Auditory Neuropathy Spectrum Disorder (ANSD) is defined as a hearing condition in which sound enters the inner ear normally, but the transmission of the signal to the brain is impaired. The characteristics of ANSD are highly variable and can manifest as any degree of hearing loss and functional ability. Some individuals with ANSD have minimal auditory complaints and do not know of their condition until adulthood. Others can have a complete lack of sound awareness, localization and auditory-based language ability (Berlin et al. , 2010).
However, most individuals fall in between these two extremes and typically complain of fluctuating hearing loss and difficulty understanding speech in noise. These vast individual differences in perception led researchers and audiologists to shift from the original name auditory neuropathy, coined by Starr et al. , (1996), to the term auditory neuropathy spectrum disorder (ANSD), which has been widely accepted. Although the prevalence of ANSD is not yet known, researchers estimate approximately 10-15% of individuals with hearing loss are affected with ANSD (Madden et al. 2002).
Risk factors such as hyperbilirubinemia, low birth weight and perinatal intracranial hemorrhage have been associated with the onset of ANSD. It is imperative that infants, especially those with these risk factor conditions, are screened for hearing loss under the Universal Newborn Hearing Screening Program. More importantly, a two-step procedure utilizing both otoacoustic emissions (OAEs) and auditory brainstem response (ABR) are necessary to identify ANSD in newborns.
ANSD is clinically diagnosed by an absent or abnormal ABR due to auditory nerve dysfunction, in the presence of outer hair cell function, indicated by present otoacoustic emissions OAEs. Speech is inherently dynamic because it involves a myriad of changes in frequency, amplitude and duration. Speech perception problems are a common feature of ANSD. The ability to perceive individual phonemes and understand speech in the presence of background noise is analogous to psychoacoustic measures, such as temporal processing.
Researchers hypothesize that an abnormality of the inner hair cells, the auditory nerve, or the synapse between them are the causes of signal disruption and poor temporal resolution in individuals with ANSD. For example, Zeng, Oba, Garde, Sininger, and Starr (1999) observed that children with ANSD performed poorly in tests of gap detection and temporal modulation transfer function (TMTF). Gap detection involves the ability to detect a small silence in between a burst of noise, while TMTF involves the identification of a change in amplitude in noise.
The researchers found that there was a correlation between these temporal findings and speech perception. The participants who had more difficulty perceiving small changes in stimuli over time, also experienced greater difficulty perceiving speech. This difficulty is exacerbated in the presence of background noise. Zeng & Liu (2006) observed that participants with ANSD performed significantly worse in sentence recognition tasks than those with normal hearing and sensorineural hearing loss (SNHL) across various signal to noise ratios (SNR).
Even in a condition of +15 dB SNR, the participants yielded an average score of 30% words correct. Commonly, audiologists are able observe an individual’s difficulty with speech perception when performing a complete audiological evaluation. The behavioral audiogram can show a hearing loss of any degree. But, the word recognition scores of those with ANSD are often poorer than what one would expect from a cochlear loss alone (Rance, Barker, Ching & Sarant, 2007). Essentially, the word recognition scores are unpredictably poor and they tend to be unrelated to the behavioral thresholds. Kraus et al. 2000) suggested even when words are presented at suprathreshold levels, there are distortions that occur in individuals with ANSD which limits their ability to identify the words correctly.
Researchers are still investigating possible treatments for people with ANSD. The two main treatments currently utilized are hearing aids and cochlear implants. However, there is much debate about the effectiveness of these devices in individuals with ANSD. In view of the differing sites of lesion and vast individual differences in auditory perception, the success rates of amplification vary. Rance et al. 2007) examined the receptive language and speech production abilities in children with ANSD managed with hearing aids.
These participants were compared to children with SNHL who were also amplified. Receptive language was measured using the Peabody Picture Vocabulary Test and speech production was measured using the Diagnostic Evaluation of Articulation and Phonology. The study revealed delays in receptive language and speech production in those with ANSD in comparison to the norms established with typically hearing children. However, there were no significant differences between the children with ANSD and SNHL.
The researchers concluded that children with ANSD could benefit from standard hearing aids and can do as well as their peers with SNHL. A limitation of the Rance et al. (2007) experiment was that the participants of the experiment had a mild form of ANSD and were high functioning. If the participants were more severely affected the hearing aids may not have demonstrated such benefit. Berlin et al. , (2010) collected data from a larger and more diverse sample (n=260) of individuals with ANSD. Eighty-five of the participants were aided and only 3. 5% reported good functional benefit with hearing aids.
Conversely, 85% of the participants reported that the hearing aids provided little to no benefit. It can be said that current hearing aid technology does not directly address the complex temporal processing deficits associated with ANSD (Jestad & Souza, 2007). There is concern that hearing aids without the appropriate technology will simply amplify an already distorted signal and background noise. However, Spirakis (2011) suggested fitting patients with slow release compression hearing aids to allow for greater perception of amplitude and temporal modulations.
Slow release compression amplification can potentially aid the acuity of “speech events, such as voice onset time, burst, and transitions, and maximize signal-to-noise ratios”, which can improve speech perception. Individuals who have tried hearing aids and have no functional benefit are usually candidates for cochlear implants. Cochlear implants bypass both the inner hair cell and synapse to directly stimulate the auditory nerve with electrical impulses, which allows for synchronized neural firing (Madden et al. 2002).
Research has demonstrated more favorable outcomes for this treatment option. The abovementioned Berlin et al. (2010) study additionally measured outcomes of participants who received cochlear implants. Of the forty-nine who were cochlear implant users, 86% reported improvement in speech and language acquisition. Zeng & Liu (2006) also noted that the implanted participants with ANSD scored significantly higher in sentence recognition in both quiet and noise listening conditions.
Though there is a clinical shift to treat ANSD with cochlear implants, audiologists and physicians should be aware that a small percentage of patients with ANSD also have absent or small auditory nerves which is a poor indication for cochlear implants (Buchman et al. , 2006). The ANSD should also be stable before making any permanent surgical because ANSD can spontaneously recover in some patients (Madden et al. , 2002). In conclusion auditory neuropathy spectrum disorder (ANSD) is a hearing condition that has a wide variety of individual perceptual effects, especially in regards to speech and language.
Typically, individuals with ANSD complain of difficulty with speech recognition in quiet and noise. In addition, research has demonstrated that individuals with ANSD have delays in receptive language and speech production. Their difficulties cannot be predicted from the behavioral audiogram, so audiologists must be vigilant when conducting assessments. The hallmark of ANSD is present otoacoustic emissions and absent or abnormal ABR findings. Newborn hearing screening should include both tests in order to increase the rate early identification and intervention.
In view of the complex pathologic processes and varying degrees of ANSD, the benefits of current treatment options are still being investigated. Hearing aid may be of limited benefit as it amplifies already distorted speech, but for those with mild ANSD it can be sufficient, and for others it is a necessity before obtaining cochlear implants. Cochlear implants show more favorable outcomes and can improve synchrony and temporal processing in individuals with ANSD. With these treatment options and rehabilitation, patients can perform as well as their peers with cochlear losses.
