The identification and management of auditory neuropathy spectrum disorder (ANSD) can be daunting, meaning that a comprehensive understanding of the condition is essential for hearing healthcare professionals.
Many professionals in the field “don't see these children on a daily basis, making diagnosis and treatment challenging when patients do present with auditory neuropathy,” said Craig A. Buchman, MD, chief of the Division of Otology/Neurotology and Skull Base Surgery, as well as director of the Ear and Hearing Center and of the Skull Base Center, at the University of North Carolina (UNC) School of Medicine.
The heterogeneity of the disorder poses potential difficulties at every step in the management process, wrote Gary Rance, PhD, Wagstaff Research Fellow in otolaryngology and associate professor at the University of Melbourne in Victoria, Australia, in an e-mail to The Hearing Journal.
Overall, a multidisciplinary approach is critical to ensuring the best care is delivered to patients, said Dr. Buchman, who works closely with his team of audiologists, speech–language pathologists, and auditory electrophysiologists at UNC to diagnosis and manage the condition.
UNDERSTANDING THE PHYSIOLOGY
In auditory neuropathy spectrum disorder, pre-neural or cochlear outer hair cell activity is preserved, but neural activity in the auditory brainstem is disrupted, Dr. Rance said.
The reported prevalence of ANSD varies across studies. One review of four available papers found that the population-based prevalence in children undergoing hearing screening was between 0.006 percent and 0.03 percent ( Int J Pediatr Otorhinolaryngol 2012;76:1708-1711 http://www.ijporlonline.com/article/S0165-5876(12)00468-5/abstract).
Among children at risk for hearing loss, a 0.94-percent prevalence has been reported ( Int J Pediatr Otorhinolaryngol 2006;70:1415-1422 http://www.ijporlonline.com/article/S0165-5876(06)00072-3/abstract).
As for children who have permanent hearing loss, around 10 percent show the ANSD pattern, said Dr. Rance, referring to the literature.
While prevalence in adults is more difficult to define, recent work in the elderly populations suggests that if we live long enough, we may all end up with a mild form of auditory neuropathy spectrum disorder, Dr. Rance said.
A number of pathological mechanisms contribute to the condition.
Some of these mechanisms are pre-neural, involving, for example, loss of cochlear inner hair cells or disruption of the synaptic connection between these cells and the auditory nerve.
Other causes, such as axonopathy or demyelination, directly involve the auditory nerve, Dr. Rance explained.
When the auditory nerve is affected, the conduction of neural inputs is disrupted, said Arnold Starr, MD, research professor in the Department of Neurology (School of Medicine) and in the Department of Neurobiology and Behavior (School of Biological Sciences) at the University of California, Irvine.
A loss of neural synchrony, called auditory dyssynchrony, occurs.
“Adult patients often say that they can hear but they cannot understand what is being said,” said Dr. Starr, author of a seminal paper on the topic ( Brain 1996;119:741-753 http://brain.oxfordjournals.org/content/119/3/741.abstract).
GENETIC OR ACQUIRED
Auditory neuropathy spectrum disorder can be inherited or acquired, Dr. Buchman said. Notably, infants who spend a lot of time in the neonatal intensive care unit (NICU) are more frequently affected by ANSD, he added.
“About 50 percent of those with auditory neuropathy have spent time there. Whether this is directly related to being in the NICU, being premature, or a combination of both is not entirely clear.”
Most infants with ANSD have a specific medical risk factor for the condition, Dr. Rance added.
“Transient neonatal insults, such as hypoxia or hyperbilirubinemia, are most common,” he said.
Other causes, including infectious processes, such as mumps or meningitis; acoustic neuroma; inflammatory conditions, such as siderosis; autoimmune disorders, such as Guillain–Barré syndrome; and genetic mutations also have been reported in the literature.
The most common genetic cause of ANSD is related to mutation of the gene encoding otoferlin, which isn't usually associated with a lot of other health conditions, Dr. Buchman said.
ANSD also can have a progressive onset, with auditory deficits often presenting in middle childhood or early adolescence, Dr. Rance said.
Progressive ANSD is typically associated with a genetic neurodegenerative disease like Friedreich ataxia, Charcot–Marie–Tooth disease, or Leber's hereditary optic neuropathy, where the auditory deficit may be part of a generalized pattern of neural deterioration.
“Interestingly, auditory symptoms often present first in these diseases,” Dr. Rance said.
Genetic disorders are prominent in ANSD patients and generally can be identified at birth with appropriate screening, Dr. Starr said.
BEST PRACTICES FOR DIAGNOSIS
When diagnosing auditory neuropathy spectrum disorder, healthcare providers look for absent or markedly abnormal auditory nerve function measures, such as auditory brainstem response (ABR), while measures of sensory hair cell function, such as otoacoustic emissions (OAEs) and cochlear microphonics (CMs), remain normal.
“The criteria for diagnosis are clear,” Dr. Starr said.
Typically, the audiogram is normal or only minimally abnormal, he said, adding that speech perception is also impaired.
When auditory neuropathy spectrum disorder was first identified in the early 1990s, the diagnostic definition was quite narrow and included demonstration of both OAE and CM responses, in conjunction with an absent ABR, Dr. Rance said.
More recently, the definition has been broadened to include cases with present cochlear microphonic responses but absent otoacoustic emissions, and situations where the auditory brainstem response is recordable but “severely abnormal,” Dr. Rance said.
More work is needed so healthcare providers can differentiate between pathology and immaturity in young infants, he added.
While OAE testing is common for hearing screening in the United States, the approach may miss auditory neuropathy spectrum disorder, Dr. Buchman said.
