Auditory neuropathy spectrum disorder (ANSD) is currently one of the most exciting and controversial topics in pediatric audiology. Our staff at Boys Town National Research Hospital in Omaha, NE, frequently receives inquiries from families audiologists, teachers, and other medical professionals about how to manage this perplexing disorder and support positive developmental outcomes. Audiologists must first accurately differentiate the disorder from other types of hearing loss to make recommendations about how to help children with ANSD. A stepwise management approach should be used following diagnosis to determine whether hearing aids or cochlear implants will be used to provide auditory access.
ANSD is quite rare, but early accurate identification is crucial for supporting positive outcomes. (Pediatrics 2011;127:269.) A significant amount of emphasis has been placed on the electrophysiological parts of assessment, which are absolutely critical for making the diagnosis. Much of the information about the child's functional auditory skills, however, must be taken from other measures to guide the intervention process.
The key components that must be present to make a diagnosis of ANSD are evidence of normal outer hair cell function in the cochlea and evidence of significantly abnormal auditory nerve function. Normal outer hair cell function is established through either normal otoacoustic emissions or the presence of cochlear microphonic in first few milliseconds of the auditory brainstem response (ABR). Abnormal auditory nerve function is documented through an ABR that is absent or significantly abnormal at high stimulus intensity levels (80-90 dB nHL). Acoustic reflexes are frequently absent, but they can be absent in children for a variety of other reasons, and an ABR should always be used as the definitive diagnostic test for auditory nerve function with ANSD. (J Am Acad Audiol 2005;16:546.) Normal outer hair cell function and abnormal auditory nerve function must be present to make a diagnosis.
Guiding the Diagnostic Process
Avoid alternating polarity for ABR, which was primarily suggested as a method of avoiding stimulus artifact back when ABR was recorded with giant circumaural headphones. Current ABR equipment shielding has improved significantly, and methods are available to differentiate stimulus artifact from cochlear microphonic. Alternating polarity combines rarefaction and condensation stimuli in the same waveform, so cochlear microphonic will not be visible because the responses from the two polarities cancel each other out when combined.
Careful investigation is needed when rapidly fluctuating electrical activity occurs in the early part of the ABR. Another waveform should be collected at the same intensity level using condensation polarity if rarefaction polarity is used for ABR assessment, which we do at Boys Town. (Figure.) One more step is needed to differentiate cochlear microphonic from stimulus artifact if the rapid fluctuations reverse direction with changes in polarity. Remove or clamp the sound tube from the inserted earphone without moving the insert transducer to prevent the acoustic signal from reaching the ear. The response is stimulus artifact if the fluctuations continue without acoustic stimulation to the ear. Cochlear microphonic will disappear when the acoustic stimulation is not routed to the ear through the insert tubing.
Cochlear microphonic, for the most part, is a good indication that ANSD is present when it occurs with a significantly abnormal ABR. I have also observed, however, cochlear microphonic on the ABR for children with normal low-frequency hearing sensitivity and high-frequency sensorineural hearing loss (SNHL). The cochlear microphonic in these cases presumably originates from normal outer hair cells at the apical end of the cochlea that are stimulated by the broadband click. This is clearly not ANSD because the latency-intensity function for wave V is normal and otoacoustic emissions are clearly absent for the high frequencies.
Initiate behavioral assessment as soon as possible. The ABR that usually provides an estimate of hearing thresholds in infants is significantly abnormal in cases of ANSD, so audiologists must wait to initiate amplification until behavioral thresholds can be reliably obtained using visual reinforcement audiometry. Many children with ANSD also have comorbid developmental concerns that can delay early behavioral hearing assessment, even though this process can begin around age 6 months in infants with typical development. Behavioral hearing assessment provides key information that will support decisions about appropriate management.
Use outcomes measures to document auditory development. Children with ANSD may be at higher risk of developmental delays because of a greater likelihood of comorbid conditions and more variability in auditory skills, even though it is important to monitor auditory skill development for all children with hearing loss. Audiologists should closely monitor auditory skills using standardized measures. The University of Western Ontario Pediatric Audiological Monitoring Protocol includes such measures and assists clinicians in documenting important aspects of hearing aid verification and use. (Trends Amplif 2010;15:57; see FastLinks.)
Assess speech understanding early and often. Children with ANSD can have speech recognition that can be equal to or poorer than children with SNHL of the same degree. (Ear Hear 2012;23:239.) Measuring speech recognition provides important information about how effectively speech information is being understood with and without hearing aids or cochlear implants. Speech recognition should be initiated as soon as the child can participate developmentally. The recent development of test materials like the Phrases in Noise Test do not require verbal response and use language that is appropriate for young children may improve the likelihood of speech recognition at younger ages or for children with developmental delays. (Ear Hear 2012 Jun 8. [Epub ahead of print].)
Assessing children with ANSD is an ongoing process that requires clinicians’ persistence to develop a full picture of the child's auditory skills. Hearing sensitivity and speech understanding can fluctuate or progress over time in children with ANSD, so more frequent audiological assessment may be required to gather additional data.
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