TEST PROTOCOL TO IDENTIFY INDIVIDUALS WITH AUDITORY NEUROPATHY

The term auditory dys-synchrony has been used to describe a form of  hearing impairment in which cochlear amplification function is normal, but neural transmission in the auditory pathways is disordered. Sininger and Oba (2001) noted that a person to be diagnosed with auditory dys synchrony must have the following features:

a)      Evidence of poor auditory functioning resulting in the patient having difficulty in understanding speech at least in some not situations regardless of the level of pure-tone hearing thresholds.

b)      Evidence of poor neural functioning leading to the patient not have ABR. and if present, it would have abnormal morphology and prolonged latency. The acoustic auditory brainstem reflex (stapedial reflex, medial olivocochlear bundle reflex or and masking level difference) are generally not present, and

c)      Evidence of moral cochlear amplification function whereby would the patient would have either cochlear microphonics or OAE

It suggests that the person with poor speech understanding ability should be assessed to rule out the presence or absence or AD. Of late it has been observed that many individuals with AD have speech identification scores proportionate to their hearing loss or even better Hence, speech Identification scores may not be good indicators of presence of AD. This has to be supplemented by many other audiological tests. This justifies the importance of a test battery approach. This could be the reason why Sininger and Oba (2001) suggested that to diagnose an individual as having auditory dys-synchrony, the client must have the three clinical Features. These include evidence of poor auditory functioning, where the patient must have difficulty in understanding speech at last in some situations regardless of pure-tone hearing thresholds; evidence of auditory neural functioning, where the patient must have abnormal or absent auditory brainstem responses and elevated/absent acoustic auditory brainstem reflexes such as stapedial reflex and medial olivocochlear Bundle reflex: and evidence of normal outer hair.cell function where the nations must show presence of either cochlear microphonics or Otoacoustic emissions. Thus, it suggests that to identify an individual with AD the test battery shown in Figure I should be used.

                                                                                                            Mamatha N.M, 2006

Figure 1: Test Battery to identify individuals having auditory neuropathy

 The recording of cochlear microphonics (CM) is not very popular as a clinical tool. This may be because audiologists lack experience in Bcording CM. Also, the availability of equipment to measure OAE, which are very simple and less time consuming to record, have made CM measurement less popular. Thus, the inclusion of cochlear microphonics in Routine clinical test battery to identify AD is rare. However, there are times when it may be difficult to record OAEs and the only way to get information regarding OHCS is by measuring CMs. One such instance when the client has a middle ear problem. In such a condition, it would difficult to record otoacoustic emissions as the middle ear problem would hamper the weak otoacoustic emissions from being measured. In such conditions, the cochlear microphonics should be recorded, if the client is suspected as having AD.

It is well established that though otoacoustic emission might be absent, of cochlear microphonics can be present in individuals with long standing AD. This suggests that, to identify individuals with AD. the measurement ce of cochlear microphonics should be the first option and measurement of to OAE may be optional. The CM can be recorded while recording ABR. Thus, the test battery should include pure tone audiometry, immittance lus measurement, ABR and measurement of cochlear microphonics. However, it is essential that suitable parameters and procedures be used to enable recording of both ABR and cochlear microphonics. Auditory nerve be abnormalities can easily be tapped by changing the rate of stimulus ng ear presentation. Often ABR is absent in individuals with AD, but in individual with mild AD. ABR recorded using high stimulus rate would the show abnormality but may be normal at a lower stimulus rate. Similarly, in it is necessary to remember that the cochlear microphonics can be sec recorded using the same electrode montage as used for ABR recording. The different recording parameters used to measure ABR and cochlear microphonics are me same. This is true for all parameters except the polarity of the stimulus. CM being an AC potential will be absent if recorded using an alternate polarity. Hence, to obtain CM while recording ABR, either a rarefaction or condensation stimulus should be used. Sometimes, CM recorded using a single polarity stimulus could be an artifact. Hence, this should be verified by reversing the polarity of the stimulus and observe for its presence. In addition, caution needs to be exerted while recording CM using supra-aural earphone. Stimulus artifacts could result as the transducer and electrodes are placed very close each. These artifacts may be misidentifies as CM. CM recorded using headphone at high stimulus levels increases the chance of stimulus artifacts. An easy way to eliminate stimulus artifacts is through the use of an insert earphone. The above mentioned test battery may be sufficient to identify the presence of AD, but may not give exact information about the severity of the condition. To determine this other audiological tests are required. It is known that ABR requires better neural synchrony to be recorded when compared to auditory late latency response (ALLR). Starr, Picton, Sininger. Hood and Berlin (1996) have reported that the ALLR could be recorded from a few individuals with AD. Absence of both ABR and ALLR is an indication of a severe degree of dys-synchrony, whereas. absence of ABR and presence of ALLR is an indication of lesser degree of dys-synchrony.

