Timothy C. Hain, MD
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An auditory neuropathy 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. 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.
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.
The diagnosis is presently made by combining an extremely abnormal BAER test (brainstem auditory evoked response, also called ABR or auditory brainstem response) with normal otoacoustic emissions, or reasonably intact hearing (e.g. absent ABR, present though impaired hearing). Wave I can be present in the ABR and the patient can still have AN. 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. Autoimmune inner ear disease is in the differential diagnosis.The extent to which individuals with sudden hearing loss actually have auditory neuropathies, is uncertain.
Treatment should logically be aimed at the underlying peripheral neuropathy. Screening tests should be performed for treatable causes and treatment selected accordingly.
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 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.
Fujikawa and Starr (2001) reported 14 patients with auditory neuropathy, of which 9 had "abnormal" caloric tests and 8 had peripheral neuropathy. The conclusion of this paper was that "asymptomatic vestibular disorders are common in patients with auditory neuropathy when a peripheral neuropathy is also present". Because the criteria for an "abnormal ENG" were rather loose, we are dubious about this.
Liange and others (2001) described 17 cases of auditory neuropathy in China. They felt that the prevalence was high.
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.
Sheykholeslami et al (2001) reported 5 patients of various ages with impaired hearing, normal OAE, and impaired ABR/ECOG.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.