Auditory neuropathy: What is it and what can we do about it

Original Material From The Hearing Journal
Volume 51, Number 8, August, 1998

By Linda J. Hood, Ph.D.
Louisiana State University Health Sciences Center
Department of Otorhinolaryngology and Biocommunication and the Kresge Laboratory

lhood@lsuhsc.edu

Phone Int + , Fax Int

1. What is auditory neuropathy?

Auditory neuropathy is a term presently used to describe a condition, found in some patients ranging in age from infants to adults, in which the patient displays auditory characteristics consistent with normal outer hair cell function and abnormal neural function at the level of the VIIIth (vestibulo-cochlear) nerve. These characteristics are observed on clinical audiologic tests as normal otoacoustic emissions (OAEs) in the presence of an absent or severely abnormal auditory brainstem response (ABR).

These patients are distinguished from patients with space-occupying lesions, such as VIIIth nerve tumors, or multiple sclerosis, in that radiological evaluation yields normal results and even the most peripheral responses from the VIIIth nerve are absent. Patients with auditory neuropathy require a different management approach to their auditory and communication problems from approaches used with patients with usual peripheral hearing losses.

2. Do patients with auditory neuropathy typically have other neural disorders?

Not all do, but the majority of patients have either overt or subtle neuropathies outside of the auditory system. Some patients will report symptoms of other non-auditory peripheral neuropathies, while neurologic dysfunction in other patients is revealed only upon clinical neurological examination. Some patients appear to have only an auditory abnormality.

Among the neurologic abnormalities identified in patients with auditory neuropathy are hereditary motor sensory neuropathy (HMSN, Charcot-Marie-Tooth syndrome), Friedreich's ataxia, gait ataxia, loss of deep tendon reflexes, and motor system disturbances. Most patients identified who are old enough to provide subjective reports complain first of hearing difficulty1.

3. Is auditory neuropathy a "new" hearing disorder?

No. What is new is our ability to clinically identify the disorder and distinguish it from other problems. That has become possible primarily because of the broader clinical use of otoacoustic emissions in recent years. OAE testing allows identification of those individuals with normal outer hair cell function despite showing abnormal ABRs.

Several past articles (e.g., Worthington and Peters, 1980; Kraus et al., 1984) presented the dilemma of patients with absent ABRs who were later found to have auditory function. A number of those patients may have been identified as auditory neuropathy, had OAE measurement been clinically at that time.

More recently, with the availability of OAE testing as well as evoked potential data, several studies have reported the paradoxical absence of ABRs in the presence of otoacoustic emissions1 (Starr et al., 1991; Berlin et al., 1993; Gravel and Stapells, 1993; Gorga et al., 1995; Sininger et al., 1995).

Some patients with auditory neuropathy appear, based on history and initial behavioral testing, to fit into the category of "central auditory processing disorder". However, evaluation of such patients with physiological measures sensitive to auditory nerve/brainstem disorders (i.e., OAEs and ABR) shows a more peripheral site consistent with auditory neuropathy. There also may be some hearing-impaired children and adults with previously undocumented normal outer hair responses who are being managed as having severe/profound hearing loss. While the incidence of this circumstance is unknown, persons who show no progress or perform poorly with amplification may fall into this category and be candidates for testing with OAEs (Berlin et al., 1996).

4. Are there different etiologies of auditory neuropathy?

The characteristics of auditory neuropathy most likely reflect more than a single etiology and thus the disorder(s) may more accurately be described as auditory neuropathies. However, while various etiologies of auditory neuropathy may exist, patients of all ages show a cohesive set of auditory symptoms. The pattern of normal outer hair cell function combined with abnormal neural responses shown by the ABR places the site of auditory neuropathy to the area including the inner hair cells, connections between the inner hair cells and the cochlear branch of the VIIIth cranial nerve, the VIIIth nerve itself, and perhaps auditory pathways of the brainstem.

Possible sites of auditory neuropathy include the inner hair cells, the tectorial membrane, the synaptic juncture between the inner hair cells, auditory neurons in the spiral ganglion, the VIIIth nerve fibers, or any compbination (Starr et al., 1996; Berlin et al., 1998). Neural problems may be axonal or demyelinating. Afferent as well as efferent pathways may be involved.

