Objectives: Hypoplasia of the auditory nerve (AN) refers to significant narrowing of the VIIIth cranial nerve which could compromise stimulation of the nerve by electrical pulses delivered from a cochlear implant (CI), thereby hindering activity in other parts of the auditory pathways. To compensate, high current levels or increased charge may be required to elicit auditory perception causing current to spread to other cranial nerves and potentially resulting in unwanted myogenic responses. Deficits in central auditory activity could reduce perception of speech and language. In the present study, we measured auditory brainstem responses in children with and without hypoplasia of the AN to answer the following questions. In children with hypoplastic ANs, (a) can CI stimulation evoke typical patterns of activity from the AN and brainstem?, (b) do brainstem responses change with CI experience?, (c) are evoked responses dependent on the size of the AN pathway?, and (d) does auditory development measured by behavioral tests of speech perception develop more slowly than in peers with normal AN diameter?
Design: Of 807 children using CIs in our program, 20 (2.5%) were identified as having AN hypoplasia using high-resolution computed tomographic scan and/or magnetic resonance imaging. An age-matched control group of children using CIs with normal AN diameter were recruited to compare electrophysiological and behavioral measures. Radiologic imaging was used to measure the diameter of the internal auditory canal (IAC), auditory nerve canal (ANC), and AN. Electrophysiological testing of the evoked compound action potential and auditory brainstem response was performed at CI activation and every 3 mo after initial testing up to 2 yr. Peak latency and waveform morphology were compared between study and control groups. Tests of speech perception and discrimination were attempted every 12 mo after device activation up to 10 yr.
Results: : Hypoplastic AN was identified as moderate to critical stenosis of the IAC, ANC, and AN. Initial electrically evoked compound action potential responses were mostly absent in children with AN hypoplasia. In the time window when electrically evoked auditory brainstem responses would be expected, some responses included single amplitude peaks at normal wave eV latencies, but the majority were abnormal, with peaks at atypical latencies or with no observable wave peaks. All evoked responses were inconsistent over time and did not reflect a typical pattern of auditory brainstem development. Speech perception scores were significantly poorer in the study group compared with controls and did not improve with CI experience. The type of abnormal evoked waveform response was independent of IAC, ANC, or AN diameter and also independent of behavioral outcome measures.
Conclusions: : Evoked responses recorded in CI children with AN hypoplasia indicate a high incidence of nonauditory activity with CI use. The range of abnormal responses was not predicted by the severity of the hypoplastic AN or associated structures. This, along with poorer auditory development compared with peers with normal AN diameters, suggests that children with hypoplasia of the AN are poor candidates for cochlear implantation.