AUDIOLOGICAL FINDINGS AND PATHO-PHYSIOLOGY COORELATES IN AUDITORY NEUROPATHY/DYS-SYNCHRONY

As mentioned in the earlier chapter, the term auditory neuropathy/ dys- synchrony (AND) has been used to describe a form of hearing impairment in which outer hair cells function is normal, but afferent neural conduction in the auditory pathway is disordered (Starr, Picton, Sininger, Food & Berlin. 1996 Clinically this condition is characterized by normal otoacoustic emissions and/or cochlear microphonics in conjunction with abnormal or absent auditory brainstem responses (Starr et al., 1996).

Auditory neuropathy/dys-synchrony is understood to demonstrate a plethora of clinical features for various audiological procedures. In the following section the landings of pure tone audiometry, immittance evaluation, speech perception), Otoacoustic emissions (OAE), auditory brainstem responses (ABR), higher order potentials, auditory steady state response (ASSR), and vestibular function evaluation are discussed.

Pure-tone thresholds

The behavioral pure-tone audiogram of individuals with auditory neuropathy/dys-synchrony are less productive than that of individuals with conductive or sensorineural hearing loss. The pure-tone thresholds of individuals with auditory dys-synchrony have been noted to vary from normal hearing sensitivity to profound hearing loss (Starr et al., 1996) Most often, the hearing loss is bilateral and symmetrical in configuration (82%). However, there are individuals who either demonstrate bilaterally normal hearing sensitivity or have a unilateral disorder. The audiogram configurations are usually flat However, a smaller but notable percentage (28%) displays a rising audiometric configuration. t has been inferred from the reverse sloping audiogram seen in patients with auditory dys-synchrony that the underlying etiology of hearing loss in this disorder is neural and not cochlear Frequent occurrence of such an audiogram may be due to anatomico-physiological make-up of the auditory nerve. The longest cochlear nerve fibers are those originating from apex of the cochlea (which mediates low frequencies). The shortest fibers are those innervating the second half of the first cochlear turn (which mediates middle frequencies). Those going to the basal parts of the cochlea have lengths in between these two extremes (which mediates high frequencies). The longest fibers are always most susceptible to the pathology. Hence. the low frequencies are more affected than the mid and high frequencies (Starr, 2001).

These reverse sloping audiograms also indicate that individuals with auditory dys-synchrony have problems in processing phase locked temporal information) According to theories of pitch perception, high frequencies above 1 kHz are signaled by excitation on the basilar membrane. This is because the auditory nerve fibers cannot fire at higher rates because of the limitations imposed by refractory periods. The fibers can fire for each phase of the signal for low frequency pure tones but not for high frequency tones. Therefore, or low frequency thresholds indicate poor auditory percepts dependent on temporal cues.

Some individuals with AN/AD also demonstrate a tuning curve audiogram'. The characteristic of these audiograms is a single normal threshold at a given frequency with a sharp slope on either side of this frequency. Such audiograms are reported to be the result of normal functioning inner hair cells located in the region sub-serving the peak frequency. At sufficiently high intensity, the same cells respond to other frequencies resulting in a complete audiogram (alpin, 2003).

It is not uncommon for hearing Threshold to fluctuate dramatically on a day-10-day basis or even during testing. Such fluctuations in hearing threshold mainly occur in temperature sensitive auditory neuropathy / dys synchrony conditions. Such individuals demonstrate elevated pure tone thresholds with impaired speech comprehension and abnormal or absent ABR's. They may also report of their symptoms worsening with even a 1⁰ raise in their core body temperature. This phenomenon could be attributed to demyelination of the auditory nerve, which results in altered nerve conduction with rise in body temperature. In general, individuals with auditory neuropathy/auditory dys-synchrony can have varying degrees of hearing loss of and day-to-day fluctuations in auditory capacity. which can be much more dramatic than what is seen in individuals with sensory loss.

Speech Perception

 It has been noted that patients with auditory dys- synchrony hvac speech perception abilities that are out of proportion with their pure-tone hearing loss (Starr et al., 1996). Speech perception abilities of individuals with auditory dys-synchrony are also seen to be highly variable. Some individuals perform at levels expected for a cochlear hearing loss of the same degree, while some others show little or no measurable speech identification despite having adequate sound detection abilities.

