OVERVIEW & ETIOLOGY OF ANSD

Auditory Neuropathy (AN) or Auditory dys-synchrony (AD) is a clinical syndrome in which outer hair cell functioning is spared, but afferent neural transmission is disordered (Starr, Picton, Sininger, Hood, & Berlin, 1996). A typical person with AN/AD has the following profile: normal/elevated thresholds on pure tone audiogram by air and bone conduction, very poor speech discrimination for degree of loss, no acoustic reflex in any configuration of hearing loss and for any stimuli, no auditory brainstem response (ABR), and presents robust otoacoustic emissions (OAE).

Incidence and Prevalence of AN

Davis and Hirsh (1979) reported that I in 200 hearing impaired children exhibit an audiological picture that is consistent with the contemporary diagnosis of auditory dys-synchrony. Rancc et al. (1999) and Tang. Mcpherson, Yuen, Wong, and Lee (2004) reported that the overall incidence rate varies from 1.83% to l1% of the hearing impaired population.

Kumar and Jayaram (2006) estimated the prevalence of auditory dys- synchrony in Mysore. The prevalence of auditory dys-synchrony was around 1 in 183 (0.54%) among individuals with sensorineural hearing loss. Around 60% of the individuals had no measurable speech identification scores.

In infants approximately 1-3 infants per 10,000 births bave A (Dolphin, 2004). This rate increased to 0.94% in the group at risk for hearing loss (Foerst, 2006). Typically AN is bilateral (96%) and shows no gender preference (Sininger, 2001). The female to male ratio of auditory dys- synchrony was reported to be 2:1 (Kumar & Jayaram, 2006).

Onset and Course

There is no consensus on the age of onset of auditory dys-synchrony. Age of not as well as the course of the disorder is variable. However, evidence available point to childhood as the age of onset in majority of the cutest. Singer and Obu (2001) studied a group of 59 individuals with auditory dys-synchrony and reported a mean age of onset of 9 years, 75% of the patients were less than 10 years of age when the first symptom of auditory dys-synchrony was seen. In fact, the age of onset of auditory dys- synchrony in the clinical population of Sininger and Oba ranged between birth to 60 years of age with the largest group reporting onset of the problem before 2 years of age. As the problem of auditory dys-synchrony recovers after the onset, or fluctuates (Berlin, 1999), the true age of the onset of the problem may never be known.

Bally unpredictable in the course of auditory dys-synchrony. The condition may resolve remain the same or even deteriorate in some individuals Berlin (1999) identified several patterns in the time course of auditory dys synchrony. Some of these are as follows:

a)      Some patients show retrograde loss of cochlear microphonics and otoacoustic emissions and become almost indistinguishableFrom patients with cochlear hearing loss.

b)      Some patients rotin cochlear microphonics and otoacoustic emissions, but not least speech language by auditory mode wone Visual information (cued speech, sign language. speech reading) is necessary

c)      Some patients show worsening of the symptoms and develop other neuropathies. Sturr, Sittinger and Praat (2000) reported peripheral neuropathy in 40% of their patients with auditory dys-synchrony. None of the children below five years of age in their group showed clinical evidence of peripheral neuropathy whereas 80% of the patients examined after the age of 15 years showed both clinical and physiological (nerve conduction) evidence of peripheral neuropathy. They hypothesized that some of the patients ay subsequently develop perip neuropathies as they grow.

d)     Some patients loose their otoacoustic emissions, but not Cochlear microphonics. Such patients may manifest score hearing impairment, while occasionally they may also show evidence of unexpected hearing abilities Withnell. 2001). Around 20% of the individuals with auditory dys-synchrony loose their otoacoustic emissions as the disease progresses (Start et al., 2000).

e)      Some patients go through life without complaining ol any problems. They develop speech and language normally and would be identified as cases of auditory dys-synchrony only an AER had to be done 1or some reason (ether as a part oL SCreening procedure of a research project) 1) Some patients with auditory dys-synchrony may show fluctuations in their hearing abilities that are temperature sensitive. Children have been reported to show variali pure tone thresholds. presence of absence of ABS cic depending on whether they were fcbric or afebrile (StarT Sininger, Winter Derebery, Oba & Michalewski, 1998). In fact, many patients with auditory dys-synchrony appear to experience momento moment fluctuations in their hearing sensitivity which may be misinterpreted as inconsistent Response during testing.

