implant patients between 1994 and 2005 was undertaken. Speech perception outcomes after cochlear implantation and electrophysiological data were collected prospectively and analyzed. Some otoacoustic emissions (OAE) data were collected retrospectively during the neonatal period. All subjects were tested using round window electrocochleography (RWEcochG), auditory brainstem responses (ABR), and implant-evoked electric auditory brainstem responses (EABR). The auditory neuropathy (AN) group consisted of 39 children (78 ears) which had present OAE and absent or grossly abnormal ABR (a broad N1 component only).

All 78 ears from the 39 AN children showed large cochlear microphonics (CM) and an abnormal positive potential (APP) using RW ECochG. A further 21 children showed large CM and APP but had not been tested for OAE. In total, 60 children were discovered to have APP among 435 pediatric patients who received a cochlear implant. Electrically evoked ABR (EABR) from the implanted ear were normal in 45 and abnormal in 15. 46 age matched patients without large CM and APP were used as a control group. Two year postimplant scores (Melbourne categories) were: 6.27 (APP and normal EABR), 2.25 (APP and abnormal EABR) and 5.37 (control group). Mann-Whitney U Test for nonparametric data was used to test for significant difference at significance level p < 0.005 (two tailed). The APP ears which provided normal EABR had significantly better outcomes after cochlear implantation than APP ears which had abnormal EABR. Furthermore, the APP ears which provided normal EABR performed significantly better after cochlear implant surgery than the control group of patients with no OAE, appropriate ABR results and normal EABR.

Ears affected by AN provide large CM and APP on RW ECochG. The presence of normal EABR may indicate a significantly better outcome after cochlear implant surgery than for those APP ears which had abnormal or absent EABR. Based on these findings it is suggested that the presence of APP and/ or OAE in 75% of the ears which have absent or abnormal ABR may not indicate a pathological condition affecting the auditory nerve or synapse but only survival of outer hair cells despite extensive loss of inner hair cells.