ERA in neuro-otologic diagnosis

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ERA in neuro-otologic diagnosis




CHOICE OF ERA TESTS


The testing strategies depend on the clinical symptomatology and suspected site of lesion. Of all ERA tests, ABR is the most suitable test for this purpose. MLR and ECochG may contribute to establishing the site of the lesion in a specific situation; however, each test on its own is less valuable than in combination. Table 15.1 shows the effect of various pathologies on the EPs.



Standardization of ERA test procedures for neuro-otological strategies is desirable. This is especially relevant for ABR, which is the most important diagnostic tool, and the clinician has to rely on specific diagnostic parameters. ERA tests and recommended parameters for neuro-otological testing are shown in Table 15.2.



ECochG is used in neuro-otological testing in order to establish the latency of the potential generated at the cochlear nerve site. It becomes especially helpful when wave I of ABR cannot be recorded in cases of moderate and severe hearing losses. The waveform of the compound action potential and SP reflect the pathophysiologic status in the cochlear, and clearest responses are obtained at high intensity click stimuli. However, some investigators found this test on its own of little use in acoustic tumour detection, or in differentiating between cochlear and retrocochlear pathologies. Testing conditions, stimulus intensities, repetition rate, and polarity are the same as for ABR, but the recording band-pass filters are not the same. For ECochG, a broader band-pass filter of 10–3000 Hz is used.


When ABR is used for neuro-otological testing, the aim is to obtain clearly defined peaks of the waves, so that the latencies can be measured accurately. For CNS lesions, useful diagnostic information may be obtained from a comparison of both ipsilateral and contralateral (to the side of stimulation) ABRs. Hence, bilateral recordings should be used routinely. Monaural stimulation is used and the results compared with the normative data. However, if the other ear is not affected, it may act as a partial control for age and sex.


High intensity clicks are used in order to obtain all components of the ABR. Clicks of 80 dBnHL may be used in the presence of hearing loss up to 40 dBnHL at 4000 Hz. For greater hearing loss, a click stimulus of 90–100 dBnHL is more appropriate.


Stimuli at a rate of approximately 10/s do not affect the latencies and waveform of ABR; therefore, a rate of 10/s is used in a standard neuro-otological test procedure. At higher repetition rates, especially in the presence of high frequency hearing loss, the components of the ABR degrade, wave I in particular, and, hence, compromise the measurement of inter-peak intervals. The auditory system may be stressed by stimulating at higher rates of 50–70/s, and this may elicit more subtle abnormalities. Abnormal responses may suggest a retrocochlear disorder (Shanon et al 1981, Pratt et al 1981). Employing alternating polarity clicks seems to be a rational way of testing, because electrical polarity does not always correspond to acoustic polarity at the eardrum (Salt 1982). In the presence of steep high frequency hearing loss, the latency differences between stimulation with condensation and rarefaction clicks may be considerable (see p. 93).


An abnormal MLR may reveal brainstem lesions. It can be recorded in addition to the ABR, using the same electrode derivation. Usually rarefaction click stimuli of high intensity, at a rate of less than 10/s, are used, and the recording band-pass is 30–300 Hz.



INTERPRETATION OF ERA TESTS


A proportion of false-positive and false-negative data is possible, and, therefore, ERA should be interpreted in conjunction with other investigations. Accurate pure-tone audiograms and impedance audiometry are desirable prior to ERA tests. For the neuro-otologic population, criteria pointing to abnormalities in ERA tests are based on assessing the morphology of the responses, the prolongation of the latencies, and abnormal amplitude ratios.


Differentiation of cochlear from retrocochlear lesions relies mainly on ABR. A large proportion of neuro-otologic patients have mild or normal hearing on the pure-tone audiogram. The useful diagnostic parameters for detecting and interpreting abnormal ABRs are listed in Table 15.3.



An interaural latency difference of greater than 0.3 ms is considered abnormal, providing there is no evidence of hearing loss due to middle ear pathology. The inter-peak intervals are independent of conductive hearing loss, and separation into I–III and III–V intervals can sometimes suggest a lesion in the lower or upper brainstem.


The subjective assessment of ABR morphology is valuable; selective absence of late waves and 50% reversal of V/I amplitude ratio are good indications of a retrocochlear lesion. The V/I amplitude ratio is a less valuable parameter for retrocochlear assessment in the presence of cochlear hearing loss.


The MLR can provide additional information, as it has been reported that there is abnormal wave morphology in some patients with multiple sclerosis (Robinson & Rudge 1980).


Abnormally enhanced SPs, recorded by ECochG, support a clinical diagnosis of Ménière’s disease. Also, AP latencies recorded at the cochlear level in cases of moderate to severe hearing loss, together with the inter-peak intervals and wave V latency of ABR, can help to separate cochlear from retrocochlear lesions.


The latencies and amplitudes can be measured, and normative values should be established for every clinic. A typical diagnostic evaluation of a mean and two standard deviations predicts a false-positive rate of 5%. However, a greater percentage of false-positive errors can be accepted when using ABR as a screening test for patient selection and for further, more expensive or invasive, diagnostic radiological CT and MRI testing. Also, ABR can reduce the number of other audiometric tests and improve site-of-lesion diagnosis.



INVESTIGATION OF UNILATERAL HEARING LOSS


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Apr 10, 2017 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on ERA in neuro-otologic diagnosis

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