Auditory neuropathy spectrum disorder (ANSD) is a unique type of hearing loss generally characterized by present outer hair cell function and dys-synchronous auditory neural activity. Clinically, the diagnosis is defined by present otoacoustic emissions and/or cochlear microphonic with absent auditory brainstem response and middle ear reflexes. The category of ANSD encompasses several etiologies that give rise to dys-synchronous neural responses, with potential sites of lesion including the inner hair cells, their synapse with the auditory nerve, and the auditory nerve fibers themselves (Starr et al. 1996; Starr et al. 2000; Berlin et al. 2005). Whether pre- or post-synaptic in origin, the dys-synchrony in ANSD primarily results in a temporal impairment.
Patients with ANSD demonstrate impaired performance on temporal tasks such as gap detection, modulation detection, and encoding of low frequency phase-locking cues compared to those with normal hearing and sensorineural hearing loss (SNHL) (Zeng et al. 1999; Rance et al. 2004; Zeng et al. 2005; Yalcinkaya et al. 2009). In addition, in ANSD the severity of temporal impairment on psychophysical measures has a direct relationship to deficits in speech perception performance (Zeng et al. 1999; Rance et al. 2002; Rance et al. 2004). In contrast, patients with SNHL have impaired loudness discrimination, high-frequency discrimination, and frequency resolution, while these same abilities are normal in patients with ANSD (Rance et al. 2004; Zeng et al. 2005). Unlike SNHL, in ANSD temporal impairment renders audiometric thresholds unreliable in estimating the degree of impairment. In addition, the degree of hearing loss in ANSD often underestimates the difficulties of speech recognition in quiet and particularly in noise (Kraus et al. 1984; Kraus et al. 2000; Zeng and Liu 2006). The converse impairment patterns in ANSD and SNHL precludes clinically managing these patients in the same way, and optimal clinical management of patients with ANSD is currently being explored.
Current clinical intervention protocols for ANSD typically include a trial with acoustic amplification. At a minimum, hearing aids fit to the severity of the hearing loss will provide audibility of acoustic input; however, this does not guarantee adequate development of auditory and speech and language skills. A subset of children with ANSD may have speech perception in quiet and spoken language abilities comparable to matched SNHL controls (Rance et al. 2002; Rance et al. 2007; Rance and Barker 2008), however speech perception performance in noise remains significantly poorer in ANSD (Rance et al. 2007). In a retrospective analysis of 260 patients with ANSD, hearing aid outcomes were reported for 85 patients (Berlin et al. 2010). It was surmised that 14% of patients gained benefit from amplification as manifested by increased functional interactions or language acquisition, while the remaining patients reportedly showed little (25%) or no benefit (61%) with hearing aids. It appears that for many patients with ANSD amplification does not provide adequate benefit, and in these cases cochlear implantation is often recommended as the next treatment option.
The implanted pediatric ANSD population is unique in that there is higher likelihood for better hearing thresholds in the contralateral ear compared to pediatric SNHL cochlear implantees, which tend to have bilateral severe-to-profound hearing loss as a criterion for candidacy. In patients with SNHL who are unilaterally implanted, contralateral acoustic input can lead to improved speech perception, particularly when low-frequency information is available through the hearing aid (Ching et al. 2004; Qin and Oxenham 2006; Kong and Carlyon 2007; Cullington and Zeng 2010; Mok et al. 2010). In cases of unilateral implantation in patients who have ANSD and who have aidable contralateral hearing, there are questions as to management of the contralateral ear. This is particularly an issue because ANSD is characterized by impaired processing of low frequency information (Zeng et al. 1999; Rance et al. 2004; Zeng et al. 2005; Yalcinkaya et al. 2009), hence there is question as to whether the bimodal advantages found in patients with SNHL would similarly be observed in those with ANSD. In ANSD, the potential exists for acoustic input to deliver misinformation that would interfere with speech perception from the cochlear implant. Currently, clinical guidelines regarding intervention of the contralateral ear are lacking, with the choices including no intervention, amplification, occlusion, or implantation.
The purpose of this study was to examine acute effects of contralateral intervention in implanted children with ANSD and aidable contralateral hearing. Specifically, the contralateral ear was either fit with a hearing aid or occluded with an ear plug. It was hypothesized that contralateral acoustic input would interfere with speech perception achieved with the cochlear implant alone; therefore speech perception performance would decline with amplification and improve with occlusion.