Indian J Pediatr 1992; 59 : 361-366

Auditory Brainstem Evoked Responses in Infants and Children Ramesh C. Deka

77~e Evoked Potential Laboratory, Department of Otodffnola~.,ngolo~,, All India b~stitute of Medical Sciences, New Delhi During the last decade auditory brainstem evoked responses (ABR) have been used extensively in clinical medicine. It has greatly contributcd in two major areas viz, audiologic evaluation in infants and children, especially in those with risk factors and ncuroaudiologic evaluation of the 8th ncrve and brainstcm function in variety of ncurologic conditions. Since Hecox and Galambos ~ first reported in their paper about the successful application of ABR in the audiologic evaluation in children, several articles on this have appeared, rio Results of the use of ABR in difficult-to-test children for auditoryevaluation were gratifying in providing early diagnosis and institution of rehabilitation measures, s Presently, besides its use in evaluation of hearing function at the earliest, several other neurologic conditions have attracted its application, T M eg. sudden infant death syndrome (SIDS) and near miss for sudden infant deaths episode (NMSID). "~'~6Recent evidences suggest that S1DS is due to failure of central respiratory control mechanisms during sleep. ~'t7 The proximily of these respiratory centres to the Reprint requests : Dr. R.C. Deka, Additional Professor, Department of Otolaryngology, All India Institute of Medical Sciences, Ansari Nagar, New Delhi-I l0 029.

auditory brainstem pathways is the basis of application of ABR in its early detection in suspect apenic infants. ABR thus can provide the objective evidence of an underlying brainstem lesion in apenic conditions. Wave Generator Site Although Sohmer and Feinmesser TM first in 1967, described electrical recordings of the 8th nerve, it was Jewetd 9 who successfully demonstrated seven upward deflections of submicrovolt amplitude during the first 10 msec period following click stimulation. Such minute electrical rcsponses were extracted with the help of the computer at the scalp from the background electroencephalogram activity. Thc electrical potcntlals generated at the cochlear nerve and the brainstem, spread through the conducting media of the brain, 'meninges, spinal fluid and skull-bones and appear at the scalp as evoked potentials. With the newer generation of computers today, it is possible to have variety of far-field scalp averaging system for clinical use. Today we have enormous animal and human experimental t as well as clinical5 7.11,19data on this subject. Clinical studies ~3'~ have corroborated the location of the brainstem generator-sites, described as follows :

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1. Auditory nerve: Wave I--Sth nerve action potential 2. Brainstcm level: Wave ll-Cochlear nucleus Wave Ill-Superior olivery nucleus Wave IV-Lateral Leminiscus, Wave V-Inferior collieulus Wave VI & VII-Not well understood. PATHOPHYSIOLOGICAL ASPECTS Latency Abnormalities Peripheral defect. In peripheral hearing loss, absolute latency of Wave I increases but the interpeak latencics (IPL) of wave I-III, wave III-V and wave I-V remain normal. Central conduction defects. Segmental demyelination, axonal and neuronal loss and degenerative changes occur as a result of diseasc or pressure effect on and/or due to poor blood supply to the 8th nerve and brainstem auditory pathways. lnterpeak Latency Abnormality (IPL) Wave I-Ill IPL abnormality. When wave l-III IPL values increase by more than mean + 2 SD, this suggests a conduction defect between the 8th nerve close to the cochlea and the lower pontine region, cg. acoustic nerve turnout.

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Absence of wave IV and/or V. Brainstem lesions such as tumour or degenerative conditions like anoxaemia, kernicterus t3 etc., can produce such ABR abnormality ipsilateraliy as well as contralatcrally. Amplitude Abnormality

Loss of amplitude. The loss of amplitude is assumed to be due to less number of fibres conducting the volley and desynchronisation of the volley, secondary to widely different velocitics. Degeneration due to kernicterus and anoxia and brainstem turnout 13,14 often produce amplitude abnormalities in wave IV and/or V. Amplitude Ratio Wave V : I amplitude ratio. It is usually more than 1 in normal condition but its alternation occurs as a result of degeneration, pressure effect and/or poor supply, m4 to the brainstem.

