Timothy C. Hain, MD Last updated: December 4, 2010
Introduction: An auditory neuropathy (AN)is a hearing loss caused by damage to the auditory portion of the eighth nerve, which is located between the inner ear (cochlea) and the brainstem. (see item 6 on figure to the right). It is, in essence, an overlap syndrome between otology and neurology.
AN was first reported in the late 1970's as paradoxical findings because of a discrepancy between absent ABR and present hearing thresholds (although with some reduction), this disorder has been referred to as central auditory dysfunction (not a good name), auditory neural synchrony disorder, and most recently, as auditory neuropathy. Auditory neuropathy may also affect vestibular function (Sheykholeslami et al. 2000).
Some of these patients may actually have a different disorder (loss of inner hair cells - -due to logical errors in the diagnostic process -- see below).
Some of these patients may also have a brainstem disorder, and no neuropathy at all.
In this regard, it has been suggested that there are two forms of AN-- a "presynaptic" type involving damage to the auditory nerve, and a "postsynaptic" version reflecting damage at the synapse of the cochlear nerve with the cochlear nucleus. The latter type of "AN", should logically be called something else than "Auditory neuropathy", as the nerve is not the site of lesion. Nevertheless, because this syndrome has been defined primarily by test findings rather than anatomic sources of data, the nomenclature has persisted of "AN" for a disorder that does not have an "N". (McMahon et al, 2008)
It seems likely that the medical profession greatly underdiagnoses auditory neuropathy. First, the syndrome itself is little known. Second, one of the tools to diagnose auditory neuropathy (otoacoustic emmissions or OAE) is generally little known to any specialty physicians other than otolaryngologists.
The prevalence is presently unknown with estimates varying from 1/200 patients with sensorineural hearing loss (SNHL) to 15% of SNHL (Krause et al, 1984). A recent paper found 22/428 children with hearing loss had AN (Madden et al, 2002), or about 1/20. Liange and others (2001) felt that the prevalence in China was high.
There probably is an analogous "vestibular neuropathy" syndrome, characterized by absent vestibular evoked potentials (VEMP tests), and peculiarities in the ENG or rotatory chair test.
The diagnosis is presently made by combining an absent or extremely abnormal ABR test (brainstem auditory evoked response, also called ABR or auditory brainstem response) with normal otoacoustic emissions (OAE), or reasonably intact hearing tests (e.g. absent ABR, present though impaired hearing). Wave I can be present in the ABR and the patient can still have "AN".
Logically, the diagnostic process does not prove that the site of lesion is the auditory nerve. Gibson and Sanli recently pointed out that a selective loss of inner hair cells might create a similar testing configuration (2007). McMahon and others pointed out that a brainstem disorder could also cause similar testing configuration (2008).
Neuroimaging studies are generally normal (Wang et al. 2003). According to Doyle(1998), these patients also typically have impaired word discrimination out of proportion to pure tone loss, which otherwise would be suggestive of a central auditory syndrome. In the present day where ABR's are no longer routinely obtained in the evaluation of hearing loss (MRI is generally preferred), this diagnosis might be easily overlooked. Diagnostic problems might also occur in persons with mild auditory neuropathy -- one might see a mildly abnormal ABR combined with normal otoacoustic emmissions.
In the authors opinion, in patients diagnosed with AN, tests for peripheral neuropathy should be performed including EMG/NCV, VEMP testing and ENG should be considered as well as testing for the more common treatable types of neuropathy.
Auditory neuropathy has been reported in several hereditary neuropathies. Berlin et al (1994) described auditory neuropathy in three patients with Charcot-Marie-Tooth disease, which is an inherited neuropathy syndrome. Doyle, Sininger and Starr (1998) reported one subject with Fredreich's ataxia, which is an inherited ataxia which is associated with neuropathy. Bamiou and others (2003) described hearing loss with abnormal ABR in Refsum's disease -- a very rare neurological disorder having retinitis pigmentosa, polyneuropathy, dermatitis, anosmia and hearing loss. Papadakis et al. (2003) reported a case of bilateral hearing loss attributed to Charcot-Marie-Tooth disease. Starr and others (1996) also reported 3 patients with hereditary neuropathy. Auditory neuropathy also can occur in an isolated familial form (Wang et al, 2003).