The problem might also be related to a biochemical abnormality involving neurotransmitter release. The specific sites and mechanisms of auditory neuropathy have yet to be determined.

5. Which clinical auditory tests are most sensitive to auditory neuropathy?

Since auditory neuropathy is characterized by normal outer hair cell function and abnormal function in the region of the inner hair cells and/or auditory nerve, the appropriate auditory tests are those sensitive to cochlear and auditory nerve function.

Outer hair cell function can be evaluated by measuring otoacoustic emissions and cochlear microphonics. Clinical tests that are specifically sensitive to auditory nerve dysfunction are middle ear muscle reflexes (ipsilateral and contralateral), auditory brainstem response, masking level difference, efferent suppression of otoacoustic emissions, and to a limited extent, word recognition with an ipsilateral competing noise or message.

Of the above measures, otoacoustic emissions and the auditory brainstem response, when used together, offer insight into preneural as well as neural function in the auditory system and thus may form the most sensitive combination.

6. Can auditory neuropathy be distinguished by testing only pure-tone thresholds and speech recognition?

Pure-tone thresholds and speech recognition scores appear the least sensitive of the above-mentioned audiologic tests sensitive to auditory neuropathy. Pure-tone thresholds are quite variable in auditory neuropathy patients and can range from normal to severe or profound hearing loss ranges. Some patients show rising or unusual configurations and threshold responses may or may not be symmetric between ears. The variability in pure tone threshold patterns limits the utility of pure-tone testing to distinguish auditory neuropathy. If pure-tone thresholds and otoacoustic emissions are compared, then disagreement between the results of the two tests may provide a clue to the presence of a retrocochlear disorder that warrants further testing.

In many but not all patients with auditory neuropathy, word recognition in quiet is poorer than one would predict from the pure-tone average. In eight patients with auditory neuropathy described by Starr et al. (1996), word recognition in 12 of 16 ears was poorer than would be predicted using the norms reported by Yellin et al. (1989). Furthermore, our experience is that those patients who show some word recognition ability in quiet have great difficulty understanding speech, even sentences, when there is even a small amount of background noise. However, results again are variable and speech understanding is similarly poor with other types of retrocochlear disorders.

7. What are the results of other auditory tests in patients with auditory neuropathy?

Cochlear responses that involve outer hair cell function, which include otoacoustic emissions and cochlear microphonics, are normal. Responses that require intact auditory nerve and/or brainstem pathways, such as the middle-ear muscle reflex (MEMR), the auditory brainstem response (ABR), masking level difference (MLD), and efferent suppression of otoacoustic emissions, are abnormal.

  Table 1 summarizes the expected results of standard baseline auditory tests, otoacoustic emissions, and measures sensitive to auditory nerve/brainstem disorders.

Efferent suppression of otoacoustic emissions involves the reduction in amplitude or change in phase of emissions resulting from addition of another stimulus (Collet et al., 1990; Berlin et al., 1995; Hood et al., 1996). Auditory neuropathy patients demonstrate a lack of suppression of otoacoustic emissions under any circumstances (Berlin et al., 1996), which may reflect efferent pathway dysfunction and/or a compromise of access to the efferent system resulting in a lack of efferent suppression of OAEs.

Middle latency responses are generally abnormal while late potentials (e.g., N1-P2, P300), where longer duration stimuli can be used, may be present.

• Pure tone thresholds: Normal to severe/profound hearing loss(Any configuration; can be asymmetric)
  
  
• Speech recognition in quiet: Variable; slightly reduced to greatly reduced
  
  
• Otoacoustic emissions: Normal
  
  
• Middle ear muscle reflexes:
  
  Ipsilateral: Absent
  Contralateral: Absent
  Non-Acoustic: Present
  
• Cochlear microphonic: Present (Inverts with stimulus polarity reversal)
  
  
• ABR: Absent (or severely abnormal)
  
  
• Masking Level Difference (MLD): No MLD (i.e., 0 dB)
  
  
• Efferent Suppression of TEOAEs: No suppression
  
  
• Speech recognition in noise: Generally poor.