Laterality of ABR Abnormality Unilateral or bilateral changes. Unilateral changes at ABR are usually more prominent as a result of unilateral diseases. Even in intrinsic brainstem lesion, unilateral ABR changes may be more discernible. Abnormality seen at ABR are however not etiologically specific, but does give indications about the site and extent of the lesion. Clinicians have to employ, detailed clinical history and examination, other investigation like ENG, attdiometry, imaging radiology and related neurological investigations, besides ABR to arrive at a diagnosis.

Wave IlI-V IPL abnormality. If wave IlI-V 1PL values increase by more than mean + 2 SD, this suggests a conduction defect between the lower pons and the midbrain. Such defcct occurs usually in brainstem lesion including tumour, or as a result of pressure effect on the brainstem by cerebellar tumour. Brainstem gliomas, ~5 anEQUIPMENTS AND MEASUREMENT oxaemia and/or kernicterus ~3 and SIDS 3'16 TECHNIQUE also produce such central conduction defect in the brainstem auditory pathways. The basic aspect of equipments, operational

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methods, stimulus types, response-evaluation and its clinical interpretation can be. read elsewhere.'.~2v~'~9 Stimulus. For audiologic cvaluation most commonly used stimuli are (a) tone-pips~ and (b) filtered clicks5-7 and for ncurologic conditions it is the clicks or the filtered clicks at 4 KI-Iz?-13

conventional pure-tone-audiometry. Hence, a factor of correction by about 10-15 dB is required in interpreting hearing function with ABR in infants and childrcn.4~':3

Stimulus rate. Most commonly employed rate is 10-20/sec. 5-7 However for audiologic purposes in infants and children, rate can be increascd upto 30-50/secr ~ which also provide better resolution. Polarity. In noisy situation, an alternate condensation and rarefaction stimulus may help to avoid noise stimulus artifact,sTa3 Sound proof room. For purely audiologic purposes, adequate sound-proof room is needed; otherwise ABR can be performed in quiet room or even at bed side in the hospital. Infant and new born testing. In infant testing, the head phone should bc kept near the side of the ear under test since it is painful to tightly keep a head-phone. This may result in reduction of about 5-10 dB intensity of the sound stimulus at the drum. s,~3 Resolution ofwaveform. To get better resolution of wave-forms stimulus, repctition may be upto 2000, and stimulus intensity may be reduced from 80 dBSL to 60 dBSL. Interpretation of latency and amplitude. Beh)rc making analysis of wave latcncy (both peak and intcrpeak latencics) and amplitude, atlcast two consistent readings must be obtained and only then data interprctcd and analysed. It must be remembered while asscssing hearing threshold in children that the ABR threshold in terms of wave V appearance in response to a minimum intensity-stimulus, is higher by about 20 dB than the one at

The cochlea and the 8th nerve, are fully developcd at birth, although myelination of the nerve and maturation of the auditory brainstem pathways are yet to be completed. By 6 months of age, thc myelinatitm becomes evident and its maturation progrosses till the age of 2 years when it bccomcs complete and comparable to adult statc (Figure 1). Sound stimulates the process of brain maturation, and dcprivation of sound in early period of life before 2 years may prevent maturation of auditory brainstem pathways and also prevent other related brain function? -~.~ Neonatal period. The ABR has been extcnsivcly used in early asscssment of hearing defect and other possible neurologic handicaps in neonates and infants in the neonalal intensive carc unit. ~~ Incidence of sensorineural loss is very high in this group of children. 5's'I~ Hence all infants in the intensive care unit should be given an ABR test. For screening purposes, a 40 dB HL, 2 and 4 KHz filtcrcd click stimulus can be givcn monoaurally and it normally shows the wave V after 6 or 7 msee depending on age of the child. Longitudinal studies a-s have provcd the value of ABR in predicting hearing threshold in at risk infants. Audiologic evaluation for early detection of hearing loss is important, since it influences the development of speech and auditory skills. On early detection, adequate rehabilitation measures like hearing aids, auditory training and spcech

