Auditory Neuropathy

The term auditory neuropathy/auditory dyssynchrony (AN/AD) describes a diagnosis that affects a small group of patients with hearing loss and speech intelligibility scores out of proportion with their presumed hearing loss. Many authors have suggested that the abnormalities that cause AN/AD reside within the lower auditory system. Specifically, the spiral ganglion cells, auditory nerve, and auditory brainstem nuclei have all been implicated. The combination of a dysfunctional auditory nerve with preservation of cochlear function can theoretically be caused at several different points along the lower auditory pathway. The following abnormalities have been proposed:

• Injury to the synaptic junction between inner hair cells of the cochlea and dendrites of spiral ganglion neurons

• Direct damage to the dendrites of the spiral ganglion neurons

• Direct injury to the spiral ganglion neurons

• Direct axonal damage to the auditory nerve that causes a cascade of damage to the lower auditory nuclei

A study by Cooper et al indicated that structural brain differences exist between children with AN/AD and healthy controls. The cortical thickness of the right temporal lobe was found to be reduced in children with AN/AD. Moreover, the subjects with AN/AD had significantly greater mean diffusivity in the corpus callosum and right occipital white matter, as determined using diffusion tensor imaging. This diffusivity change may be evidence for density loss in  the white matter microstructure of youngsters with AN/AD. ^[4]

Several risk factors have been speculated to contribute to AN/AD, including the following ^[5] :

• Neonatal history of anoxia;

• Neonatal history of hyperbilirubinemia;

• Neonatal history of mechanical ventilation, hypoxia, or both;

• Congenital brain abnormalities;

• Low birth weight

• Extremely premature birth (< 28 wk);

• Genetics or family history of AN/AD

In addition, AN/AD has been reported in association with viral diseases, seizure disorders, and high fever.

AN/AD can occur with or without accompanying neurologic disorders. Friedrich ataxia, Stevens-Johnson syndrome, Ehlers-Danlos syndrome, and Charcot-Marie-Tooth syndrome are all disorders with peripheral neuropathies that have been associated with AN/AD. Although a complicated perinatal history is common among most patients with AN/AD, one third of patients have no predisposing factors that led to the development of AN/AD.

A study by Kitao et al suggested that adults with AN/AD have a higher rate of decrease in distortion product otoacoustic emissions (DPOAEs) than do pediatric patients with the disease (72% of ears vs 45% of ears, respectively), but a lower rate of DPOAE loss (6% of ears vs 27% of ears, respectively). ^[6]

A study by Apeksha and Kumar reported that compared with persons with normal hearing, patients with AN/AD spectrum disorder have more diffuse brain activations in response to detection and discrimination of speech sounds. The investigators also found evidence, through behavioral data, that sensitivity is lower and reaction times are longer when these patients are exposed to oddball stimuli. ^[7]

Current research is aimed at the development of animal models to help decipher the exact etiology of AN/AD. Several attempts have been made to replicate the electrophysiologic findings of normal OAEs and abnormal ABRs in an animal model for AN. The first was a chinchilla model that used carboplatin ototoxicity. In this model, intravenous carboplatin treatment produces a selective, although variable, loss of inner hair cells. This animal model displayed the electrophysiologic correlate of normal OAEs and cochlear microphonics (CMs) but elevated thresholds of the ABRs.

However, the magnitude of inner hair cell loss (>50% at the apex of the cochlea and a higher percentage at the base) in this animal model indicates that if inner hair cell loss due to cochlear hypoxia were a significant factor in accounting for the paradoxical finding of normal OAEs and absent or abnormal ABRs, pure tone thresholds would be considerably more elevated than is consistent with the normal to moderate hearing loss characteristic of most patients with AN.

In another model, chronic infusion of ouabain to the round window of gerbils has also shown some physiological similarities to the findings associated with AN/AD and has the potential to be an animal model for AN/AD. This animal model shows the direct destruction of the spiral ganglion cells and produces the same constellation of symptoms found in humans with AN/AD. Although the diagnostic testing results were similar to those found in patients with AN/AD, the destruction of spiral ganglion cells may not be representative of the true pathology of AN/AD.

Another emerging animal model is the Gunn rat. This mutant of the Wistar rat is a well-known animal model for hyperbilirubinemia. Early studies have shown preservation of the CMs with severely abnormal ABRs. Immunohistochemical brainstem studies have shown a reduction in the synaptic inputs to the lower auditory brainstem nuclei, which receive inputs from the spiral ganglion cells. Preliminary results involving the auditory nerve of these hyperbilirubinemic animals demonstrate a selective loss of the large-caliber axons in the auditory nerve with complete preservation of the outer hair cells.

The low threshold of activation and high spontaneous discharge rate of large-diameter axons that innervate inner hair cells in the cochlea are the electrophysiologic properties ideally suited for the temporal coding of auditory information, particularly as it relates to neural synchrony and temporally dependent auditory events, such as speech comprehension. Because hyperbilirubinemia has a direct association with AN/AD, this animal model, if proven, will have significant potential in helping investigators pinpoint the pathophysiology of AN/AD.

 

Frequency

United States

The exact frequency of auditory neuropathy/auditory dyssynchrony (AN/AD) has varied among different publications. Although the prevalence among all children is extremely low, when children with known hearing loss are examined, the rate increases dramatically. Some authors have suggested that the prevalence is 2-15% of children with known hearing loss. In a 2002 review of the prevalence, Sininger suggested that approximately 1 in 10 children with hearing loss and severely affected ABR test results have AN/AD. Overall, AN/AD is rare and can be found in an estimated 1-3 children per 10,000 births. Most patients with AN/AD have other significant perinatal risk factors. However, one third of the patients have no definable etiologic cause.

Race

Auditory neuropathy/auditory dyssynchrony (AN/AD) has no racial bias.

Sex

Auditory neuropathy/auditory dyssynchrony (AN/AD) occurs with near-equal frequency in males (55%) and females (45%).

Age

Although the insults that cause auditory neuropathy/auditory dyssynchrony (AN/AD) are thought to arise in the perinatal period, this disorder can also be diagnosed in adults. Because AN/AD is a relatively newly described condition, many adults may have not obtained the proper audiologic testing to reach a diagnosis of AN/AD. With the advent of newborn hearing screening tests, the delay in diagnosis of AN/AD should be minimized, which will expedite intervention.