Auditory brainstem evoked response

Aust. Paediatr. J. (1985) 21, 73-74

Annotation Auditory brainstem evoked response JOHN BENCH School of Communication Disorders, Lirucoln Institute, Carlton, Victoria

The last twenty years have seen a surge of interest in auditory evoked physiological responses. This interest has in large part stemmed from a continuing wish by clinicians to have available objective audiometric tests for difficult-to-test patients in a climate where sophisticated developments in electronics and computing offered a prospect of making such tests valid, reliable and easy to administer. Some early tests explored auditory evoked encephalographic and galvanic skin activity. These tests proved generally unsatisfactory as standard clinical instruments, but in the early 1970s attention was focused on electro-cochleography. This appeared to offer real promise, as it sought to measure more directly the electrical activity of the proximal part of the auditory nervous system. When sound stimuli with fast onset times (such as 'clicks' or high frequency tones) are presented to a patient, electrodes located in various sites around the ear will record the cochlear microphonic, the compound action potential of the Vlll nerve, and a long duration summating potential. This, the basic technique of electro-cochleography, finds its best expression when a fine transtympanic electrode is placed near the round window. It is of course an invasive method, which renders it unsuitable for certain applications. However, it was expected that electro-cochleography would help clinicians to diagnose acoustic neuromas and other cerebello-pontine angle tumours, to distinguish between peripheral hearing disorders and neural hearing loss, and t o aid in the assessment of hearing loss in patients difficult to assess with conventional audiometry, such as infants and young children. For reasons for which space does not allow of discussion here, these aims have been met only partly. To a considerable extent the role anticipated for the rather direct method of electro-cochleography has been taken more recently by auditory brainstem evoked response (BSER). Here, surface electrodes on the scalp record brainstem potential changes t o click stimuli. Usually the averaged potential from several hundred up to one thousand or even more stimuli are obtained. The potentials last some 400 ms. By convention the potentials are usually recorded between the vertex and the ipsilateral mastoid. They are classified according to their respective latencies. Of the three main epochs of events (early, middle and late) detected by this technique, the early components, with latencies of about 2-1 0 ms and amplitudes in Correspondence: J. Bench, School of Communicatlon Disorders, Lincoln Institute, Carlton. Victoria.

the pV range, consist of seven waves. At least the first five of thlese seven wave components are thought to indicate the serial activity of brainstem auditory nuclei, and may be said to constitute the auditory brainstem evoked response proper. The latencies of the waves vary with the amplitude of the sound stimulus, with shorter latencies for stimuli of high amplitude. From animal studies, wave I is thought to arise from the fibres of: nerve VIII. Component II seemingly has its origin in the cochlear nucleus, and component 111 is considered to involve the superior olivary complex on the contralateral side (it disappears on midline section). The neurological locus of components IV to VII is less certain; it is possible that wave IV refers to activity in the ventral nucleus of the lateral lemniscus. Wave V is generally the largest BSER component, and it is fortmate for measurement that this wave is involved with most abnormal BSER. There is some variation between individuals in the response pattern of the components such that, at present, specific pathology cannot generally be ascertained reliably from inspecting the relationships between the components of the overall waveform. BSER is a non-invasive technique. It is relatively independent of changes in the state of overall arousal or activity, and is relatively unaffected by a range of sedatives and anaesthetics. In particular, these variables have little effect on the latency of wave V. BSER thusoffers the clinician the possibilityof reliable, artefact-free measures of auditory response not readily available with other measures of auditory evoked potentials. It is of particular interest to those who work with infants and young children, with the mentally handicapped, and with those patients for whom conventional audiometry produces confused, ambiguous or conflicting findings. It is generally accepted that hearing loss in infancy and early childhood may have serious effects on the development of language and on social, emotional and cognitive development. It is thus important to have available methods for the early detection of hearing loss which will lead to diagnosis of hearing imlpairment much earlier than at the age of three years, an all too common age at present.' BSER has been successful in assessing auditory responses in neonates, but this very suiccess has given rise to questions of cost effectiveness when, folr example, it is applied as a standard test to 'at risk' babies. The cost of professional time expended on analysing response traces2 is a particular matter of concern. Attention is now tuirning towards fully objective method^,^ in which presentation of sound stimuli, and the recording and analysis of responses may be automated.

J. Bench

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A particularly problematical area in work with infants and young children is that aspect of aural rehabilitation where the appropriate gain for a hearing aid needs to be determined. Unless the infant's hearing loss is known, the gain may be set to high or too low, with a risk of excessive or inadequate amplification. BSER shows considerable promise for this area since it offers the prospect of assessing the infant's hearing loss with a precision difficult t o achieve previously. BSER has also found favour amongst audiometric tests for the detection of both brainstem neoplasms and demyelinating disease (though at present BSER cannot reliably distinguish between them). There is clear advantage to the detection of tumours before they grow to cause neurological signs. Tumours removed while less than 2 cm in diameter involve negligible surgical morbidity and little postoperative d i ~ a b i l i t y . ~ BSER appears to be the best single test of its type employed so far as regards validity and reliability for the detection of acoustic tumours since, if there is compression of the auditory nerve, there is an increase in the latency of the response which is usually detected readily. An effective measure is to assess the time interval between components 111 and V. Whereas normally this interval is of the order of 1.9kO.1 ms, an interval of 2.1 to 2.8 ms has been noted in some 70% of one group of 55 patients with acoustic tumours of 3 cm or Note, it is important to exclude the possibility.of conductive hearing loss in assessing the effects of an acoustic tumour, since conductive losses produce shifts in latency similar to those originating with a tumour.

Research in auditory BSER proceeds apace, and many useful applications may be expected. For example, it has been suggested that BSER could be used to assess brain death.6 Here, BSER would be used together with middle ear acoustic reflex tests and otological examination, to exclude the possibility that absent BSER responses are due to hearing

loss. The reader is referred to Dobie (1980)7 for a lucid summary of the auditory BSER field. REFERENCES 1 Martin J.A.M. Childhood Deafness in the European Community. Commission of the European Communities. Luxembourg 1979. 2 Despland P A . . Galambos R. The brainstem auditory evoked potential is a useful diagnostic tool in evaluating risk factors for hearing loss in neonatology. In Courjon J.. Mauguiere F.. Revol M. eds. Clinical Applications of Evoked Potentials in Neurology. Raven Press, New York. 1982; 241-7. 3 McCleiland R.J.. Sayers B.McA. Towards fully objective evoked response audiometry. Br. J. Audiol. 1983; 17: 263-70. 4 Prasher 0.K , Gibson W.P.R. Brainstem audltory-evoked potentials and electrocochleography: Comparison of dlfferent criteria for the detection of acoustic neuroma and other Cerebello-pontlne angle tumours. Br. J. Audiol. 17: 163-74. 5 Brackmann D.E.. Sheehy J.L. The neuro-otologic evaluation. in Keith R.W. ed., Audiology for the Physician. Williams 8 Wilkins, BaltirnoreILondon. 1980; 199-212. 6 Starr A S. Auditory brain stem responses in brain death. Brain 1976; 99: 543-54. 7 Dobie R.A. Physiological techniques used in the assessment of the auditory system. In Keith R.W. ed.. Audiology for the Physician Williams and Wilkins. BaltimorelLondon. 1980; 109-42.