Newborn screening with auditory brainstem response testing will identify the problem, but children who don't spend time in the NICU are less likely to undergo the test, he said.
Moreover, providers may miss an ANSD diagnosis if they don't properly conduct ABR audiometry, Dr. Buchman said.
To identify auditory neuropathy, diagnostic ABR testing needs to include single-polarity stimuli that are reversed in polarity to detect inverting responses. The sound tube has to be crimped or disconnected at some point to rule out electrical artifact.
ANSD diagnosis also must include imaging studies, such as MRI, to evaluate anatomy and behavioral tests to assess hearing, Dr. Buchman said.
The first major challenge to contend with in managing infants who have auditory neuropathy spectrum disorder involves sound detection, Dr. Rance said.
People with ANSD can present with hearing levels anywhere from the normal range to profound loss, and, unlike their counterparts with sensorineural hearing loss, ANSD patients’ thresholds can't be predicted using standard evoked potential assessment, he said.
Over time, clinicians have become more confident in revisiting older behavioral observation audiometry (BOA) techniques to estimate hearing levels in youngsters with ANSD, but, in most cases, intervention decisions tend to be put on hold until the child can provide conditioned audiometric thresholds, Dr. Rance said.
The second major challenge is that the audiogram does not predict functional hearing or speech perception ability in patients with auditory neuropathy spectrum disorder, Dr. Rance said.
All listeners with severe to profound hearing loss do poorly on the assessment. In those with lesser degrees of hearing loss, speech understanding is often determined by the degree of temporal distortion introduced by the neuropathy, rather than audibility, he said.
WHAT TO DO
Potential interventions for auditory neuropathy include observation if the child's hearing appears to be relatively normal, Dr. Buchman said.
The next step is to give the patient a hearing aid. If speech and language don't improve, then a cochlear implant is an option.
Since ANSD was first identified, opinion has fluctuated regarding the efficacy of acoustic amplification and cochlear implantation as management options, Dr. Rance said.
As more data have become available, there has been general, if not universal, acceptance that some patients will derive the most benefit from hearing aids, while others will be best served by implantation, he said.
Hearing aids allow improved access for ANSD listeners with impaired hearing thresholds, but they can't dramatically improve the clarity of a distorted signal, Dr. Rance noted.
As a result, outcomes with hearing aids have been mixed, he said. Some patients, typically children with lesser degrees of temporal disruption, respond well and show aided perception abilities consistent with those of their counterparts who have sensorineural hearing loss.
Generally, cochlear implants are beneficial in both presynaptic and postsynaptic forms of auditory neuropathy spectrum disorder, said Dr. Starr, adding that the device activates the auditory nerve to produce neural synchrony.
“The vast majority of patients do well with an implant,” Dr. Buchman noted.
While cochlear implantation is currently the intervention of choice, a number of recent studies have reported poorer-than-expected results, raising the possibility that some forms of ANSD can affect the integrity of the cochlear implant-elicited signal as it passes through the auditory pathway, Dr. Rance said.
These cases highlight the need for diagnostic procedures that can better differentiate lesion sites in ANSD.
In certain cases of auditory neuropathy spectrum disorder, MRI indicates that the patient doesn't have any auditory nerves present or that these nerves are too small for a cochlear implant to work, Dr. Buchman said.
In infants with this condition, researchers are evaluating implants that bypass the auditory nerve and deliver stimulation directly to the brainstem so patients can detect sound.
The use of auditory brainstem implants for infants and young children is relatively new in the United States, Dr. Buchman said. (To learn more about the approach, read the Journal Club article http://journals.lww.com/thehearingjournal/Fulltext/2013/09000/Journal_Club___Auditory_Brainstem_Implant_Sees.2.aspx from HJ's Sept. 2013 issue, page 4.)
WATCHING AND WAITING
Overall, finding the most appropriate intervention for very young patients with auditory neuropathy spectrum disorder can be a frustrating process, as practitioners can't assess their hearing levels, Dr. Buchman said. As a result, these patients can't be fit with hearing aids.
“We watch and wait to test them,” he said.
Then, once a child can be fit with hearing aids, the professional goes into watch-and-wait mode once again to see if the patient has any improvement in speech perception and production before a cochlear implant is considered.
“This is why we need some objective tests to evaluate hearing in these patients,” he said.
To address these challenges, Dr. Buchman and his colleagues are using electrophysiology testing to evaluate the cerebral auditory cortex and its activation following sound input.
“It's emerging, but it's been really exciting. If we can apply this to babies, we can fit [hearing aids] at a much earlier stage.”
Such testing can also be used to determine patients’ ability to discriminate various stimuli, which helps predict whether they'd benefit from hearing aids or how much cochlear implants would help, Dr. Buchman said.
“It could ultimately take a lot of the guesswork out of auditory neuropathy.”
This type of electrophysiological testing is “particularly exciting for children who are unable to carry out behavioral testing because of cognitive delays,” Dr. Buchman said.
Ultimately, if healthcare professionals understand the etiology of auditory neuropathy spectrum disorder, they can better predict what treatment children should receive, Dr. Buchman said.
“We'll be able to categorize patients better and move them through the evaluation process more quickly and accurately.”
Research examining the genetic causes of the disease may one day lead to the use of genetic modification for treatment, Dr. Starr said.
“We're [also] very interested in whether stem cells can correct the abnormality of the cochlea.”
Auditory neuropathy spectrum disorder is one of the most vigorously researched topics in audiology, and for good reason, Dr. Rance said.
“It is an inherently interesting area, at the nexus between audiology and neurology.”