Objectives: 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. Methods: 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). Results: 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 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. (Ear & Hearing 2007;28;102S-106S) For many years clinicians have been aware that a few hearing impaired children have a disproportionate loss of speech perception leading to a poor performance using conventional hearing aids. The term "auditory neuropathy" was originally coined in 1995 (Sininger, Hood, & Starr, 1995) after the first longitudinal study of 10 subjects who were discovered to have absent or grossly abnormal auditory brainstem. responses (ABR), but present otoacoustic emissions (OAE). These findings led to the concept that the dysfunction primarily affected the neural auditory pathway (Starr, Picton, Sininger, et al., 1996); however, this concept failed to account for the unexpectedly good performance of most of these ears using a cochlear implant (Miyamoto, Kirk, Renshaw, et al., 1999; Trautwein, Sininger, & Nelson, 2000; Shallop, Peterson, Facer, et al., 2001; Gibson & Sanli, 2002; Madden, Hilbert, Rutter, et al., 2002; Buss, Labadie, Brown, Grosset, et al., 2002; Mason, De Michelle, & Stevens, 2003). This paper reviews the electrophysiological findings obtained from children affected by AN. OAE are assumed to be derived from the motor activity of outer hair cells (OHC) and do not appear to depend in any important way of the functioning of inner hair cells (IHC) (Mills, 2006). IHC are not connected to the overlying tectorial membrane and cannot exert an effective force back on the organ of Corti and cannot be a significant source of OAE (Trautwein, Hofstetter, Wang, et al., 1996). Abnormal positive potentials (APP) are an early positive summating potential (SP) with respect to an active electrode placed in the round window niche (O'Leary, Mitchell, Gibson, et al., 2000). The APP or early positive SP is probably generated by a slight asymmetry of production of CM. Conclusions: 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.