Discrepancy between sound detection and speech identification has been observed to be related to suprathreshold distortion of temporal cues rather than audibility. Ramirez and Mann (2005) reported that individuals with auditory dys-synchrony perceive high frequency consonants (/s/, /sh/. Ich) better than other consonants. Glides and nasals were better perceived than stops consonants. I was also observed that a majority of these individuals had significant difficulty in perceiving place of articulation cues when compared to manner and voicing cues.

Further, it has been noticed that they have more difficulty in perceiving stop and liquids when compared to fricatives, affricates and nasals (Narne & Vanaja, 2008). It has been found that individuals with auditory dys- synchrony cannot use phase locking cues to the same extent as normal hearing listeners due to dyssynchronous firing of auditory nerve fibers. However, detection of high frequencies does not depend on phase locking cues as much as low frequencies do (Kumar & Jayaram, 2005). On the other hand, it depends on information on the place of excitation on the basilar membrane. It has been hypothesized that hearing sensitivity at low frequencies in these individuals may indicate the extent of temporal disruption in the auditory system (Kumar & Jayaram, 2005). Therefore, the greater the loss in the low frequencies, the greater is the severity of temporal asynchrony which in turn reduces speech perception abilities.

Middle ear muscle reflex

 In individuals with auditory neuropathy lays synchrony the middle ear muscle reflex is typically absent or abnormal. A few individuals with AN/AD may show presence of ipsilateral and contralateral reflexes, elicited bilaterally at 95 dB HL or below at 1 kHz and 2 kHz. Other individuals with AD/AN may have either absent or elevated reflexes above 100 dB HL which is incongruous with the finding of normal otoacoustic emissions throughout frequency bands (Berlin et al., 2005). Interestingly, individuals with unilateral AN/AD may have normal reflexes in the affected ear, on stimulating the normal ear, while the same may be absent in the normal ear, when the tone is presented to the affected ear while measuring contralateral reflexes. This can be explained based on the anatomical structures of the auditory system. Neurons of the spiral ganglion do not project only to the central auditory pathway. They also project to the ipsi- and contra-lateral facial nuclei in the pons whose axons form the facial (7") cranial nerve, a small branch of which innervates the stapedius muscle of the middle ear. Thus, in an ear with a normal middle ear function with absence of the acoustic middle ear muscle reflex can be due to severe pathology of the inner hair cells (IHCs), spiral ganglion neurons, auditory nerve or brainstem (Rapin & Gravel, 2003). The damage to the brainstem could affect the cochlear nuclei, facial nucleus or facial nerve. Despite acoustic reflexes being absent in these individuals, non-acoustic middle ear reflexes may be present. An absent middle ear reflex to sound is unlikely to be due to brain stem pathology if there is a tensor tympani response to tactile stimulation of the face or cornea, as this reflex (tactile middle ear muscle reflex) is mediated by the trigeminal (5) nerve whose sensory and motor nuclei are also located in the brainstem (Rapin & Gravel, 2003).

Otoacoustic Emissions

 (Individuals with AN/AD are found to have normal outer hair cell function, therefore resulting in them having preserved otoacoustic emissions) (Sininger, & Oba, 2001). Interestingly, there has been a report of more robust Otoacoustic emissions in these individuals. It has been reported by Kumar and Jayaram (2006) that in individuals with AD the mean amplitude of TEOAE was around 16 dB SPL, while it was just 11.5 dB SPL in normal hearing adults studied by them. The researchers attributed this finding to the lack of efferent suppression of otoacoustic emissions.

It is not necessary that all individuals with AD demonstrate higher Amplitude of otoacoustic emissions. Many can have normal TEOAE amplitudes. "There also accounts of the absence of OAE's in individuals with AD. A few studies have also reported that the OAEs are present initially in individuals with auditory neuropathy / dys-synchrony but may disappear over time (Starr, Sininger & Pratt, 2001). This finding suggests that AD might be progressive in nature, resulting in the involvement of outer hair cells at a later stage.

