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

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8. Do OAEs and ABRs test hearing?

That's an important question. Neither OAEs nor ABRs are direct tests of hearing! OAEs, which are used to evaluate outer hair cell function, represent preneural phenomena related to mechanical processes in the cochlea. The presence of OAEs in an otherwise intact auditory system is most commonly consistent with normal or near-normal peripheral hearing sensitivity. Presence of an ABR to low-intensity stimuli also is most typically consistent with good hearing sensitivity.

The ABR is actually a test of neural synchrony and its use in evaluating hearing is dependent upon the ability of neurons to maintain precise timing and respond synchronously to external stimuli. Presence of an ABR to low-intensity stimuli is most typically consistent with good hearing sensitivity. If there is a loss of timing or onset sensitive neural units, demyelination, or a loss of cues for temporal onset of signals, then responses may be desynchronized, compromising the ability to record an ABR (Starr et al., 1991; Berlin et al./ 1996).

Thus, while neither OAEs or the ABR is really hearing tests, under appropriate , both can give us information about function of the peripheral auditory system. This is especially useful in cases when behavioral testing is impossible. Both OAEs and ABRs are very useful in the early identification of peripheral hearing loss, as well as auditory neuropathy, and allow initiation of appropriate clinical intervention prior to behavioral confirmation of hearing sensitivity.

9. How can otoacoustic emissions be normal and yet patient reports hearing difficulty?

Again, it is important to remember that otoacoustic emissions relate to the mechanical function of the outer hair cell system. While mechanical cochlear function is important to the normal function of the cochlea, it is insufficient by itself for hearing to occur. Inner hair cell function is also necessary to activate the sensory processes that transmit incoming information to the auditory nerve and central auditory system.

10. If absence of the ABR is characteristic of auditory neuropathy patients, then why are "waves" sometimes present in ABR recordings?

The cochlea generates electrical responses which are most commonly measured using electrocochleography (ECochG). One of the cochlear potentials is the cochlear microphonic, an electrical response occurring just prior to the ABR. This response is generally small in surface-recorded responses (e.g., ABR), but is more evident when insert earphones are used and the stimulus artifact is separated in time from the biological response. In patients without an ABR, the cochlear microphonic may be larger and in infants the cochlear microphonic may even continue over several milliseconds (Berlin et al., 1998).

11. How can I distinguish the cochlear microphonic from the ABR?

Cochlear microphonics follow the characteristics of the external stimulus. The direction of the cochlear microphonic will reverse with changes in polarity of the stimulus. Comparison of responses obtained with positive (condensation) and negative (rarefaction) polarity stimuli shows an inversion of the peaks of the cochlear microphonic waveform. Neural responses such as the ABR may show very slight latency shifts with polarity changes but they will not invert. Thus, cochlear and neural components can be distinguished based on whether or not the peaks reverse with the stimulus. Use of alternating polarity stimuli is not helpful since the cochlear microphonic will cancel and not be visible in the averaged response.

Another difference between cochlear and neural responses is the effect of intensity on response latency. ABR waves increase in latency and decrease in amplitude with stimulus intensity decreases. In contrast, the cochlear microphonic does NOT increase in latency as the stimulus intensity decreases. Thus comparison of response latency at various intensities can be used to distinguish cochlear from neural responses.

A third difference between cochlear and neural responses relates to the effects of masking on the response. Cochlear microphonics do not change in latency with masking presented to the same ear while the compound action potential (CAP; Wave I of the ABR) shows amplitude reduction and latency increases during simultaneous masking to the same ear (Dallos, 1973). For an in-depth discussion of this topic and examples of responses showing these characteristics, the reader is referred to Berlin et al. (1998).

12. Are there certain risk factors for auditory neuropathy?

Currently, specific risk factors for auditory neuropathy are not clearly understood. As more patients are identified and reported, patterns may become more evident. A number of infants with auditory neuropathy have a history of major neonatal illnesses including hyperbilirubinemia and other risk factors (Stein et al., 1997; Deltenre et al., 1997; Berlin et al., 1998). Auditory neuropathy is also associated with other non-auditory peripheral neuropathies. Siblings have been identified with auditory neuropathy, suggesting underlying genetic factors as well.

13. Can auditory neuropathy be unilateral?

While most cases of auditory neuropathy identified to date are bilateral (though often asymmetric), a few patients have been reported with unilateral auditory neuropathy. These patients display normal auditory function in one ear and the pattern of results consistent with auditory neuropathy in the other ear. Functionally, patients with unilateral auditory neuropathy appear similar to patients with other types of unilateral hearing loss. At present, the management approach in these cases is similar to that used in other more common types of unilateral hearing loss, such as directing speech to the normal ear and maximizing the signal-to-noise ratio.

14. Is the hearing loss progressive or does hearing ability ever fluctuate?

Progression in hearing thresholds is observed in some patients, though it is not a characteristic of all patients. We have noted progressive hearing loss particularly in some of our patients with hereditary motor sensory neuropathy (Charcot-Marie-Tooth disease) (Berlin et al., 1994). Other patients demonstrate stable threshold responses over many years. So, progressive hearing loss is not necessarily a characteristic of auditory neuropathy. Whether or not progression occurs may depend on the underlying etiology.

Some cases of fluctuating hearing loss associated with auditory neuropathy have been reported. Starr and Sininger (personal communication) are following two siblings who show symptoms of auditory neuropathy accompanying fever with normal auditory function between periods of increased temperature. Gorga et al. (1995) reported a patient with fluctuations in hearing sensitivity, felt to be related to an auto-immune disorder, where OAEs remained intact while the ABR was affected.

15. How is auditory neuropathy different than other retrocochlear or central auditory disorders?

While any disorder of the auditory neural pathways from to VIIIth nerve to the cortex might be defined as an auditory neuropathy, the current use of the term relates specifically to more peripheral portions of the auditory pathways in the area between the outer hair cells and brainstem. Auditory neuropathy differs from other disorders affecting the VIIIth nerve, such as a vestibular Schwannoma, in that there is no space occupying lesion and radiological findings are normal.

While patients may display characteristics of central auditory processing problems (e.g., inattention, missing some information, inconsistencies in responses, etc.), peripheral measures such as middle-ear muscle reflexes and the ABR are abnormal in auditory neuropathy while function at the brainstem level is more often normal in patients with classic central auditory processing disorders.