Auditory neuropathy spectrum disorder is among the most vigorously discussed topics in audiology — for good reason.
Fledgling auditory screening programs first used to diagnose infants with ANSD in the late 1980s and early 1990s led audiologists to assume that absent auditory brainstem responses meant that the stimuli weren't loud enough to elicit a response, and we were alarmed when children started presenting with no auditory brainstem responses but relatively normal sound detection. Not surprisingly, initial research focused on the mechanisms that might produce this disturbing result.
But interest has shifted recently to important clinical questions: What are the functional consequences of ANSD for perception and speech and language development? What are the best intervention strategies?
This would be a simple story if the effects of ANSD were the same as cochlear hearing loss. Patients with this form of hearing loss have normal cochlear outer hair cells, but neural transmission in the eighth nerve and higher is abnormal. This disorder presents with typical preneural responses, such as otoacoustic emissions and cochlear microphonics, and absent or severely distorted neural reactions, including auditory brainstem responses.
Management strategies would have been essentially the same, apart from not being able to predict the audiogram from evoked potential thresholds. But it has recently become clear that the perceptual consequences of ANSD are different from those caused by cochlear hearing loss. ANSD mainly affects the processing of temporal (timing) information through disruption of neural firing patterns while cochlear hearing loss primarily disrupts the resolution of frequency cues. These fundamental differences have implications for management, and have led to spirited debates over whether hearing aids or cochlear implants are the best option for affected children.
Hearing aids deliver increased access to the speech signal, provided the child has sufficient residual hearing and the aids have adequate gain, but they can also damage normal cochlear structures. Conventional amplification is not designed to improve the clarity of temporally distorted signals, providing louder but equally unintelligible sounds. Cochlear implant's, on the other hand, can provide improved access to speech signals, and electrical stimulation may bypass peripheral lesion sites. The cochlear implant's generated signal, however, may still need to pass through a disordered neural system.
Generating evidence to address these issues has proven challenging. Despite the significant interest in research over the past decade, the relatively low prevalence of ANSD has meant that exploratory research involving individual cases or studies with small numbers has dominated the literature. A recent study by Roush et al has attempted to tackle this problem, producing a meta-analysis of all the available intervention data for children with ANSD.
Audiologic Management of Auditory Neuropathy Spectrum Disorder in Children: A Systematic Review of the Literature
Roush P, Frymark T, et al Am J Audiol 2011;20:159
Roush et al summarized the current evidence through July 2010 related to the audiologic management of children with ANSD. They focused particularly on the effects of acoustic amplification and cochlear implantation in auditory function (e.g., sound detection and speech perception), speech and language development, academic progress, and social and emotional status.
The literature search yielded 202 citations, and 18 met methodological standards that involved appropriate participants (0-18 years), and addressed the target questions. Findings for 115 participants with ANSD were included.
The study found that many standard outcome criteria had not been adequately addressed in ANSD literature. No data were published on academic, social, and emotional outcomes in children with ANSD. Similarly, speech and language progress have been given scant attention, with only one paper presenting pre- and postoperative cochlear implant data in four children. Subsequent work has addressed this issue to a limited extent, but this important area is certainly in need of further research.
The effects of intervention on auditory function received more attention. Four papers involving 28 participants fit with conventional amplification were included in the meta-analysis. The participants all showed improved sound detection thresholds with hearing aids, but they varied markedly in their capacity to use what they heard. Fifty percent of participants showed a significant speech perception advantage when aided, and the other 50 percent demonstrated little or no ability to make sense of the amplified signal.
The researchers did point out that most of the participants had audiograms in the profound range and that incomplete access to the speech spectrum may have skewed the findings. The majority of participants, though, did have reasonably aided access to normal speech, so performance variability most likely reflected differing degrees of neural distortion rather than sound detection.
Fifteen studies involving 88 participants with cochlear implants were included in the review. Again, all participants showed improved sound detection. Postoperative speech perception was significantly better in most cases, with many participants performing at levels similar to their implanted peers with sensorineural loss. The data did, however, reveal a broad performance range with 27 percent of ANSD participants showing open-set word scores of less than 30 percent, which is below the level necessary to support conversational speech. I found this particularly interesting because opinions over the past decade have strongly supported cochlear implants as the primary option for children with ANSD.
Roush et al diplomatically and correctly concluded that the findings from this review do not resolve the controversies surrounding audiologic treatment of ANSD in children. I found this review enlightening and a little sobering in the way it highlighted gaps in the ANSD canon. The field has certainly progressed since the review was conducted, but big questions still remain. While it's now reasonably well accepted that hearing aids and cochlear implants have a place in ANSD management, the long-term benefits and limitations of each strategy and their applicability for individual children has yet to be determined.
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