PEDIATRIC APPLICATION OF ABR l)evelopment o1"Auditory Pathways

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Fig. 1. Effect of myelination and maturation on cochlea, 8th nerve and auditory brainstem during the early 2 years of life (latency-age cu~,e) as observcd at ABRs. Sample ABRs at different ages are also shown. skills, can be instituted. It helps in selection of patients for hearing aids and cochlear implantation. Several conditions during new born period may be subjected to ABR, they include: (a) hyperbilirubinemia (b) kernicterus (c) perinatal anoxia (d) prcmaturity (e) cerebral palsy (f) suspected mental retardation (g) hydrocephaly (h) microcephaly (i) congenital rubella and (j) sever',d metabolic disorders like non-kctotic hyperglycemia, maple syrup urine disease, phcnyl ketonuria and thiamine metabolism disorderJ 3 ABRs in many of these conditions not only help us to determine the hearing rune-

tion, it also establishes the neuropathological changes that might be associated with the disease or disorders. ABRs are employed in the following commonly seen neurologic conditions in children: Brainstem gliomas. This is a common tumour in children. Although it is a midline lesion, it produces more prominent ABR findings unilaterally. 4a5 Opposite side may also show ABR abnormality. Cerebellar meningiomas. This disease can produce pressure effect on the brainstem and results in abnormal ABRs. Large turnout can also produce ABR abnormality on tile contralatcral side. 7''4

I)EKA : ABR IN CHILDREN Conm. Sanders 2~ studied 17 comatose children with different ctiologies, gross ABR abnormality was reported and its predictive value was emphasiscd. Epilepsy. ABR abnormalities in the auditory brainstem pathways were reported in epilepsy. Effect of anticonvulsants like carbamazepine were also studied. 13 Drug effects and ototoxicity. Effects of arninoglycoside, barbiturate, alcohol etc., on the cochlea and auditory brainstem pathways are known ~3 with ABR, it becomes possible to monitor such drug effects. Animal and human studies have confirmed these effects. ~3 Since in pediatrics such antibiotics are used to contain septaeemia, ABR can be employed to monitor its toxic effects on the auditory system and the drug can be withdrawn immediately at the event of ABR abnormality being noticed. Adrenoleucodystrophy. The disease process and its progress can be monitored with ABR. z3 ltead injnry. Effccts of head injury in the inner ear and brainstem can be evaluated by A B R . 7't3 Sudden infant death syndrome and apnea. ABRs have provided better insight into the pathophysiology of this otherwise ill understood condition) Episodes of prolonged sleep apnea without any clinically detected factors, and producing at the same time clinical evidence of hypoxemia in the form of cyanosis and flaccidity, warranting resuscilation measure in an infant, bcfore spontaneous ventilation prevails, are termed "near miss-sudden infant death (NMSID) syndrome". There are today, considerable evidence that such an infant may go into SIDS rcsulting in a crib-death) Proximity of the auditory brainslcm pathways to the respiratory centres in the pons becomes the basis of employing ABR to elucidate the

.%5 brainstem lesion that may cause apenic episodes in infants. On the basis of availablc data on ABR abnormalities in NMS1D infants,3aeav it can be assumed that ABR should prove useful in identifying infants at risks for SIDS and thus attempt is on to prevent crib death due to SIDS. Although other methods like assessment of alveolar ventilation 2L and polygraphic evaluation 22 are reported to hold more promise in terms of therapeutic implication i.e. prevention, ABRs do provide valuable data in NMS1D syndrome and it is contributing to the belter understanding of the disease. ACWO~EtmEMEt,"rs The author gratefully thanks Professor S.K. Kacker, for his kind encouragement. This work was partly supported by AIIMS research grant and also a grant from the Neuro-Otological and Equilibriometric Society of India. Thanks are also due to Mr. E. Soren for his help in the laboratory. REFERENCES 1. Hecox K, Glambos R. Brainstem auditory cwoked responses in human infants and adults. Atvh Otolatyngol 1974; 99 : 30-33. 2. Starr A, Amile RN, Martin WH. Sanders S. Development of auditory, function in new born infants revealed by auditory brainstem potentials. Pediat/ics 1977; 60 : 831-839. 3. Stockard JJ. Brainstem auditory evoked potentials in adult and infant sleep apnea syndromes, including sudden infant death syndrome and near-miss for sudden infanl death. Ann N Y Acad'Sci 1982; 388 : 443465. 4. Suzuki Jl, Kodera K, Kaga K. Auditou.'y evoked brainstem response assessment in Otolaryngology. Ann N Y Acad Sci 1982; 388 : 487-500. 5. Deka RC, Deka D, Kacker SK. Auditory