Auditory neuropathy has also been reported in AIDS (HIV infection). Not all patients with AIDS develop hearing loss,but rates from 20.9% to 49% have reported (Sooy 1987; Bell, Atkins et al. 1988; Kantu, Lee et al. 1996). This syndrome is typified by high frequency hearing loss. AIDS auditory neuropathy is generally diagnosed by prolonged ABR latencies and atypical waveforms (Belman, Ultmann et al. 1985; Geltma and Schupbach 1986; Smith, Jakobsen et al. 1988)
Autoimmune inner ear disease is in the differential diagnosis.The extent to which individuals with sudden hearing loss actually have AN, is uncertain. Of course, this could be diagnosed by OAE testing.
Auditory neuropathy may also affect vestibular function (Sheykholeslami et al. 2000; Fujikawa and Starr, 2001). When auditory neuropathy is combined with vestibular neuropathy, the diagnosis is made by combining the usual criteria for auditory neuropathy with abnormalities in vestibular function tests. It seems likely that some persons with bilateral vestibular impairment might have this due to auditory neuropathy, and greater use of ABR testing and VEMP testing might be useful in this population.
One might expect that diabetes, which is common and associated with cranial nerve neuropathy might also be associated with auditory neuropathy. This association, so far, does not seem documented in the literature. Similarly, Guillain Barre syndrome and Miller Fisher syndrome (two types of acquired neuropathies) probably have some incidence of accompanying auditory neuropathy. Greater use of ABR and otoacoustic emmision testing (OAE) in this context might be useful. VEMP testing (a vestibular evoked response test) might also play a useful role.
Treatment should logically be aimed at the underlying peripheral neuropathy. As treatment is often not available for peripheral neuropathy, this often translates into no treatment. Nevertheless, there are treatments for Guillain Barre, porphria, and many other generalized neuropathies. Screening tests should be performed for treatable causes and treatment selected accordingly.
Doyle, Sininger and Starr (1998) reported 8 pediatric patients having hearing deficits which they attributed to auditory neuropathy. They comment that word discrimination was impaired out of proportion to pure tone performance. Their subject 8 also had Fredreich's ataxia, which is an inherited ataxia which is associated with neuropathy.
Gibson and Sanli (2007) studied 39 children with auditory neuropathy and concluded that the more likely mechanism was selective loss of inner hair cells.
Madden and others (2001) described 22 cases of auditory neuropathy from a pediatric otology clinic. 50% had a history of hyperbilirubinemia. 45% had a history of prematurity, 45% ototoxic drug exposure, 36% a family history of hearing loss, and 36% had a history of neonatal ventilator dependence. This would seem to indicate that AN is multifactorial. Cochlear implantation was successful in 4 children.
Starr and others (1996) reported 10 patients, all children or young adults. Cochlear microphonics and otoacoustic emissions were preserved in all, but auditory brainstem response were normal. Auditory brainstem reflexes were also negative. The shape of the pure tone loss varied, being mainly low frequency in 5, flat in 3, and high frequency in 3. Speech was affected out of proportion to that expected from a pure tone loss, which might lead to some confusion with a central hearing loss pattern. subsequently eight of these patients developed evidence of a peripheral neuropathy, which was hereditary in 3. Starr and others (1998) also reported a variant in three children in which transient deafness occurred when the children were febrile. This pattern is reminescent of the typical exacerbation of neurological symptoms in individuals with demyelinating disease (e.g. MS).
Stein and others (1997) identified infants who failed hearing screening on ABR, but passed their otoacoustic emission test. Four of five had hyperbilirubinemia.