Auditory Brainstem Responses

 The auditory brainstem response is usually absent in individuals with auditory neuropathy / dys-synchrony. ABR abnormalities are out of proportion to the pure-tone hearing loss. There are reports saying that ABR components I through V are absent in 73 % of individuals with AN/AD, whereas 21 % of the subjects may have wave V with prolonged latency and reduced amplitude. About 6% of individuals with AN/AD have been observed to have wave V with wave Il of poor morphology This occurs since normal auditory brainstem responses can be recorded only when multiple neurons fire synchronously at onset. Even a minor variation in the timing of the neural discharge after each stimulus can make the auditory brainstem responses unrecognizable (Kraus et al., 2000).

remember that an absent or grossly abnormal ABR is not always associated with deafness (Not every child with an absent ABR will actor grow up deaf. For example, 7% to 10% of children that have been diagnosed with AN/AD had no observable symptoms other than an absent ABR, and have continued to develop normal speech and Language will only complaints of mild difficulty in language learning or poor hearing in noise (Berlin et al., 2003). Secondly, some newborns with normal GIRL and absent ABR my show movement if neuromaturation in the underlying problem. In these cases, as the neural system matures, the ABR may improve. Still other infants and young children have shown a progressive decrease in auditory responsiveness. In contrast, some patients go through life without complaining of any problem, developing speech and language normally, and would never have been discovered if no one had done an ABR as part of either a screening or research project for these reasons, professionals should cross-check every abnormal ABR with reflexes and OAEs periodically, and plan a management strategy that is appropriate for that particular patient

Auditory Late latency Responses

Presence or absence of LLR depends upon the amount of dyssynchrony present in that particular individual has been reported that the subject with absent cortical potentials have poorer speech identification score, compared to the subject who have presence of cortical potentials. Presence or absence of cortical potentials has also been correlated with the hearing aid benefits in these individuals There are reports that individuals who have higher amplitude in cortical potentials have better speech identification scores and also benefit more from hearing aids than those with lesser amplitude. Cortical potentials have been observed to reflect a different form of neural response wiser compared to ABR (Kraus et al., 2000). While ABR peaks reflect synchronous spike discharges generated in nerve tracts. the peaks in cortical responses reflect the summation of excitatory postsynaptic potentials. These differences are evident in the spectra of these evoked potentials: the dominant peak in the ABR is-1 KHz, whercas the peak for cortical potentials is on the order of tens of hertz. As unit contributions to the ABR are biphasic and of short duration, ABR peaks tend to cancel when discharges are separated by fractions of a millisecond. In contrast cortical potentials are so slow that waves separated by several milliseconds enhance these later waves (Kraus et al.2000). Thus, if the nerve fibers have dys-synchrous firing which is not very severe, LLR may be present, whereas the ABR will be absent.

Auditory Steady State Response (ASSR)

 Steady State Auditory Evoked Potentials (SSAEP) allow the use of steady-state stimuli such as continuous tones with sinusoidal modulated amplitude. Two modulation rates have been used for recording the Auditory Steady State responses 40 H & 80 Hz). The generation site of auditory steady state response for lese (wo modulation rates is different The generator site for 40 Hz ASSR is the cortical structure whereas the generator site of 80 Hz ASSR is the brainstem structure. The presence or absence of ASSR may depend upon the modulation rate used in the ASSR (80 Hz ASSR will be absent in individuals with auditory neuropathy/dys-synchrony whereas the 40 Hz ASSR may be present depending upon the amount of dys-synchrous firing in the nerve,

Electrocochleography (ECochG)

Cochlear microphonics, through their ability to reflect the integrity of the cochlear hair cells, has been observed to play a significant role in the identification of ears with auditory neuropathy/dys-synchrony. The presence of cochlear microphonics. assured through ECochG, has been considered indicative of at least some degree of outer hair cell function. Cochlear microphonics is usually robust and is present for several milliseconds in individuals with auditory dys-synchrony (Sinha, 2006). Cochlear microphonics ringing has been recorded up to 3.5 msec in individuals with AN/AD (Sinha & Vanaja, 2009). In approximately 50% of the individuals with auditory dys- synchrony, the amplitude of cochlear microphonics is more compared to those with normal hearing. It is speculated that this finding of increased cochlear microphonics in patients with auditory dys-synchrony, reflect specific outer hair cell changes that are secondary to alterations of the auditory nerve input The results of studies investigating summating potential, in subjects with AN AD are equivocal. Some of the studies have reported presence of summating potentials with normal amplitude, whereas, other have reported abnormal or absent summating potentials. For recording of summating potentials in cases with AN/ADS, an intratympanic method should be employed as the signal-to-noise ratio is better in intratympanic method of recording. Catherene et al. (2008) attempted to record summating potentials in these individuals. They respond a presynaptic mechanism and a postsynaptic mechanism of AN AD based on the presence or absence of summating potentials. If the summating potential was present and action potential was absent, they called it as postsynaptic mechanism and if the summating potential was absent and action potential was present they called it as presynaptic mechanism of auditory neuropathy Usually it is speculated that the cochlear implant benefit is better in individuals with presynaptic mechanism compared to a postsynaptic mechanism. The measures of (summating potentials are important, because the generators for summating potentials include both types of hair oils i.e. IHCs and OHCs, with IHCS the principle generator Further, the presence of summating potentials in individuals with auditory neuropathy dys Synchrony leads one to conclude that THE Retains a normal function in ese patients. However, additional studies of summating potentials are Tequired to conclude that this cock car event is normal in all the auditory dys-synchrony subjects.