Due to inconsistency of responses in individuals with AD, threshold estimation may be difficult. Kumar and Jayaram (2006) observed that many of their clients with auditory dys-synchrony appeared to have moment-to-moment fluctuations in the hearing sensitivity that could create the illusion of inconsistent responses during testing. Hence, predicting threshold based on pure-tone audiometry may not be reliable in such individuals. Further, threshold estimation using ABR may not always possible as it is often absent. However, in individuals with AD in whom ALLR is present, this test can be used to estimate threshold. The protocol for threshold estimation is provided in Figure-2. 

Figure 2: Test protocol to identify individuals having auditory neuropathy

The protocol depicted in figure 2 is more suitable for adult population who can provide voluntary responses. The test battery included Identifying AD in infants or younger children would have to be different The difficulty is more in case of infants and children from whom voluntary responses are difficult to obtain. In such individual's behavioural observation audiometry (BOA) or visual reinforcement audiometry (VRA are used as alternating test, instead of conventional pure-tone audiometry Thus, the protocol for infants or younger children would be BONVITA Immittance evaluation, ABR along with CM and or OAE and ALLR.

The findings of K. Gupta and Anand (1991) caution that certain infants with risk factors dike hyperbilirubinemia, very low birth weight, asphyxia night have symptoms similar to that of auditory dys-synchrony initial states but might come normal in follow up evaluation. Infants with such audiological findings should be cautiously diagnosed as having auditory, maturational delay (AMD) mother than diagnosing the as having AD.

Thus, it is important to differentiate AMD from AD, which is most often irreversible. Use of an appropriate to protocol wherein ALLR is also included is necessary to differentiate these conditions Several Movies have vouched for the utility in identifying AD in infants and children

McPherson, Lee, Yuen and Wong (2001) reported that fragment of LLR o complete LLR can obtained in infant even at birth. However, most of the studies using LLR have been carried out in adults and there is very little literature about LLR in infants to Identify AD. Though the literature on AD in adults suggests that may not  be sent to identify AD, not present in all adults with AD, it still might be a sensitive tool for infant to  Identity such a condition. McPherson, Lee, Yuen and Wong note that during neural development, dendritic arborization and synaptogenesis occur.  Due to auditory deprivation the synapse lose their function or die which might lead to absence of EUR n most of the adults with AD However, it is Like that measured during synaptogenesis one  might observe LLR in most of the infants.

Lee McPherson, Yuen and Wong (2001) reported of two school going  children with AD, one with ME absent and LLR present, while the other had presence of both MI RALLIN/23. They opined that I could be a useful tool to identity and differentiate different conditions in me children with risk factors

Coles and Mason (1984) used LLR for threshold estimation and fine tone bursts and they report a discrepancy as much 75 dB for an average of S00 Hz. 1 k12 and 2 kHz between the subjective and objective (LLR) threshold. ABR-based protocol (use of click rather than one list) ls so been used on the difficult to lose population under sedation by lytic (1997) It was suggested that 15 and conditions should be taken which plotting the behavioral thresholds. if tone is used Thus, the con LLR might help not only to solve paradox of differentiating AMD AD, but also help in establishing appropriate behavior threshold least with some degree of accuracy.

Thus, findings of studies reported in literature suggest that ALLR can be recorded in infant. However, ALLR should not only be used to predict threshold, it should also be used to differentiate individuals with normal hearing, permanent hearing loss, reversible hearing loss (AMD) and AD.

The absence of ALLR and ABR in at-risk infants can indicate auditor maturational delay AND as it might suggest delay in neural development due to risk factor Presence of ALLR with the of ADR le indicative AD. As the peripheral structure should develop prior m central auditory system Thus use of appropriate test protocol would enable this even in the younger population.

Singh Darman have described the importance of various test results which would help identify different conditions in infants and younger children It is evident from Table I that the test results in a AMD and AD are identical OR results are not included without the We of LLR and would have to wait until the maturation Tully occurred to make diagnosis of the actual condition.

Table 1: different test results and diagnosis based onthem.

					DIAGNOSIS
	AUDIOLOGICAL TEST
BOA	Immittance	OAE	ABR	LLR
Normal	A type	Present	Present	Present	Normal Hearing
Normal	A type	Present	Absent	Present	AD
Normal	A type	Present	Present/Absent /Abnormal	Absent	AMD
Abnormal	A type	Absent	Absent	Absent	Severe Hearing Loss

Conclusion:

The difficulty of differential diagnosis having symptoms similar to AD can be overcome when a test battery is used which includes pure-tone evaluation, ABR, OAE, CM and ALLR. The former three tests are more commonly used. Though recording of CM is rarely used, it is useful in certain conditions like the presence of middle car disorders and also while assessing individuals with long standing AD.

The absence of ALLR & ABR  can indicate auditory maturational delay (AMD), while presence of ALLR with the absence of ABR can be indicate of AD. Thus, the LLR can prove to be of immense importance in differential diagnosis of AD and AMD. Though routine audiological tests may indicate hearing sensitivity within normal limits, there still may be delayed maturation at higher cortical centers. This can be ruled out by the presence of ALLR. The ALLR can also be used a substitute for ABR to obtain threshold especially if ABR morphology is poor or if it is completely absent as in cases with AD.