Etiology

 The characteristics of auditory neuropathy most likely reflect more than single etiology and thus the disorders) may more accurately be described as auditory neuropathy. Reviewing a large body of patients. Slam Sininger and Pratt (2000) reported that AN appears to consist of a number of varieties, with different etiologies and sites affected. All varieties are a relatively spared receptor function, and an impaired neural response with diminished ability to follow fast temporal changes in the stimulus.

but different varieties in this general scheme can be distinguished. The pattern of nomal 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 VII cranial nerve, the VII nerve itself, and perhaps auditory pathways of the brainstem. The following etiologies have been attributed to AN Medical conditions encompassed by the AN umbrella are:

·         Anoxia

·         Hyperbilirubinemia

·         Infectious processes Ex Mumps)

·         Immune disorders (ex: Guillain-Barre syndrome) Genetic & Syndrome (Autosomal dominant, Autosomal recessive or X-linked)

·         Neurological disorders (Ex. Friedreich's ataxia)

·         perinatal diseases Hyperbilirubinemia

·         Hyperbilirubinemia

·         Ischemic Insult

·         Prematurity

Neurological disorders

·         Demyelinating diseases

·         hydrocephalus

·         immune disorders (Guillian-Barre Syndrome)

·         Inflammatory neuropathy

·         Severe developmental delay

Genetic & hereditary etiologies

·         Refsum's disease

·         Friedreich's ataxia

·         Connexin mutations

·         Otoferlin (OTOF) gene

·         Wartenberg's syndrome

·         Leber’s heredity optic neuropathy

·         Heredity sensory motor neuropathies

Pathophysiology of Auditory Dys-synchrony

Auditory dys-synchrony may affect the functioning of inner hair cells. synaptic Junctions between the inner hair cells and auditory nerve, or the auditory nerve itself (Starr . 1996). The physiological changes paying auditory dys-synchrony may lead to a reduction number of conducting fibers due to axonal loss. In a histopathological study of cochlea and auditory nerve in an individual with auditory dys shorty. Starr et al. (2003) found that the organ of corti was nomad out the cochlea except at the apical turn where a 30% loss o veer 62 cells was observed. But, the inner hair cells were normal throughout the length of the cochlea, However, there was a profound 1055 o ganglion cells (> 95%). The auditory nerve adjacent to the Cochlear nucleus showed a marked reduction in the number of auditory fibers Furthermore, the myelin sheath on the surviving auditory BTC fiters was thin indicating incomplete myelination. Therefore, il may esate that reduced neural input due to axonal loss could result in the 105 of acoustic reflexes, that is, middle ear muscle and olivocochlear reflexes Star. Pics & Kim, 2001). These reflexes invoked using suprathreshold signals that activate large number of auditory nerve fibers and this may not be possible in persons with auditory dys-synchrony because of large axonal loss.

Persons with auditory dys-synchrony may also manifest asynchronous ring of the auditory nerve fibers due to demyelination (Stawial. 2001). Demyelination alexis saltatory conduction and thereby slows down the conduction velocity of the nerve fibers. If the extent of slowing varies from one fiber to next (due to different degrees of demyelination). then it leads to temporal asynchrony in the firing of the auditory nerve fibers thereby reducing the compound action potential of the auditory nerve. Asynchronization not only affects ABRs, but also influences auditory perception dependent on temporal cues (Kraus et al. 2000; Rance, Mckay & Grayden, 2004: Starr et al., 1991; Starr et al., 1996: Zeng, Oba, Garde. Sininger & Starr, 1999; Zeng, Kong, Michalewsk, & Starr, 2005). Axonal loss and demyelination can occur together.

Assessment

The diagnosis of AD is based on the results of a battery of audiological tests. The audiological tests would include pure tone audiogram, speech audiometry. immittance audiometry, auditory brainstem responses. otoacoustic emissions and/or electrocochleography, middle and late latency responses. Although not necessary for the diagnosis, to further characterize the processing and perceptual deficits, psychophysical tests like random gap detection test, temporal modulation transfer function and threshold equalizing noise (TEN) test can be used.

The diagnosis of AN/AD would also require inputs from other professionals like Neurologist, Neurologist and Pediatrician. A neurologist would provide information about the presence/absence of space occupying lesion and the involvement of the other peripheral nerves. Whereas, otologists and pediatrician help in understanding whether the condition is syndrome or nonsyndromic.

Rehabilitation

Till date, one of the currently available rehabilitative options have been satisfactory in individuals with AD. The treatment options attempted include hearing aids, FM system, IR systems, Cochlear implantation, auditory training etc. Although some positive findings have been reported with cochlear implantation (Wattoo, Gibson, Sanli & Prolog, 2008), it is proved successful only in a small group of individuals with AD.

Summary

From the information given above, it is clear that, although AD can from a variety of causes, the clinical manifestation is same across me individuals. The communicative handicap is for more in individuals with AD compared to those with typical cochlear pathology. This warrants greater need for understanding this clinical condition in terms of its cause, pathophysiology,clinical profile and management.