ERA in non-organic hearing loss (NOHL)

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ERA in non-organic hearing loss (NOHL)



CLINICAL PROBLEM


A subject may exhibit deliberately false hearing thresholds or have an unconsciously raised threshold of response. Such subjects are categorized as having non-organic hearing loss (NOHL). Adult patients exhibiting NOHL tend to be exaggerators and malingerers, but they commonly exhibit a functional hearing loss which may have been unconsciously elevated; that is, the NOHL may be superimposed on a genuine hearing impairment. Not uncommonly children aged between 6 and 14 years can exhibit NOHL and the cause may generally be traced to family and/or school problems.


Verification of hearing loss and validation of the pure-tone audiogram is of special relevance in dealing with compensation claimants in medicolegal practice. Hearing threshold may be affected by numerous factors, including poor comprehension of the test requirements as a result of low intellectual or linguistic ability, fatigue, and temporary threshold shift due to recent noise exposure.


An important factor in detecting a bogus NOHL is a high initial index of suspicion on the part of the clinician or audiologist. A patient’s history, especially in medicolegal cases concerning compensation, may alert the otologist. Suspicion of a claimed NOHL can be based on the patient’s behaviour and discrepancies in the results of auditory tests such as pure-tone, speech, and impedance audiometry. Suspicious audiometric findings may include pure-tone and speech reception threshold differences greater than 10 dB and pure-tone thresholds close to or above the acoustic stapedial reflex threshold.


Having suspected the NOHL, one can administer a variety of tests to detect a bogus NOHL. These include the Lombard test, the Von Békésy test, and some other more complicated methods, which, in any case, do not reveal true hearing threshold. Threshold estimation from acoustic reflex measurement is not sufficiently accurate. Stenger’s test is useful when there is considerable unilateral NOHL.


The need for more precise assessment of the threshold at audiometric frequencies has drawn attention to ERA, of which SVR is the test of choice. Having suspected a patient of bogus NOHL after observing clinical and audiometric discrepancies, one may use numerous confirmatory tests for detection. Accurate estimation of audiometric threshold is the next stage in assessing NOHL, and ERA has a definite role to play.



ERA TESTS FOR NOHL


ERA tests for threshold estimation depend on subjective judgement of response presence or absence; therefore, proper technical experience is essential. SVR, MLR, ABR, and CAP recorded using electrocochleography can be employed in hearing threshold assessment. However, the tests differ in their frequency specificity, accuracy, and convenience.


SVR is the test of choice for verification of the pure-tone audiogram in order to exclude NOHL in adults and adolescents. This ERA method is the nearest to conventional audiometry. It is frequency specific and relatively insensitive to neurological dysfunction. Monaural stimulation is used. Usually air-conduction testing is done, and bone-conduction testing is rarely required. By testing low frequencies at high intensities, especially with a bone conductor on the skin, one can record a response to a vibro-tactile stimulus. SVR is thought to be enhanced by stimulus-orientated attention, and, therefore, the patient is instructed to pay attention. The deliberate exaggerator usually increases his attention because of anxiety, and SVRs are often very clear in patients who deliberately exaggerate hearing loss.


Using fairly long tone bursts of about 100 ms duration with a rise–fall time of 10 ms, one can construct an audiogram at selected low and high audiometric frequencies.


Stimuli are presented at a rate of 1/2 s, with the number of sweeps ranging from 20 to 40. At least two averages should be recorded at a threshold. Replication at a threshold will help to identify the peaks of the SVR and avoid false-positive response detection. High levels of EEG alpha rhythm make it very difficult to detect SVR reliably in about 5% of cases (Hyde et al 1986).


SVR-ERA is a lengthy procedure, and it is best to limit the number of frequencies tested. However, in most cases an audiogram at 500, 1000, 2000, and 3000 Hz can be estimated within an hour. For assessment, in a medicolegal case of an individual claiming compensation for occupational hearing loss, the frequencies 500, 1000, 2000, and 3000 Hz are essential, as combinations of them have been used to calculate the degree of disability.


Recording MLR using a stimulation rate of 40/s instead of the usual rate of 10/s, one can achieve superimposition of the peaks of MLR and augmentation of the response. When SVR measurement conditions are poor, the 40 Hz MLR gives more reasonable threshold estimates. However, when SVR conditions are good, 40 Hz MLR threshold estimates are the more variable of the two (Hyde et al 1986).


ABRs using clicks and short tone bursts have significant drawbacks as regards frequency specificity, and, therefore, their use in audiometric verification is limited. Some improvement in frequency specificity is possible by using ipsilateral notch masking or high-pass masking derived-response techniques. Only in a few instances where SVR is not obtained is ABR used for detection of NOHL. The averaging time is comparable to that of SVR.


Transtympanic ECochG gives the best signal-to-noise ratio. However, its limitation as regards frequency specificity is similar to that of ABR. Being an invasive procedure, ECochG should be avoided in assessment of NOHL in adults, especially in compensation claimants.

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Apr 10, 2017 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on ERA in non-organic hearing loss (NOHL)

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