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6.

7.

8.

9.

10.

11.

12.

13.

14.

TIlE INDIAN JOURNAl. OF PEDIATRICS brainstem evoked response in at-risk children. h~dian J Pediatr 1986; 53 : 651-656. Deka RC, Deka D, Kackcr SK. Maturation of cochlea, auditory, nerve and brainstem as observed by auditory evoked potentials in human infants. Indian J Otolaryngol 1986; 38 : 56-58. Deka RC. Auditory brainstem evoked potentials. In : Tandon PN, Bijlani V, Wadhwa S, eds. Lectures bt Neutr~biolog); New Delhi, Bombay, Calcutta : Wiley Eastern Ltd., 1989; pp 100-103. Kaga K, Kitazumi E, Kodama K. Auditory. brainstcrn responses of kernicterus infants. h,erJPediatric 1979; 1:255-2+64. Davis H. Principles of electric respon~ audiometry. Ann Otol Rhh~ol Latyngol 1976; 85 ; (Suppl 28) 4-96. Galambos R. The BER in the ICU. in : Nautal RF, Fernandez C, eds. Evoked Electrical Activity hz Auditory Nervous System, New York, SanFrancisco, London: Academic Press, 1978 : pp 405-408. Starr A, Achor I.d. Auditor)" brainstcm responses in neurological disease. Atvh Neurol 1975; 32 : 761-768. Brackmann DE. Electric response audiomctry in a clinical practice. Lalyngoscope 1977; 37 : (Suppl 15) 1-33. Chiappa KH. Evoked Potentials hl Clinical Medicine. New York : Raven Press, 1983; pp. 106-202. Deka RC, Kacker SK, Tandon PN. Auditor), brainstem evoked responses in cere-

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15.

16.

17.

18.

19.

20.

21.

22.

bellopontile angle tumours. Aivh Otolao,p~gol ltead Neck Surg 1987; 113:647-650. Nodar RH, Hahn J, Levine HL. Brainstern auditory evoked potemial in determiniog the site of lesion of brainstem gliomas in children. Laryngoscope 1980; 90 : 258-266. Nodar R, Lonsdale D, Orlowski J. Abnormal brainstem auditor), evoked potentials in infants with threatened sudden infant death syndrome. Otolao,ngol ltead Neck Surg 1980; 88 : 619-621. Weilzman ED, Graziani L. Sleep and sudden infant death syndrome. A new hypothesis. Adv Sleep Res 1974; 1 : 327344. Sobmer H, Feinmesser M. Cochlear action potentials recorded from the external ear in man../Inn Otol Rhh~ol Lao,ngol 1967; 67 : 427-435. Jeweet DE, Williston .IS. Auditor)" evoked far-field averaged potentials from scalp of humans. Brahz 1971; 94 : (~1-696. Sanders RA, Smriga D J, McCullough DW, Duncan PG. Auditory brainstem responses in patients v,fith global cerebral insults. J NeuJosurg 1981; 55 : 227-236. Shannon DC, Kelly DH, O'Connel K. Abnormal regulation of ventilation in infants at risk for sudden-risk death syndrome. New EnglJ Med 1977; 297 : 747-750. Guilleminault C, Ariagno R, Souquet M, Demant WC. Abnormal polygraphic findings in near miss sudden infant death. Lancet 1976; i : 1326-1327.