Efferent activity in subjects with auditory neuropathy dys- synchrony

 Otoacoustic emissions have been widely used to assess the functional integrity of the efferent system in the auditory neuropathy / dys synchrony subjects. Subjects with auditory dys-synchrony consistently shows no or minimal suppression of TEOAE and DPOAES. The result for TEOAE suppression is consistent using ipsilateral, binaural and contralateral noise. The lack of efferent suppression may be due to an [10:00, 23/02/2020] Shrutinathbanerjee171097: afferent deficit rather than a efferent deficit. The afferent deficit in subjects with auditory dys synchrony has been supported by findings in subjects with unilateral auditory dys-synchrony. The deficit in afferent system rather than efferent system has also been supported by the presence or absence of middle ear muscle reflex in subjects with auditory dys- synchrony. Berlin et al. (2005) reported 8 subjects with unilateral auditory dys synchrony in whom there was a clear middle ear muscle reflex at nomal level when the normal ear was stimulated but it was absent when the abnormal ear was stimulated. This supports the hypothesis of Starr (2001) that in subjects with auditory dys-synchrony the auditory nerve may not achieve a sufficient high rate of discharge to activate crossed olivocochlear reflex.

Vestibular Tests findings

 individuals with AND may have hearing and balance problems simultaneously. Clinical tests of the balance system in these patients have indicated abnormalities on the Romberg test, the Mann test and the stepping tests. Ice water calorie stimulation of the labyrinth failed to elicit nystagmus or dizziness in these patients. Strong rotational testing gave results consistent with bilaterally impaired function of the horizontal semicircular canals and or vestibular nerves. Recordings of pursuit, saccadic and optokinetic induced eye movements indicated that the central oculomotor system tracts were intact and functional in all patients (Sheykholeslami et al., 2000).

In an attempt to find out inferior vestibular nerve involvement and the Otolith organs involvement, Kumar et al. (2007) evaluated ten subjects with auditory dys-synchrony with vestibular evoked myogenic potentials. The results revealed that: ( nine out of the 10 subjects showed abnormal or absent VEMP. (2) there was no one-to-one correlation between the abnormal or absent VEMP and the vestibular symptoms that these subjects present; and (3) 80% of the east with auditory neuropathy showed abnormal VEMP results giving an indication of high incidence of vestibular involvement in the auditory neuropathy population. In individuals with isolated auditory neuropathy, the vestibular branch of the Viii cranial nerve and its innervated structures bas also been found to be affected. Kumar et al. (2007) also suggested to use the term acoustic neuropathy' to be used to indicate those patients in whom only the acoustic nerve is affected and vestibuloacoustic neuropathy to Jebel those patients who also show involvement of the vestibular system.

Summary

 To summarize, in subjects with auditory neuropathy /dys-synchrony, the behavioral pure-tone thresholds varies from normal hearing sensitivity to profound hearing loss. Sometime the pure-tone thresholds may fluctuate dramatically from day-to-day or even while tests are being administered In addition, the middle ear muscle reflexes are absent in subjects with auditory neuropathy / dys-synchrony. Otoacoustic emissions are typically present in all the subjects with auditory neuropathy / dys-synchrony whereas auditory brainstem responses are typically absent. Cochlear microphonics is always present, but there are equivocal findings regarding summating potentials and action potentials in subjects with auditory neuropathy / dys-synchrony. The above information needs to be kept in mind while diagnosing auditory neuropathy /dys-synchrony